package checker

import "../ast"
import "../hir"
import "../source"
import "../symbol"
import "../target"
import "../types"
import "core:fmt"
import "core:mem"
import "core:slice"
import "core:strings"

Spec_Id :: distinct u32
INVALID_SPEC :: Spec_Id(0xffff_ffff)

spec_id :: proc(index: int) -> Spec_Id {
	assert(index >= 0 && u64(index) < u64(INVALID_SPEC))
	return Spec_Id(index)
}

spec_index :: proc(id: Spec_Id, count: int) -> (int, bool) {
	index := int(id)
	return index, id != INVALID_SPEC && index < count
}

Spec :: struct {
	template: ast.Function_Id,
	args:     []types.Type,
	comptime_values: []Comptime_Value,
	result:   types.Type,
	hir_id:   hir.Function_Id,
}

Infer_Local :: struct {
	name:      symbol.Id,
	type:      types.Type,
	declared:  types.Type,
	statement: ast.Stmt_Id,
	mutable:   bool,
	// open_const/open_float mark a local whose initializer is an unannotated numeric
	// constant: like an open-constant global, it can adopt a backward demand from use.
	open_const:  bool,
	open_float:  bool,
	const_value: i128,
	demanded:    bool,
}

Build_Local :: struct {
	name:    symbol.Id,
	type:    types.Type,
	mutable: bool,
	id:      hir.Local_Id,
}

Static_Binding :: struct {
	name:  symbol.Id,
	type:  types.Type,
	value: Ct_Value_Id,
}

Expand_Expansion :: struct {
	statement: ast.Stmt_Id,
	index:     u32,
}

Entry_Point_Kind :: enum u8 {
	Invalid,
	Plain,
	Process,
}

// Build_Ctx threads the per-function accumulators through build_block so that
// nested control-flow blocks (if/else) can be built recursively. `locals` is a
// scope stack: each block records its entry length and truncates back to it on
// exit, while `hir_locals` keeps every allocated slot for the function.
// A labeled value-loop currently being built. A `yield :label x` inside the loop
// body assigns `x` to the loop's result `slot` (typed `slot_type`) and `break`s.
// Pushed by `build_value_loop` while its body is built; innermost is last.
Yield_Target :: struct {
	label:           symbol.Id,
	slot:            hir.Local_Id,
	slot_type:       types.Type,
	// True when the loop also yields `none` (a `{T, none}` set → `?T`); set from a
	// pure-AST scan, used to pick the slot's element type on the first concrete yield.
	result_optional: bool,
	// `len(defers)` when this target's body began; a `yield :label` flushes defers down
	// to here before breaking, so an outer-loop / value-block yield runs inner defers too.
	defer_floor:     int,
}

Defer_Entry :: struct {
	body:       []hir.Stmt_Id,
	error_only: bool,
	capture:    hir.Local_Id,
}

Build_Ctx :: struct {
	checker:      ^Checker,
	pkg:          ast.Package_Id,
	file:         ast.File_Id,
	result:       types.Type,
	local_types:  []types.Type,
	locals:       ^[dynamic]Build_Local,
	hir_locals:   ^[dynamic]hir.Local,
	local_spans:  ^[dynamic]source.Span,
	local_used:   ^[dynamic]bool,
	local_warnable: ^[dynamic]bool,
	global_reads: ^[dynamic]hir.Global_Id,
	calls:        ^[dynamic]hir.Function_Id,
	problematic:  ^bool,
	// Stack of labeled value-loops being built (innermost last); see Yield_Target.
	yield_targets: ^[dynamic]Yield_Target,
	// `defer` lowering. Deferred statements are built once at the `defer` site and
	// their hir stmt ids stored here as a flat stack across scopes (one entry per
	// deferred statement); they are replayed (appended) at each scope exit in LIFO
	// order. `loop_defer_starts` records `len(defers)` at each enclosing loop body
	// entry: `break`/`continue` flush down to that mark (and need `len > loop_floor`
	// to be valid). `defer_depth`/`loop_floor` guard control flow inside a deferred
	// statement: `return` is rejected while `defer_depth > 0`, and `break`/`continue`
	// only see loops opened within the defer (those past `loop_floor`).
	defers:            ^[dynamic]Defer_Entry,
	loop_defer_starts: ^[dynamic]int,
	// Parallel to `loop_defer_starts`: the label of each enclosing break target (INVALID
	// when unlabeled), so a `break :L` / `continue :L` can target an outer one. A labeled
	// block statement is a break target too; `loop_is_loop` distinguishes loops (which
	// `continue` and unlabeled `break`/`continue` target) from value/labeled blocks.
	loop_labels:       ^[dynamic]symbol.Id,
	loop_is_loop:      ^[dynamic]bool,
	defer_depth:       int,
	loop_floor:        int,
}

Function_Index_Entry :: struct {
	scope: ast.Package_Id,
	file:   ast.File_Id,
	hidden: bool,
	name:  symbol.Id,
	id:    ast.Function_Id,
}

Global_Index_Entry :: struct {
	scope: ast.Package_Id,
	file:   ast.File_Id,
	hidden: bool,
	name:  symbol.Id,
	id:    ast.Global_Id,
}

Import_Index_Entry :: struct {
	scope: ast.File_Id,
	name:  symbol.Id,
	id:    ast.Import_Id,
}

Type_Factory_Entry :: struct {
	template:  ast.Function_Id,
	values:    []Comptime_Value,
	result:    types.Type,
	resolving: bool,
}

Generated_Type_Entry :: struct {
	expr:   ast.Expr_Id,
	values: []Comptime_Value,
	result: types.Type,
	pkg:    ast.Package_Id,
	file:   ast.File_Id,
	defaults: []Ct_Value_Id,
}

Resolved_Field_Default :: struct {
	expr:         ast.Expr_Id,
	pkg:          ast.Package_Id,
	file:         ast.File_Id,
	static_value: Ct_Value_Id,
	span:         source.Span,
}

Type_Factory_Origin :: struct {
	result:   types.Type,
	template: ast.Function_Id,
	values:   []Comptime_Value,
}

Call_Resolution :: struct {
	expr:            ast.Expr_Id,
	ctx:             []Comptime_Value,
	expand_ctx:      []Expand_Expansion,
	mapping:         []int,
	comptime_values: []Comptime_Value,
	runtime_types:   []types.Type,
}

Checker :: struct {
	ast_module:           ^ast.Module,
	diagnostics:          ^source.Diagnostics,
	symbols:              ^symbol.Table,
	module:               hir.Module,
	specs:                [dynamic]Spec,
	function_index:       []Function_Index_Entry,
	global_index:         []Global_Index_Entry,
	import_index:         []Import_Index_Entry,
	global_types:         []types.Type,
	// Backward type-demand state for open-constant globals (milestone 14). global_demands
	// accumulates demands reachable from any use (other globals' initializers and function
	// bodies); global_demands_dirty lets a demand pushed from a function body re-trigger the
	// inference fixpoint.
	global_demands:       []types.Type,
	global_open_const:    []bool,
	global_open_float:    []bool,
	global_const_value:   []i128,
	global_demands_dirty: bool,
	// Program-wide inference for direct constraint fields in named native records.
	// The arrays use Store.fields' existing dense indices; resolved concrete types
	// live directly in module.types.fields so layout and lowering need no side table.
	record_field_constraints: []types.Type,
	record_field_defaults:    []types.Type,
	record_field_conflicts:   []types.Type,
	record_field_conflict_spans: []source.Span,
	record_field_demands_dirty: bool,
	poisoned_packages: []bool,
	external_global_canonical: []ast.Global_Id,
	external_global_diagnostics: []source.Diagnostic_Id,
	constants:            []Constant,
	template_diagnostics: []source.Diagnostic_Id,
	constant_stack:       [dynamic]Constant_Frame,
	ast_expr_stack:       [dynamic]ast.Expr_Id,
	hir_expr_stack:       [dynamic]hir.Expr_Id,
	infer_stack:          [dynamic]Infer_Frame,
	build_stack:          [dynamic]Build_Expr_Frame,
	cycle_stack:          [dynamic]Cycle_Frame,
	// Anonymous globals synthesized for `&<array literal>` (Zig's `&.{...}`). Staged
	// here during global/function building and flushed into module.globals AFTER
	// build_globals, so the 1:1 module.globals <-> ast.globals index identity holds.
	anon_globals:         [dynamic]hir.Global,
	main_symbol:          symbol.Id,
	entry_point:          Entry_Point_Kind,
	io_provider_template: ast.Function_Id,
	sink_symbol:          symbol.Id,
	type_symbol:          symbol.Id,
	current_result:       types.Type,
	inferred_test_error:  ^types.Type,
	current_build_ctx:    ^Build_Ctx,
	current_comptime_values: []Comptime_Value,
	static_state:            Ct_State,
	static_bindings:         [dynamic]Static_Binding,
	comptime_keys:           [dynamic]string,
	comptime_static_values:  [dynamic]Ct_Value_Id,
	expand_context:          [dynamic]Expand_Expansion,
	type_factories:       [dynamic]Type_Factory_Entry,
	generated_types:      [dynamic]Generated_Type_Entry,
	type_factory_origins: [dynamic]Type_Factory_Origin,
	call_resolutions:     [dynamic]Call_Resolution,
	target:               target.Target,
	allocator:            mem.Allocator,
}

symbol_text :: proc(checker: ^Checker, id: symbol.Id) -> string {
	return symbol.resolve(checker.symbols, id)
}

append_tracked_local :: proc(
	locals: ^[dynamic]hir.Local,
	spans: ^[dynamic]source.Span,
	used: ^[dynamic]bool,
	warnable: ^[dynamic]bool,
	local: hir.Local,
	span: source.Span,
) -> hir.Local_Id {
	id := hir.local_id(len(locals^))
	append(locals, local)
	append(spans, span)
	append(used, false)
	append(warnable, true)
	return id
}

append_build_local :: proc(
	ctx: ^Build_Ctx,
	name: symbol.Id,
	type: types.Type,
	mutable: bool,
	span: source.Span,
) -> hir.Local_Id {
	id := append_tracked_local(
		ctx.hir_locals,
		ctx.local_spans,
		ctx.local_used,
		ctx.local_warnable,
		hir.Local{name=name, type=type, mutable=mutable},
		span,
	)
	append(ctx.locals, Build_Local{name=name, type=type, mutable=mutable, id=id})
	return id
}

ignore_tracked_locals :: proc(ctx: ^Build_Ctx, start: int) {
	for i := start; i < len(ctx.local_warnable^); i += 1 {
		ctx.local_warnable^[i] = false
	}
}

mark_local_used :: proc(checker: ^Checker, id: hir.Local_Id) {
	ctx := checker.current_build_ctx
	if ctx == nil || id == hir.INVALID_LOCAL {
		return
	}
	index := int(id)
	if index >= 0 && index < len(ctx.local_used^) {
		ctx.local_used^[index] = true
	}
}

build_local_expr :: proc(checker: ^Checker, local: Build_Local, span: source.Span) -> hir.Expr_Id {
	mark_local_used(checker, local.id)
	return add_hir_expr(checker, hir.Expr{
		kind=.Local, span=span, type=local.type, target=hir.local_ref(local.id),
		left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

block_reads_name :: proc(checker: ^Checker, statements: []ast.Stmt_Id, name: symbol.Id) -> bool {
	// Imported C expressions can be deeply nested, so walk the source graph iteratively.
	statement_stack: [dynamic]ast.Stmt_Id
	statement_stack.allocator = checker.allocator
	defer delete(statement_stack)
	expr_stack: [dynamic]ast.Expr_Id
	expr_stack.allocator = checker.allocator
	defer delete(expr_stack)
	append(&statement_stack, ..statements)

	for len(statement_stack) > 0 || len(expr_stack) > 0 {
		if len(statement_stack) > 0 {
			statement_id := pop(&statement_stack)
			if statement_id == ast.INVALID_STMT || int(statement_id) >= len(checker.ast_module.statements) {
				continue
			}
			statement := checker.ast_module.statements[statement_id]
			#partial switch statement.kind {
			case .Declaration, .Assignment, .Return, .Expression, .Yield:
				append(&expr_stack, statement.expr, statement.target)
				append(&statement_stack, ..statement.body)
			case .If:
				append(&expr_stack, statement.expr, statement.guard)
				append(&statement_stack, ..statement.body)
				append(&statement_stack, ..statement.else_body)
			case .While:
				append(&expr_stack, statement.expr)
				append(&statement_stack, ..statement.body)
				if statement.update != ast.INVALID_STMT {
					append(&statement_stack, statement.update)
				}
			case .For:
				append(&expr_stack, statement.expr)
				append(&statement_stack, ..statement.body)
			case .Block:
				append(&statement_stack, ..statement.body)
			case .Defer:
				if statement.update != ast.INVALID_STMT {
					append(&statement_stack, statement.update)
				}
			case .Match, .Match_Arm:
				append(&expr_stack, statement.expr)
				append(&expr_stack, ..statement.patterns)
				append(&statement_stack, ..statement.body)
			case .Break, .Continue, .Invalid:
			}
			continue
		}

		expr_id := pop(&expr_stack)
		if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
			continue
		}
		expr := checker.ast_module.exprs[expr_id]
		if expr.kind == .Name || expr.kind == .Call {
			if !symbol.is_valid(expr.qualifier) && expr.name == name || expr.qualifier == name {
				return true
			}
		}
		switch expr.kind {
		case .Call, .Array, .Struct_Literal, .Slice:
			append(&expr_stack, ..expr.args)
			append(&expr_stack, expr.left)
		case .Negate, .Not, .Bit_Not, .Address, .Deref, .Field, .Unwrap, .Try, .Keyed, .Enum_Literal, .Cast:
			append(&expr_stack, expr.left)
		case .Comptime:
			append(&expr_stack, expr.left)
			append(&statement_stack, ..expr.body)
		case .Catch:
			append(&expr_stack, expr.left, expr.right)
			append(&statement_stack, ..expr.body)
		case .Add, .Sub, .Mul, .Div, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left, .Shift_Right,
		     .Shift_Left_Saturating, .Index, .Orelse, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
			append(&expr_stack, expr.left, expr.right)
		case .Invalid, .Integer, .Float, .String, .Bool, .None, .Undefined, .Inference_Hole,
		     .Type, .Name, .Function_Literal, .Anonymous_Struct_Type:
		}
	}
	return false
}

record_unused_locals :: proc(
	checker: ^Checker,
	locals: []hir.Local,
	spans: []source.Span,
	used: []bool,
	warnable: []bool,
) {
	for local, index in locals {
		if local.name == checker.sink_symbol ||
		   !symbol.is_valid(local.name) ||
		   index >= len(warnable) ||
		   !warnable[index] ||
		   index >= len(used) ||
		   used[index] {
			continue
		}
		span := source.Span{}
		if index < len(spans) {
			span = spans[index]
		}
		if local.parameter {
			source.addf_warning(checker.diagnostics, span, "unused parameter '%s'", symbol_text(checker, local.name))
		} else {
			source.addf_warning(checker.diagnostics, span, "unused local '%s'", symbol_text(checker, local.name))
		}
	}
}

write_type_label :: proc(checker: ^Checker, builder: ^strings.Builder, value: types.Type) {
	store := &checker.module.types
	item, ok := types.node(store, value)
	if !ok {
		strings.write_string(builder, types.name(value))
		return
	}
	if item.name != 0 {
		strings.write_string(builder, symbol_text(checker, symbol.Id(item.name)))
		return
	}

	switch item.kind {
	case .Array:
		strings.write_byte(builder, '[')
		if item.inferred_count {
			strings.write_byte(builder, '_')
		} else {
			fmt.sbprintf(builder, "%d", item.count)
		}
		if item.has_sentinel {
			fmt.sbprintf(builder, ";%d", item.sentinel)
		}
		strings.write_byte(builder, ']')
		if item.mutable {
			strings.write_string(builder, "mut ")
		}
		write_type_label(checker, builder, item.child)
	case .Pointer:
		if item.has_sentinel {
			fmt.sbprintf(builder, "[*;%d]", item.sentinel)
		} else {
			strings.write_byte(builder, '*' if item.many else '@')
		}
		if item.mutable {
			strings.write_string(builder, "mut ")
		}
		write_type_label(checker, builder, item.child)
	case .Slice:
		if item.has_sentinel {
			fmt.sbprintf(builder, "[;%d]", item.sentinel)
		} else {
			strings.write_string(builder, "[]")
		}
		if item.mutable {
			strings.write_string(builder, "mut ")
		}
		write_type_label(checker, builder, item.child)
	case .Range:
		strings.write_string(builder, "range(")
		write_type_label(checker, builder, item.child)
		strings.write_byte(builder, ')')
	case .Optional:
		strings.write_byte(builder, '?')
		write_type_label(checker, builder, item.child)
	case .Function:
		strings.write_string(builder, "c_func(" if item.c_abi else "func(")
		for param, index in types.params_for(store, value) {
			if index > 0 {
				strings.write_string(builder, ", ")
			}
			write_type_label(checker, builder, param.type)
		}
		if item.variadic {
			if item.field_count > 0 {
				strings.write_string(builder, ", ")
			}
			strings.write_string(builder, "...")
		}
		strings.write_string(builder, ") ")
		write_type_label(checker, builder, item.child)
	case .Fallible:
		write_type_label(checker, builder, item.child)
		strings.write_string(builder, " ! ")
		write_type_label(checker, builder, item.extra)
	case .Sum:
		write_type_label(checker, builder, item.child)
		strings.write_string(builder, " | ")
		write_type_label(checker, builder, item.extra)
	case .Type_Call:
		strings.write_string(builder, "<type factory call>")
	case .Struct:
		strings.write_string(builder, "struct")
	case .Union:
		strings.write_string(builder, "union")
	case .Enum:
		strings.write_string(builder, "enum")
	case .Alias, .Distinct, .Named:
		write_type_label(checker, builder, item.child)
	case .Invalid, .Void, .Anyopaque, .Int_Constraint, .Uint_Constraint, .Float_Constraint, .Range_Constraint, .Scalar:
		strings.write_string(builder, types.name(value))
	}
}

// Render dynamic types using source syntax so diagnostics never expose internal
// type-store ids such as `<type 230>`.
type_label :: proc(checker: ^Checker, value: types.Type) -> string {
	builder := strings.builder_make(context.temp_allocator)
	write_type_label(checker, &builder, value)
	return strings.to_string(builder)
}

is_ptrcast_call :: proc(checker: ^Checker, expr: ast.Expr) -> bool {
	return expr.intrinsic &&
	       expr.left == ast.INVALID_EXPR &&
	       !symbol.is_valid(expr.qualifier) &&
	       symbol_text(checker, expr.name) == "ptrcast"
}

is_intrinsic_call :: proc(checker: ^Checker, expr: ast.Expr, name: string) -> bool {
	return expr.intrinsic && expr.kind == .Call && expr.left == ast.INVALID_EXPR &&
	       !symbol.is_valid(expr.qualifier) && symbol_text(checker, expr.name) == name
}

static_field_value :: proc(checker: ^Checker, base, name: symbol.Id) -> (Ct_Value, bool) {
	binding, ok := current_static_binding(checker, base)
	if !ok || binding.value == INVALID_CT_VALUE || int(binding.value) >= len(checker.static_state.values) {
		return {}, false
	}
	return persistent_field_value(checker, binding.value, name)
}

persistent_field_value :: proc(checker: ^Checker, root: Ct_Value_Id, name: symbol.Id) -> (Ct_Value, bool) {
	if root == INVALID_CT_VALUE || int(root) >= len(checker.static_state.values) {
		return {}, false
	}
	value := checker.static_state.values[root]
	index, _, found := find_struct_field(checker, value.type, name)
	children := ct_child_slice(&checker.static_state, value)
	if !found || index < 0 || index >= len(children) || children[index] == INVALID_CT_VALUE ||
	   int(children[index]) >= len(checker.static_state.values) {
		return {}, false
	}
	return checker.static_state.values[children[index]], true
}

build_static_value :: proc(checker: ^Checker, value: Ct_Value, span: source.Span, expected: types.Type) -> hir.Expr_Id {
	if value.kind == .Void {
		return add_hir_expr(checker, hir.Expr{
			kind=.Void, span=span, type=types.VOID,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if value.kind == .Integer {
		if types.is_enum(value.type, &checker.module.types) {
			return add_hir_expr(checker, hir.Expr{
				kind=.Integer, span=span, type=value.type, integer=i64(value.integer),
				target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		expr := ast.Expr{
			kind=.Integer, span=span, integer=u64(value.integer),
			left=ast.INVALID_EXPR, right=ast.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		}
		return build_constant_expr(checker, expr, Constant{kind=.Value, value=value.integer}, value.type)
	}
	if value.kind == .Float {
		bits := transmute(i64)value.float
		if types.bits(value.type, checker.target) == 32 {
			bits = i64(transmute(u32)f32(value.float))
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Float, span=span, type=value.type, integer=bits,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if value.kind == .Bool {
		return add_hir_expr(checker, hir.Expr{
			kind=.Bool, span=span, type=types.BOOL, integer=i64(value.integer),
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if value.kind == .String && value.index < u64(len(checker.ast_module.strings)) {
		return add_hir_expr(checker, hir.Expr{
			kind=.String, span=span, type=string_literal_type(checker, value.index), integer=i64(value.index),
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if value.kind == .Function {
		value_expected := expected if types.is_valid(expected) else value.type
		return build_function_value(checker, ast.Function_Id(u32(value.index)), span, value_expected)
	}
	if value.kind == .Array || value.kind == .Struct || value.kind == .Range {
		children := ct_child_slice(&checker.static_state, value)
		args := make([]hir.Expr_Id, len(children), checker.allocator)
		for child, index in children {
			if child == INVALID_CT_VALUE || int(child) >= len(checker.static_state.values) {
				delete(args, checker.allocator)
				id := source.add(checker.diagnostics, span, "invalid persistent compile-time aggregate")
				return invalid_hir_expr(checker, span, id, expected)
			}
			args[index] = build_static_value(checker, checker.static_state.values[child], span, types.INVALID)
		}
		kind := hir.Expr_Kind.Array
		if value.kind == .Struct {
			kind = .Struct
		} else if value.kind == .Range {
			kind = .Range
		}
		return add_hir_expr(checker, hir.Expr{
			kind=kind, span=span, type=value.type, integer=value.active, args=args,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if value.kind == .None {
		return add_hir_expr(checker, hir.Expr{
			kind=.None, span=span, type=value.type,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if value.kind == .Optional_Some {
		children := ct_child_slice(&checker.static_state, value)
		if len(children) == 1 && children[0] != INVALID_CT_VALUE &&
		   int(children[0]) < len(checker.static_state.values) {
			child_type := types.child_type(value.type, &checker.module.types)
			child := build_static_value(checker, checker.static_state.values[children[0]], span, child_type)
			return add_hir_expr(checker, hir.Expr{
				kind=.Optional_Some, span=span, type=value.type, left=child,
				target=hir.INVALID_REF, right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
	}
	id := source.add(checker.diagnostics, span, "compile-time-only value cannot be used at runtime")
	return invalid_hir_expr(checker, span, id, expected)
}

push_static_integer_binding :: proc(checker: ^Checker, name: symbol.Id, value_type: types.Type, value: i128) -> int {
	start := len(checker.static_bindings)
	if symbol.is_valid(name) && name != checker.sink_symbol {
		id := ct_add_value(&checker.static_state, Ct_Value{kind=.Integer, type=value_type, integer=value})
		append(&checker.static_bindings, Static_Binding{name=name, type=value_type, value=id})
	}
	return start
}

push_static_void_binding :: proc(checker: ^Checker, name: symbol.Id) -> int {
	start := len(checker.static_bindings)
	if symbol.is_valid(name) && name != checker.sink_symbol {
		id := ct_add_value(&checker.static_state, Ct_Value{kind=.Void, type=types.VOID})
		append(&checker.static_bindings, Static_Binding{name=name, type=types.VOID, value=id})
	}
	return start
}

pop_static_bindings :: proc(checker: ^Checker, start: int) {
	resize(&checker.static_bindings, start)
}

comptime_string_argument :: proc(
	checker: ^Checker,
	id: ast.Expr_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (string, bool) {
	if id == ast.INVALID_EXPR || int(id) >= len(checker.ast_module.exprs) {
		return "", false
	}
	expr := checker.ast_module.exprs[id]
	if expr.kind == .String && expr.integer < u64(len(checker.ast_module.strings)) {
		return checker.ast_module.strings[expr.integer], true
	}
	if expr.kind == .Name && !symbol.is_valid(expr.qualifier) {
		value, ok := current_comptime_value(checker, expr.name)
		if ok && value.kind == .String {
			return value.text, true
		}
	}
	if expr.kind == .Name && symbol.is_valid(expr.qualifier) &&
	   symbol_text(checker, expr.name) == "name" {
		if value, ok := static_field_value(checker, expr.qualifier, expr.name); ok &&
		   value.kind == .String && value.index < u64(len(checker.ast_module.strings)) {
			return checker.ast_module.strings[value.index], true
		}
	}
	state := ct_state_make(checker, pkg, file, diagnose=false)
	defer ct_state_destroy(&state)
	value, flow, ok := ct_eval_expr(&state, id, types.INVALID, 0)
	if ok && flow.kind == .Normal {
		return ct_value_bytes(&state, value)
	}
	return "", false
}

comptime_slice_bytes :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (string, bool) {
	state := ct_state_make(checker, pkg, file, diagnose=false)
	defer ct_state_destroy(&state)
	value, flow, ok := ct_eval_slice_expr(&state, expr, 0)
	if !ok || flow.kind != .Normal {
		return "", false
	}
	return ct_value_bytes(&state, value)
}

canonical_decimal_index :: proc(text: string) -> (u64, bool) {
	if len(text) == 0 || len(text) > 1 && text[0] == '0' {
		return 0, false
	}
	value: u64
	for byte in text {
		if byte < '0' || byte > '9' {
			return 0, false
		}
		digit := u64(byte-'0')
		if value > (0xffff_ffff_ffff_ffff-digit)/10 {
			return 0, false
		}
		value = value*10+digit
	}
	return value, true
}

field_intrinsic_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (ast.Expr, bool) {
	if !is_intrinsic_call(checker, expr, "field") || len(expr.args) != 2 {
		return {}, false
	}
	name, ok := comptime_string_argument(checker, expr.args[1], pkg, file)
	if !ok {
		return {}, false
	}
	result := ast.Expr{
		kind=.Field, span=expr.span, left=expr.args[0], right=ast.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	}
	if index, numeric := canonical_decimal_index(name); numeric {
		result.integer = index
		result.name = symbol.INVALID
	} else {
		result.name = symbol.intern(checker.symbols, name)
	}
	return result, true
}

std_named_type :: proc(checker: ^Checker, path, name: string) -> types.Type {
	name_id := symbol.intern(checker.symbols, name)
	result := types.INVALID
	for &import_item in checker.ast_module.imports {
		if import_item.valid && import_item.path == path {
			import_item.used = true
			if !types.is_valid(result) {
				result = types.find_named(&checker.module.types, u32(import_item.target), u32(name_id))
			}
		}
	}
	return result
}

Type_Builtin :: enum u8 {
	None,
	Size_Of,
	Align_Of,
	Min_Value,
	Max_Value,
}

Division_Builtin :: enum u8 {
	None,
	Trunc,
	Floor,
	Exact,
	Ceil,
	Rem,
	Mod,
}

division_builtin_call :: proc(checker: ^Checker, expr: ast.Expr) -> Division_Builtin {
	if !expr.intrinsic || expr.kind != .Call || expr.left != ast.INVALID_EXPR || symbol.is_valid(expr.qualifier) {
		return .None
	}
	switch symbol_text(checker, expr.name) {
	case "divtrunc": return .Trunc
	case "divfloor": return .Floor
	case "divexact": return .Exact
	case "divceil":  return .Ceil
	case "rem":       return .Rem
	case "mod":       return .Mod
	}
	return .None
}

type_builtin_call :: proc(checker: ^Checker, expr: ast.Expr) -> Type_Builtin {
	if !expr.intrinsic || expr.kind != .Call || expr.left != ast.INVALID_EXPR || symbol.is_valid(expr.qualifier) {
		return .None
	}
	name := symbol_text(checker, expr.name)
	if name == "sizeof" {
		return .Size_Of
	}
	if name == "alignof" {
		return .Align_Of
	}
	if name == "minval" {
		return .Min_Value
	}
	if name == "maxval" {
		return .Max_Value
	}
	return .None
}

valid_ptrcast_child :: proc(checker: ^Checker, value: types.Type) -> bool {
	return types.is_valid(value) &&
	       !types.is_void(value) &&
	       !types.is_anyopaque(value) &&
	       !types.is_function(value, &checker.module.types) &&
	       (types.is_runtime_value(value, &checker.module.types) ||
	        types.is_opaque_struct(value, &checker.module.types))
}

valid_layout_type :: proc(checker: ^Checker, value: types.Type) -> bool {
	return types.is_runtime_value(value, &checker.module.types)
}

type_builtin_value :: proc(checker: ^Checker, kind: Type_Builtin, value: types.Type) -> i128 {
	#partial switch kind {
	case .Size_Of:
		return i128(types.size(value, &checker.module.types, checker.target))
	case .Align_Of:
		return i128(types.alignment_of(value, &checker.module.types, checker.target))
	case .Min_Value:
		if types.is_unsigned(value, checker.target) {
			return 0
		}
		return -(i128(1) << u32(types.bits(value, checker.target)-1))
	case .Max_Value:
		bit_count := types.bits(value, checker.target)
		sign_bit_count := 1 if types.is_signed(value, checker.target) else 0
		return (i128(1) << u32(bit_count-sign_bit_count))-1
	case:
		return 0
	}
}

build_type_builtin :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	kind: Type_Builtin,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> hir.Expr_Id {
	if len(expr.args) != 1 {
		id := source.addf(checker.diagnostics, expr.span, "%s! expects 1 argument, got %d", symbol_text(checker, expr.name), len(expr.args))
		return invalid_hir_expr(checker, expr.span, id, types.USIZE)
	}
	target, target_ok := resolve_type_argument(checker, expr.args[0], pkg, file)
	if !target_ok {
		label := "layout" if kind == .Size_Of || kind == .Align_Of else "integer bound"
		id := source.addf(checker.diagnostics, checker.ast_module.exprs[expr.args[0]].span, "%s target must be a type", label)
		return invalid_hir_expr(checker, expr.span, id, types.USIZE)
	}
	if (kind == .Size_Of || kind == .Align_Of) && !valid_layout_type(checker, target) {
		id := source.addf(checker.diagnostics, checker.ast_module.exprs[expr.args[0]].span, "layout target must be a sized runtime value type, got %s", type_label(checker, target))
		return invalid_hir_expr(checker, expr.span, id, types.USIZE)
	}
	if (kind == .Min_Value || kind == .Max_Value) && !types.is_concrete_integer(target) {
		id := source.addf(checker.diagnostics, checker.ast_module.exprs[expr.args[0]].span, "integer bound target must be a concrete integer type, got %s", type_label(checker, target))
		return invalid_hir_expr(checker, expr.span, id, types.USIZE)
	}
	result_type := types.USIZE if kind == .Size_Of || kind == .Align_Of else target
	return build_constant_expr(
		checker,
		expr,
		Constant{kind=.Value, value=type_builtin_value(checker, kind, target)},
		result_type,
	)
}

tag_result_type :: proc(checker: ^Checker, value: types.Type) -> (types.Type, bool) {
	if !types.is_tagged_union(value, &checker.module.types) {
		return types.INVALID, false
	}
	return types.union_tag_enum(value, &checker.module.types), true
}

enum_member_name_from_value :: proc(checker: ^Checker, enum_type: types.Type, value: i128) -> (string, bool) {
	if !types.is_enum(enum_type, &checker.module.types) {
		return "", false
	}
	for member in types.enum_members_for(&checker.module.types, enum_type) {
		if member.value == value {
			return symbol_text(checker, symbol.Id(member.name)), true
		}
	}
	return "", false
}

build_tag_intrinsic :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Build_Local,
	global_reads: ^[dynamic]hir.Global_Id,
	calls: ^[dynamic]hir.Function_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> hir.Expr_Id {
	if len(expr.args) != 1 {
		id := source.addf(checker.diagnostics, expr.span, "tag! expects 1 argument, got %d", len(expr.args))
		return invalid_hir_expr(checker, expr.span, id)
	}
	state := ct_state_make(checker, pkg, file, values=checker.current_comptime_values, diagnose=false)
	value_id, flow, comptime_ok := ct_eval_expr(&state, expr.args[0], types.INVALID, 0)
	if comptime_ok && flow.kind == .Normal && value_id != INVALID_CT_VALUE && int(value_id) < len(state.values) {
		value := state.values[value_id]
		if tag_type, tagged := tag_result_type(checker, value.type); tagged && value.kind == .Struct &&
		   value.active >= 0 {
			fields := types.fields_for(&checker.module.types, value.type)
			if int(value.active) < len(fields) {
				if member, found := find_enum_member(checker, tag_type, symbol.Id(fields[value.active].name)); found {
					ct_state_destroy(&state)
					return add_hir_expr(checker, hir.Expr{
						kind=.Integer, span=expr.span, type=tag_type, integer=i64(member.value),
						target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
						diagnostic=source.INVALID_DIAGNOSTIC,
					})
				}
			}
		}
	}
	ct_state_destroy(&state)
	value := build_nested_expr(checker, expr.args[0], locals, global_reads, calls, types.INVALID, pkg, file)
	if checker.module.exprs[value].kind == .Invalid {
		return value
	}
	tag_type, ok := tag_result_type(checker, checker.module.exprs[value].type)
	if !ok {
		id := source.add(checker.diagnostics, expr.span, "tag! requires a tagged-union value")
		return invalid_hir_expr(checker, expr.span, id)
	}
	return add_hir_expr(checker, hir.Expr{
		kind=.Union_Tag, span=expr.span, type=tag_type, left=value,
		target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

build_tagname_intrinsic :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> hir.Expr_Id {
	if len(expr.args) != 1 {
		id := source.addf(checker.diagnostics, expr.span, "tagname! expects 1 argument, got %d", len(expr.args))
		return invalid_hir_expr(checker, expr.span, id)
	}
	state := ct_state_make(checker, pkg, file, values=checker.current_comptime_values, diagnose=false)
	defer ct_state_destroy(&state)
	value_id, flow, ok := ct_eval_expr(&state, expr.args[0], types.INVALID, 0)
	if !ok || flow.kind != .Normal || value_id == INVALID_CT_VALUE || int(value_id) >= len(state.values) {
		id := source.add(checker.diagnostics, expr.span, "tagname! requires a comptime-known enum value")
		return invalid_hir_expr(checker, expr.span, id)
	}
	value := state.values[value_id]
	name, name_ok := enum_member_name_from_value(checker, value.type, value.integer)
	if value.kind != .Integer || !name_ok {
		id := source.add(checker.diagnostics, expr.span, "tagname! requires a comptime-known enum value")
		return invalid_hir_expr(checker, expr.span, id)
	}
	string_id := intern_comptime_string(checker, name)
	return add_hir_expr(checker, hir.Expr{
		kind=.String, span=expr.span, type=string_literal_type(checker, string_id), integer=i64(string_id),
		target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

is_type_metatype_syntax :: proc(checker: ^Checker, value: ast.Type_Syntax) -> bool {
	item, ok := types.node(&checker.module.types, value)
	return ok && item.name == u32(checker.type_symbol) && item.qualifier == 0
}

is_comptime_type_param :: proc(checker: ^Checker, param: ast.Param) -> bool {
	return param.comptime_value && is_type_metatype_syntax(checker, param.type)
}

function_has_comptime_params :: proc(function: ast.Function) -> bool {
	for param in function.params {
		if param.comptime_value {
			return true
		}
	}
	return false
}

runtime_param_count :: proc(function: ast.Function) -> int {
	count := 0
	for param in function.params {
		if !param.comptime_value {
			count += 1
		}
	}
	return count
}

comptime_param_count :: proc(function: ast.Function) -> int {
	count := 0
	for param in function.params {
		if param.comptime_value {
			count += 1
		}
	}
	return count
}

fits_signed_type :: proc(value: i128, value_type: types.Type, selected := target.DEFAULT) -> bool {
	if !types.is_signed(value_type, selected) {
		return false
	}
	limit := i128(1) << u32(types.bits(value_type, selected) - 1)
	return value >= -limit && value < limit
}

fits_unsigned_type :: proc(value: i128, value_type: types.Type, selected := target.DEFAULT) -> bool {
	if !types.is_unsigned(value_type, selected) || value < 0 {
		return false
	}
	limit := i128(1) << u32(types.bits(value_type, selected))
	return value < limit
}

fits_integer_type :: proc(value: i128, value_type: types.Type, selected := target.DEFAULT) -> bool {
	return fits_signed_type(value, value_type, selected) || fits_unsigned_type(value, value_type, selected)
}

fits_i64 :: proc(value: i128) -> bool {
	return fits_signed_type(value, types.I64)
}

fits_u64 :: proc(value: i128) -> bool {
	return value >= 0 && value <= i128(0xffff_ffff_ffff_ffff)
}

constraint_integer_literal_type :: proc(constraint: types.Type, value: i128) -> types.Type {
	if constraint == types.UINT {
		return types.smallest_unsigned_for_literal(u64(value)) if fits_u64(value) else types.INVALID
	}
	return types.smallest_signed_for_literal(i64(value)) if fits_i64(value) else types.INVALID
}

constraint_recovery_type :: proc(checker: ^Checker, constraint: types.Type) -> types.Type {
	switch constraint {
	case types.UINT:  return types.U64
	case types.FLOAT: return types.F64
	case types.RANGE: return types.range(&checker.module.types, types.I64)
	case:             return types.I64
	}
}

type_from_syntax :: proc(
	checker: ^Checker,
	value: ast.Type_Syntax,
	pkg := ast.Package_Id(0),
	file := ast.File_Id(0),
	depth := 0,
	active_state: ^Ct_State = nil,
) -> types.Type {
	if depth > 64 {
		return types.INVALID
	}
	item, ok := types.node(&checker.module.types, value)
	if !ok {
		return value
	}
	if item.qualifier == 0 && item.name != 0 {
		if actual, ok := current_comptime_type(checker, symbol.Id(item.name)); ok {
			return actual
		}
	}
	store := &checker.module.types
	changed := false
	#partial switch item.kind {
	case .Alias:
		return type_from_syntax(checker, item.child, pkg, file, depth+1, active_state)
	case .Array:
		child := type_from_syntax(checker, item.child, pkg, file, depth+1, active_state)
		changed = changed || child != item.child
		item.child = child
		if item.unresolved_count {
			expr_id := ast.Expr_Id(item.count_expr)
			span := source.Span{}
			if expr_id != ast.INVALID_EXPR && int(expr_id) < len(checker.ast_module.exprs) {
				span = checker.ast_module.exprs[expr_id].span
			}
			constant := eval_integer_constant_in_state(active_state, expr_id) if active_state != nil else
			            eval_integer_constant_in_context(checker, expr_id, pkg, file)
			if constant.kind == .Value {
				switch {
				case constant.value < 0:
					source.add(checker.diagnostics, span, "array count must be non-negative")
					return types.INVALID
				case constant.value > i128(0xffff_ffff_ffff_ffff):
					source.add(checker.diagnostics, span, "array count does not fit in u64")
					return types.INVALID
				case:
					item.count = u64(constant.value)
					item.unresolved_count = false
					item.count_expr = 0
					changed = true
				}
			} else {
				if constant.kind == .Integer_Division {
					source.add(checker.diagnostics, span, "integer '/' is not allowed; use divtrunc!, divfloor!, divexact!, or divceil!")
					return types.INVALID
				}
				source.add(checker.diagnostics, span, "array count must be a compile-time integer expression")
				return types.INVALID
			}
		}
		return types.intern(store, item)
	case .Pointer, .Slice, .Optional, .Range, .Fallible:
		child := type_from_syntax(checker, item.child, pkg, file, depth+1, active_state)
		extra := type_from_syntax(checker, item.extra, pkg, file, depth+1, active_state)
		item.child = child
		item.extra = extra
		return types.intern(store, item)
	case .Distinct, .Enum:
		child := type_from_syntax(checker, item.child, pkg, file, depth+1, active_state)
		extra := type_from_syntax(checker, item.extra, pkg, file, depth+1, active_state)
		changed = child != item.child || extra != item.extra
		item.child = child
		item.extra = extra
	case .Function:
		params := types.params_for(store, value)
		resolved_params := make([]types.Type, len(params), checker.allocator)
		defer delete(resolved_params, checker.allocator)
		for param, index in params {
			resolved_params[index] = type_from_syntax(checker, param.type, pkg, file, depth+1, active_state)
		}
		result := type_from_syntax(checker, item.child, pkg, file, depth+1, active_state)
		return types.function(store, resolved_params, result, item.c_abi, item.variadic)
	case .Sum:
		left := type_from_syntax(checker, item.child, pkg, file, depth+1, active_state)
		right := type_from_syntax(checker, item.extra, pkg, file, depth+1, active_state)
		composed, compose_error := types.compose_sum(store, left, right)
		if compose_error == .Unsupported {
			source.add(checker.diagnostics, source.Span{}, "only native unbacked enums and tagged unions can be composed with '|'")
			return types.INVALID
		}
		if compose_error == .Conflict {
			source.add(checker.diagnostics, source.Span{}, "sum composition contains the same variant name with different payload types")
			return types.INVALID
		}
		return composed
	case .Type_Call:
		expr_id := ast.Expr_Id(item.count_expr)
		call_file := file
		if expr_id != ast.INVALID_EXPR && int(expr_id) < len(checker.ast_module.exprs) {
			call_file = ast.File_Id(checker.ast_module.exprs[expr_id].span.file)
		}
		return resolve_type_factory_call(checker, expr_id, pkg, call_file)
	}
	if changed {
		return types.intern(store, item)
	}
	return value
}

function_channel_type :: proc(checker: ^Checker, function: ast.Function) -> types.Type {
	result := type_from_syntax(checker, function.result, function.pkg, function.file)
	if types.is_valid(function.error) {
		return types.fallible(&checker.module.types, result, type_from_syntax(checker, function.error, function.pkg, function.file))
	}
	return result
}

is_runtime_type :: proc(checker: ^Checker, value: types.Type) -> bool {
	return types.is_runtime_value(value, &checker.module.types)
}

type_contains_unresolved_named :: proc(checker: ^Checker, value: types.Type, depth := 0) -> bool {
	if depth > 64 {
		return true
	}
	item, ok := types.node(&checker.module.types, value)
	if !ok {
		return false
	}
	if item.kind == .Named && !item.declared {
		return true
	}
	if type_contains_unresolved_named(checker, item.child, depth+1) ||
	   type_contains_unresolved_named(checker, item.extra, depth+1) {
		return true
	}
	if item.kind == .Function {
		for param in types.params_for(&checker.module.types, value) {
			if type_contains_unresolved_named(checker, param.type, depth+1) {
				return true
			}
		}
	}
	return false
}

is_comptime_value_type :: proc(checker: ^Checker, value: types.Type, depth := 0) -> bool {
	if depth > 256 || !types.is_valid(value) {
		return false
	}
	if is_type_metatype_syntax(checker, value) {
		return true
	}
	if is_runtime_type(checker, value) {
		return true
	}
	item, ok := types.node(&checker.module.types, value)
	if !ok {
		return false
	}
	if item.kind == .Function {
		return true
	}
	if item.kind == .Array || item.kind == .Optional || item.kind == .Alias || item.kind == .Distinct {
		return is_comptime_value_type(checker, item.child, depth+1)
	}
	if item.kind == .Struct || item.kind == .Union {
		if !item.declared || item.opaque || item.c_layout ||
		   (item.kind == .Union && !types.is_tagged_union(value, &checker.module.types)) {
			return false
		}
		for field in types.fields_for(&checker.module.types, value) {
			if !types.is_void(field.type) && !is_comptime_value_type(checker, field.type, depth+1) {
				return false
			}
		}
		return true
	}
	return false
}

is_undefined_expr :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> bool {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	return checker.ast_module.exprs[expr_id].kind == .Undefined
}

is_float_constant_expr :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> bool {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	expr := checker.ast_module.exprs[expr_id]
	if expr.kind == .Float {
		return true
	}
	return expr.kind == .Negate && is_float_constant_expr(checker, expr.left)
}

is_numeric_arithmetic_expr :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> bool {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	#partial switch checker.ast_module.exprs[expr_id].kind {
	case .Add, .Sub, .Mul, .Div, .Negate:
		return true
	}
	return false
}

is_numeric_constant_expr :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> bool {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	return eval_constant(checker, expr_id).kind == .Value || is_float_constant_expr(checker, expr_id)
}

is_typed_integer_fold_candidate :: proc(checker: ^Checker, expr_id: ast.Expr_Id, depth := 0) -> bool {
	if depth > 64 || expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	expr := checker.ast_module.exprs[expr_id]
	#partial switch expr.kind {
	case .Integer:
		return true
	case .Negate, .Bit_Not, .Cast:
		return is_typed_integer_fold_candidate(checker, expr.left, depth+1)
	case .Add, .Sub, .Mul, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left, .Shift_Right, .Shift_Left_Saturating:
		return is_typed_integer_fold_candidate(checker, expr.left, depth+1) &&
		       is_typed_integer_fold_candidate(checker, expr.right, depth+1)
	}
	return false
}

is_numeric_demand :: proc(value: types.Type, selected := target.DEFAULT) -> bool {
	return types.is_concrete_scalar(value) && !types.is_bool(value) ||
	       types.is_float(value, selected)
}

string_literal_type :: proc(checker: ^Checker, string_id: u64) -> types.Type {
	length: u64
	if string_id < u64(len(checker.ast_module.strings)) {
		length = u64(len(checker.ast_module.strings[string_id]))
	}
	array := types.array(&checker.module.types, types.U8, length, false, true, 0)
	return types.pointer(&checker.module.types, array, false, false)
}

container_pointer_type :: proc(store: ^types.Store, item: types.Node) -> types.Type {
	return types.pointer(store, item.child, item.mutable, true, item.has_sentinel, item.sentinel)
}

resolve_inferred_array :: proc(checker: ^Checker, value: types.Type, expr_id: ast.Expr_Id) -> types.Type {
	item, ok := types.node(&checker.module.types, value)
	if !ok || item.kind != .Array || !item.inferred_count ||
	   expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return value
	}
	expr := checker.ast_module.exprs[expr_id]
	if expr.kind != .Array {
		return value
	}
	return types.with_array_count(&checker.module.types, value, u64(len(expr.args)))
}

has_inferred_array_count :: proc(checker: ^Checker, value: types.Type) -> bool {
	item, ok := types.node(&checker.module.types, value)
	return ok && item.kind == .Array && item.inferred_count
}

resolve_inferred_array_from_type :: proc(checker: ^Checker, value, inferred: types.Type) -> types.Type {
	item, ok := types.node(&checker.module.types, value)
	actual, actual_ok := types.node(&checker.module.types, inferred)
	if !ok || !actual_ok || item.kind != .Array || actual.kind != .Array || !item.inferred_count {
		return value
	}
	if item.child != actual.child || item.mutable != actual.mutable ||
	   item.has_sentinel != actual.has_sentinel ||
	   (item.has_sentinel && item.sentinel != actual.sentinel) {
		return value
	}
	return types.with_array_count(&checker.module.types, value, actual.count)
}

function_index_less :: proc(left, right: Function_Index_Entry) -> bool {
	if left.scope != right.scope {
		return left.scope < right.scope
	}
	if left.name != right.name {
		return int(left.name) < int(right.name)
	}
	return left.id < right.id
}

global_index_less :: proc(left, right: Global_Index_Entry) -> bool {
	if left.scope != right.scope {
		return left.scope < right.scope
	}
	if left.name != right.name {
		return int(left.name) < int(right.name)
	}
	return left.id < right.id
}

import_index_less :: proc(left, right: Import_Index_Entry) -> bool {
	if left.scope != right.scope {
		return left.scope < right.scope
	}
	if left.name != right.name {
		return int(left.name) < int(right.name)
	}
	return left.id < right.id
}

find_function_symbol :: proc(index: []Function_Index_Entry, scope: ast.Package_Id, name: symbol.Id, file := ast.INVALID_FILE) -> ast.Function_Id {
	low := 0
	high := len(index)
	for low < high {
		middle := low + (high-low)/2
		entry := index[middle]
		if entry.scope < scope || entry.scope == scope && int(entry.name) < int(name) {
			low = middle + 1
		} else {
			high = middle
		}
	}
	visible := ast.INVALID_FUNCTION
	for low < len(index) && index[low].scope == scope && index[low].name == name {
		entry := index[low]
		if entry.hidden {
			if entry.file == file {
				return entry.id
			}
		} else {
			visible = entry.id
		}
		low += 1
	}
	return visible
}

find_global_symbol :: proc(index: []Global_Index_Entry, scope: ast.Package_Id, name: symbol.Id, file := ast.INVALID_FILE) -> ast.Global_Id {
	low := 0
	high := len(index)
	for low < high {
		middle := low + (high-low)/2
		entry := index[middle]
		if entry.scope < scope || entry.scope == scope && int(entry.name) < int(name) {
			low = middle + 1
		} else {
			high = middle
		}
	}
	visible := ast.INVALID_GLOBAL
	for low < len(index) && index[low].scope == scope && index[low].name == name {
		entry := index[low]
		if entry.hidden {
			if entry.file == file {
				return entry.id
			}
		} else {
			visible = entry.id
		}
		low += 1
	}
	return visible
}

find_import_symbol :: proc(index: []Import_Index_Entry, scope: ast.File_Id, name: symbol.Id) -> ast.Import_Id {
	low := 0
	high := len(index)
	for low < high {
		middle := low + (high-low)/2
		entry := index[middle]
		if entry.scope < scope || entry.scope == scope && int(entry.name) < int(name) {
			low = middle + 1
		} else {
			high = middle
		}
	}
	if low < len(index) && index[low].scope == scope && index[low].name == name {
		return index[low].id
	}
	return ast.INVALID_IMPORT
}

build_symbol_indexes :: proc(checker: ^Checker) {
	function_count := 0
	for function in checker.ast_module.functions {
		if !function.generated {
			function_count += 1
		}
	}
	for alias in checker.ast_module.aliases {
		if alias.valid && alias.kind == .Function {
			function_count += 1
		}
	}
	checker.function_index = make([]Function_Index_Entry, function_count, checker.allocator)
	function_index := 0
	for function, id in checker.ast_module.functions {
		if function.generated {
			continue
		}
		checker.function_index[function_index] = Function_Index_Entry{scope=function.pkg, file=function.file, hidden=function.file_hidden, name=function.name, id=ast.function_id(id)}
		function_index += 1
	}
	for alias in checker.ast_module.aliases {
		if alias.valid && alias.kind == .Function {
			checker.function_index[function_index] = Function_Index_Entry{scope=alias.pkg, file=alias.file, hidden=alias.file_hidden, name=alias.name, id=ast.Function_Id(alias.target)}
			function_index += 1
		}
	}
	slice.sort_by(checker.function_index, function_index_less)

	global_count := len(checker.ast_module.globals)
	for alias in checker.ast_module.aliases {
		if alias.valid && alias.kind == .Global {
			global_count += 1
		}
	}
	checker.global_index = make([]Global_Index_Entry, global_count, checker.allocator)
	for global, id in checker.ast_module.globals {
		checker.global_index[id] = Global_Index_Entry{scope=global.pkg, file=global.file, hidden=global.file_hidden, name=global.name, id=ast.global_id(id)}
	}
	global_index := len(checker.ast_module.globals)
	for alias in checker.ast_module.aliases {
		if alias.valid && alias.kind == .Global {
			checker.global_index[global_index] = Global_Index_Entry{scope=alias.pkg, file=alias.file, hidden=alias.file_hidden, name=alias.name, id=ast.Global_Id(alias.target)}
			global_index += 1
		}
	}
	slice.sort_by(checker.global_index, global_index_less)

	import_count := 0
	for import_item in checker.ast_module.imports {
		if !import_item.test_only {
			import_count += 1
		}
	}
	checker.import_index = make([]Import_Index_Entry, import_count, checker.allocator)
	import_index := 0
	for import_item, id in checker.ast_module.imports {
		if import_item.test_only {
			continue
		}
		checker.import_index[import_index] = Import_Index_Entry{scope=import_item.file, name=import_item.alias, id=ast.import_id(id)}
		import_index += 1
	}
	slice.sort_by(checker.import_index, import_index_less)
}

find_template :: proc(checker: ^Checker, name: symbol.Id, pkg := ast.Package_Id(0), file := ast.INVALID_FILE) -> ast.Function_Id {
	return find_function_symbol(checker.function_index, pkg, name, file)
}

configure_entry_point :: proc(checker: ^Checker) {
	main_template := find_template(checker, checker.main_symbol, 0)
	if main_template == ast.INVALID_FUNCTION {
		return
	}
	main := checker.ast_module.functions[main_template]
	if len(main.params) == 0 {
		checker.entry_point = .Plain
		return
	}
	if len(main.params) != 1 || main.params[0].comptime_value {
		return
	}

	parameter_type := types.resolve_alias(
		type_from_syntax(checker, main.params[0].type, main.pkg, main.file),
		&checker.module.types,
	)
	init_name := symbol.intern(checker.symbols, "Init")
	process_package := ast.INVALID_PACKAGE
	for import_item in checker.ast_module.imports {
		if import_item.valid && import_item.path == "@std/process" {
			candidate := types.find_named(&checker.module.types, u32(import_item.target), u32(init_name))
			if types.equal(parameter_type, candidate) {
				process_package = import_item.target
				break
			}
		}
	}
	if process_package == ast.INVALID_PACKAGE {
		return
	}

	io_name := symbol.intern(checker.symbols, "Io")
	io_package := ast.INVALID_PACKAGE
	io_type := types.INVALID
	for import_item in checker.ast_module.imports {
		if import_item.valid && import_item.path == "@std/io" {
			candidate := types.find_named(&checker.module.types, u32(import_item.target), u32(io_name))
			io_package = import_item.target
			io_type = candidate
			break
		}
	}
	if io_package == ast.INVALID_PACKAGE {
		return
	}
	init_item, init_ok := types.node(&checker.module.types, parameter_type)
	init_fields := types.fields_for(&checker.module.types, parameter_type)
	io_field_name := symbol.intern(checker.symbols, "io")
	if !init_ok || init_item.kind != .Struct || init_item.tuple || init_item.c_layout ||
	   len(init_fields) != 1 || init_fields[0].name != u32(io_field_name) ||
	   !types.equal(types.resolve_alias(init_fields[0].type, &checker.module.types), io_type) {
		checker.template_diagnostics[main_template] = source.add(
			checker.diagnostics,
			main.span,
			"@std/process Init must be an auto-layout record containing exactly 'io io.Io'",
		)
		return
	}

	provider_name := symbol.intern(checker.symbols, "system")
	provider := ast.INVALID_FUNCTION
	provider_count := 0
	for function, function_id in checker.ast_module.functions {
		if function.pkg != io_package || function.name != provider_name {
			continue
		}
		provider_count += 1
		result := types.resolve_alias(
			type_from_syntax(checker, function.result, function.pkg, function.file),
			&checker.module.types,
		)
		if function.file_hidden && function.has_body && !function.c_abi && len(function.params) == 0 &&
		   !types.is_valid(function.error) && types.equal(result, io_type) {
			provider = ast.function_id(function_id)
		}
	}
	if provider_count != 1 || provider == ast.INVALID_FUNCTION {
		checker.template_diagnostics[main_template] = source.add(
			checker.diagnostics,
			main.span,
			"@std/io does not provide the required 'hide system func() Io' startup implementation",
		)
		return
	}

	checker.entry_point = .Process
	checker.io_provider_template = provider
}

find_global :: proc(checker: ^Checker, name: symbol.Id, pkg := ast.Package_Id(0), file := ast.INVALID_FILE) -> ast.Global_Id {
	return find_global_symbol(checker.global_index, pkg, name, file)
}

find_import :: proc(checker: ^Checker, file: ast.File_Id, alias: symbol.Id, mark_used := false) -> ast.Import_Id {
	id := find_import_symbol(checker.import_index, file, alias)
	if id != ast.INVALID_IMPORT && mark_used {
		checker.ast_module.imports[id].used = true
	}
	return id
}

declared_type_named :: proc(checker: ^Checker, pkg: ast.Package_Id, name: symbol.Id, file := ast.INVALID_FILE) -> bool {
	id := types.find_named(&checker.module.types, u32(pkg), u32(name), file=u32(file))
	item, ok := types.node(&checker.module.types, id)
	return ok && item.declared
}

declarations_conflict :: proc(left_file: ast.File_Id, left_hidden: bool, right_file: ast.File_Id, right_hidden: bool) -> bool {
	return left_file == right_file if left_hidden && right_hidden else true
}

type_declaration_conflicts :: proc(checker: ^Checker, pkg: ast.Package_Id, name: symbol.Id, file: ast.File_Id, hidden: bool) -> bool {
	for item in checker.module.types.nodes {
		if item.declared && item.pkg == u32(pkg) && item.name == u32(name) &&
		   declarations_conflict(file, hidden, ast.File_Id(item.file), item.file_hidden) {
			return true
		}
	}
	return false
}

visible_name_kind :: proc(
	checker: ^Checker,
	name: symbol.Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	locals: []Build_Local = nil,
	labels: []symbol.Id = nil,
	yield_targets: []Yield_Target = nil,
) -> string {
	if !symbol.is_valid(name) || name == checker.sink_symbol {
		return ""
	}
	if _, ok := current_comptime_value(checker, name); ok {
		return "comptime parameter"
	}
	if _, ok := find_build_local(locals, name); ok {
		return "local"
	}
	for label in labels {
		if label == name {
			return "label"
		}
	}
	for target in yield_targets {
		if target.label == name {
			return "label"
		}
	}
	if find_import(checker, file, name) != ast.INVALID_IMPORT {
		return "import"
	}
	if find_global(checker, name, pkg, file) != ast.INVALID_GLOBAL {
		return "global"
	}
	if find_template(checker, name, pkg, file) != ast.INVALID_FUNCTION {
		return "function"
	}
	if declared_type_named(checker, pkg, name, file) {
		return "type"
	}
	return ""
}

add_shadow_diagnostic :: proc(
	checker: ^Checker,
	span: source.Span,
	name: symbol.Id,
	decl_kind: string,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	locals: []Build_Local = nil,
	labels: []symbol.Id = nil,
	yield_targets: []Yield_Target = nil,
) -> source.Diagnostic_Id {
	kind := visible_name_kind(checker, name, pkg, file, locals, labels, yield_targets)
	if len(kind) == 0 {
		return source.INVALID_DIAGNOSTIC
	}
	return source.addf(
		checker.diagnostics,
		span,
		"%s '%s' shadows visible %s",
		decl_kind,
		symbol_text(checker, name),
		kind,
	)
}

add_label_shadow_diagnostic :: proc(ctx: ^Build_Ctx, span: source.Span, label: symbol.Id) -> source.Diagnostic_Id {
	if !symbol.is_valid(label) {
		return source.INVALID_DIAGNOSTIC
	}
	yield_targets := ctx.yield_targets^[:]
	if len(yield_targets) > 0 && yield_targets[len(yield_targets) - 1].label == label {
		label_is_active_loop := false
		for loop_label in ctx.loop_labels^[:] {
			if loop_label == label {
				label_is_active_loop = true
				break
			}
		}
		if !label_is_active_loop {
			yield_targets = yield_targets[:len(yield_targets) - 1]
		}
	}
	return add_shadow_diagnostic(
		ctx.checker, span, label, "label",
		ctx.pkg, ctx.file, ctx.locals^[:], ctx.loop_labels^[:], yield_targets,
	)
}

expr_package :: proc(checker: ^Checker, expr: ast.Expr, pkg: ast.Package_Id, file: ast.File_Id, mark_used := false) -> (ast.Package_Id, bool) {
	if !symbol.is_valid(expr.qualifier) {
		return pkg, true
	}
	import_id := find_import(checker, file, expr.qualifier, mark_used)
	if import_id == ast.INVALID_IMPORT {
		return ast.INVALID_PACKAGE, false
	}
	import_item := checker.ast_module.imports[import_id]
	if import_item.target == ast.INVALID_PACKAGE || int(import_item.target) >= len(checker.ast_module.packages) ||
	   !checker.ast_module.packages[import_item.target].available {
		return import_item.target, false
	}
	return import_item.target, true
}

expr_lookup_file :: proc(expr: ast.Expr, file: ast.File_Id) -> ast.File_Id {
	return ast.INVALID_FILE if symbol.is_valid(expr.qualifier) else file
}

expr_symbol_span :: proc(checker: ^Checker, expr: ast.Expr, name: symbol.Id) -> source.Span {
	span := expr.span
	span.end = span.start+source.Offset(len(symbol_text(checker, name)))
	return span
}

add_package_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, file: ast.File_Id) -> source.Diagnostic_Id {
	if find_import(checker, file, expr.qualifier) == ast.INVALID_IMPORT {
		span := expr_symbol_span(checker, expr, expr.qualifier)
		id := source.addf(checker.diagnostics, span, "unknown symbol '%s'", symbol_text(checker, expr.qualifier))
		source.set_primary_label(checker.diagnostics, id, "unknown symbol")
		return id
	}
	return source.addf(checker.diagnostics, expr.span, "unavailable imported package '%s'", symbol_text(checker, expr.qualifier))
}

add_name_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target_pkg: ast.Package_Id, file: ast.File_Id) -> source.Diagnostic_Id {
	if find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file)) != ast.INVALID_FUNCTION {
		template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
		id := source.addf(checker.diagnostics, expr_symbol_span(checker, expr, expr.name), "'%s' is a function, not a global value", symbol_text(checker, expr.name))
		if !checker.ast_module.functions[template].imported {
			source.add_secondary_label(checker.diagnostics, id, checker.ast_module.functions[template].span, "function declared here")
		}
		return id
	}
	if symbol.is_valid(expr.qualifier) {
		return source.addf(
			checker.diagnostics,
			expr.span,
			"package '%s' has no member '%s'",
			symbol_text(checker, expr.qualifier),
			symbol_text(checker, expr.name),
		)
	}
	span := expr_symbol_span(checker, expr, expr.name)
	id := source.addf(checker.diagnostics, span, "unknown symbol '%s'", symbol_text(checker, expr.name))
	source.set_primary_label(checker.diagnostics, id, "unknown symbol")
	return id
}

add_call_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target_pkg: ast.Package_Id, file: ast.File_Id) -> source.Diagnostic_Id {
	if find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file)) != ast.INVALID_GLOBAL {
		global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
		id := source.addf(checker.diagnostics, expr_symbol_span(checker, expr, expr.name), "'%s' is a global, not a function", symbol_text(checker, expr.name))
		if !checker.ast_module.globals[global].external {
			source.add_secondary_label(checker.diagnostics, id, checker.ast_module.globals[global].span, "global declared here")
		}
		return id
	}
	if symbol.is_valid(expr.qualifier) {
		return source.addf(
			checker.diagnostics,
			expr.span,
			"package '%s' has no member '%s'",
			symbol_text(checker, expr.qualifier),
			symbol_text(checker, expr.name),
		)
	}
	span := expr_symbol_span(checker, expr, expr.name)
	id := source.addf(checker.diagnostics, span, "unknown symbol '%s'", symbol_text(checker, expr.name))
	source.set_primary_label(checker.diagnostics, id, "unknown symbol")
	return id
}

find_unsupported :: proc(checker: ^Checker, pkg: ast.Package_Id, name: symbol.Id) -> (ast.Unsupported, bool) {
	for item in checker.ast_module.unsupported {
		if item.pkg == pkg && item.name == name {
			return item, true
		}
	}
	return {}, false
}

add_unsupported_diagnostic :: proc(checker: ^Checker, span: source.Span, pkg: ast.Package_Id, name: symbol.Id) -> source.Diagnostic_Id {
	if item, ok := find_unsupported(checker, pkg, name); ok {
		return source.addf(
			checker.diagnostics,
			span,
			"C declaration '%s' is unavailable: %s",
			symbol_text(checker, name),
			item.reason,
		)
	}
	return source.INVALID_DIAGNOSTIC
}

add_unsupported_type_diagnostic :: proc(
	checker: ^Checker,
	span: source.Span,
	value: types.Type,
	depth := 0,
) -> source.Diagnostic_Id {
	if depth > 64 {
		return source.INVALID_DIAGNOSTIC
	}
	item, ok := types.node(&checker.module.types, value)
	if !ok {
		return source.INVALID_DIAGNOSTIC
	}
	if item.kind == .Alias {
		return add_unsupported_diagnostic(checker, span, ast.Package_Id(item.pkg), symbol.Id(item.name))
	}
	if types.is_valid(item.child) {
		return add_unsupported_type_diagnostic(checker, span, item.child, depth+1)
	}
	if types.is_valid(item.extra) {
		return add_unsupported_type_diagnostic(checker, span, item.extra, depth+1)
	}
	return source.INVALID_DIAGNOSTIC
}

function_signatures_equal :: proc(left, right: ast.Function) -> bool {
	if left.result != right.result || left.error != right.error ||
	   left.variadic != right.variadic || len(left.params) != len(right.params) {
		return false
	}
	for param, index in left.params {
		if param.type != right.params[index].type ||
		   param.comptime_value != right.params[index].comptime_value {
			return false
		}
	}
	return true
}

valid_call_arity :: proc(function: ast.Function, count: int) -> bool {
	return count >= len(function.params) if function.variadic else count == len(function.params)
}

Call_Argument_Mode :: enum u8 {
	Invalid,
	Explicit,
	Inferred,
}

call_param_index :: proc(mapping: []int, source_index: int) -> int {
	if source_index < 0 || source_index >= len(mapping) {
		return -1
	}
	return mapping[source_index]
}

next_runtime_call_arg :: proc(function: ast.Function, mapping: []int, start, source_count: int) -> int {
	index := start
	for index < source_count {
		param_index := call_param_index(mapping, index)
		if param_index >= len(function.params) || !function.params[param_index].comptime_value {
			break
		}
		index += 1
	}
	return index
}

call_mapping_mode :: proc(function: ast.Function, mapping: []int) -> Call_Argument_Mode {
	if len(mapping) != len(function.params) {
		return .Inferred
	}
	for value, index in mapping {
		if value != index {
			return .Inferred
		}
	}
	return .Explicit
}

comptime_binding_index :: proc(function: ast.Function, prefix: int, name: symbol.Id) -> (int, bool) {
	ordinal := 0
	for param in function.params {
		if !param.comptime_value {
			continue
		}
		if ordinal >= prefix {
			return -1, false
		}
		if param.name == name {
			return ordinal, true
		}
		ordinal += 1
	}
	return -1, false
}

comptime_param_for_name :: proc(function: ast.Function, name: symbol.Id) -> (ast.Param, bool) {
	for param in function.params {
		if param.comptime_value && param.name == name {
			return param, true
		}
	}
	return {}, false
}

Call_Mapping_Search :: struct {
	checker: ^Checker,
	function: ^ast.Function,
	args: []ast.Expr_Id,
	current: []int,
	candidates: ^[dynamic][]int,
}

is_comptime_string_param :: proc(checker: ^Checker, param: ast.Param, function: ast.Function) -> bool {
	declared := types.resolve_alias(type_from_syntax(checker, param.type, function.pkg, function.file), &checker.module.types)
	item, ok := types.container(declared, &checker.module.types)
	return ok && item.kind == .Slice && !item.mutable && item.child == types.U8
}

explicit_comptime_argument_valid :: proc(
	checker: ^Checker,
	function: ast.Function,
	param: ast.Param,
	arg: ast.Expr_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> bool {
	if arg == ast.INVALID_EXPR || int(arg) >= len(checker.ast_module.exprs) {
		return false
	}
	expr := checker.ast_module.exprs[arg]
	if expr.kind == .Inference_Hole {
		return true
	}
	if is_comptime_type_param(checker, param) {
		// Avoid asking the ordinary type resolver to diagnose while candidates are
		// being probed. A call can be a type argument only when its declaration is
		// a type factory.
		if expr.kind == .Call {
			target_pkg, available := expr_package(checker, expr, pkg, file, false)
			if !available {
				return false
			}
			template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
			if template == ast.INVALID_FUNCTION ||
			   !is_type_metatype_syntax(checker, checker.ast_module.functions[template].result) {
				return false
			}
		}
		_, ok := resolve_type_argument(checker, arg, pkg, file)
		return ok
	}
	if is_comptime_string_param(checker, param, function) {
		_, ok := comptime_string_argument(checker, arg, pkg, file)
		return ok
	}
	if types.is_concrete_integer(type_from_syntax(checker, param.type, function.pkg, function.file)) {
		return eval_integer_constant_in_context(checker, arg, pkg, file).kind == .Value
	}
	if type_pattern_mentions_comptime(checker, function, comptime_param_count(function), param.type) {
		return true
	}
	declared := type_from_syntax(checker, param.type, function.pkg, function.file)
	_, ok := eval_static_comptime_value(checker, param.name, arg, declared, pkg, file)
	return ok
}

search_call_mappings :: proc(search: ^Call_Mapping_Search, param_index, source_index: int) {
	if len(search.candidates^) >= COMPTIME_EVAL_QUOTA {
		return
	}
	function := search.function^
	if param_index == len(function.params) {
		if source_index != len(search.args) {
			return
		}
		candidate := make([]int, len(search.current), search.checker.allocator)
		copy(candidate, search.current)
		append(search.candidates, candidate)
		return
	}
	param := function.params[param_index]
	if param.comptime_value {
		if source_index < len(search.args) {
			search.current[source_index] = param_index
			search_call_mappings(search, param_index+1, source_index+1)
			search.current[source_index] = -1
		}
		search_call_mappings(search, param_index+1, source_index)
		return
	}
	if source_index >= len(search.args) {
		return
	}
	search.current[source_index] = param_index
	search_call_mappings(search, param_index+1, source_index+1)
	search.current[source_index] = -1
}

call_mapping_semantically_valid :: proc(
	checker: ^Checker,
	function: ast.Function,
	mapping: []int,
	args: []ast.Expr_Id,
	expected: types.Type,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (bool, string) {
	actual_args := make([]types.Type, len(function.params), checker.allocator)
	defer delete(actual_args, checker.allocator)
	for source_index in 0..<len(args) {
		param_index := call_param_index(mapping, source_index)
		if param_index < 0 || param_index >= len(function.params) {
			continue
		}
		if function.params[param_index].comptime_value {
			if !explicit_comptime_argument_valid(
				checker, function, function.params[param_index], args[source_index], pkg, file,
			) {
				return false, fmt.aprintf(
					"argument %d is not a valid comptime value for parameter '%s'",
					source_index+1, symbol_text(checker, function.params[param_index].name),
					allocator=checker.allocator,
				)
			}
			continue
		}
		actual_args[param_index] = infer_expr(checker, args[source_index], locals, pkg, file)
	}
	inference_failure := ""
	values, ok := infer_call_comptime_values(
		checker, function, comptime_param_count(function), mapping, args, actual_args,
		expected, pkg, file, diagnose=false, failure=&inference_failure,
	)
	defer delete(values, checker.allocator)
	if !ok {
		if len(inference_failure) > 0 {
			return false, inference_failure
		}
		return false, fmt.aprintf("could not infer or evaluate every comptime parameter", allocator=checker.allocator)
	}
	delete(inference_failure, checker.allocator)
	previous := checker.current_comptime_values
	checker.current_comptime_values = values
	defer checker.current_comptime_values = previous
	for param, index in function.params {
		if param.comptime_value {
			continue
		}
		actual := actual_args[index]
		declared := type_from_syntax(checker, param.type, function.pkg, function.file)
		source_index := -1
		for mapped_param, candidate_source in mapping {
			if mapped_param == index {
				source_index = candidate_source
				break
			}
		}
		if !is_runtime_type(checker, actual) && !can_implicitly_convert_type(checker, actual, declared) {
			return false, fmt.aprintf(
				"argument %d is not a runtime value", source_index+1,
				allocator=checker.allocator,
			)
		}
		if types.is_constraint(declared) {
			if !types.is_valid(types.constraint_target(declared, actual, &checker.module.types)) {
				return false, fmt.aprintf(
					"argument %d of type %s does not satisfy %s",
					source_index+1, type_label(checker, actual), type_label(checker, declared),
					allocator=checker.allocator,
				)
			}
		} else if !can_implicitly_convert_type(checker, actual, declared) {
			if source_index < 0 ||
			   !is_numeric_constant_expr(checker, args[source_index]) ||
			   !expr_accepts_numeric_demand(checker, args[source_index], declared, locals, pkg, file) {
				return false, fmt.aprintf(
					"argument %d of type %s cannot convert to %s",
					source_index+1, type_label(checker, actual), type_label(checker, declared),
					allocator=checker.allocator,
				)
			}
		}
	}
	if is_runtime_type(checker, expected) {
		result := function_channel_type(checker, function)
		if !is_runtime_type(checker, result) || !can_implicitly_convert_type(checker, result, expected) {
			return false, fmt.aprintf(
				"result type %s cannot convert to expected type %s",
				type_label(checker, result), type_label(checker, expected),
				allocator=checker.allocator,
			)
		}
	}
	return true, ""
}

call_argument_mapping :: proc(
	checker: ^Checker,
	function: ^ast.Function,
	args: []ast.Expr_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	expected := types.INVALID,
	locals: []Infer_Local = nil,
) -> (mapping: []int, mode: Call_Argument_Mode, comptime_count: int, failure: string) {
	if function.variadic || function.c_abi {
		if !valid_call_arity(function^, len(args)) {
			return nil, .Invalid, 0, ""
		}
		mapping = make([]int, len(args), checker.allocator)
		for &value, index in mapping {
			value = index
		}
		return mapping, .Explicit, 0, ""
	}
	current := make([]int, len(args), checker.allocator)
	defer delete(current, checker.allocator)
	for &value in current {
		value = -1
	}
	candidates: [dynamic][]int
	candidates.allocator = checker.allocator
	defer {
		for candidate in candidates {
			delete(candidate, checker.allocator)
		}
		delete(candidates)
	}
	search := Call_Mapping_Search{
		checker=checker,
		function=function,
		args=args,
		current=current,
		candidates=&candidates,
	}
	search_call_mappings(&search, 0, 0)
	selected_index := -1
	failures: [dynamic]string
	failures.allocator = checker.allocator
	defer {
		for item in failures {
			delete(item, checker.allocator)
		}
		delete(failures)
	}
	if len(candidates) == 1 {
		selected_index = 0
	}
	if selected_index < 0 && len(candidates) > 1 {
		for candidate, index in candidates {
			valid, reason := call_mapping_semantically_valid(
				checker, function^, candidate, args, expected, locals, pkg, file,
			)
			if valid {
				if selected_index >= 0 {
					return nil, .Invalid, comptime_param_count(function^), fmt.aprintf(
						"multiple complete argument mappings satisfy the call",
						allocator=checker.allocator,
					)
				}
				selected_index = index
			} else {
				append(&failures, reason)
			}
		}
	}
	if selected_index < 0 {
		if len(failures) > 0 {
			builder := strings.builder_make(checker.allocator)
			defer strings.builder_destroy(&builder)
			for reason, index in failures {
				if index > 0 {
					strings.write_string(&builder, "; ")
				}
				fmt.sbprintf(&builder, "candidate %d: %s", index+1, reason)
			}
			return nil, .Invalid, comptime_param_count(function^), fmt.aprintf(
				"%s", strings.to_string(builder), allocator=checker.allocator,
			)
		}
		return nil, .Invalid, comptime_param_count(function^), ""
	}
	selected := make([]int, len(args), checker.allocator)
	copy(selected, candidates[selected_index])
	identity := len(args) == len(function.params)
	if identity {
		for value, index in selected {
			if value != index {
				identity = false
				break
			}
		}
	}
	return selected, .Explicit if identity else .Inferred, comptime_param_count(function^), ""
}

bind_inferred_comptime :: proc(
	checker: ^Checker,
	function: ast.Function,
	prefix: int,
	values: []Comptime_Value,
	bound: []bool,
	name: symbol.Id,
	candidate: Comptime_Value,
	span: source.Span,
	diagnose: bool,
) -> bool {
	index, ok := comptime_binding_index(function, prefix, name)
	if !ok {
		return false
	}
	value := candidate
	value.name = name
	if !bound[index] {
		values[index] = value
		bound[index] = true
		return true
	}
	existing := values[index]
	matches := existing.kind == value.kind
	if matches {
		if existing.kind == .Type {
			matches = types.equal(types.resolve_alias(existing.type, &checker.module.types), types.resolve_alias(value.type, &checker.module.types))
		} else if existing.kind == .Static {
			matches = existing.fingerprint == value.fingerprint && existing.key == value.key && types.equal(existing.type, value.type)
		} else {
			matches = existing.value == value.value && types.equal(existing.type, value.type)
		}
	}
	if !matches && diagnose {
		left := type_label(checker, existing.type) if existing.kind == .Type else fmt.aprintf("<comptime value>", allocator=checker.allocator) if existing.kind == .Static else fmt.aprintf("%d", existing.value, allocator=checker.allocator)
		right := type_label(checker, value.type) if value.kind == .Type else fmt.aprintf("<comptime value>", allocator=checker.allocator) if value.kind == .Static else fmt.aprintf("%d", value.value, allocator=checker.allocator)
		source.addf(checker.diagnostics, span, "conflicting inference for comptime parameter '%s': %s and %s", symbol_text(checker, name), left, right)
		if existing.kind != .Type {
			delete(left, checker.allocator)
		}
		if value.kind != .Type {
			delete(right, checker.allocator)
		}
	}
	return matches
}

type_pattern_mentions_comptime :: proc(
	checker: ^Checker,
	function: ast.Function,
	prefix: int,
	pattern: types.Type,
	depth := 0,
	type_params_only := false,
) -> bool {
	if depth > 64 {
		return false
	}
	item, ok := types.node(&checker.module.types, pattern)
	if !ok {
		return false
	}
	if item.qualifier == 0 && item.name != 0 {
		if _, found := comptime_binding_index(function, prefix, symbol.Id(item.name)); found {
			if !type_params_only {
				return true
			}
			param, param_ok := comptime_param_for_name(function, symbol.Id(item.name))
			if param_ok && is_comptime_type_param(checker, param) {
				return true
			}
		}
	}
	if item.kind == .Array && item.unresolved_count {
		expr_id := ast.Expr_Id(item.count_expr)
		if expr_id != ast.INVALID_EXPR && int(expr_id) < len(checker.ast_module.exprs) {
			expr := checker.ast_module.exprs[expr_id]
			if expr.kind == .Name && !symbol.is_valid(expr.qualifier) {
				if _, found := comptime_binding_index(function, prefix, expr.name); found {
					if !type_params_only {
						return true
					}
					param, param_ok := comptime_param_for_name(function, expr.name)
					if param_ok && is_comptime_type_param(checker, param) {
						return true
					}
				}
			}
		}
	}
	if item.kind == .Type_Call {
		expr_id := ast.Expr_Id(item.count_expr)
		if expr_id != ast.INVALID_EXPR && int(expr_id) < len(checker.ast_module.exprs) {
			for arg_id in checker.ast_module.exprs[expr_id].args {
				if arg_id == ast.INVALID_EXPR || int(arg_id) >= len(checker.ast_module.exprs) {
					continue
				}
				arg := checker.ast_module.exprs[arg_id]
				if arg.kind == .Name && !symbol.is_valid(arg.qualifier) {
					if _, found := comptime_binding_index(function, prefix, arg.name); found {
						if !type_params_only {
							return true
						}
						param, param_ok := comptime_param_for_name(function, arg.name)
						if param_ok && is_comptime_type_param(checker, param) {
							return true
						}
					}
				}
			}
		}
	}
	if types.is_valid(item.child) && type_pattern_mentions_comptime(checker, function, prefix, item.child, depth+1, type_params_only) {
		return true
	}
	if types.is_valid(item.extra) && type_pattern_mentions_comptime(checker, function, prefix, item.extra, depth+1, type_params_only) {
		return true
	}
	if item.kind == .Function {
		for field in types.params_for(&checker.module.types, pattern) {
			if type_pattern_mentions_comptime(checker, function, prefix, field.type, depth+1, type_params_only) {
				return true
			}
		}
	}
	return false
}

match_inferred_type_pattern :: proc(
	checker: ^Checker,
	function: ast.Function,
	prefix: int,
	pattern, actual: types.Type,
	values: []Comptime_Value,
	bound: []bool,
	span: source.Span,
	diagnose: bool,
	depth := 0,
) -> bool {
	if depth > 64 || !types.is_valid(actual) {
		return false
	}
	store := &checker.module.types
	actual_type := types.resolve_alias(actual, store)
	pattern_item, pattern_ok := types.node(store, pattern)
	if pattern_ok && pattern_item.qualifier == 0 && pattern_item.name != 0 {
		name := symbol.Id(pattern_item.name)
		if binding_index, is_binding := comptime_binding_index(function, prefix, name); is_binding {
			param, param_ok := comptime_param_for_name(function, name)
			if !param_ok || !is_comptime_type_param(checker, param) {
				return false
			}
			if bound[binding_index] && can_implicitly_convert_type(checker, actual_type, values[binding_index].type) {
				return true
			}
			return bind_inferred_comptime(
				checker, function, prefix, values, bound, name,
				Comptime_Value{type=actual_type, kind=.Type}, span, diagnose,
			)
		}
	}
	if !pattern_ok {
		return types.equal(pattern, actual_type)
	}
	if pattern_item.kind == .Type_Call {
		expr_id := ast.Expr_Id(pattern_item.count_expr)
		if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
			return false
		}
		expr := checker.ast_module.exprs[expr_id]
		target_pkg, available := expr_package(checker, expr, function.pkg, function.file, true)
		if !available {
			return false
		}
		template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, function.file))
		origin: Type_Factory_Origin
		found := false
		for candidate in checker.type_factory_origins {
			if candidate.template == template && types.equal(candidate.result, actual_type) {
				origin = candidate
				found = true
				break
			}
		}
		if !found || len(expr.args) != len(origin.values) {
			return false
		}
		matched := true
		for arg_id, index in expr.args {
			if arg_id == ast.INVALID_EXPR || int(arg_id) >= len(checker.ast_module.exprs) {
				matched = false
				continue
			}
			arg := checker.ast_module.exprs[arg_id]
			if arg.kind == .Name && !symbol.is_valid(arg.qualifier) {
				if _, is_binding := comptime_binding_index(function, prefix, arg.name); is_binding {
					matched = bind_inferred_comptime(
						checker, function, prefix, values, bound, arg.name,
						origin.values[index], span, diagnose,
					) && matched
				}
			}
		}
		return matched
	}
	actual_item, actual_ok := types.node(store, actual_type)
	if pattern_item.kind == .Slice {
		actual_pointer, actual_array, array_pointer_ok := types.array_pointer(actual_type, store)
		if array_pointer_ok {
			mutable := actual_pointer.mutable && actual_array.mutable
			if pattern_item.mutable && !mutable ||
			   pattern_item.has_sentinel && (!actual_array.has_sentinel || pattern_item.sentinel != actual_array.sentinel) {
				return false
			}
			return match_inferred_type_pattern(
				checker, function, prefix, pattern_item.child, actual_array.child,
				values, bound, span, diagnose, depth+1,
			)
		}
	}
	if !actual_ok || pattern_item.kind != actual_item.kind {
		resolved := type_from_syntax(checker, pattern, function.pkg, function.file)
		return types.is_valid(resolved) && types.equal(types.resolve_alias(resolved, store), actual_type)
	}
	if pattern_item.many != actual_item.many || pattern_item.has_sentinel != actual_item.has_sentinel ||
	   pattern_item.has_sentinel && pattern_item.sentinel != actual_item.sentinel {
		return false
	}
	matched := true
	if pattern_item.kind == .Array {
		if pattern_item.unresolved_count {
			count_expr := ast.Expr_Id(pattern_item.count_expr)
			if count_expr != ast.INVALID_EXPR && int(count_expr) < len(checker.ast_module.exprs) {
				expr := checker.ast_module.exprs[count_expr]
				if expr.kind == .Name && !symbol.is_valid(expr.qualifier) {
					if binding_index, is_binding := comptime_binding_index(function, prefix, expr.name); is_binding {
						param, param_ok := comptime_param_for_name(function, expr.name)
						if param_ok && !is_comptime_type_param(checker, param) {
							matched = bind_inferred_comptime(
								checker, function, prefix, values, bound, expr.name,
								Comptime_Value{type=values[binding_index].type, value=i128(actual_item.count), kind=.Integer},
								span, diagnose,
							) && matched
						}
					}
				}
			}
		} else if pattern_item.count != actual_item.count {
			matched = false
		}
	}
	if types.is_valid(pattern_item.child) {
		matched = match_inferred_type_pattern(
			checker, function, prefix, pattern_item.child, actual_item.child,
			values, bound, span, diagnose, depth+1,
		) && matched
	}
	if types.is_valid(pattern_item.extra) {
		matched = match_inferred_type_pattern(
			checker, function, prefix, pattern_item.extra, actual_item.extra,
			values, bound, span, diagnose, depth+1,
		) && matched
	}
	if pattern_item.kind == .Function {
		pattern_params := types.params_for(store, pattern)
		actual_params := types.params_for(store, actual_type)
		if len(pattern_params) != len(actual_params) || pattern_item.c_abi != actual_item.c_abi || pattern_item.variadic != actual_item.variadic {
			return false
		}
		for field, index in pattern_params {
			matched = match_inferred_type_pattern(
				checker, function, prefix, field.type, actual_params[index].type,
				values, bound, span, diagnose, depth+1,
			) && matched
		}
	}
	return matched
}

infer_call_comptime_values :: proc(
	checker: ^Checker,
	function: ast.Function,
	prefix: int,
	mapping: []int,
	args: []ast.Expr_Id,
	actual_args: []types.Type,
	expected: types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	diagnose := false,
	failure: ^string = nil,
) -> ([]Comptime_Value, bool) {
	values := make([]Comptime_Value, prefix, checker.allocator)
	bound := make([]bool, prefix, checker.allocator)
	defer delete(bound, checker.allocator)
	ordinal := 0
	for param in function.params {
		if !param.comptime_value {
			continue
		}
		values[ordinal].name = param.name
		if is_comptime_type_param(checker, param) {
			values[ordinal].kind = .Type
		} else if is_comptime_string_param(checker, param, function) {
			values[ordinal].kind = .String
			values[ordinal].type = type_from_syntax(checker, param.type, function.pkg, function.file)
		} else if types.is_concrete_integer(type_from_syntax(checker, param.type, function.pkg, function.file)) {
			values[ordinal].kind = .Integer
			values[ordinal].type = type_from_syntax(checker, param.type, function.pkg, function.file)
		} else {
			values[ordinal].kind = .Static
			values[ordinal].type = type_from_syntax(checker, param.type, function.pkg, function.file)
		}
		ordinal += 1
	}
	matched := true
	for arg_id, source_index in args {
		param_index := call_param_index(mapping, source_index)
		if param_index < 0 || param_index >= len(function.params) || !function.params[param_index].comptime_value ||
		   arg_id == ast.INVALID_EXPR || int(arg_id) >= len(checker.ast_module.exprs) {
			continue
		}
		expr := checker.ast_module.exprs[arg_id]
		if expr.kind == .Inference_Hole {
			continue
		}
		param := function.params[param_index]
		binding_index, binding_ok := comptime_binding_index(function, prefix, param.name)
		if !binding_ok {
			matched = false
			continue
		}
		if is_comptime_type_param(checker, param) {
			actual, ok := resolve_type_argument(checker, arg_id, pkg, file)
			if !ok {
				if failure != nil && len(failure^) == 0 {
					failure^ = fmt.aprintf(
						"argument %d for comptime type parameter '%s' is not a type",
						source_index+1, symbol_text(checker, param.name), allocator=checker.allocator,
					)
				}
				if diagnose {
					source.addf(checker.diagnostics, expr.span,
						"argument for comptime type parameter '%s' must be a type",
						symbol_text(checker, param.name))
				}
				matched = false
				continue
			}
			values[binding_index] = Comptime_Value{name=param.name, type=actual, kind=.Type}
			bound[binding_index] = true
		} else if is_comptime_string_param(checker, param, function) {
			text, text_ok := comptime_string_argument(checker, arg_id, pkg, file)
			if !text_ok {
				if failure != nil && len(failure^) == 0 {
					failure^ = fmt.aprintf(
						"argument %d for comptime string parameter '%s' does not evaluate to immutable bytes",
						source_index+1, symbol_text(checker, param.name), allocator=checker.allocator,
					)
				}
				if diagnose {
					source.addf(checker.diagnostics, expr.span,
						"argument for comptime string parameter '%s' must evaluate to immutable bytes",
						symbol_text(checker, param.name))
				}
				matched = false
				continue
			}
			values[binding_index] = Comptime_Value{
				name=param.name,
				type=type_from_syntax(checker, param.type, function.pkg, function.file),
				text=text,
				kind=.String,
			}
			bound[binding_index] = true
		} else if types.is_concrete_integer(type_from_syntax(checker, param.type, function.pkg, function.file)) {
			declared := type_from_syntax(checker, param.type, function.pkg, function.file)
			constant := eval_integer_constant_in_context(checker, arg_id, pkg, file)
			if constant.kind != .Value {
				if failure != nil && len(failure^) == 0 {
					failure^ = fmt.aprintf(
						"argument %d for comptime parameter '%s' is not a compile-time integer expression",
						source_index+1, symbol_text(checker, param.name), allocator=checker.allocator,
					)
				}
				if diagnose {
					source.addf(checker.diagnostics, expr.span,
						"argument for comptime parameter '%s' must be a compile-time integer expression",
						symbol_text(checker, param.name))
				}
				matched = false
				continue
			}
			if !fits_integer_type(constant.value, declared, checker.target) {
				if failure != nil && len(failure^) == 0 {
					failure^ = fmt.aprintf(
						"argument %d integer constant %d does not fit in %s",
						source_index+1, constant.value, types.name(declared), allocator=checker.allocator,
					)
				}
				if diagnose {
					source.addf(checker.diagnostics, expr.span,
						"integer constant %d does not fit in %s", constant.value, types.name(declared))
				}
				matched = false
				continue
			}
			values[binding_index] = Comptime_Value{name=param.name, type=declared, value=constant.value, kind=.Integer}
			bound[binding_index] = true
		} else {
			available: [dynamic]Comptime_Value
			available.allocator = checker.allocator
			append(&available, ..checker.current_comptime_values)
			for prior, prior_index in values[:binding_index] {
				if bound[prior_index] {
					append(&available, prior)
				}
			}
			previous := checker.current_comptime_values
			checker.current_comptime_values = available[:]
			declared := type_from_syntax(checker, param.type, function.pkg, function.file)
			checker.current_comptime_values = previous
			value, value_ok := eval_static_comptime_value(
				checker, param.name, arg_id, declared, pkg, file, available[:], diagnose,
			)
			delete(available)
			if !value_ok {
				if failure != nil && len(failure^) == 0 {
					failure^ = fmt.aprintf(
						"argument %d for comptime parameter '%s' has no stable comptime identity",
						source_index+1, symbol_text(checker, param.name), allocator=checker.allocator,
					)
				}
				matched = false
				continue
			}
			values[binding_index] = value
			bound[binding_index] = true
		}
	}
	all_bound := true
	for value_bound in bound {
		all_bound = all_bound && value_bound
	}
	if !all_bound && is_runtime_type(checker, expected) {
		if types.is_valid(function.error) && types.kind(expected, &checker.module.types) == .Fallible {
			matched = match_inferred_type_pattern(
				checker, function, prefix, function.result,
				types.fallible_success(expected, &checker.module.types), values, bound,
				source.Span{}, diagnose,
			) && matched
			matched = match_inferred_type_pattern(
				checker, function, prefix, function.error,
				types.fallible_error(expected, &checker.module.types), values, bound,
				source.Span{}, diagnose,
			) && matched
		} else if type_pattern_mentions_comptime(checker, function, prefix, function.result) {
			matched = match_inferred_type_pattern(
				checker, function, prefix, function.result, expected, values, bound,
				source.Span{}, diagnose,
			) && matched
		}
	}
	all_bound = true
	for value_bound in bound {
		all_bound = all_bound && value_bound
	}
	// Concrete arguments bind first. Numeric constants and `none` are contextual and
	// therefore only contribute after stronger evidence has had a chance to bind the
	// parameter type.
	weak_passes := [2]bool{false, true}
	for weak in weak_passes {
		for arg_id, source_index in args {
			param_index := call_param_index(mapping, source_index)
			if param_index >= len(function.params) || param_index >= len(actual_args) {
				continue
			}
			if param_index < 0 || function.params[param_index].comptime_value {
				continue
			}
			arg_expr := checker.ast_module.exprs[arg_id]
			is_none := arg_expr.kind == .None
			is_weak := is_numeric_constant_expr(checker, arg_id) || is_none
			if is_weak != weak {
				continue
			}
			if !type_pattern_mentions_comptime(checker, function, prefix, function.params[param_index].type) {
				continue
			}
			// Once the result type has fixed every comptime parameter, an anonymous
			// keyed record must be checked against the specialized parameter type.
			// Its provisional structural type intentionally contains only the supplied
			// fields, so comparing that type here would reject omitted defaulted fields.
			if all_bound && arg_expr.kind == .Struct_Literal && !arg_expr.tuple && !symbol.is_valid(arg_expr.name) {
				continue
			}
			if weak {
				if item, ok := types.node(&checker.module.types, function.params[param_index].type); ok &&
				   item.qualifier == 0 && item.name != 0 {
					if binding_index, is_binding := comptime_binding_index(function, prefix, symbol.Id(item.name));
					   is_binding && bound[binding_index] {
						continue
					}
				}
			}
			actual := actual_args[param_index]
			if is_none {
				previous := checker.current_comptime_values
				checker.current_comptime_values = values
				contextual := type_from_syntax(
					checker, function.params[param_index].type, function.pkg, function.file,
				)
				checker.current_comptime_values = previous
				if types.is_optional(contextual, &checker.module.types) {
					actual = contextual
					actual_args[param_index] = contextual
				}
			}
			matched = match_inferred_type_pattern(
				checker, function, prefix, function.params[param_index].type,
				actual, values, bound,
				checker.ast_module.exprs[arg_id].span, diagnose,
			) && matched
		}
	}
	ordinal = 0
	for param in function.params {
		if !param.comptime_value {
			continue
		}
		if bound[ordinal] {
			ordinal += 1
			continue
		}
		matched = false
		if failure != nil && len(failure^) == 0 {
			failure^ = fmt.aprintf(
				"cannot infer comptime parameter '%s'",
				symbol_text(checker, param.name), allocator=checker.allocator,
			)
		}
		if diagnose {
			source.addf(
				checker.diagnostics, param.span,
				"cannot infer comptime parameter '%s'; pass it explicitly",
				symbol_text(checker, param.name),
			)
		}
		ordinal += 1
	}
	if !matched {
		if failure != nil && len(failure^) == 0 {
			failure^ = fmt.aprintf("comptime inference produced conflicting bindings", allocator=checker.allocator)
		}
		delete(values, checker.allocator)
		return nil, false
	}
	return values, true
}

call_arg_expected :: proc(checker: ^Checker, function: ast.Function, index: int) -> types.Type {
	if index < 0 || index >= len(function.params) {
		return types.INVALID
	}
	if is_comptime_type_param(checker, function.params[index]) {
		return types.INVALID
	}
	for param in function.params {
		if !param.comptime_value {
			continue
		}
		if _, ok := current_comptime_value(checker, param.name); !ok {
			return types.INVALID
		}
	}
	declared := type_from_syntax(checker, function.params[index].type, function.pkg, function.file)
	// A `float` param defaults to f64 so an integer-literal argument builds as a
	// float constant (e.g. `f(3)` -> 3.0), mirroring `pi float = 3` for locals.
	// Integer/range constraints keep building naturally from their literals.
	if declared == types.FLOAT {
		return types.F64
	}
	return declared
}

resolve_type_argument :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (types.Type, bool) {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return types.INVALID, false
	}
	expr := checker.ast_module.exprs[expr_id]
	#partial switch expr.kind {
	case .Type:
		resolved := type_from_syntax(checker, expr.type, pkg, file)
		return resolved, types.is_valid(resolved)
	case .Name:
		if symbol.is_valid(expr.qualifier) && symbol_text(checker, expr.name) == "type" {
			if value, ok := static_field_value(checker, expr.qualifier, expr.name); ok && value.kind == .Type {
				return types.Type(value.index), true
			}
		}
		if !symbol.is_valid(expr.qualifier) {
			if actual, ok := current_comptime_type(checker, expr.name); ok {
				return actual, true
			}
		}
		target_pkg, available := expr_package(checker, expr, pkg, file)
		if !available {
			return types.INVALID, false
		}
		value := types.find_named(&checker.module.types, u32(target_pkg), u32(expr.name), file=u32(expr_lookup_file(expr, file)))
		value = types.resolve_alias(value, &checker.module.types)
		return value, types.is_valid(value)
	case .Call:
		value := resolve_type_factory_call(checker, expr_id, pkg, file)
		return value, types.is_valid(value)
	}
	return types.INVALID, false
}

clone_comptime_values :: proc(values: []Comptime_Value, allocator: mem.Allocator) -> []Comptime_Value {
	result := make([]Comptime_Value, len(values), allocator)
	copy(result, values)
	return result
}

expand_expansions_equal :: proc(left, right: []Expand_Expansion) -> bool {
	if len(left) != len(right) {
		return false
	}
	for value, index in left {
		if value != right[index] {
			return false
		}
	}
	return true
}

find_call_resolution :: proc(
	checker: ^Checker,
	expr: ast.Expr_Id,
) -> (int, bool) {
	for index := len(checker.call_resolutions)-1; index >= 0; index -= 1 {
		entry := checker.call_resolutions[index]
		if entry.expr == expr &&
		   comptime_values_equal(entry.ctx, checker.current_comptime_values) &&
		   expand_expansions_equal(entry.expand_ctx, checker.expand_context[:]) {
			return index, true
		}
	}
	return -1, false
}

store_call_resolution :: proc(
	checker: ^Checker,
	expr: ast.Expr_Id,
	mapping: []int,
	comptime_values: []Comptime_Value,
	runtime_types: []types.Type,
) {
	entry := Call_Resolution{
		expr=expr,
		ctx=clone_comptime_values(checker.current_comptime_values, checker.allocator),
		expand_ctx=slice.clone(checker.expand_context[:], checker.allocator),
		mapping=slice.clone(mapping, checker.allocator),
		comptime_values=clone_comptime_values(comptime_values, checker.allocator),
		runtime_types=slice.clone(runtime_types, checker.allocator),
	}
	if index, ok := find_call_resolution(checker, expr); ok {
		previous := checker.call_resolutions[index]
		delete(previous.ctx, checker.allocator)
		delete(previous.expand_ctx, checker.allocator)
		delete(previous.mapping, checker.allocator)
		delete(previous.comptime_values, checker.allocator)
		delete(previous.runtime_types, checker.allocator)
		checker.call_resolutions[index] = entry
		return
	}
	append(&checker.call_resolutions, entry)
}

resolved_call_arg_expected :: proc(
	checker: ^Checker,
	function: ast.Function,
	param_index: int,
	resolution_index: int,
) -> types.Type {
	if resolution_index < 0 || resolution_index >= len(checker.call_resolutions) {
		return call_arg_expected(checker, function, param_index)
	}
	previous := checker.current_comptime_values
	checker.current_comptime_values = checker.call_resolutions[resolution_index].comptime_values
	result := call_arg_expected(checker, function, param_index)
	checker.current_comptime_values = previous
	return result
}

resolve_generated_struct_type :: proc(checker: ^Checker, expr_id: ast.Expr_Id, pkg: ast.Package_Id, file: ast.File_Id, active_state: ^Ct_State) -> types.Type {
	for entry in checker.generated_types {
		if entry.expr == expr_id && comptime_values_equal(entry.values, checker.current_comptime_values) {
			return entry.result
		}
	}
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return types.INVALID
	}
	expr := checker.ast_module.exprs[expr_id]
	field_start := int(u32(expr.integer>>32))
	field_count := int(u32(expr.integer))
	if field_start < 0 || field_count < 0 || field_start+field_count > len(checker.ast_module.type_fields) {
		return types.INVALID
	}
	template_fields := checker.ast_module.type_fields[field_start:field_start+field_count]
	fields := make([]types.Field, len(template_fields), checker.allocator)
	defer delete(fields, checker.allocator)
	for field, index in template_fields {
		resolved := type_from_syntax(checker, field.type, pkg, file, active_state=active_state)
		if (!is_runtime_type(checker, resolved) && !is_comptime_value_type(checker, resolved)) || types.is_void(resolved) {
			if expr.tuple {
				source.addf(checker.diagnostics, expr.span, "tuple element %d requires a concrete runtime type, got %s", index, type_label(checker, resolved))
			} else {
				source.addf(checker.diagnostics, expr.span, "anonymous struct field '%s' requires a concrete runtime type, got %s", symbol_text(checker, symbol.Id(field.name)), type_label(checker, resolved))
			}
			return types.INVALID
		}
		fields[index] = types.Field{name=field.name, type=resolved}
	}
	result := types.struct_generated(&checker.module.types, fields, expr.tuple)
	append(&checker.generated_types, Generated_Type_Entry{
		expr=expr_id,
		values=clone_comptime_values(checker.current_comptime_values, checker.allocator),
		result=result,
		pkg=pkg,
		file=file,
	})
	return result
}

resolve_type_factory_call :: proc(checker: ^Checker, expr_id: ast.Expr_Id, pkg: ast.Package_Id, file: ast.File_Id) -> types.Type {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return types.INVALID
	}
	expr := checker.ast_module.exprs[expr_id]
	if expr.kind != .Call || expr.left != ast.INVALID_EXPR {
		source.add(checker.diagnostics, expr.span, "type position requires a direct type-factory call")
		return types.INVALID
	}
	if expr.intrinsic {
		if !is_intrinsic_call(checker, expr, "struct_type") {
			return types.INVALID
		}
		state := ct_state_make(checker, pkg, file, values=checker.current_comptime_values)
		value, flow, ok := ct_eval_call_expr(&state, expr, types.INVALID, 0, expr_id)
		result := types.INVALID
		if ok && flow.kind == .Normal && value != INVALID_CT_VALUE && int(value) < len(state.values) && state.values[value].kind == .Type {
			result = types.Type(state.values[value].index)
		}
		ct_state_destroy(&state)
		return result
	}
	target_pkg, available := expr_package(checker, expr, pkg, file, true)
	if !available {
		_ = add_package_resolution_diagnostic(checker, expr, file)
		return types.INVALID
	}
	template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
	if template == ast.INVALID_FUNCTION || int(template) >= len(checker.ast_module.functions) {
		source.addf(checker.diagnostics, expr.span, "unknown type factory '%s'", symbol_text(checker, expr.name))
		return types.INVALID
	}
	function := checker.ast_module.functions[template]
	if !is_type_metatype_syntax(checker, function.result) || types.is_valid(function.error) {
		source.addf(checker.diagnostics, expr.span, "function '%s' does not return a type", symbol_text(checker, expr.name))
		return types.INVALID
	}
	for param in function.params {
		if !param.comptime_value {
			source.addf(checker.diagnostics, param.span, "type-factory parameter '%s' must be comptime", symbol_text(checker, param.name))
			return types.INVALID
		}
	}
	if !valid_call_arity(function, len(expr.args)) {
		source.addf(checker.diagnostics, expr.span, "type factory '%s' expects %d arguments, got %d", symbol_text(checker, expr.name), len(function.params), len(expr.args))
		return types.INVALID
	}
	values, ok := collect_comptime_values(checker, function, expr.args, pkg, file, true, checker.current_comptime_values)
	defer delete(values, checker.allocator)
	if !ok {
		return types.INVALID
	}
	// A generic function's declaration is validated before it has a specialization.
	// Leave calls containing its unresolved type parameters pending until then.
	for value in values {
		if value.kind != .Type {
			continue
		}
		if item, item_ok := types.node(&checker.module.types, value.type); item_ok && item.kind == .Named && !item.declared {
			return types.INVALID
		}
	}
	for &entry in checker.type_factories {
		if entry.template != template || !comptime_values_equal(entry.values, values) {
			continue
		}
		if entry.resolving {
			source.addf(checker.diagnostics, expr.span, "recursive type-factory specialization of '%s'", symbol_text(checker, expr.name))
			return types.INVALID
		}
		return entry.result
	}
	entry_index := len(checker.type_factories)
	append(&checker.type_factories, Type_Factory_Entry{
		template=template,
		values=clone_comptime_values(values, checker.allocator),
		result=types.INVALID,
		resolving=true,
	})
	state := ct_state_make(checker, pkg, file)
	value, flow, eval_ok := ct_eval_call_expr(&state, expr, function.result, 0, expr_id)
	result := types.INVALID
	if eval_ok && flow.kind == .Normal && value != INVALID_CT_VALUE && int(value) < len(state.values) && state.values[value].kind == .Type {
		result = types.Type(state.values[value].index)
	} else if state.diagnostic == source.INVALID_DIAGNOSTIC {
		source.addf(checker.diagnostics, expr.span, "type factory '%s' did not return a type", symbol_text(checker, expr.name))
	}
	ct_state_destroy(&state)
	checker.type_factories[entry_index].result = result
	checker.type_factories[entry_index].resolving = false
	record_type_factory_origin(checker, result, template, values)
	return result
}

record_type_factory_origin :: proc(
	checker: ^Checker,
	result: types.Type,
	template: ast.Function_Id,
	values: []Comptime_Value,
) {
	if !types.is_valid(result) {
		return
	}
	generated := false
	for entry in checker.generated_types {
		if types.equal(entry.result, result) {
			generated = true
			break
		}
	}
	if !generated {
		return
	}
	for origin in checker.type_factory_origins {
		if origin.template == template && types.equal(origin.result, result) &&
		   comptime_values_equal(origin.values, values) {
			return
		}
	}
	append(&checker.type_factory_origins, Type_Factory_Origin{
		result=result,
		template=template,
		values=clone_comptime_values(values, checker.allocator),
	})
}

collect_comptime_values :: proc(
	checker: ^Checker,
	function: ast.Function,
	args: []ast.Expr_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	diagnose := false,
	extra_values: []Comptime_Value = nil,
) -> ([]Comptime_Value, bool) {
	if !function_has_comptime_params(function) {
		return nil, true
	}
	values: [dynamic]Comptime_Value
	values.allocator = checker.allocator
	ok := true
	for param, index in function.params {
		if !param.comptime_value {
			continue
		}
		span := param.span
		if index < len(args) && args[index] != ast.INVALID_EXPR && int(args[index]) < len(checker.ast_module.exprs) {
			span = checker.ast_module.exprs[args[index]].span
		}
		if is_comptime_type_param(checker, param) {
			actual, actual_ok := types.INVALID, false
			if index < len(args) {
				actual, actual_ok = resolve_type_argument(checker, args[index], pkg, file)
			}
			if !actual_ok {
				if diagnose {
					source.addf(
						checker.diagnostics,
						span,
						"argument for comptime type parameter '%s' must be a type",
						symbol_text(checker, param.name),
					)
				}
				ok = false
				continue
			}
			append(&values, Comptime_Value{name=param.name, type=actual, kind=.Type})
			continue
		}
		if is_comptime_string_param(checker, param, function) {
			text, text_ok := "", false
			if index < len(args) {
				text, text_ok = comptime_string_argument(checker, args[index], pkg, file)
			}
			if !text_ok {
				if diagnose {
					source.addf(
						checker.diagnostics, span,
						"argument for comptime string parameter '%s' must evaluate to immutable bytes",
						symbol_text(checker, param.name),
					)
				}
				ok = false
				continue
			}
			append(&values, Comptime_Value{
				name=param.name,
				type=type_from_syntax(checker, param.type, function.pkg, function.file),
				text=text,
				kind=.String,
			})
			continue
		}
		resolution_values: [dynamic]Comptime_Value
		resolution_values.allocator = checker.allocator
		append(&resolution_values, ..extra_values)
		append(&resolution_values, ..values[:])
		previous := checker.current_comptime_values
		checker.current_comptime_values = resolution_values[:]
		declared := type_from_syntax(checker, param.type, function.pkg, function.file)
		checker.current_comptime_values = previous
		delete(resolution_values)
		if types.is_concrete_integer(declared) {
			constant := Constant{kind = .Not_Constant}
			if index < len(args) {
				constant = eval_integer_constant_in_context(checker, args[index], pkg, file, values=extra_values)
			}
			if constant.kind != .Value {
				if diagnose {
					source.addf(
						checker.diagnostics,
						span,
						"argument for comptime parameter '%s' must be a compile-time integer expression",
						symbol_text(checker, param.name),
					)
				}
				ok = false
				continue
			}
			if !fits_integer_type(constant.value, declared, checker.target) {
				if diagnose {
					source.addf(
						checker.diagnostics,
						span,
						"integer constant %d does not fit in %s",
						constant.value,
						types.name(declared),
					)
				}
				ok = false
				continue
			}
			append(&values, Comptime_Value{name=param.name, type=declared, value=constant.value})
			continue
		}
		if index >= len(args) || args[index] == ast.INVALID_EXPR {
			ok = false
			continue
		}
		available: [dynamic]Comptime_Value
		available.allocator = checker.allocator
		append(&available, ..extra_values)
		append(&available, ..values[:])
		value, value_ok := eval_static_comptime_value(
			checker, param.name, args[index], declared, pkg, file, available[:], diagnose,
		)
		delete(available)
		if !value_ok {
			ok = false
			continue
		}
		append(&values, value)
	}
	if !ok {
		delete(values)
		return nil, false
	}
	return values[:], true
}

callable_arg_expected :: proc(function_type: types.Type, function_item: types.Node, store: ^types.Store, index: int) -> types.Type {
	if index < 0 || index >= int(function_item.field_count) {
		return types.INVALID
	}
	params := types.params_for(store, function_type)
	if index >= len(params) {
		return types.INVALID
	}
	return params[index].type
}

valid_callable_arity :: proc(function_item: types.Node, count: int) -> bool {
	return count >= int(function_item.field_count) if function_item.variadic else count == int(function_item.field_count)
}

function_value_signature :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
) -> (params: []types.Type, result: types.Type, ok: bool) {
	if template == ast.INVALID_FUNCTION || int(template) >= len(checker.ast_module.functions) {
		return nil, types.INVALID, false
	}
	function := checker.ast_module.functions[template]
	if function_has_comptime_params(function) {
		return nil, types.INVALID, false
	}
	if function.c_abi && types.is_valid(function.error) {
		return nil, types.INVALID, false
	}
	if !function.c_abi && (!function.has_body || function.variadic) {
		return nil, types.INVALID, false
	}
	result = function_channel_type(checker, function)
	if !types.is_void(result) && !is_runtime_type(checker, result) {
		return nil, types.INVALID, false
	}
	params = make([]types.Type, len(function.params), checker.allocator)
	for param, index in function.params {
		param_type := type_from_syntax(checker, param.type, function.pkg, function.file)
		if !is_runtime_type(checker, param_type) {
			delete(params, checker.allocator)
			return nil, types.INVALID, false
		}
		params[index] = param_type
	}
	return params, result, true
}

function_type_for_template :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	demanded: ^[dynamic]Spec_Id = nil,
	demand_spec := true,
) -> (types.Type, Spec_Id, bool) {
	params, result, ok := function_value_signature(checker, template)
	if !ok {
		return types.INVALID, INVALID_SPEC, false
	}
	defer delete(params, checker.allocator)
	function := checker.ast_module.functions[template]
	function_type := types.function(&checker.module.types, params, result, function.c_abi, function.variadic)
	spec := INVALID_SPEC
	if demanded == nil {
		if demand_spec {
			spec = ensure_spec(checker, template, params)
		} else {
			spec = find_spec(checker, template, params)
		}
	} else {
		spec = find_spec(checker, template, params)
		mark_spec_demanded(checker, spec, demanded)
	}
	return function_type, spec, spec != INVALID_SPEC || !demand_spec
}

function_pointer_type_for_template :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	demanded: ^[dynamic]Spec_Id = nil,
	demand_spec := true,
) -> (types.Type, Spec_Id, bool) {
	function_type, spec, ok := function_type_for_template(checker, template, demanded, demand_spec)
	if !ok {
		return types.INVALID, spec, false
	}
	return types.pointer(&checker.module.types, function_type, false, false), spec, true
}

function_expr_type_for_template :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	expected: types.Type,
	demanded: ^[dynamic]Spec_Id = nil,
) -> (types.Type, bool) {
	function_type, _, ok := function_type_for_template(checker, template, demanded)
	if !ok {
		return types.INVALID, false
	}
	pointer_expected := expected
	if types.is_optional(pointer_expected, &checker.module.types) {
		pointer_expected = types.child_type(pointer_expected, &checker.module.types)
	}
	if types.can_coerce_function_pointer(function_type, pointer_expected, &checker.module.types) {
		return pointer_expected, true
	}
	return function_type, true
}

contains_name :: proc(names: []symbol.Id, name: symbol.Id) -> bool {
	for existing in names {
		if existing == name {
			return true
		}
	}
	return false
}

mark_expr_imports_used :: proc(checker: ^Checker, expr_id: ast.Expr_Id, file: ast.File_Id) {
	stack := checker.ast_expr_stack
	clear_dynamic_array(&stack)
	defer {
		clear_dynamic_array(&stack)
		checker.ast_expr_stack = stack
	}
	append(&stack, expr_id)
	for len(stack) > 0 {
		id := pop(&stack)
		if id == ast.INVALID_EXPR || int(id) >= len(checker.ast_module.exprs) {
			continue
		}
		expr := checker.ast_module.exprs[id]
		if is_intrinsic_call(checker, expr, "typeinfo") {
			for &import_item in checker.ast_module.imports {
				if import_item.valid && import_item.path == "@std/meta" {
					import_item.used = true
				}
			}
		}
		if (expr.kind == .Name || expr.kind == .Call) && symbol.is_valid(expr.qualifier) {
			_ = find_import(checker, file, expr.qualifier, true)
		}
		switch expr.kind {
		case .Call:
			append(&stack, ..expr.args)
			if expr.left != ast.INVALID_EXPR {
				append(&stack, expr.left)
			}
		case .Array, .Struct_Literal, .Slice:
			append(&stack, ..expr.args)
			if expr.left != ast.INVALID_EXPR {
				append(&stack, expr.left)
			}
		case .Negate, .Not, .Bit_Not, .Address, .Deref, .Field, .Unwrap, .Try, .Keyed, .Enum_Literal, .Cast:
			append(&stack, expr.left)
		case .Comptime:
			if expr.left != ast.INVALID_EXPR {
				append(&stack, expr.left)
			}
			mark_block_imports_used(checker, expr.body, file)
		case .Catch:
			append(&stack, expr.left)
			if expr.right != ast.INVALID_EXPR {
				append(&stack, expr.right)
			}
			mark_block_imports_used(checker, expr.body, file)
		case .Function_Literal:
			function_id := ast.Function_Id(u32(expr.integer))
			if function_id != ast.INVALID_FUNCTION && int(function_id) < len(checker.ast_module.functions) {
				function := checker.ast_module.functions[function_id]
				mark_block_imports_used(checker, function.body, function.file)
			}
		case .Add, .Sub, .Mul, .Div, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left, .Shift_Right,
		     .Shift_Left_Saturating, .Index, .Orelse, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
			append(&stack, expr.left, expr.right)
		case .Invalid, .Integer, .Float, .String, .Bool, .None, .Undefined, .Inference_Hole, .Type, .Name, .Anonymous_Struct_Type:
		}
	}
}

mark_block_imports_used :: proc(checker: ^Checker, statements: []ast.Stmt_Id, file: ast.File_Id) {
	for statement_id in statements {
		statement := checker.ast_module.statements[statement_id]
		#partial switch statement.kind {
		case .Declaration, .Assignment, .Return, .Expression, .Yield:
			mark_expr_imports_used(checker, statement.expr, file)
			if statement.target != ast.INVALID_EXPR {
				mark_expr_imports_used(checker, statement.target, file)
			}
			// A value-block declaration/assignment carries its block in `body`.
			mark_block_imports_used(checker, statement.body, file)
		case .If:
			mark_expr_imports_used(checker, statement.expr, file)
			if statement.guard != ast.INVALID_EXPR {
				mark_expr_imports_used(checker, statement.guard, file)
			}
			mark_block_imports_used(checker, statement.body, file)
			mark_block_imports_used(checker, statement.else_body, file)
		case .While:
			mark_expr_imports_used(checker, statement.expr, file)
			mark_block_imports_used(checker, statement.body, file)
			if statement.update != ast.INVALID_STMT {
				update := [1]ast.Stmt_Id{statement.update}
				mark_block_imports_used(checker, update[:], file)
			}
		case .For:
			mark_expr_imports_used(checker, statement.expr, file)
			mark_block_imports_used(checker, statement.body, file)
		case .Block:
			mark_block_imports_used(checker, statement.body, file)
		case .Defer:
			deferred := [1]ast.Stmt_Id{statement.update}
			mark_block_imports_used(checker, deferred[:], file)
		case .Match, .Match_Arm:
			// `Match` carries the subject in `expr` and arms in `body`; each `Match_Arm`
			// carries its patterns in `patterns` and the arm body in `body`.
			mark_expr_imports_used(checker, statement.expr, file)
			for pattern in statement.patterns {
				mark_expr_imports_used(checker, pattern, file)
			}
			mark_block_imports_used(checker, statement.body, file)
		case .Break, .Continue:
		case .Invalid:
		}
	}
}

validate_external_globals :: proc(checker: ^Checker) {
	for global, global_index in checker.ast_module.globals {
		checker.external_global_canonical[global_index] = ast.global_id(global_index)
		if !global.external {
			continue
		}
		switch global.link_name {
		case "main":
			checker.external_global_diagnostics[global_index] = source.add(
				checker.diagnostics,
				global.span,
				"external C variable 'main' conflicts with the program entry point",
			)
		case "write":
			checker.external_global_diagnostics[global_index] = source.add(
				checker.diagnostics,
				global.span,
				"external C variable 'write' conflicts with the compiler runtime",
			)
		}
		for previous, previous_index in checker.ast_module.globals[:global_index] {
			if !previous.external || previous.link_name != global.link_name {
				continue
			}
			canonical := checker.external_global_canonical[previous_index]
			if canonical == ast.INVALID_GLOBAL {
				canonical = ast.global_id(previous_index)
			}
			canonical_index := int(canonical)
			if canonical_index < 0 || canonical_index >= len(checker.ast_module.globals) {
				canonical = ast.global_id(previous_index)
				canonical_index = previous_index
			}
			checker.external_global_canonical[global_index] = canonical
			canonical_global := checker.ast_module.globals[canonical_index]
			canonical_type := checker.global_types[canonical_index]
			if !types.equal(checker.global_types[global_index], canonical_type) ||
			   global.writable != canonical_global.writable {
				checker.external_global_diagnostics[global_index] = source.addf(
					checker.diagnostics,
					global.span,
					"conflicting external C variable declarations for '%s'",
					global.link_name,
				)
			}
			checker.global_types[global_index] = canonical_type
			break
		}
		for function in checker.ast_module.functions {
			if !function.c_abi || function.has_body || len(function.unsupported_reason) > 0 ||
			   symbol_text(checker, function.name) != global.link_name {
				continue
			}
			if checker.external_global_diagnostics[global_index] == source.INVALID_DIAGNOSTIC {
				checker.external_global_diagnostics[global_index] = source.addf(
					checker.diagnostics,
					global.span,
					"external C variable '%s' conflicts with a C function declaration",
					global.link_name,
				)
			}
			break
		}
	}
}

runtime_write_declaration_matches :: proc(checker: ^Checker, function: ast.Function) -> bool {
	if function.variadic || len(function.params) != 3 || types.is_valid(function.error) {
		return false
	}
	store := &checker.module.types
	buffer := types.optional(store, types.pointer(store, types.ANYOPAQUE, false, true))
	return type_from_syntax(checker, function.params[0].type, function.pkg, function.file) == types.C_INT &&
	       type_from_syntax(checker, function.params[1].type, function.pkg, function.file) == buffer &&
	       type_from_syntax(checker, function.params[2].type, function.pkg, function.file) == types.C_ULONG &&
	       type_from_syntax(checker, function.result, function.pkg, function.file) == types.C_LONG
}

validate_declarations :: proc(checker: ^Checker) {
	for function, function_id in checker.ast_module.functions {
		if len(function.unsupported_reason) > 0 {
			continue
		}
		has_comptime := function_has_comptime_params(function)
		signature_poisoned := function.diagnostic != source.INVALID_DIAGNOSTIC
		locals: [dynamic]symbol.Id
		locals.allocator = checker.allocator
		comptime_prefix := 0
		for param in function.params {
			param_type := types.INVALID
			if param.comptime_value || !has_comptime {
				param_type = type_from_syntax(checker, param.type, function.pkg, function.file)
			}
			if param.comptime_value && !signature_poisoned {
				dependent := type_pattern_mentions_comptime(
					checker, function, comptime_prefix, param.type, type_params_only=true,
				)
				if function.c_abi {
					checker.template_diagnostics[function_id] = source.add(
						checker.diagnostics,
						param.span,
						"comptime parameters require 'func', not 'c_func'",
					)
				}
				if !is_type_metatype_syntax(checker, param.type) &&
				   !dependent && !is_comptime_value_type(checker, param_type) && param_type != types.RANGE {
					checker.template_diagnostics[function_id] = source.addf(
						checker.diagnostics,
						param.span,
						"comptime parameter '%s' requires type or a concrete value type",
						symbol_text(checker, param.name),
					)
				}
			} else if !has_comptime && !signature_poisoned && types.is_comptime_only(param_type, &checker.module.types) {
				checker.template_diagnostics[function_id] = source.addf(
					checker.diagnostics,
					param.span,
					"parameter '%s' has a comptime-only type; prefix it with '$'",
					symbol_text(checker, param.name),
				)
			} else if !has_comptime && !signature_poisoned {
				if diagnostic := add_unsupported_type_diagnostic(checker, param.span, param_type);
				   diagnostic != source.INVALID_DIAGNOSTIC {
					checker.template_diagnostics[function_id] = diagnostic
					continue
				}
			}
			if param.comptime_value {
				comptime_prefix += 1
			}
			if param.type == types.VOID {
				source.add(
					checker.diagnostics,
					param.span,
					"void is only valid as a function result type",
				)
			}
			if param.name != checker.sink_symbol && contains_name(locals[:], param.name) {
				source.addf(
					checker.diagnostics,
					param.span,
					"duplicate parameter '%s'",
					symbol_text(checker, param.name),
				)
			} else {
				_ = add_shadow_diagnostic(
					checker, param.span, param.name, "parameter",
					function.pkg, function.file,
				)
			}
			append(&locals, param.name)
			if !has_comptime && !signature_poisoned && types.contains_c_struct_by_value(param_type, &checker.module.types) {
				checker.template_diagnostics[function_id] = source.addf(
					checker.diagnostics,
					param.span,
					"C records cannot be passed by value to '%s'",
					symbol_text(checker, function.name),
				)
			}
		}
		if !has_comptime && !signature_poisoned {
			result_type := type_from_syntax(checker, function.result, function.pkg, function.file)
			if diagnostic := add_unsupported_type_diagnostic(checker, function.span, result_type);
			   diagnostic != source.INVALID_DIAGNOSTIC {
				checker.template_diagnostics[function_id] = diagnostic
			}
			if types.contains_c_struct_by_value(result_type, &checker.module.types) {
				checker.template_diagnostics[function_id] = source.addf(
					checker.diagnostics,
					function.span,
					"C records cannot be returned by value from '%s'",
					symbol_text(checker, function.name),
				)
			}
		}
		if types.is_valid(function.error) && !signature_poisoned {
			error_type := type_from_syntax(checker, function.error, function.pkg, function.file)
			error_sum := types.is_enum(error_type, &checker.module.types) ||
			             types.is_tagged_union(error_type, &checker.module.types)
			if function.c_abi {
				checker.template_diagnostics[function_id] = source.add(
					checker.diagnostics,
					function.span,
					"fallible functions must use 'func', not 'c_func'",
				)
			} else if !error_sum {
				checker.template_diagnostics[function_id] = source.add(
					checker.diagnostics,
					function.span,
					"fallible function error type must be a native enum or tagged union",
				)
			}
		}
		if !function.has_body && !function.c_abi && !signature_poisoned {
			checker.template_diagnostics[function_id] = source.addf(
				checker.diagnostics,
				function.span,
				"bodyless function '%s' must use 'c_func'",
				symbol_text(checker, function.name),
			)
		}
		if function.variadic && (!function.c_abi || function.has_body) && !signature_poisoned {
			checker.template_diagnostics[function_id] = source.addf(
				checker.diagnostics,
				function.span,
				"variadic function '%s' must be a bodyless 'c_func' declaration",
				symbol_text(checker, function.name),
			)
		}
		if !function.has_body && function.c_abi && !signature_poisoned {
			for param in function.params {
				param_type := type_from_syntax(checker, param.type, function.pkg, function.file)
				if add_unsupported_type_diagnostic(checker, param.span, param_type) !=
				   source.INVALID_DIAGNOSTIC {
					continue
				}
				if types.contains_c_struct_by_value(param_type, &checker.module.types) {
					continue
				}
				if !types.is_c_signature_type(param_type, &checker.module.types) {
					checker.template_diagnostics[function_id] = source.addf(
						checker.diagnostics,
						param.span,
						"foreign function '%s' requires concrete parameter types",
						symbol_text(checker, function.name),
					)
				}
			}
			result := type_from_syntax(checker, function.result, function.pkg, function.file)
			if add_unsupported_type_diagnostic(checker, function.span, result) == source.INVALID_DIAGNOSTIC &&
			   !types.contains_c_struct_by_value(result, &checker.module.types) &&
			   !types.is_c_signature_type(result, &checker.module.types, true) {
				checker.template_diagnostics[function_id] = source.addf(
					checker.diagnostics,
					function.span,
					"foreign function '%s' requires a concrete or void result type",
					symbol_text(checker, function.name),
				)
			}
			if function.pkg == 0 && function.name == checker.main_symbol {
				checker.template_diagnostics[function_id] = source.add(
					checker.diagnostics,
					function.span,
					"main must have a body",
				)
			}
			external_name := function.link_name if len(function.link_name) > 0 else symbol_text(checker, function.name)
			if external_name == "write" && !runtime_write_declaration_matches(checker, function) {
				checker.template_diagnostics[function_id] = source.add(
					checker.diagnostics,
					function.span,
					"external C function 'write' conflicts with the compiler runtime declaration",
				)
			}
		}
		mark_block_imports_used(checker, function.body, function.file)
		delete(locals)
	}
	for function, function_id in checker.ast_module.functions {
		if function.has_body || !function.c_abi {
			continue
		}
		for other, other_id in checker.ast_module.functions {
			if other_id == function_id || other.has_body || !other.c_abi || other.name != function.name {
				continue
			}
			if function.imported && other.imported && function_signatures_equal(function, other) {
				continue
			}
			checker.template_diagnostics[function_id] = source.addf(
				checker.diagnostics,
				function.span,
				"duplicate foreign symbol '%s'",
				symbol_text(checker, function.name),
			)
			break
		}
	}
}

init_record_field_inference :: proc(checker: ^Checker) {
	store := &checker.module.types
	for item in store.nodes {
		if !(item.declared && (item.kind == .Struct || item.kind == .Union) &&
		     !item.c_layout && symbol.is_valid(symbol.Id(item.name))) {
			continue
		}
		start := int(item.field_start)
		end := start+int(item.field_count)
		if start < 0 || end > len(store.fields) {
			continue
		}
		for slot in start..<end {
			if types.is_constraint(store.fields[slot].type) {
				checker.record_field_constraints[slot] = store.fields[slot].type
			}
		}
	}
}

is_inferred_record_field :: proc(checker: ^Checker, slot: int) -> bool {
	return slot >= 0 && slot < len(checker.record_field_constraints) &&
	       types.is_constraint(checker.record_field_constraints[slot])
}

record_field_owner :: proc(checker: ^Checker, slot: int) -> (symbol.Id, symbol.Id, ast.Package_Id, bool) {
	for item in checker.module.types.nodes {
		if !(item.declared && (item.kind == .Struct || item.kind == .Union) &&
		     !item.c_layout && symbol.is_valid(symbol.Id(item.name))) {
			continue
		}
		start := int(item.field_start)
		if slot >= start && slot < start+int(item.field_count) &&
		   slot >= 0 && slot < len(checker.module.types.fields) {
			return symbol.Id(item.name), symbol.Id(checker.module.types.fields[slot].name), ast.Package_Id(item.pkg), true
		}
	}
	return symbol.INVALID, symbol.INVALID, ast.INVALID_PACKAGE, false
}

record_field_conflict :: proc(checker: ^Checker, slot: int, actual: types.Type, span: source.Span) {
	if !is_inferred_record_field(checker, slot) ||
	   types.is_valid(checker.record_field_conflicts[slot]) {
		return
	}
	checker.record_field_conflicts[slot] = actual
	checker.record_field_conflict_spans[slot] = span
}

merge_record_field_default :: proc(checker: ^Checker, slot: int, candidate: types.Type, span: source.Span) -> bool {
	if !is_inferred_record_field(checker, slot) || !is_runtime_type(checker, candidate) {
		return false
	}
	constraint := checker.record_field_constraints[slot]
	if !types.constraint_accepts(constraint, candidate, &checker.module.types) {
		record_field_conflict(checker, slot, candidate, span)
		return false
	}
	current := checker.record_field_defaults[slot]
	if !is_runtime_type(checker, current) {
		checker.record_field_defaults[slot] = candidate
		checker.record_field_demands_dirty = true
		return true
	}
	if types.equal(current, candidate) {
		return false
	}
	merged := types.widest(current, candidate)
	if types.is_concrete_scalar(merged) {
		if !types.equal(current, merged) {
			checker.record_field_defaults[slot] = merged
			checker.record_field_demands_dirty = true
			return true
		}
		return false
	}
	record_field_conflict(checker, slot, candidate, span)
	return false
}

merge_record_field_demand :: proc(checker: ^Checker, slot: int, demand: types.Type, span: source.Span) -> bool {
	if !is_inferred_record_field(checker, slot) || !is_runtime_type(checker, demand) {
		return false
	}
	constraint := checker.record_field_constraints[slot]
	if !types.constraint_accepts(constraint, demand, &checker.module.types) {
		record_field_conflict(checker, slot, demand, span)
		return false
	}
	current := checker.module.types.fields[slot].type
	if types.is_constraint(current) {
		checker.module.types.fields[slot].type = demand
		checker.record_field_demands_dirty = true
		return true
	}
	if types.equal(current, demand) {
		return false
	}
	merged := types.widest(current, demand)
	if types.is_concrete_scalar(merged) {
		if !types.equal(current, merged) {
			checker.module.types.fields[slot].type = merged
			checker.record_field_demands_dirty = true
			return true
		}
		return false
	}
	record_field_conflict(checker, slot, demand, span)
	return false
}

record_field_expr_candidate :: proc(
	checker: ^Checker,
	slot: int,
	expr_id: ast.Expr_Id,
	inferred: types.Type,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> bool {
	if !is_inferred_record_field(checker, slot) || expr_id == ast.INVALID_EXPR {
		return false
	}
	expr := checker.ast_module.exprs[expr_id]
	constraint := checker.record_field_constraints[slot]
	if !numeric_operand_is_open(checker, expr_id, locals, pkg, file) {
		concrete := types.constraint_target(constraint, inferred, &checker.module.types)
		if is_runtime_type(checker, concrete) {
			return merge_record_field_demand(checker, slot, concrete, expr.span)
		}
	}
	if constant := eval_integer_constant_in_context(checker, expr_id, pkg, file);
	   constant.kind == .Value {
		candidate := constraint_integer_literal_type(constraint, constant.value)
		if constraint == types.FLOAT {
			candidate = types.F64 if fits_i64(constant.value) else types.INVALID
		}
		if types.is_valid(candidate) {
			return merge_record_field_default(checker, slot, candidate, expr.span)
		}
	}
	if is_float_constant_expr(checker, expr_id) {
		return merge_record_field_default(checker, slot, types.F64, expr.span)
	}
	if numeric_operand_is_open(checker, expr_id, locals, pkg, file) {
		candidate := inferred
		if constraint == types.FLOAT && types.is_concrete_integer(candidate) {
			candidate = types.F64
		}
		return merge_record_field_default(checker, slot, candidate, expr.span)
	}
	concrete := types.constraint_target(constraint, inferred, &checker.module.types)
	if !is_runtime_type(checker, concrete) {
		if is_runtime_type(checker, inferred) {
			record_field_conflict(checker, slot, inferred, expr.span)
		}
		return false
	}
	return merge_record_field_demand(checker, slot, concrete, expr.span)
}

finalize_record_field_inference :: proc(checker: ^Checker) {
	for constraint, slot in checker.record_field_constraints {
		if !types.is_constraint(constraint) {
			continue
		}
		record_name, field_name, record_pkg, ok := record_field_owner(checker, slot)
		if !ok {
			continue
		}
		current := checker.module.types.fields[slot].type
		conflict := checker.record_field_conflicts[slot]
		if types.is_valid(conflict) {
			if is_runtime_type(checker, current) {
				source.addf(
					checker.diagnostics,
					checker.record_field_conflict_spans[slot],
					"conflicting types %s and %s for field '%s.%s' declared as '%s'",
					types.name(current), types.name(conflict),
					symbol_text(checker, record_name), symbol_text(checker, field_name),
					types.name(constraint),
				)
			} else {
				source.addf(
					checker.diagnostics,
					checker.record_field_conflict_spans[slot],
					"type %s does not satisfy the '%s' constraint for field '%s.%s'",
					types.name(conflict), types.name(constraint),
					symbol_text(checker, record_name), symbol_text(checker, field_name),
				)
			}
		} else if types.is_constraint(current) &&
		          !(int(record_pkg) < len(checker.poisoned_packages) && checker.poisoned_packages[record_pkg]) {
			source.addf(
				checker.diagnostics,
				source.Span{},
				"could not resolve the '%s' constraint for field '%s.%s'",
				types.name(constraint),
				symbol_text(checker, record_name), symbol_text(checker, field_name),
			)
		}
		if types.is_constraint(current) {
			checker.module.types.fields[slot].type = constraint_recovery_type(checker, constraint)
		}
	}
}

is_immutable_u8_slice :: proc(checker: ^Checker, value: types.Type) -> bool {
	item, ok := types.node(&checker.module.types, value)
	return ok && item.kind == .Slice && !item.mutable && item.child == types.U8
}

validate_meta_schema :: proc(checker: ^Checker) {
	meta_package := ast.INVALID_PACKAGE
	for import_item in checker.ast_module.imports {
		if import_item.valid && import_item.path == "@std/meta" {
			meta_package = import_item.target
			break
		}
	}
	if meta_package == ast.INVALID_PACKAGE {
		return
	}
	find := proc(checker: ^Checker, pkg: ast.Package_Id, name: string) -> types.Type {
		return types.find_named(&checker.module.types, u32(pkg), u32(symbol.intern(checker.symbols, name)))
	}
	field_info := find(checker, meta_package, "FieldInfo")
	record_info := find(checker, meta_package, "RecordInfo")
	enum_info := find(checker, meta_package, "EnumInfo")
	type_info := find(checker, meta_package, "TypeInfo")
	layout := find(checker, meta_package, "Layout")
	valid := types.is_valid(field_info) && types.is_valid(record_info) && types.is_valid(enum_info) &&
	         types.is_valid(type_info) && types.is_valid(layout)
	layout_item, layout_ok := types.node(&checker.module.types, layout)
	layout_members := types.enum_members_for(&checker.module.types, layout)
	valid = valid && layout_ok && layout_item.kind == .Enum && len(layout_members) == 2
	if valid {
		valid = symbol_text(checker, symbol.Id(layout_members[0].name)) == "auto" &&
		        symbol_text(checker, symbol.Id(layout_members[1].name)) == "c"
	}
	field_item, field_ok := types.node(&checker.module.types, field_info)
	record_item, record_ok := types.node(&checker.module.types, record_info)
	enum_item, enum_ok := types.node(&checker.module.types, enum_info)
	valid = valid && field_ok && field_item.kind == .Struct && !field_item.tuple && !field_item.c_layout &&
	        record_ok && record_item.kind == .Struct && !record_item.tuple && !record_item.c_layout &&
	        enum_ok && enum_item.kind == .Struct && !enum_item.tuple && !enum_item.c_layout
	field_fields := types.fields_for(&checker.module.types, field_info)
	valid = valid && len(field_fields) == 3
	if valid {
		valid = symbol_text(checker, symbol.Id(field_fields[0].name)) == "name" &&
		        is_immutable_u8_slice(checker, field_fields[0].type) &&
		        symbol_text(checker, symbol.Id(field_fields[1].name)) == "type" &&
		        is_type_metatype_syntax(checker, field_fields[1].type) &&
		        symbol_text(checker, symbol.Id(field_fields[2].name)) == "index" &&
		        types.equal(field_fields[2].type, types.USIZE)
	}
	enum_fields := types.fields_for(&checker.module.types, enum_info)
	valid = valid && len(enum_fields) == 1
	if valid {
		fields_item, fields_ok := types.node(&checker.module.types, enum_fields[0].type)
		valid = symbol_text(checker, symbol.Id(enum_fields[0].name)) == "fields" && fields_ok &&
		        fields_item.kind == .Slice && !fields_item.mutable && types.equal(fields_item.child, field_info)
	}
	record_fields := types.fields_for(&checker.module.types, record_info)
	valid = valid && len(record_fields) == 4
	if valid {
		name_type := types.child_type(record_fields[0].type, &checker.module.types)
		fields_item, fields_ok := types.node(&checker.module.types, record_fields[1].type)
		valid = symbol_text(checker, symbol.Id(record_fields[0].name)) == "name" &&
		        types.is_optional(record_fields[0].type, &checker.module.types) && is_immutable_u8_slice(checker, name_type) &&
		        symbol_text(checker, symbol.Id(record_fields[1].name)) == "fields" && fields_ok &&
		        fields_item.kind == .Slice && !fields_item.mutable && types.equal(fields_item.child, field_info) &&
		        symbol_text(checker, symbol.Id(record_fields[2].name)) == "is_tuple" && types.is_bool(record_fields[2].type) &&
		        symbol_text(checker, symbol.Id(record_fields[3].name)) == "layout" && types.equal(record_fields[3].type, layout)
	}
	type_item, type_ok := types.node(&checker.module.types, type_info)
	type_fields := types.fields_for(&checker.module.types, type_info)
	expected_tags := []string{
		"invalid", "void", "anyopaque", "bool", "integer", "float", "array", "pointer", "slice",
		"range", "optional", "function", "enum", "record", "union", "fallible", "distinct",
	}
	valid = valid && type_ok && type_item.kind == .Union &&
	        types.is_enum(type_item.child, &checker.module.types) && len(type_fields) == len(expected_tags)
	if valid {
		for tag, index in expected_tags {
			field := type_fields[index]
			if symbol_text(checker, symbol.Id(field.name)) != tag ||
			   (tag == "record" && !types.equal(field.type, record_info)) ||
			   (tag == "enum" && !types.equal(field.type, enum_info)) ||
			   (tag != "record" && tag != "enum" && !types.is_void(field.type)) {
				valid = false
				break
			}
		}
	}
	if !valid {
		source.add(checker.diagnostics, source.Span{}, "@std/meta declarations do not match the compiler reflection ABI")
	}
}

validate_type_nodes :: proc(checker: ^Checker) {
	node_count := len(checker.module.types.nodes)
	for index in 0..<node_count {
		item := checker.module.types.nodes[index]
		if item.kind != .Struct && item.kind != .Union {
			continue
		}
		start := int(item.field_start)
		end := start+int(item.field_count)
		if start < 0 || end > len(checker.module.types.fields) {
			continue
		}
		for slot in start..<end {
			resolved := type_from_syntax(
				checker, checker.module.types.fields[slot].type,
				ast.Package_Id(item.pkg), ast.File_Id(item.file),
			)
			checker.module.types.fields[slot].type = resolved
		}
	}
	validate_meta_schema(checker)
	for item, index in checker.module.types.nodes {
		id := types.DYNAMIC_START+types.Type(index)
		if item.kind == .Distinct &&
		   (!item.declared || !types.is_runtime_value(item.child, &checker.module.types)) {
			source.addf(
				checker.diagnostics,
				source.Span{},
				"distinct type '%s' requires a concrete runtime backing type",
				symbol_text(checker, symbol.Id(item.name)),
			)
		}
		if item.kind == .Enum {
			if !item.declared || !types.is_concrete_integer(item.child) {
				source.addf(
					checker.diagnostics,
					source.Span{},
					"enum type '%s' requires a concrete integer backing type",
					symbol_text(checker, symbol.Id(item.name)),
				)
			} else {
				for member in types.enum_members_for(&checker.module.types, id) {
					if !fits_integer_type(member.value, item.child, checker.target) {
						source.addf(
							checker.diagnostics,
							source.Span{},
							"enum value %d does not fit in %s",
							member.value,
							types.name(item.child),
						)
					}
				}
			}
		}
		if item.has_sentinel {
			value := i128(item.sentinel)
			if types.is_signed(item.child, checker.target) {
				value = i128(i64(item.sentinel))
			}
			if !types.is_concrete_integer(item.child) || !fits_integer_type(value, item.child, checker.target) {
				source.addf(
					checker.diagnostics,
					source.Span{},
					"sentinel value does not fit array, slice, or pointer element type %s",
					types.name(item.child),
				)
			}
		}
		if item.kind == .Struct || item.kind == .Union {
			meta_package := false
			for import_item in checker.ast_module.imports {
				if import_item.valid && import_item.target == ast.Package_Id(item.pkg) && import_item.path == "@std/meta" {
					meta_package = true
					break
				}
			}
			item_name := symbol_text(checker, symbol.Id(item.name))
			comptime_meta := meta_package &&
				                 (item_name == "FieldInfo" || item_name == "RecordInfo" || item_name == "EnumInfo" || item_name == "TypeInfo")
			if item.c_layout && !item.opaque && item.field_count == 0 {
				source.add(
					checker.diagnostics,
					source.Span{},
					"c_struct definitions require at least one field",
				)
			}
			// A tagged union may carry `void`-payload variants (`.quit void`): the
			// variant has no runtime value, only a tag. Allowed only here, not for
			// structs, untagged unions, or c_structs.
			tagged_union := item.kind == .Union && types.is_enum(item.child, &checker.module.types)
			for field, field_index in types.fields_for(&checker.module.types, id) {
				field_slot := int(item.field_start)+field_index
				if tagged_union && types.is_void(field.type) {
					// void variant: no payload to validate.
				} else if is_inferred_record_field(checker, field_slot) {
					// Direct constraints in named native records are validated after the
					// program-wide inference fixpoint. C, nested, and anonymous fields do
					// not enter this state and retain the existing validation below.
				} else if comptime_meta {
					// Reflection metadata is compile-time-only and may contain `type`.
				} else if item.c_layout && !types.is_runtime_value(field.type, &checker.module.types) {
					source.add(
						checker.diagnostics,
						source.Span{},
						"c_struct fields must have C-layout-compatible types",
					)
				} else if !types.is_runtime_value(field.type, &checker.module.types) &&
				          !is_comptime_value_type(checker, field.type) {
					source.add(
						checker.diagnostics,
						source.Span{},
						"record fields must have runtime value types",
					)
				} else if item.c_layout && !types.is_c_record_field_type(field.type, &checker.module.types) {
					source.add(
						checker.diagnostics,
						source.Span{},
						"c_struct fields must have C-layout-compatible types",
					)
				}
			}
			// A tagged union stores a hidden runtime tag enum keyed by global variant IDs.
			// An explicit `union(Enum)` keeps that declared enum only for validation.
			if item.kind == .Union && types.is_valid(item.child) {
				declared_tag := types.union_declared_tag_enum(id, &checker.module.types)
				if !types.is_enum(declared_tag, &checker.module.types) {
					source.add(checker.diagnostics, source.Span{}, "a tagged union's tag must be an enum")
				} else {
					for field in types.fields_for(&checker.module.types, id) {
						if _, ok := find_enum_member(checker, declared_tag, symbol.Id(field.name)); !ok {
							source.addf(
								checker.diagnostics,
								source.Span{},
								"union variant '%s' is not a member of the tag enum",
								symbol_text(checker, symbol.Id(field.name)),
							)
						}
					}
				}
			}
		}
		if item.kind == .Function {
			unresolved := type_contains_unresolved_named(checker, item.child)
			for param in types.params_for(&checker.module.types, id) {
				unresolved = unresolved || type_contains_unresolved_named(checker, param.type)
			}
			if unresolved {
				continue
			}
			if item.c_abi {
				if types.kind(item.child, &checker.module.types) == .Fallible {
					source.add(checker.diagnostics, source.Span{}, "c_func pointer results cannot be fallible")
				}
				for param in types.params_for(&checker.module.types, id) {
					if types.is_void(param.type) || !types.is_c_signature_type(param.type, &checker.module.types) {
						source.add(checker.diagnostics, source.Span{}, "c_func pointer parameters must be concrete C signature types")
					}
				}
				if !types.is_c_signature_type(item.child, &checker.module.types, true) {
					source.add(checker.diagnostics, source.Span{}, "c_func pointer results must be concrete C signature types or void")
				}
			} else {
				if item.variadic {
					source.add(checker.diagnostics, source.Span{}, "native function pointer types do not support variadic parameters")
				}
				for param in types.params_for(&checker.module.types, id) {
					if types.is_void(param.type) || !is_runtime_type(checker, param.type) {
						source.add(checker.diagnostics, source.Span{}, "native function pointer parameters must be concrete runtime types")
					}
				}
				if !types.is_void(item.child) && !is_runtime_type(checker, item.child) {
					source.add(checker.diagnostics, source.Span{}, "native function pointer results must be concrete runtime types or void")
				}
			}
		}
	}
}

find_infer_local :: proc(locals: []Infer_Local, name: symbol.Id) -> types.Type {
	if index, ok := find_infer_local_index(locals, name); ok {
		return locals[index].type
	}
	return types.INVALID
}

find_infer_local_index :: proc(locals: []Infer_Local, name: symbol.Id) -> (int, bool) {
	for index := len(locals) - 1; index >= 0; index -= 1 {
		if locals[index].name == name {
			return index, true
		}
	}
	return -1, false
}

find_spec :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	actual_args: []types.Type,
	comptime_values: []Comptime_Value = nil,
) -> Spec_Id {
	function := checker.ast_module.functions[template]
	for spec, index in checker.specs {
		if spec.template != template || len(spec.args) != runtime_param_count(function) ||
		   !comptime_values_equal(spec.comptime_values, comptime_values) {
			continue
		}
		matches := true
		runtime_index := 0
		previous_comptime := checker.current_comptime_values
		checker.current_comptime_values = comptime_values
		for param, param_index in function.params {
			if param.comptime_value {
				continue
			}
			actual := types.INVALID
			if param_index < len(actual_args) {
				actual = actual_args[param_index]
			}
			if runtime_index >= len(spec.args) ||
			   !types.equal(spec.args[runtime_index], specialized_param_type(checker, param.type, actual, function.pkg, function.file)) {
				matches = false
				break
			}
			runtime_index += 1
		}
		checker.current_comptime_values = previous_comptime
		if matches {
			return spec_id(index)
		}
	}
	return INVALID_SPEC
}

// specialized_param_type maps a parameter's declared type to its monomorphized
// type for a given actual argument. A constraint param (`int`/`uint`/`float`/`range`)
// resolves to the actual's family member (INVALID if out of family), so a call
// passing an out-of-family argument fails to specialize and is rejected.
specialized_param_type :: proc(
	checker: ^Checker,
	syntax: ast.Type_Syntax,
	actual: types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> types.Type {
	declared := type_from_syntax(checker, syntax, pkg, file)
	if types.is_constraint(declared) {
		return types.constraint_target(declared, actual, &checker.module.types)
	}
	return declared
}

can_specialize :: proc(
	checker: ^Checker,
	function: ast.Function,
	actual_args: []types.Type,
	comptime_values: []Comptime_Value = nil,
) -> bool {
	previous_comptime := checker.current_comptime_values
	checker.current_comptime_values = comptime_values
	defer checker.current_comptime_values = previous_comptime
	for param, index in function.params {
		if param.comptime_value {
			continue
		}
		actual := types.INVALID
		if index < len(actual_args) {
			actual = actual_args[index]
		}
		if !is_runtime_type(checker, specialized_param_type(checker, param.type, actual, function.pkg, function.file)) {
			return false
		}
	}
	return true
}

ensure_spec :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	actual_args: []types.Type,
	comptime_values: []Comptime_Value = nil,
) -> Spec_Id {
	if existing := find_spec(checker, template, actual_args, comptime_values); existing != INVALID_SPEC {
		return existing
	}
	function := checker.ast_module.functions[template]
	previous_comptime := checker.current_comptime_values
	checker.current_comptime_values = comptime_values
	defer checker.current_comptime_values = previous_comptime
	signature: [dynamic]types.Type
	signature.allocator = checker.allocator
	for param, index in function.params {
		if param.comptime_value {
			continue
		}
		actual := types.INVALID
		if index < len(actual_args) {
			actual = actual_args[index]
		}
		append(&signature, specialized_param_type(checker, param.type, actual, function.pkg, function.file))
	}
	comptime_signature: [dynamic]Comptime_Value
	comptime_signature.allocator = checker.allocator
	append(&comptime_signature, ..comptime_values)
	result := function_channel_type(checker, function)
	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT &&
	   !types.is_valid(function.error) {
		result = types.I32
	}
	index := spec_id(len(checker.specs))
	append(
		&checker.specs,
		Spec{
			template = template,
			args = signature[:],
			comptime_values = comptime_signature[:],
			result = result,
			hir_id = hir.INVALID_FUNCTION,
		},
	)
	return index
}

mark_spec_demanded :: proc(checker: ^Checker, id: Spec_Id, stack: ^[dynamic]Spec_Id) {
	if id == INVALID_SPEC || checker.specs[id].hir_id != hir.INVALID_FUNCTION {
		return
	}
	checker.specs[id].hir_id = hir.Function_Id(0)
	append(stack, id)
}

Infer_Frame :: struct {
	expr:      ast.Expr_Id,
	expected:  types.Type,
	stage:     u8,
	left:      types.Type,
	arg_index: int,
	arg_mode:  Call_Argument_Mode,
	prefix:    int,
	mapping:   []int,
	args:      []types.Type,
	template:  ast.Function_Id,
}

merge_inferred_test_error :: proc(checker: ^Checker, incoming: types.Type) {
	current := checker.inferred_test_error
	if current == nil || !types.is_valid(incoming) ||
	   types.can_sum_widen(incoming, current^, &checker.module.types) {
		return
	}
	if merged, err := types.compose_sum(&checker.module.types, current^, incoming); err == .None {
		current^ = merged
	}
}

infer_nested_expr :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	demanded: ^[dynamic]Spec_Id,
	local_types: []types.Type = nil,
	expected := types.INVALID,
) -> types.Type {
	outer := checker.infer_stack
	checker.infer_stack = nil
	checker.infer_stack.allocator = checker.allocator
	result := infer_expr(checker, expr_id, locals, pkg, file, demanded, local_types, expected)
	delete(checker.infer_stack)
	checker.infer_stack = outer
	return result
}

infer_division_builtin :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	demanded: ^[dynamic]Spec_Id,
	local_types: []types.Type,
	expected: types.Type,
) -> types.Type {
	if len(expr.args) != 2 {
		return types.INVALID
	}
	hint := expected if types.is_concrete_scalar(expected) && !types.is_bool(expected) else types.INVALID
	left_const := is_numeric_constant_expr(checker, expr.args[0])
	right_const := is_numeric_constant_expr(checker, expr.args[1])
	left, right := types.INVALID, types.INVALID
	if left_const && !right_const && !types.is_valid(hint) {
		right = infer_nested_expr(checker, expr.args[1], locals, pkg, file, demanded, local_types)
		left = infer_nested_expr(checker, expr.args[0], locals, pkg, file, demanded, local_types, right)
	} else {
		left = infer_nested_expr(checker, expr.args[0], locals, pkg, file, demanded, local_types, hint)
		right_hint := hint if types.is_valid(hint) else left
		right = infer_nested_expr(checker, expr.args[1], locals, pkg, file, demanded, local_types, right_hint)
	}
	result := types.widest(left, right)
	return result if types.is_concrete_scalar(result) && !types.is_bool(result) else types.INVALID
}

infer_compound_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	demanded: ^[dynamic]Spec_Id,
	local_types: []types.Type = nil,
	expected := types.INVALID,
) -> types.Type {
	store := &checker.module.types
	#partial switch expr.kind {
	case .Comptime:
		return infer_comptime_expr_type(checker, expr, pkg, file, demanded)
	case .Bool:
		return types.BOOL
	case .Not:
		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		return types.BOOL
	case .Bit_Not:
		hint := expected if types.is_concrete_integer(expected) else types.INVALID
		operand := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, hint)
		return operand if types.is_concrete_integer(operand) else types.INVALID
	case .Bit_And, .Bit_Or, .Bit_Xor:
		hint := expected if types.is_concrete_integer(expected) else types.INVALID
		left_const := is_numeric_constant_expr(checker, expr.left)
		right_const := is_numeric_constant_expr(checker, expr.right)
		left, right := types.INVALID, types.INVALID
		if left_const && !right_const && !types.is_valid(hint) {
			right = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
			left = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, right)
		} else {
			left = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, hint)
			right = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types, hint if types.is_valid(hint) else left)
		}
		result := types.widest(left, right)
		return result if types.is_concrete_integer(result) else types.INVALID
	case .Shift_Left, .Shift_Right, .Shift_Left_Saturating:
		hint := expected if types.is_concrete_integer(expected) else types.INVALID
		left := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, hint)
		right := infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types, types.U64)
		return left if types.is_concrete_integer(left) && types.is_unsigned(right, checker.target) else types.INVALID
	case .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or:
		left_expr := checker.ast_module.exprs[expr.left]
		right_expr := checker.ast_module.exprs[expr.right]
		if left_expr.kind == .None && right_expr.kind != .None {
			right := infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
			_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, right)
		} else if right_expr.kind == .None && left_expr.kind != .None {
			left := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
			_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types, left)
		} else {
			_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
			_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
		}
		return types.BOOL
	case .Range:
		left := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		right := infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
		left_const := eval_constant(checker, expr.left)
		right_const := eval_constant(checker, expr.right)
		child := types.INVALID
		if left_const.kind == .Value && right_const.kind != .Value {
			child = right
		} else if right_const.kind == .Value && left_const.kind != .Value {
			child = left
		} else {
			child = types.widest(left, right)
		}
		if !types.is_concrete_integer(child) {
			return types.INVALID
		}
		return types.range(store, child)
	case .String:
		return string_literal_type(checker, expr.integer)
	case .Array:
		element := types.INVALID
		for arg in expr.args {
			actual := infer_nested_expr(checker, arg, locals, pkg, file, demanded, local_types)
			if !types.is_valid(element) {
				element = actual
			} else if !types.equal(element, actual) {
				element = types.widest(element, actual)
			}
		}
		if !types.is_valid(element) {
			element = types.I64
		}
		return types.array(store, element, u64(len(expr.args)), false)
	case .None:
		return expected if types.is_optional(expected, store) else types.INVALID
	case .Undefined:
		return types.INVALID
	case .Enum_Literal:
		if expr.left != ast.INVALID_EXPR {
			_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		}
		return types.INVALID
	case .Cast:
		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		return type_from_syntax(checker, expr.type, pkg, file)
	case .Address:
		child := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		return types.pointer(store, child, false, false)
	case .Deref:
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		return types.child_type(value, store) if types.is_pointer(value, store) else types.INVALID
	case .Index:
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
		item, ok := types.container(value, store)
		return item.child if ok else types.INVALID
	case .Slice:
		text, comptime_ok := comptime_slice_bytes(checker, expr, pkg, file)
		if comptime_ok {
			string_id := intern_comptime_string(checker, text)
			return string_literal_type(checker, string_id)
		}
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		item, ok := types.container(value, store)
		if !ok || (item.kind == .Pointer && expr.args[1] == ast.INVALID_EXPR) {
			return types.INVALID
		}
		for bound in expr.args {
			if bound != ast.INVALID_EXPR {
				_ = infer_nested_expr(checker, bound, locals, pkg, file, demanded, local_types)
			}
		}
		preserve := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
		return types.slice(store, item.child, item.mutable, preserve, item.sentinel)
	case .Field:
		if enum_type, enum_ok := enum_type_from_field_expr(checker, expr, pkg, file); enum_ok {
			_, member_ok := find_enum_member(checker, enum_type, expr.name)
			return enum_type if member_ok else types.INVALID
		}
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		if !symbol.is_valid(expr.name) {
			return field_type_from_value(checker, expr, value)
		}
		field_name := symbol_text(checker, expr.name)
		item, has_item := types.container(value, store)
		if has_item && (item.kind == .Array || item.kind == .Slice) {
			if field_name == "len" {
				return types.USIZE
			}
			if field_name == "ptr" &&
			   (item.kind == .Slice || types.is_pointer(value, store)) {
				return container_pointer_type(store, item)
			}
		}
		if types.is_pointer(value, store) {
			value = types.child_type(value, store)
		}
		slot, field, ok := find_struct_field_slot(checker, value, expr.name)
		if ok && is_inferred_record_field(checker, slot) && is_runtime_type(checker, expected) {
			_ = merge_record_field_demand(checker, slot, expected, expr.span)
			field = checker.module.types.fields[slot]
		}
		return field.type if ok else types.INVALID
	case .Unwrap:
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		return types.child_type(value, store) if types.is_optional(value, store) else types.INVALID
	case .Orelse:
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
		return types.child_type(value, store) if types.is_optional(value, store) else types.INVALID
	case .Try:
		left_expected := expected if checker.ast_module.exprs[expr.left].kind == .Call else types.INVALID
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, left_expected)
		merge_inferred_test_error(checker, types.fallible_error(value, store))
		return types.fallible_success(value, store) if types.kind(value, store) == .Fallible else types.INVALID
	case .Catch:
		left_expected := expected if checker.ast_module.exprs[expr.left].kind == .Call else types.INVALID
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types, left_expected)
		success := types.fallible_success(value, store)
		error_type := types.fallible_error(value, store)
		if expr.right != ast.INVALID_EXPR {
			fallback := infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
			if types.is_valid(success) && types.is_valid(fallback) && !types.equal(success, fallback) {
				return types.widest(success, fallback)
			}
			return success if types.is_valid(success) else fallback
		}
		block_locals: [dynamic]Infer_Local
		block_locals.allocator = checker.allocator
		defer delete(block_locals)
		append(&block_locals, ..locals)
		capture_start := len(block_locals)
		if symbol.is_valid(expr.name) && expr.name != checker.sink_symbol && types.is_valid(error_type) {
			append(&block_locals, Infer_Local{name=expr.name, type=error_type, declared=error_type, statement=ast.INVALID_STMT})
		}
		infer_statements(checker, expr.body, &block_locals, local_types, pkg, file, demanded, &success, success)
		resize(&block_locals, capture_start)
		return success
	case .Struct_Literal:
		value := types.INVALID
		if expr.left != ast.INVALID_EXPR {
			value, _ = resolve_type_argument(checker, expr.left, pkg, file)
		} else if symbol.is_valid(expr.name) {
			target_pkg, available := expr_package(checker, expr, pkg, file)
			value = types.find_named(store, u32(target_pkg), u32(expr.name), file=u32(expr_lookup_file(expr, file))) if available else types.INVALID
		} else {
			value = expected
		}
		value = types.resolve_alias(value, store)
		tuple_literal := expr.tuple
		if tuple_literal && len(expr.args) == 0 && types.is_valid(value) {
			item, ok := types.node(store, value)
			tuple_literal = !ok || item.kind != .Struct || item.tuple
		}
		if tuple_literal {
			if !types.is_valid(value) {
				fields := make([]types.Field, len(expr.args), checker.allocator)
				defer delete(fields, checker.allocator)
				for arg, index in expr.args {
					fields[index].type = infer_nested_expr(checker, arg, locals, pkg, file, demanded, local_types)
				}
				return types.struct_anonymous(store, fields, true)
			}
			item, ok := types.node(store, value)
			if !ok || item.kind != .Struct || !item.tuple {
				for arg in expr.args {
					_ = infer_nested_expr(checker, arg, locals, pkg, file, demanded, local_types)
				}
				return types.INVALID
			}
			fields := types.fields_for(store, value)
			for arg, index in expr.args {
				expected_field := fields[index].type if index < len(fields) else types.INVALID
				_ = infer_nested_expr(checker, arg, locals, pkg, file, demanded, local_types, expected_field)
			}
			return value
		}
		if !types.is_valid(value) {
			fields := make([]types.Field, len(expr.args), checker.allocator)
			defer delete(fields, checker.allocator)
			valid := true
			for keyed, index in expr.args {
				keyed_expr := checker.ast_module.exprs[keyed]
				fields[index].name = u32(keyed_expr.name)
				if keyed_expr.left == ast.INVALID_EXPR {
					valid = false
					continue
				}
				for previous in fields[:index] {
					valid = valid && previous.name != fields[index].name
				}
				fields[index].type = infer_nested_expr(
					checker, keyed_expr.left, locals, pkg, file, demanded, local_types,
				)
			}
			return types.struct_anonymous(store, fields) if valid else types.INVALID
		}
		fields := types.fields_for(store, value)
		item, item_ok := types.node(store, value)
		initialized := make([]bool, len(fields), checker.allocator)
		defer delete(initialized, checker.allocator)
		for keyed in expr.args {
			keyed_expr := checker.ast_module.exprs[keyed]
			if keyed_expr.left == ast.INVALID_EXPR {
				continue
			}
			slot, field, ok := find_struct_field_slot(checker, value, keyed_expr.name)
			if ok && item_ok {
				initialized[slot-int(item.field_start)] = true
			}
			if !ok || !is_inferred_record_field(checker, slot) {
				_ = infer_nested_expr(checker, keyed_expr.left, locals, pkg, file, demanded, local_types)
				continue
			}
			field_expected := field.type if is_runtime_type(checker, field.type) else types.INVALID
			actual := infer_nested_expr(checker, keyed_expr.left, locals, pkg, file, demanded, local_types, field_expected)
			_ = record_field_expr_candidate(checker, slot, keyed_expr.left, actual, locals, pkg, file)
			if is_runtime_type(checker, checker.module.types.fields[slot].type) {
				_ = record_demand(
					checker, keyed_expr.left, checker.module.types.fields[slot].type,
					locals, local_types, pkg, file,
				)
			}
		}
		if item_ok && item.kind == .Struct && !item.tuple {
			for field, index in fields {
				if initialized[index] {
					continue
				}
				if field_default, default_values, ok := find_struct_field_default(checker, value, symbol.Id(field.name)); ok {
					if field_default.static_value != INVALID_CT_VALUE {
						continue
					}
					previous := checker.current_comptime_values
					checker.current_comptime_values = default_values
					_ = infer_nested_expr(
						checker, field_default.expr, nil, field_default.pkg, field_default.file,
						demanded, expected=field.type,
					)
					checker.current_comptime_values = previous
				}
			}
		}
		return value
	case .Keyed:
		return infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
	case:
		return types.INVALID
	}
}

infer_expr :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	locals: []Infer_Local,
	pkg := ast.Package_Id(0),
	file := ast.File_Id(0),
	demanded: ^[dynamic]Spec_Id = nil,
	local_types: []types.Type = nil,
	expected := types.INVALID,
) -> types.Type {
	stack := checker.infer_stack
	checker.infer_stack = nil
	clear_dynamic_array(&stack)
	defer {
		for frame in stack {
			delete(frame.args, checker.allocator)
			delete(frame.mapping, checker.allocator)
		}
		clear_dynamic_array(&stack)
		if checker.infer_stack == nil {
			checker.infer_stack = stack
		} else {
			delete(stack)
		}
	}
	append(&stack, Infer_Frame{expr=expr_id, expected=expected, template=ast.INVALID_FUNCTION})
	last := types.INVALID

	for len(stack) > 0 {
		frame_index := len(stack)-1
		frame := stack[frame_index]
		if frame.expr == ast.INVALID_EXPR || int(frame.expr) >= len(checker.ast_module.exprs) {
			last = types.INVALID
			_ = pop(&stack)
			continue
		}
		expr := checker.ast_module.exprs[frame.expr]
		if frame.stage == 0 {
			constant := Constant{}
			_, static_name := current_static_binding(checker, expr.name)
			if expr.kind != .Name || symbol.is_valid(expr.qualifier) || !static_name {
				constant = eval_constant(checker, frame.expr)
			}
			if constant.kind == .Overflow || constant.kind == .Div_By_Zero {
				last = types.I64
				_ = pop(&stack)
				continue
			}
			if constant.kind == .Value {
				last = constraint_integer_literal_type(frame.expected, constant.value)
				if !types.is_valid(last) {
					last = types.I64
				}
				_ = pop(&stack)
				continue
			}
			switch expr.kind {
			case .Invalid:
				last = types.INVALID
				_ = pop(&stack)
			case .Inference_Hole:
				last = types.INVALID
				_ = pop(&stack)
			case .Type:
				last = types.INVALID
				_ = pop(&stack)
			case .Anonymous_Struct_Type:
				last = types.INVALID
				_ = pop(&stack)
			case .Integer:
				last = types.I64
				if expr.integer <= 0x7fff_ffff_ffff_ffff {
					last = types.smallest_signed_for_literal(i64(expr.integer))
				}
				_ = pop(&stack)
			case .Float:
				last = types.F64
				_ = pop(&stack)
			case .String, .Array, .None, .Undefined, .Address, .Deref, .Index, .Slice,
			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed, .Enum_Literal, .Cast,
			     .Comptime, .Bool, .Not, .Bit_Not, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left,
			     .Shift_Right, .Shift_Left_Saturating, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
				last = infer_compound_expr(checker, expr, locals, pkg, file, demanded, local_types, frame.expected)
				_ = pop(&stack)
			case .Function_Literal:
				template := ast.Function_Id(u32(expr.integer))
				function_type, ok := function_expr_type_for_template(checker, template, frame.expected, demanded)
				last = function_type if ok else types.INVALID
				_ = pop(&stack)
			case .Name:
				last = types.INVALID
				if !symbol.is_valid(expr.qualifier) {
					last = find_infer_local(locals, expr.name)
				} else {
					base_type := find_infer_local(locals, expr.qualifier)
					item, has_item := types.container(base_type, &checker.module.types)
					field_name := symbol_text(checker, expr.name)
					if has_item && (item.kind == .Array || item.kind == .Slice) {
						if field_name == "len" {
							last = types.USIZE
						} else if field_name == "ptr" &&
						          (item.kind == .Slice || types.is_pointer(base_type, &checker.module.types)) {
							last = container_pointer_type(&checker.module.types, item)
						}
					}
					if types.is_pointer(base_type, &checker.module.types) {
						base_type = types.child_type(base_type, &checker.module.types)
					}
					if !types.is_valid(last) {
						slot, field, ok := find_struct_field_slot(checker, base_type, expr.name)
						if ok {
							if is_inferred_record_field(checker, slot) && is_runtime_type(checker, frame.expected) {
								_ = merge_record_field_demand(checker, slot, frame.expected, expr.span)
								field = checker.module.types.fields[slot]
							}
							last = field.type
						}
					}
				}
				if !types.is_valid(last) && symbol.is_valid(expr.qualifier) &&
				   find_import(checker, file, expr.qualifier) == ast.INVALID_IMPORT {
					if value, ok := current_comptime_value(checker, expr.qualifier);
					   ok && value.kind == .Static {
						if _, field, found := find_struct_field(checker, value.type, expr.name); found {
							last = field.type
						}
					}
					if value, ok := current_static_binding(checker, expr.qualifier); ok {
						if _, field, found := find_struct_field(checker, value.type, expr.name); found {
							last = field.type
						}
					}
					if global := find_global(checker, expr.qualifier, pkg, file); global != ast.INVALID_GLOBAL {
						base_type := checker.global_types[global]
						item, has_item := types.container(base_type, &checker.module.types)
						field_name := symbol_text(checker, expr.name)
						if has_item && (item.kind == .Array || item.kind == .Slice) {
							if field_name == "len" {
								last = types.USIZE
							} else if field_name == "ptr" &&
							          (item.kind == .Slice || types.is_pointer(base_type, &checker.module.types)) {
								last = container_pointer_type(&checker.module.types, item)
							}
						}
						if types.is_pointer(base_type, &checker.module.types) {
							base_type = types.child_type(base_type, &checker.module.types)
						}
						if !types.is_valid(last) {
							slot, field, ok := find_struct_field_slot(checker, base_type, expr.name)
							if ok {
								if is_inferred_record_field(checker, slot) && is_runtime_type(checker, frame.expected) {
									_ = merge_record_field_demand(checker, slot, frame.expected, expr.span)
									field = checker.module.types.fields[slot]
								}
								last = field.type
							}
						}
					}
				}
				if !types.is_valid(last) {
					if !symbol.is_valid(expr.qualifier) {
						if value, ok := current_comptime_value(checker, expr.name); ok {
							if value.kind == .Integer || value.kind == .String || value.kind == .Static {
								last = value.type
							}
						}
						if binding, ok := current_static_binding(checker, expr.name); ok {
							last = binding.type
						}
					}
				}
				if !types.is_valid(last) {
					if enum_type, enum_ok := enum_type_from_name_expr(checker, expr, pkg, file); enum_ok {
						_, member_ok := find_enum_member(checker, enum_type, expr.name)
						if member_ok {
							last = enum_type
						}
					}
				}
				if !types.is_valid(last) {
					target_pkg, available := expr_package(checker, expr, pkg, file)
					if available {
						global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
						if global != ast.INVALID_GLOBAL {
							last = checker.global_types[global]
						}
					}
				}
				if !types.is_valid(last) {
					target_pkg, available := expr_package(checker, expr, pkg, file)
					if available {
						template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
						if template != ast.INVALID_FUNCTION &&
						   len(checker.ast_module.functions[template].unsupported_reason) == 0 &&
						   checker.template_diagnostics[template] == source.INVALID_DIAGNOSTIC {
							function_type, ok := function_expr_type_for_template(checker, template, frame.expected, demanded)
							if ok {
								last = function_type
							}
						}
					}
				}
				_ = pop(&stack)
			case .Negate:
				stack[frame_index].stage = 5
				append(&stack, Infer_Frame{expr=expr.left, template=ast.INVALID_FUNCTION})
			case .Add, .Sub, .Mul, .Div:
				stack[frame_index].stage = 1
				append(&stack, Infer_Frame{expr=expr.left, template=ast.INVALID_FUNCTION})
			case .Call:
				if expr.left != ast.INVALID_EXPR {
					callee_type := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
					_, function_item, function_type, ok := types.callable_function(callee_type, &checker.module.types)
					if !ok {
						last = types.INVALID
						_ = pop(&stack)
						continue
					}
					stack[frame_index].left = function_type
					stack[frame_index].args = make([]types.Type, len(expr.args), checker.allocator)
					stack[frame_index].stage = 6
					if len(expr.args) > 0 {
						append(&stack, Infer_Frame{expr=expr.args[0], template=ast.INVALID_FUNCTION})
					} else if valid_callable_arity(function_item, 0) {
						last = function_item.child
						delete(stack[frame_index].args, checker.allocator)
						stack[frame_index].args = nil
						_ = pop(&stack)
					}
					continue
				}
				if builtin := type_builtin_call(checker, expr); builtin != .None {
					if builtin == .Size_Of || builtin == .Align_Of {
						last = types.USIZE
					} else if len(expr.args) == 1 {
						target, ok := resolve_type_argument(checker, expr.args[0], pkg, file)
						last = target if ok && types.is_concrete_integer(target) else types.INVALID
					} else {
						last = types.INVALID
					}
					_ = pop(&stack)
					continue
				}
				if division_builtin_call(checker, expr) != .None {
					last = infer_division_builtin(checker, expr, locals, pkg, file, demanded, local_types, frame.expected)
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "some") {
					if len(expr.args) == 1 && types.is_optional(frame.expected, &checker.module.types) {
						child := types.child_type(frame.expected, &checker.module.types)
						_ = infer_nested_expr(checker, expr.args[0], locals, pkg, file, demanded, local_types, child)
						last = frame.expected
					} else {
						last = types.INVALID
					}
					_ = pop(&stack)
					continue
				}
				if field_expr, handled := field_intrinsic_expr(checker, expr, pkg, file); handled {
					if enum_type, ok := resolve_type_argument(checker, field_expr.left, pkg, file);
					   ok && types.is_enum(enum_type, &checker.module.types) {
						_, member_ok := find_enum_member(checker, enum_type, field_expr.name)
						last = enum_type if member_ok else types.INVALID
						_ = pop(&stack)
						continue
					}
					base_type := infer_nested_expr(
						checker, field_expr.left, locals, pkg, file, demanded, local_types,
					)
					last = field_type_from_value(checker, field_expr, base_type)
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "typeinfo") {
					_, target_ok := resolve_type_argument(
						checker, expr.args[0] if len(expr.args) == 1 else ast.INVALID_EXPR, pkg, file,
					)
					last = std_named_type(checker, "@std/meta", "TypeInfo") if target_ok else types.INVALID
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "tag") {
					if len(expr.args) == 1 {
						value_type := infer_nested_expr(checker, expr.args[0], locals, pkg, file, demanded, local_types)
						last, _ = tag_result_type(checker, value_type)
					} else {
						last = types.INVALID
					}
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "tagname") {
					last = types.slice(&checker.module.types, types.U8, false)
					_ = pop(&stack)
					continue
				}
				if is_ptrcast_call(checker, expr) {
					if len(expr.args) != 2 {
						last = types.INVALID
						_ = pop(&stack)
						continue
					}
					child, child_ok := resolve_type_argument(checker, expr.args[0], pkg, file)
					operand := infer_nested_expr(checker, expr.args[1], locals, pkg, file, demanded, local_types)
					result := types.INVALID
					if child_ok && valid_ptrcast_child(checker, child) {
						result, _ = types.replace_pointer_child(&checker.module.types, operand, child)
					}
					last = result
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "compile_error") {
					last = types.VOID
					_ = pop(&stack)
					continue
				}
				if expr.intrinsic {
					last = types.INVALID
					_ = pop(&stack)
					continue
				}
				if callee_type, handled := infer_qualified_value_field_type(checker, expr, locals, pkg, file, demanded); handled {
					_, function_item, function_type, ok := types.callable_function(callee_type, &checker.module.types)
					if !ok {
						last = types.INVALID
						_ = pop(&stack)
						continue
					}
					stack[frame_index].left = function_type
					stack[frame_index].args = make([]types.Type, len(expr.args), checker.allocator)
					stack[frame_index].stage = 6
					if len(expr.args) > 0 {
						append(&stack, Infer_Frame{expr=expr.args[0], template=ast.INVALID_FUNCTION})
					} else if valid_callable_arity(function_item, 0) {
						last = function_item.child
						delete(stack[frame_index].args, checker.allocator)
						stack[frame_index].args = nil
						_ = pop(&stack)
					}
					continue
				}
				target_pkg, available := expr_package(checker, expr, pkg, file)
				template := ast.INVALID_FUNCTION
				if available {
					template = find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
				}
				if template == ast.INVALID_FUNCTION {
					callee_type := types.INVALID
					if !symbol.is_valid(expr.qualifier) {
						callee_type = find_infer_local(locals, expr.name)
						if !types.is_valid(callee_type) {
							if binding, ok := current_static_binding(checker, expr.name); ok {
								callee_type = binding.type
								if binding.value != INVALID_CT_VALUE && int(binding.value) < len(checker.static_state.values) {
									function_value := checker.static_state.values[binding.value]
									if function_value.kind == .Function {
										_, _, _ = function_pointer_type_for_template(
											checker, ast.Function_Id(u32(function_value.index)), demanded,
										)
									}
								}
							}
						}
						if !types.is_valid(callee_type) {
							if value, ok := current_comptime_value(checker, expr.name); ok && value.kind == .Static {
								callee_type = value.type
								if value.static_value != INVALID_CT_VALUE && int(value.static_value) < len(checker.static_state.values) {
									function_value := checker.static_state.values[value.static_value]
									if function_value.kind == .Function {
										_, _, _ = function_pointer_type_for_template(
											checker, ast.Function_Id(u32(function_value.index)), demanded,
										)
									}
								}
							}
						}
					}
					if !types.is_valid(callee_type) && available {
						global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
						if global != ast.INVALID_GLOBAL {
							callee_type = checker.global_types[global]
						}
					}
					_, function_item, function_type, ok := types.callable_function(callee_type, &checker.module.types)
					if !ok {
						distinct_type := types.find_named(&checker.module.types, u32(target_pkg), u32(expr.name), file=u32(expr_lookup_file(expr, file)))
						distinct_item, distinct_ok := types.node(&checker.module.types, distinct_type)
						if available && distinct_ok && distinct_item.kind == .Distinct && len(expr.args) == 1 {
							stack[frame_index].left = distinct_type
							stack[frame_index].stage = 7
							append(&stack, Infer_Frame{expr=expr.args[0], template=ast.INVALID_FUNCTION})
							continue
						}
						last = types.INVALID
						_ = pop(&stack)
						continue
					}
					stack[frame_index].left = function_type
					stack[frame_index].args = make([]types.Type, len(expr.args), checker.allocator)
					stack[frame_index].stage = 6
					if len(expr.args) > 0 {
						append(&stack, Infer_Frame{expr=expr.args[0], template=ast.INVALID_FUNCTION})
					} else if valid_callable_arity(function_item, 0) {
						last = function_item.child
						delete(stack[frame_index].args, checker.allocator)
						stack[frame_index].args = nil
						_ = pop(&stack)
					}
					continue
				}
				if len(checker.ast_module.functions[template].unsupported_reason) > 0 {
					last = types.INVALID
					_ = pop(&stack)
					continue
				}
				if checker.template_diagnostics[template] != source.INVALID_DIAGNOSTIC {
					declared := function_channel_type(checker, checker.ast_module.functions[template])
					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
					_ = pop(&stack)
					continue
				}
				function := checker.ast_module.functions[template]
				mapping, mode, prefix, mapping_failure := call_argument_mapping(
					checker, &function, expr.args, pkg, file, frame.expected, locals,
				)
				delete(mapping_failure, checker.allocator)
				if mode == .Invalid {
					last = types.INVALID
					_ = pop(&stack)
					continue
				}
				stack[frame_index].template = template
				stack[frame_index].arg_mode = mode
				stack[frame_index].prefix = prefix
				stack[frame_index].mapping = mapping
				stack[frame_index].args = make([]types.Type, len(function.params), checker.allocator)
				stack[frame_index].arg_index = next_runtime_call_arg(function, mapping, 0, len(expr.args))
				stack[frame_index].stage = 3
				if stack[frame_index].arg_index < len(expr.args) {
					param_index := call_param_index(mapping, stack[frame_index].arg_index)
					arg_expected := call_arg_expected(checker, function, param_index)
					if !is_runtime_type(checker, arg_expected) {
						arg_expected = types.INVALID
					}
					append(&stack, Infer_Frame{expr=expr.args[stack[frame_index].arg_index], expected=arg_expected, template=ast.INVALID_FUNCTION})
				}
			}
			continue
		}
		if frame.stage == 5 {
			if !types.is_signed(last, checker.target) && !types.is_float(last, checker.target) {
				last = types.INVALID
			}
			_ = pop(&stack)
			continue
		}
		if frame.stage == 1 {
			stack[frame_index].left = last
			stack[frame_index].stage = 2
			append(&stack, Infer_Frame{expr=expr.right, template=ast.INVALID_FUNCTION})
			continue
		}
		if frame.stage == 2 {
			right := last
			if expr.kind == .Add && types.is_many_pointer(frame.left, &checker.module.types) && types.is_concrete_integer(right) {
				last = frame.left
			} else if is_numeric_constant_expr(checker, expr.right) &&
			          is_numeric_demand(frame.left, checker.target) &&
			          expr_accepts_numeric_demand(checker, expr.right, frame.left, locals, pkg, file) {
				last = frame.left
			} else if is_numeric_constant_expr(checker, expr.left) &&
			          is_numeric_demand(right, checker.target) &&
			          expr_accepts_numeric_demand(checker, expr.left, right, locals, pkg, file) {
				last = right
			} else if is_numeric_demand(frame.left, checker.target) &&
			          !numeric_operand_is_open(checker, expr.left, locals, pkg, file) &&
			          expr_accepts_numeric_demand(checker, expr.right, frame.left, locals, pkg, file) {
				// Propagate only from an authoritative (fixed-type) left operand. A left
				// operand that is still a provisional open constant carries only its
				// smallest-signed default, which must not poison the sibling's family;
				// two provisional operands are resolved together by the backward demand
				// from the declaration/use.
				_ = record_demand(checker, expr.right, frame.left, locals, local_types, pkg, file)
				last = frame.left
			} else if is_numeric_demand(right, checker.target) &&
			          !numeric_operand_is_open(checker, expr.right, locals, pkg, file) &&
			          expr_accepts_numeric_demand(checker, expr.left, right, locals, pkg, file) {
				_ = record_demand(checker, expr.left, right, locals, local_types, pkg, file)
				last = right
			} else {
				last = types.widest(frame.left, right)
			}
			_ = pop(&stack)
			continue
		}
		if frame.stage == 3 {
			if frame.arg_index < len(expr.args) {
				param_index := call_param_index(frame.mapping, frame.arg_index)
				if param_index < len(stack[frame_index].args) {
					stack[frame_index].args[param_index] = last
				}
				next := next_runtime_call_arg(
					checker.ast_module.functions[frame.template], frame.mapping,
					frame.arg_index+1, len(expr.args),
				)
				stack[frame_index].arg_index = next
				if next < len(expr.args) {
					next_param := call_param_index(frame.mapping, next)
					arg_expected := call_arg_expected(checker, checker.ast_module.functions[frame.template], next_param)
					if !is_runtime_type(checker, arg_expected) {
						arg_expected = types.INVALID
					}
					append(&stack, Infer_Frame{expr=expr.args[next], expected=arg_expected, template=ast.INVALID_FUNCTION})
					continue
				}
			}
			function := checker.ast_module.functions[frame.template]
			// Deferred defaulting leaves an undemanded open constant typeless; give such an
			// argument its default so the call can still monomorphize (the default feeds only
			// the spec arg vector, not a demand).
			for source_index in 0..<len(expr.args) {
				param_index := call_param_index(frame.mapping, source_index)
				if param_index < len(stack[frame_index].args) && !is_runtime_type(checker, stack[frame_index].args[param_index]) {
					fallback := open_const_default_type(checker, expr.args[source_index], locals, pkg, file)
					if is_runtime_type(checker, fallback) {
						stack[frame_index].args[param_index] = fallback
					}
				}
			}
			comptime_values: []Comptime_Value
			comptime_ok := false
			comptime_values, comptime_ok = infer_call_comptime_values(
				checker, function, frame.prefix, frame.mapping, expr.args, stack[frame_index].args,
				frame.expected, pkg, file,
			)
			defer delete(comptime_values, checker.allocator)
			if comptime_ok &&
			   can_specialize(checker, function, stack[frame_index].args, comptime_values) {
				store_call_resolution(
					checker, frame.expr, frame.mapping,
					comptime_values, stack[frame_index].args,
				)
				previous_comptime := checker.current_comptime_values
				checker.current_comptime_values = comptime_values
				for source_index in 0..<len(expr.args) {
					param_index := call_param_index(frame.mapping, source_index)
					if param_index >= len(function.params) || function.params[param_index].comptime_value {
						continue
					}
					demand := call_arg_expected(checker, function, param_index)
					record_demand(checker, expr.args[source_index], demand, locals, local_types, pkg, file)
				}
				checker.current_comptime_values = previous_comptime
				spec := INVALID_SPEC
				if demanded == nil {
					spec = ensure_spec(checker, frame.template, stack[frame_index].args, comptime_values)
				} else {
					spec = find_spec(checker, frame.template, stack[frame_index].args, comptime_values)
					mark_spec_demanded(checker, spec, demanded)
				}
				if spec != INVALID_SPEC {
					last = checker.specs[spec].result
				} else {
					previous_comptime := checker.current_comptime_values
					checker.current_comptime_values = comptime_values
					declared := function_channel_type(checker, function)
					checker.current_comptime_values = previous_comptime
					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
				}
			} else {
				declared := function_channel_type(checker, function)
				if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT &&
				   !types.is_valid(function.error) {
					last = types.I32
				} else {
					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
				}
			}
			delete(stack[frame_index].args, checker.allocator)
			stack[frame_index].args = nil
			delete(stack[frame_index].mapping, checker.allocator)
			stack[frame_index].mapping = nil
			_ = pop(&stack)
		}
		if frame.stage == 6 {
			if frame.arg_index < len(expr.args) {
				stack[frame_index].args[frame.arg_index] = last
				stack[frame_index].arg_index += 1
				if frame.arg_index+1 < len(expr.args) {
					append(&stack, Infer_Frame{expr=expr.args[frame.arg_index+1], template=ast.INVALID_FUNCTION})
					continue
				}
			}
			function_item, ok := types.node(&checker.module.types, frame.left)
			if ok && function_item.kind == .Function && valid_callable_arity(function_item, len(expr.args)) {
				last = function_item.child
			} else {
				last = types.INVALID
			}
			delete(stack[frame_index].args, checker.allocator)
			stack[frame_index].args = nil
			_ = pop(&stack)
		}
		if frame.stage == 7 {
			last = frame.left
			_ = pop(&stack)
		}
	}
	return last
}

flatten_conditional_unwrap_operands :: proc(
	module: ^ast.Module,
	expr_id: ast.Expr_Id,
	operands: ^[dynamic]ast.Expr_Id,
) {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(module.exprs) {
		append(operands, expr_id)
		return
	}
	expr := module.exprs[expr_id]
	if expr.kind == .And {
		flatten_conditional_unwrap_operands(module, expr.left, operands)
		flatten_conditional_unwrap_operands(module, expr.right, operands)
		return
	}
	append(operands, expr_id)
}

record_infer_local_type :: proc(local: Infer_Local, local_types: []types.Type) {
	if local.statement != ast.INVALID_STMT && int(local.statement) < len(local_types) {
		local_types[local.statement] = local.type
	}
}

merge_infer_local_type :: proc(
	checker: ^Checker,
	local: ^Infer_Local,
	inferred: types.Type,
	local_types: []types.Type,
) -> bool {
	if !is_runtime_type(checker, inferred) {
		return false
	}
	if types.is_constraint(local.declared) {
		if !types.constraint_accepts(local.declared, inferred, &checker.module.types) {
			return false
		}
		if !is_runtime_type(checker, local.type) {
			local.type = inferred
			record_infer_local_type(local^, local_types)
			return true
		}
		if types.equal(local.type, inferred) {
			return false
		}
		merged := types.widest(local.type, inferred)
		if types.constraint_accepts(local.declared, merged, &checker.module.types) {
			local.type = merged
			record_infer_local_type(local^, local_types)
			return true
		}
		return false
	}
	if is_runtime_type(checker, local.declared) {
		local.type = local.declared
		record_infer_local_type(local^, local_types)
		return false
	}
	if !is_runtime_type(checker, local.type) {
		local.type = inferred
		record_infer_local_type(local^, local_types)
		return true
	}
	if types.equal(local.type, inferred) {
		return false
	}
	merged := types.widest(local.type, inferred)
	if types.is_concrete_scalar(merged) {
		local.type = merged
		record_infer_local_type(local^, local_types)
		return true
	}
	return false
}

infer_statements :: proc(
	checker: ^Checker,
	statements: []ast.Stmt_Id,
	locals: ^[dynamic]Infer_Local,
	local_types: []types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	demanded: ^[dynamic]Spec_Id,
	result: ^types.Type,
	result_hint := types.INVALID,
) {
	scope_start := len(locals^)
	for statement_id in statements {
		statement := checker.ast_module.statements[statement_id]
		#partial switch statement.kind {
		case .Declaration:
			if statement.expr == ast.INVALID_EXPR {
				// Value block (`x :: { ... yield v }` / `x T = { ... }`): register the
				// binding (its declared type when annotated, else left open) and walk
				// the block body. The build pass resolves the yielded value's type
				// independently — value blocks don't join the demand fixpoint.
				declared_block := type_from_syntax(checker, statement.type, pkg, file)
				block_type := declared_block if is_runtime_type(checker, declared_block) else types.INVALID
				local := Infer_Local{
					name=statement.name, type=block_type, declared=declared_block,
					statement=statement_id, mutable=!statement.immutable,
				}
				append(locals, local)
				record_infer_local_type(local, local_types)
				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
				continue
			}
			declared_local := resolve_inferred_array(checker, type_from_syntax(checker, statement.type, pkg, file), statement.expr)
			value_type := types.INVALID
			if !is_undefined_expr(checker, statement.expr) {
				expected := declared_local if is_runtime_type(checker, declared_local) || declared_local == types.UINT else types.INVALID
				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types, expected)
				declared_local = resolve_inferred_array_from_type(checker, declared_local, value_type)
			}
			if is_runtime_type(checker, declared_local) && !has_inferred_array_count(checker, declared_local) {
				value_type = declared_local
			} else if types.is_constraint(declared_local) {
				// Seed the binding in-family (INVALID on mismatch, which
				// build_block reports). FLOAT defaults integers to f64.
				value_type = types.constraint_target(declared_local, value_type, &checker.module.types)
			}
			open := false
			open_float := false
			const_val := i128(0)
			if !is_runtime_type(checker, declared_local) && !is_undefined_expr(checker, statement.expr) {
				constant := eval_integer_constant_in_context(checker, statement.expr, pkg, file)
				if constant.kind == .Value &&
				   (fits_i64(constant.value) || declared_local == types.UINT && fits_u64(constant.value)) {
					open = true
					const_val = constant.value
				} else if is_float_constant_expr(checker, statement.expr) {
					open_float = true
				}
			}
			local := Infer_Local{
				name=statement.name,
				type=value_type,
				declared=declared_local,
				statement=statement_id,
				mutable=!statement.immutable,
				open_const=open,
				open_float=open_float,
				const_value=const_val,
			}
			append(locals, local)
			record_infer_local_type(local, local_types)
			// A typed/constraint declaration initialized by a bare name pushes its resolved
			// type backward onto that name (mirrors the `Y int :: X; Z i32 :: Y` global chain).
			// Skip this for a constraint/untyped declaration whose initializer is arithmetic:
			// value_type is then only a provisional default (e.g. `x int = a - b` resolving to
			// i16 before the operands' real uses are seen) and would poison the open-constant
			// operands' family. The operands resolve from their own authoritative uses, and the
			// local adopts their resolved type forward. Concrete-declared arithmetic (e.g.
			// `b u16 :: a + 2`) still pushes, since value_type is the concrete declared type.
			init_is_arith := is_arith_kind(checker.ast_module.exprs[statement.expr].kind)
			if is_runtime_type(checker, declared_local) || !init_is_arith {
				record_demand(checker, statement.expr, value_type, locals^[:], local_types, pkg, file)
			}
		case .Assignment:
			if statement.expr == ast.INVALID_EXPR {
				// Value block assigned to a target: walk the block body; the build
				// pass handles the target coercion.
				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
				continue
			}
			expected_assignment := types.INVALID
			if statement.target != ast.INVALID_EXPR {
				expected_assignment = infer_expr(checker, statement.target, locals^[:], pkg, file, demanded, local_types)
				target_expr := checker.ast_module.exprs[statement.target]
				if target_expr.kind == .Name && !symbol.is_valid(target_expr.qualifier) {
					if local_index, ok := find_infer_local_index(locals^[:], target_expr.name); ok &&
					   locals^[local_index].declared == types.UINT {
						expected_assignment = types.UINT
					} else if global := find_global(checker, target_expr.name, pkg, file); global != ast.INVALID_GLOBAL {
						ast_global := checker.ast_module.globals[global]
						if type_from_syntax(checker, ast_global.type, ast_global.pkg, ast_global.file) == types.UINT {
							expected_assignment = types.UINT
						}
					}
				}
			} else if statement.name != checker.sink_symbol {
				if local_index, ok := find_infer_local_index(locals^[:], statement.name); ok {
					expected_assignment = types.UINT if locals^[local_index].declared == types.UINT else locals^[local_index].type
				} else if global := find_global(checker, statement.name, pkg, file); global != ast.INVALID_GLOBAL {
					ast_global := checker.ast_module.globals[global]
					declared_global := type_from_syntax(checker, ast_global.type, ast_global.pkg, ast_global.file)
					expected_assignment = types.UINT if declared_global == types.UINT else checker.global_types[global]
				}
			}
			value_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types, expected_assignment)
			// Only push the target's type back onto a bare-name RHS (e.g. `x += speed`):
			// pushing through an arithmetic RHS would feed the target's (often provisional)
			// type onto open-constant operands and poison their family. Operands of an
			// arithmetic RHS resolve from their own authoritative uses.
			rhs_is_arith := is_arith_kind(checker.ast_module.exprs[statement.expr].kind)
			if statement.target != ast.INVALID_EXPR {
				target_type := expected_assignment
				target_expr := checker.ast_module.exprs[statement.target]
				if target_expr.kind == .Name && !symbol.is_valid(target_expr.qualifier) {
					if local_index, ok := find_infer_local_index(locals^[:], target_expr.name); ok &&
					   locals^[local_index].mutable {
						_ = merge_infer_local_type(checker, &locals^[local_index], value_type, local_types)
					}
				} else if target_expr.kind == .Field ||
				          target_expr.kind == .Name && symbol.is_valid(target_expr.qualifier) {
					if slot, ok := inferred_record_field_slot_from_expr(
						checker, target_expr, locals^[:], local_types, pkg, file,
					); ok {
						_ = record_field_expr_candidate(
							checker, slot, statement.expr, value_type, locals^[:], pkg, file,
						)
						target_type = checker.module.types.fields[slot].type
					}
				}
				if !rhs_is_arith {
					_ = record_demand(checker, statement.expr, target_type, locals^[:], local_types, pkg, file)
				}
			} else if statement.name != checker.sink_symbol {
				if local_index, ok := find_infer_local_index(locals^[:], statement.name); ok &&
				   locals^[local_index].mutable {
					_ = merge_infer_local_type(checker, &locals^[local_index], value_type, local_types)
					if !rhs_is_arith {
						_ = record_demand(checker, statement.expr, locals^[local_index].type, locals^[:], local_types, pkg, file)
					}
				} else if global := find_global(checker, statement.name, pkg, file); global != ast.INVALID_GLOBAL {
					_ = merge_global_demand(checker, global, value_type)
					if !rhs_is_arith {
						_ = record_demand(checker, statement.expr, checker.global_types[global], locals^[:], local_types, pkg, file)
					}
				}
			}
		case .Expression:
			_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
		case .Yield:
			if statement.expr != ast.INVALID_EXPR {
				_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
			}
		case .Return:
			if statement.expr != ast.INVALID_EXPR {
				returned := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types, result_hint)
				if is_runtime_type(checker, result_hint) {
					_ = record_demand_shallow(checker, statement.expr, result_hint, locals^[:], local_types, pkg, file)
					expr := checker.ast_module.exprs[statement.expr]
					if expr.kind == .Name && !symbol.is_valid(expr.qualifier) {
						if local_index, ok := find_infer_local_index(locals^[:], expr.name); ok {
							// Open constants can adopt the result type cross-family; other
							// locals widen within family as before.
							if !merge_local_demand(checker, &locals^[local_index], result_hint, local_types) {
								_ = merge_infer_local_type(checker, &locals^[local_index], result_hint, local_types)
							}
							returned = result_hint
						} else {
							// `return G` for a global const: demand the result type onto it.
							record_demand_shallow(checker, statement.expr, result_hint, locals^[:], local_types, pkg, file)
						}
					}
				}
				if !types.is_valid(result^) {
					result^ = returned
				} else if !types.equal(result^, returned) {
					result^ = types.widest(result^, returned)
				}
			}
		case .If:
			if len(statement.captures) > 0 {
				operands: [dynamic]ast.Expr_Id
				operands.allocator = checker.allocator
				flatten_conditional_unwrap_operands(checker.ast_module, statement.expr, &operands)
				operand_types := make([]types.Type, len(operands), checker.allocator)
				for operand, index in operands {
					operand_types[index] = infer_expr(checker, operand, locals^[:], pkg, file, demanded, local_types)
				}
				capture_start := len(locals^)
				for capture, index in statement.captures {
					if capture == checker.sink_symbol {
						continue
					}
					capture_type := types.INVALID
					if index < len(operand_types) &&
					   types.is_optional(operand_types[index], &checker.module.types) {
						capture_type = types.child_type(operand_types[index], &checker.module.types)
					}
					append(locals, Infer_Local{name=capture, type=capture_type, declared=capture_type, statement=ast.INVALID_STMT})
				}
				if statement.guard != ast.INVALID_EXPR {
					_ = infer_expr(checker, statement.guard, locals^[:], pkg, file, demanded, local_types)
				}
				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
				resize(locals, capture_start)
				infer_statements(checker, statement.else_body, locals, local_types, pkg, file, demanded, result, result_hint)
				delete(operand_types, checker.allocator)
				delete(operands)
			} else {
				_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
				if selected, comptime_ok := specialization_bool(checker, statement.expr, pkg, file); comptime_ok {
					selected_body := statement.body if selected else statement.else_body
					infer_statements(checker, selected_body, locals, local_types, pkg, file, demanded, result, result_hint)
				} else {
					infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
					infer_statements(checker, statement.else_body, locals, local_types, pkg, file, demanded, result, result_hint)
				}
			}
		case .While:
			_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
			infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
			if statement.update != ast.INVALID_STMT {
				update := [1]ast.Stmt_Id{statement.update}
				infer_statements(checker, update[:], locals, local_types, pkg, file, demanded, result, result_hint)
			}
		case .For:
			if statement.expand {
				bindings, expand_error := expand_field_bindings(checker, statement.expr, statement.name, pkg, file)
				if expand_error == .None {
					for binding, expand_index in bindings {
						binding_start := push_expand_binding(checker, binding, statement.index_name, expand_index, statement_id)
						iteration: [dynamic]ast.Stmt_Id
						iteration.allocator = checker.allocator
						control := flatten_expand_iteration(
							checker, statement.body, pkg, file, &iteration, statement.label, nil,
						)
						if control != .Invalid {
							infer_statements(checker, iteration[:], locals, local_types, pkg, file, demanded, result, result_hint)
						}
						delete(iteration)
						pop_expand_binding(checker, binding_start)
						if control == .Break || control == .Invalid {
							break
						}
					}
				}
				delete(bindings, checker.allocator)
				continue
			}
			iterable_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
			capture_start := len(locals^)
			capture_type := types.INVALID
			if types.is_range(iterable_type, &checker.module.types) {
				capture_type = types.child_type(iterable_type, &checker.module.types)
			} else {
				item, ok := sequence_item(iterable_type, &checker.module.types)
				if ok {
					capture_type = item.child
					if statement.pointer_capture {
						capture_type = types.pointer(&checker.module.types, item.child, item.mutable, false)
					}
				}
			}
			if symbol.is_valid(statement.name) {
				append(locals, Infer_Local{name=statement.name, type=capture_type, declared=capture_type, statement=ast.INVALID_STMT})
			}
			if symbol.is_valid(statement.index_name) {
				append(locals, Infer_Local{name=statement.index_name, type=types.USIZE, declared=types.USIZE, statement=ast.INVALID_STMT})
			}
			infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
			resize(locals, capture_start)
		case .Block:
			infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
		case .Defer:
			capture_start := len(locals^)
			if statement.error_only && len(statement.captures) > 0 &&
			   statement.captures[0] != checker.sink_symbol {
				append(locals, Infer_Local{
					name=statement.captures[0],
					type=types.fallible_error(result^, &checker.module.types),
					declared=types.fallible_error(result^, &checker.module.types),
					statement=ast.INVALID_STMT,
				})
			}
			deferred := [1]ast.Stmt_Id{statement.update}
			infer_statements(checker, deferred[:], locals, local_types, pkg, file, demanded, result, result_hint)
			resize(locals, capture_start)
		case .Match:
			if selected_body, capture, has_capture, comptime_ok := specialization_match_body(checker, statement, pkg, file); comptime_ok {
				if has_capture {
					append(&checker.static_bindings, capture)
				}
				infer_statements(checker, selected_body, locals, local_types, pkg, file, demanded, result, result_hint)
				if has_capture {
					_ = pop(&checker.static_bindings)
				}
				continue
			}
			// The build pass desugars `match` to an if/else chain, but inference runs first
			// and must still visit the subject and arm bodies so calls there get specialized
			// (e.g. `match get()`). Mirror the `.For`/unwrap-`.If` capture handling.
			subject_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
			is_tagged := types.is_tagged_union(subject_type, &checker.module.types)
			is_enum_subject := types.is_enum(subject_type, &checker.module.types)
			covered: [dynamic]symbol.Id
			covered.allocator = checker.allocator
			for arm_id in statement.body {
				arm := checker.ast_module.statements[arm_id]
				if arm.kind != .Match_Arm {
					continue
				}
				if arm.expand && (is_tagged || is_enum_subject) {
					tag_type := types.union_tag_enum(subject_type, &checker.module.types) if is_tagged else subject_type
					if is_tagged {
						for field in types.fields_for(&checker.module.types, subject_type) {
							name := symbol.Id(field.name)
							if contains_name(covered[:], name) {
								continue
							}
							member, found := find_enum_member(checker, tag_type, name)
							if !found {
								continue
							}
							capture_start := len(locals^)
							static_start := len(checker.static_bindings)
							if len(arm.captures) > 1 {
								_ = push_static_integer_binding(checker, arm.captures[1], tag_type, member.value)
							}
							if len(arm.captures) > 0 && arm.captures[0] != checker.sink_symbol {
								if types.is_void(field.type) && !arm.pointer_capture {
									_ = push_static_void_binding(checker, arm.captures[0])
								} else {
									capture_type := field.type
									if arm.pointer_capture {
										capture_type = types.pointer(&checker.module.types, field.type, true, false)
									}
									append(locals, Infer_Local{name=arm.captures[0], type=capture_type, declared=capture_type, statement=ast.INVALID_STMT})
								}
							}
							infer_statements(checker, arm.body, locals, local_types, pkg, file, demanded, result, result_hint)
							resize(locals, capture_start)
							pop_static_bindings(checker, static_start)
						}
					} else {
						for member in types.enum_members_for(&checker.module.types, subject_type) {
							name := symbol.Id(member.name)
							if contains_name(covered[:], name) {
								continue
							}
							static_start := push_static_integer_binding(checker, arm.captures[0] if len(arm.captures) > 0 else symbol.INVALID, tag_type, member.value)
							infer_statements(checker, arm.body, locals, local_types, pkg, file, demanded, result, result_hint)
							pop_static_bindings(checker, static_start)
						}
					}
					continue
				}
				for pattern in arm.patterns {
					_ = infer_expr(checker, pattern, locals^[:], pkg, file, demanded, local_types)
					pattern_expr := checker.ast_module.exprs[pattern]
					if pattern_expr.kind == .Enum_Literal {
						append(&covered, pattern_expr.name)
					}
				}
				capture_start := len(locals^)
				if len(arm.captures) > 0 && is_tagged && len(arm.patterns) > 0 {
					capture := arm.captures[0]
					if capture != checker.sink_symbol {
						capture_type := types.INVALID
						pattern := checker.ast_module.exprs[arm.patterns[0]]
						if pattern.kind == .Enum_Literal {
							if _, field, ok := find_struct_field(checker, subject_type, pattern.name); ok {
								capture_type = field.type
								if arm.pointer_capture {
									// Mutability is best-effort here; the build pass finalizes
									// the exact pointer type and coerces the captured value.
									capture_type = types.pointer(&checker.module.types, field.type, true, false)
								}
							}
						}
						append(locals, Infer_Local{name=capture, type=capture_type, declared=capture_type, statement=ast.INVALID_STMT})
					}
				}
				infer_statements(checker, arm.body, locals, local_types, pkg, file, demanded, result, result_hint)
				resize(locals, capture_start)
			}
			delete(covered)
		}
	}
	resize(locals, scope_start)
}

infer_spec_locals_and_result :: proc(
	checker: ^Checker,
	id: Spec_Id,
	demanded: ^[dynamic]Spec_Id = nil,
) -> ([]types.Type, types.Type) {
	spec := checker.specs[id]
	function := checker.ast_module.functions[spec.template]
	previous_comptime := checker.current_comptime_values
	checker.current_comptime_values = spec.comptime_values
	defer checker.current_comptime_values = previous_comptime
	declared := type_from_syntax(checker, function.result, function.pkg, function.file)
	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT {
		declared = types.I32
	}
	result_hint := declared if is_runtime_type(checker, declared) || declared == types.UINT else types.INVALID

	locals: [dynamic]Infer_Local
	locals.allocator = checker.allocator
	defer delete(locals)
	local_types := make([]types.Type, len(checker.ast_module.statements), checker.allocator)
	runtime_index := 0
	for param, index in function.params {
		if param.comptime_value {
			continue
		}
		param_type := types.INVALID
		if runtime_index < len(spec.args) {
			param_type = spec.args[runtime_index]
		}
		append(&locals, Infer_Local{name=param.name, type=param_type, declared=param_type, statement=ast.INVALID_STMT})
		runtime_index += 1
	}

	result := types.INVALID
	test_error := function.error
	previous_test_error := checker.inferred_test_error
	checker.inferred_test_error = &test_error if function.test else nil
	infer_statements(checker, function.body, &locals, local_types, function.pkg, function.file, demanded, &result, result_hint)
	checker.inferred_test_error = previous_test_error
	if function.test && !types.equal(function.error, test_error) {
		checker.ast_module.functions[spec.template].error = test_error
		success := types.fallible_success(checker.specs[id].result, &checker.module.types)
		checker.specs[id].result = types.fallible(&checker.module.types, success, test_error)
	}
	if types.is_constraint(declared) {
		// Narrow the inferred result to the constraint's family; an out-of-family
		// result (e.g. returning a non-integer from an `int` function) yields
		// INVALID and is rejected downstream.
		return local_types, types.constraint_target(declared, result, &checker.module.types)
	}
	return local_types, declared
}

infer_spec_result :: proc(checker: ^Checker, id: Spec_Id, demanded: ^[dynamic]Spec_Id = nil) -> types.Type {
	local_types, result := infer_spec_locals_and_result(checker, id, demanded)
	delete(local_types, checker.allocator)
	return result
}

merge_inferred_type :: proc(store: ^types.Store, current: ^types.Type, inferred: types.Type) -> bool {
	if !types.is_runtime_value(inferred, store) {
		return false
	}
	if !types.is_runtime_value(current^, store) {
		current^ = inferred
		return true
	}
	if types.equal(current^, inferred) {
		return false
	}
	merged := types.widest(current^, inferred)
	if types.is_concrete_scalar(merged) && !types.equal(current^, merged) {
		current^ = merged
		return true
	}
	return false
}

open_integer_accepts_demand :: proc(checker: ^Checker, value: i128, demand: types.Type) -> bool {
	if types.is_concrete_integer(demand) {
		return fits_integer_type(value, demand, checker.target)
	}
	return types.is_float(demand, checker.target)
}

open_float_accepts_demand :: proc(checker: ^Checker, demand: types.Type) -> bool {
	return types.is_float(demand, checker.target)
}

// merge_open_const_demand records a concrete numeric demand onto an open numeric
// global's slot. Integer constants may adopt integer or float demands; float
// constants may adopt float demands. Later demands only widen within the chosen family.
merge_open_const_demand :: proc(
	checker: ^Checker,
	slot: ^types.Type,
	demand: types.Type,
	int_open: bool,
	float_open: bool,
	value: i128,
) -> bool {
	if !(int_open && open_integer_accepts_demand(checker, value, demand) ||
	     float_open && open_float_accepts_demand(checker, demand)) {
		return false
	}
	if !is_runtime_type(checker, slot^) {
		slot^ = demand
		return true
	}
	if types.equal(slot^, demand) {
		return false
	}
	merged := types.widest(slot^, demand)
	if types.is_concrete_scalar(merged) && !types.equal(slot^, merged) {
		slot^ = merged
		return true
	}
	return false
}

// merge_global_demand routes a concrete demand onto a global's slot: open constants
// adopt any fitting family, other referents widen within family. Sets a dirty flag so
// a demand pushed from a function body re-triggers the inference fixpoint.
merge_global_demand :: proc(checker: ^Checker, global: ast.Global_Id, demand: types.Type) -> bool {
	index := int(global)
	if index < 0 || index >= len(checker.global_demands) {
		return false
	}
	ast_global := checker.ast_module.globals[global]
	declared := type_from_syntax(checker, ast_global.type, ast_global.pkg, ast_global.file)
	if types.is_constraint(declared) && !types.constraint_accepts(declared, demand, &checker.module.types) {
		return false
	}
	changed: bool
	if checker.global_open_const[index] || checker.global_open_float[index] {
		changed = merge_open_const_demand(
			checker,
			&checker.global_demands[index],
			demand,
			checker.global_open_const[index],
			checker.global_open_float[index],
			checker.global_const_value[index],
		)
	} else {
		changed = merge_inferred_type(&checker.module.types, &checker.global_demands[index], demand)
	}
	checker.global_demands_dirty = checker.global_demands_dirty || changed
	return changed
}

// merge_local_demand records a concrete numeric demand onto an open-constant local.
// The first demand replaces the literal's default type; later demands may only widen
// within the chosen family.
merge_local_demand :: proc(checker: ^Checker, local: ^Infer_Local, demand: types.Type, local_types: []types.Type) -> bool {
	if !(local.open_const && open_integer_accepts_demand(checker, local.const_value, demand) ||
	     local.open_float && open_float_accepts_demand(checker, demand)) {
		return false
	}
	if types.is_constraint(local.declared) &&
	   !types.constraint_accepts(local.declared, demand, &checker.module.types) {
		return false
	}
	if !local.demanded {
		local.type = demand
		local.demanded = true
		record_infer_local_type(local^, local_types)
		return true
	}
	if types.equal(local.type, demand) {
		return false
	}
	merged := types.widest(local.type, demand)
	if types.is_concrete_scalar(merged) && !types.equal(local.type, merged) {
		local.type = merged
		record_infer_local_type(local^, local_types)
		return true
	}
	return false
}

is_arith_kind :: proc(k: ast.Expr_Kind) -> bool {
	return k == .Add || k == .Sub || k == .Mul || k == .Div || k == .Negate
}

// open_const_default_type returns the fallback type a bare open-constant reference
// (local or global) would take if no use ever demands it: the smallest signed type
// that holds an integer constant, or f64 for a float constant. Used to give a call's
// argument a concrete type for monomorphization when deferred defaulting has left the
// open constant typeless (its param is a constraint, so the call records no demand on
// it). The fallback feeds only the specialization's arg vector, never a demand, so it
// cannot leak back onto the constant or a sibling.
open_const_default_type :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> types.Type {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return types.INVALID
	}
	expr := checker.ast_module.exprs[expr_id]
	if expr.kind != .Name {
		return types.INVALID
	}
	if !symbol.is_valid(expr.qualifier) {
		if index, ok := find_infer_local_index(locals, expr.name); ok {
			local := locals[index]
			if local.open_const {
				return constraint_integer_literal_type(local.declared, local.const_value)
			}
			if local.open_float {
				return types.F64
			}
			return types.INVALID
		}
	}
	target_pkg, available := expr_package(checker, expr, pkg, file)
	if !available {
		return types.INVALID
	}
	global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
	index := int(global)
	if global == ast.INVALID_GLOBAL || index < 0 || index >= len(checker.global_open_const) {
		return types.INVALID
	}
	if checker.global_open_const[index] {
		ast_global := checker.ast_module.globals[global]
		declared := type_from_syntax(checker, ast_global.type, ast_global.pkg, ast_global.file)
		return constraint_integer_literal_type(declared, checker.global_const_value[index])
	}
	if checker.global_open_float[index] {
		return types.F64
	}
	return types.INVALID
}

// numeric_operand_is_open reports whether an arithmetic operand still carries a
// provisional type (an open constant at its smallest-signed default, or a bare
// numeric literal) rather than an authoritative one. A provisional operand must not
// propagate its type onto a sibling open constant: doing so locks the sibling into a
// default family and blocks the real backward demand from the declaration/use. Two
// provisional operands are instead resolved together by that backward demand.
numeric_operand_is_open :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> bool {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	expr := checker.ast_module.exprs[expr_id]
	#partial switch expr.kind {
	case .Name:
		// A bare open-constant name is open iff it has a deferred default to assign.
		return is_runtime_type(checker, open_const_default_type(checker, expr_id, locals, pkg, file))
	case .Negate:
		return numeric_operand_is_open(checker, expr.left, locals, pkg, file)
	case .Add, .Sub, .Mul, .Div:
		return numeric_operand_is_open(checker, expr.left, locals, pkg, file) ||
		       numeric_operand_is_open(checker, expr.right, locals, pkg, file)
	}
	// A bare integer/float literal adapts freely, so it too is provisional.
	return is_numeric_constant_expr(checker, expr_id)
}

expr_accepts_numeric_demand :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	demand: types.Type,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> bool {
	if !is_numeric_demand(demand, checker.target) ||
	   expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	if constant := eval_integer_constant_in_context(checker, expr_id, pkg, file); constant.kind == .Value {
		return open_integer_accepts_demand(checker, constant.value, demand)
	}
	if is_float_constant_expr(checker, expr_id) {
		return open_float_accepts_demand(checker, demand)
	}
	expr := checker.ast_module.exprs[expr_id]
	#partial switch expr.kind {
	case .Name:
		if !symbol.is_valid(expr.qualifier) {
			if index, ok := find_infer_local_index(locals, expr.name); ok {
				local := locals[index]
				return local.open_const && open_integer_accepts_demand(checker, local.const_value, demand) ||
				       local.open_float && open_float_accepts_demand(checker, demand)
			}
		}
		target_pkg, available := expr_package(checker, expr, pkg, file)
		if !available {
			return false
		}
		global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
		index := int(global)
		if global == ast.INVALID_GLOBAL || index < 0 || index >= len(checker.global_open_const) {
			return false
		}
		return checker.global_open_const[index] &&
		       open_integer_accepts_demand(checker, checker.global_const_value[index], demand) ||
		       checker.global_open_float[index] && open_float_accepts_demand(checker, demand)
	case .Negate:
		if !types.is_signed(demand, checker.target) && !types.is_float(demand, checker.target) {
			return false
		}
		return expr_accepts_numeric_demand(checker, expr.left, demand, locals, pkg, file)
	case .Add, .Sub, .Mul, .Div:
		return expr_accepts_numeric_demand(checker, expr.left, demand, locals, pkg, file) &&
		       expr_accepts_numeric_demand(checker, expr.right, demand, locals, pkg, file)
	}
	return false
}

DEMAND_RECURSION_LIMIT :: 4096

record_demand_too_deep :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> bool {
	stack: [dynamic]ast.Expr_Id
	stack.allocator = checker.allocator
	defer delete(stack)
	append(&stack, expr_id)
	seen := 0
	for len(stack) > 0 {
		current := pop(&stack)
		if current == ast.INVALID_EXPR || int(current) >= len(checker.ast_module.exprs) {
			continue
		}
		seen += 1
		if seen > DEMAND_RECURSION_LIMIT {
			return true
		}
		expr := checker.ast_module.exprs[current]
		#partial switch expr.kind {
		case .Negate:
			append(&stack, expr.left)
		case .Add, .Sub, .Mul, .Div:
			append(&stack, expr.left, expr.right)
		}
	}
	return false
}

record_demand_shallow :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	demand: types.Type,
	locals: []Infer_Local,
	local_types: []types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> bool {
	if record_demand_too_deep(checker, expr_id) {
		return false
	}
	return record_demand(checker, expr_id, demand, locals, local_types, pkg, file)
}

inferred_record_field_slot_from_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Infer_Local,
	local_types: []types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (int, bool) {
	base := types.INVALID
	field_name := expr.name
	if expr.kind == .Field {
		base = infer_nested_expr(checker, expr.left, locals, pkg, file, nil, local_types)
	} else if expr.kind == .Name && symbol.is_valid(expr.qualifier) &&
	          find_import(checker, file, expr.qualifier) == ast.INVALID_IMPORT {
		base = find_infer_local(locals, expr.qualifier)
		if !types.is_valid(base) {
			if global := find_global(checker, expr.qualifier, pkg, file); global != ast.INVALID_GLOBAL {
				base = checker.global_types[global]
			}
		}
	} else {
		return -1, false
	}
	if types.is_pointer(base, &checker.module.types) {
		base = types.child_type(base, &checker.module.types)
	}
	slot, _, ok := find_struct_field_slot(checker, base, field_name)
	return slot, ok && is_inferred_record_field(checker, slot)
}

// record_demand pushes a concrete type demand onto open numeric slots reachable
// through bare names, inferred record fields, and numeric arithmetic. Calls remain
// a boundary (milestone 14.5).
record_demand :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	demand: types.Type,
	locals: []Infer_Local,
	local_types: []types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> bool {
	if !is_runtime_type(checker, demand) ||
	   expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return false
	}
	expr := checker.ast_module.exprs[expr_id]
	#partial switch expr.kind {
	case .Name:
		if symbol.is_valid(expr.qualifier) {
			if slot, ok := inferred_record_field_slot_from_expr(
				checker, expr, locals, local_types, pkg, file,
			); ok {
				return merge_record_field_demand(checker, slot, demand, expr.span)
			}
		}
		if !symbol.is_valid(expr.qualifier) {
			if index, ok := find_infer_local_index(locals, expr.name); ok {
				return merge_local_demand(checker, &locals[index], demand, local_types)
			}
		}
		target_pkg, available := expr_package(checker, expr, pkg, file)
		if !available {
			return false
		}
		global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
		if global != ast.INVALID_GLOBAL {
			return merge_global_demand(checker, global, demand)
		}
	case .Field:
		if slot, ok := inferred_record_field_slot_from_expr(
			checker, expr, locals, local_types, pkg, file,
		); ok {
			return merge_record_field_demand(checker, slot, demand, expr.span)
		}
	case .Negate:
		if types.is_signed(demand, checker.target) || types.is_float(demand, checker.target) {
			return record_demand(checker, expr.left, demand, locals, local_types, pkg, file)
		}
	case .Add, .Sub, .Mul, .Div:
		if is_numeric_demand(demand, checker.target) {
			left := record_demand(checker, expr.left, demand, locals, local_types, pkg, file)
			right := record_demand(checker, expr.right, demand, locals, local_types, pkg, file)
			return left || right
		}
	case .Call:
		if division_builtin_call(checker, expr) != .None && len(expr.args) == 2 && is_numeric_demand(demand, checker.target) {
			left := record_demand(checker, expr.args[0], demand, locals, local_types, pkg, file)
			right := record_demand(checker, expr.args[1], demand, locals, local_types, pkg, file)
			return left || right
		}
	}
	return false
}

infer_all :: proc(checker: ^Checker) {
	// An "open constant" global has no concrete declared type and a compile-time
	// numeric initializer. Its slot can adopt a backward demand from any reachable use.
	// Demands accumulate in global_demands so the default never blocks a later
	// cross-family demand (e.g. integer literal -> unsigned or float).
	for global, index in checker.ast_module.globals {
		if global.diagnostic != source.INVALID_DIAGNOSTIC {
			checker.global_types[index] = types.I64
			continue
		}
		declared := resolve_inferred_array(
			checker,
			type_from_syntax(checker, global.type, global.pkg, global.file),
			global.expr,
		)
		if is_runtime_type(checker, declared) && !has_inferred_array_count(checker, declared) {
			checker.global_types[index] = declared
			continue
		}
		if global.external {
			continue
		}
		if global.expr != ast.INVALID_EXPR && int(global.expr) < len(checker.ast_module.exprs) {
			expr := checker.ast_module.exprs[global.expr]
			if builtin := type_builtin_call(checker, expr); builtin != .None {
				if builtin == .Size_Of || builtin == .Align_Of {
					checker.global_types[index] = types.USIZE
				} else if len(expr.args) == 1 {
					target, ok := resolve_type_argument(checker, expr.args[0], global.pkg, global.file)
					if ok && types.is_concrete_integer(target) {
						checker.global_types[index] = target
					}
				}
				continue
			}
		}
		constant := eval_integer_constant_in_context(checker, global.expr, global.pkg, global.file)
		if constant.kind == .Value &&
		   (fits_i64(constant.value) || declared == types.UINT && fits_u64(constant.value)) {
			checker.global_open_const[index] = true
			checker.global_const_value[index] = constant.value
		} else if is_float_constant_expr(checker, global.expr) {
			checker.global_open_float[index] = true
		}
	}

	for function, index in checker.ast_module.functions {
		if function.analysis_root {
			ensure_spec(checker, ast.function_id(index), nil)
		}
	}
	main_template := find_template(checker, checker.main_symbol, 0)
	if main_template != ast.INVALID_FUNCTION {
		ensure_spec(checker, main_template, nil)
	}
	if checker.entry_point == .Process {
		ensure_spec(checker, checker.io_provider_template, nil)
	}

	defaults_applied := false
	for {
		changed := false
		checker.global_demands_dirty = false
		checker.record_field_demands_dirty = false
		spec_count := len(checker.specs)

		for field_default in checker.ast_module.struct_field_defaults {
			slot, field, ok := find_struct_field_slot(checker, field_default.record, field_default.field)
			if !ok {
				continue
			}
			expected := field.type if is_runtime_type(checker, field.type) else types.INVALID
			inferred := infer_expr(
				checker, field_default.expr, nil, field_default.pkg, field_default.file,
				expected=expected,
			)
			_ = record_field_expr_candidate(
				checker, slot, field_default.expr, inferred, nil, field_default.pkg, field_default.file,
			)
			if is_runtime_type(checker, checker.module.types.fields[slot].type) {
				_ = record_demand(
					checker, field_default.expr, checker.module.types.fields[slot].type,
					nil, nil, field_default.pkg, field_default.file,
				)
			}
		}

		// Backward demands: a global pushes its own (declared or already-resolved) type
		// onto open numeric slots reachable through names and numeric arithmetic.
		for global, index in checker.ast_module.globals {
			if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC {
				continue
			}
			demand := checker.global_types[index]
			if !is_runtime_type(checker, demand) {
				continue
			}
			_ = record_demand(checker, global.expr, demand, nil, nil, global.pkg, global.file)
		}

		// Forward / resolution. infer_expr runs for every non-external global (even
		// concrete-typed ones) for its side effect of specializing called functions and
		// recording demands from call arguments in their initializers.
		for global, index in checker.ast_module.globals {
			if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC {
				continue
			}
			declared := resolve_inferred_array(
				checker,
				type_from_syntax(checker, global.type, global.pkg, global.file),
				global.expr,
			)
			expected := declared if is_runtime_type(checker, declared) || declared == types.UINT else types.INVALID
			inferred := infer_expr(checker, global.expr, nil, global.pkg, global.file, expected=expected)
			if resolved := resolve_inferred_array_from_type(checker, declared, inferred);
			   resolved != declared {
				if !types.equal(checker.global_types[index], resolved) {
					checker.global_types[index] = resolved
					changed = true
				}
				continue
			}
			if is_runtime_type(checker, declared) && !has_inferred_array_count(checker, declared) {
				continue
			}
			if types.is_constraint(declared) {
				resolved := types.INVALID
				if is_runtime_type(checker, checker.global_demands[index]) &&
				   types.constraint_accepts(declared, checker.global_demands[index], &checker.module.types) {
					resolved = checker.global_demands[index]
				} else {
					resolved = types.constraint_target(declared, inferred, &checker.module.types)
				}
				if is_runtime_type(checker, resolved) && !types.equal(checker.global_types[index], resolved) {
					checker.global_types[index] = resolved
					changed = true
				}
				continue
			}
			if is_runtime_type(checker, checker.global_demands[index]) {
				// A backward demand is authoritative; assign directly (it may cross the
				// signed/unsigned family that widening would reject).
				if !types.equal(checker.global_types[index], checker.global_demands[index]) {
					checker.global_types[index] = checker.global_demands[index]
					changed = true
				}
			} else if checker.global_open_const[index] || checker.global_open_float[index] {
				// Defer defaulting: an undemanded open constant stays typeless during the
				// fixpoint so its provisional smallest-signed default never leaks as a
				// demand and poisons a sibling open constant used in the same arithmetic
				// (e.g. the lagging type in `x += speed`). The pass after this loop assigns
				// the default once the fixpoint settles and no further demand can arrive.
			} else {
				changed = merge_inferred_type(&checker.module.types, &checker.global_types[index], inferred) || changed
			}
		}

		for index := 0; index < len(checker.specs); index += 1 {
			id := spec_id(index)
			before := checker.specs[id].result
			inferred := infer_spec_result(checker, id)
			changed = !types.equal(before, checker.specs[id].result) || changed
			changed = merge_inferred_type(&checker.module.types, &checker.specs[id].result, inferred) || changed
		}
		if len(checker.specs) != spec_count {
			changed = true
		}
		// A demand pushed onto a global from inside a function body (via the spec loop)
		// is picked up by the next pass's resolution, so keep iterating for it.
		if checker.global_demands_dirty {
			changed = true
		}
		if checker.record_field_demands_dirty {
			changed = true
		}
		if !changed {
			if !defaults_applied {
				defaults_applied = true
				defaulted := false
				// No authoritative demand can still arrive. Assign final defaults, then
				// continue the same fixpoint so dependent globals/specs observe them.
				for global, index in checker.ast_module.globals {
					if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC ||
					   is_runtime_type(checker, checker.global_types[index]) {
						continue
					}
					if checker.global_open_const[index] {
						declared := type_from_syntax(checker, global.type, global.pkg, global.file)
						checker.global_types[index] = constraint_integer_literal_type(declared, checker.global_const_value[index])
						defaulted = true
					} else if checker.global_open_float[index] {
						checker.global_types[index] = types.F64
						defaulted = true
					}
				}
				for fallback, slot in checker.record_field_defaults {
					if !is_inferred_record_field(checker, slot) ||
					   !types.is_constraint(checker.module.types.fields[slot].type) ||
					   !is_runtime_type(checker, fallback) {
						continue
					}
					checker.module.types.fields[slot].type = fallback
					defaulted = true
				}
				if defaulted {
					continue
				}
			}
			break
		}
	}
}

prune_specs :: proc(checker: ^Checker) {
	stack: [dynamic]Spec_Id
	stack.allocator = checker.allocator
	defer delete(stack)

	main_template := find_template(checker, checker.main_symbol, 0)
	if main_template != ast.INVALID_FUNCTION {
		mark_spec_demanded(checker, find_spec(checker, main_template, nil), &stack)
	}
	if checker.entry_point == .Process {
		mark_spec_demanded(checker, find_spec(checker, checker.io_provider_template, nil), &stack)
	}
	for global, index in checker.ast_module.globals {
		if global.external || global.diagnostic != source.INVALID_DIAGNOSTIC {
			continue
		}
		expected := checker.global_types[index] if is_runtime_type(checker, checker.global_types[index]) else types.INVALID
		_ = infer_expr(checker, global.expr, nil, global.pkg, global.file, &stack, expected=expected)
	}
	for len(stack) > 0 {
		id := pop(&stack)
		_ = infer_spec_result(checker, id, &stack)
	}

	retained := 0
	for spec in checker.specs {
		if spec.hir_id == hir.INVALID_FUNCTION {
			delete(spec.args, checker.allocator)
			delete(spec.comptime_values, checker.allocator)
			continue
		}
		checker.specs[retained] = spec
		checker.specs[retained].hir_id = hir.function_id(retained)
		retained += 1
	}
	for len(checker.specs) > retained {
		_ = pop(&checker.specs)
	}
}

add_hir_expr :: proc(checker: ^Checker, expr: hir.Expr) -> hir.Expr_Id {
	id := hir.expr_id(len(checker.module.exprs))
	append(&checker.module.exprs, expr)
	return id
}

invalid_hir_expr :: proc(
	checker: ^Checker,
	span: source.Span,
	diagnostic: source.Diagnostic_Id,
	recovery_type := types.INVALID,
) -> hir.Expr_Id {
	return add_hir_expr(
		checker,
		hir.Expr {
			kind = .Invalid,
			span = span,
			type = recovery_type,
			target = hir.INVALID_REF,
			left = hir.INVALID_EXPR,
			right = hir.INVALID_EXPR,
			diagnostic = diagnostic,
		},
	)
}

invalid_expr_diagnostic :: proc(checker: ^Checker, id: hir.Expr_Id) -> (source.Diagnostic_Id, bool) {
	if id == hir.INVALID_EXPR || int(id) >= len(checker.module.exprs) {
		return source.INVALID_DIAGNOSTIC, false
	}
	expr := checker.module.exprs[id]
	return expr.diagnostic, expr.kind == .Invalid && expr.diagnostic != source.INVALID_DIAGNOSTIC
}

propagate_invalid_expr :: proc(checker: ^Checker, span: source.Span, values: ..hir.Expr_Id) -> (hir.Expr_Id, bool) {
	for value in values {
		if diagnostic, invalid := invalid_expr_diagnostic(checker, value); invalid {
			return invalid_hir_expr(checker, span, diagnostic), true
		}
	}
	return hir.INVALID_EXPR, false
}

add_unique_global :: proc(values: ^[dynamic]hir.Global_Id, value: hir.Global_Id) {
	for existing in values {
		if existing == value {
			return
		}
	}
	append(values, value)
}

build_global_reference :: proc(
	checker: ^Checker,
	global: ast.Global_Id,
	span: source.Span,
	global_reads: ^[dynamic]hir.Global_Id,
) -> hir.Expr_Id {
	if int(global) < len(checker.external_global_diagnostics) {
		diagnostic := checker.external_global_diagnostics[global]
		if diagnostic != source.INVALID_DIAGNOSTIC {
			return invalid_hir_expr(checker, span, diagnostic, checker.global_types[global])
		}
	}
	hir_global := hir.Global_Id(global)
	add_unique_global(global_reads, hir_global)
	return add_hir_expr(checker, hir.Expr{
		kind=.Global, span=span, type=checker.global_types[global],
		target=hir.global_ref(hir_global), left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

add_unique_function :: proc(values: ^[dynamic]hir.Function_Id, value: hir.Function_Id) {
	for existing in values {
		if existing == value {
			return
		}
	}
	append(values, value)
}

find_build_local :: proc(locals: []Build_Local, name: symbol.Id) -> (Build_Local, bool) {
	for index := len(locals) - 1; index >= 0; index -= 1 {
		if locals[index].name == name {
			return locals[index], true
		}
	}
	return Build_Local{}, false
}

can_implicitly_convert_type :: proc(checker: ^Checker, actual, expected: types.Type) -> bool {
	store := &checker.module.types
	if types.equal(actual, expected) ||
	   types.can_widen(actual, expected) ||
	   types.can_coerce_c_integer(actual, expected, checker.target) ||
	   types.can_coerce_c_scalar(actual, expected, checker.target) ||
	   types.can_weaken_pointer(actual, expected, store) ||
	   types.can_coerce_function_pointer(actual, expected, store) ||
	   types.can_weaken_slice(actual, expected, store) ||
	   types.can_decay_slice_c_string(actual, expected, store) ||
	   types.can_decay_array_pointer(actual, expected, store) ||
	   types.can_sum_widen(actual, expected, store) {
		return true
	}
	if types.is_optional(expected, store) {
		return can_implicitly_convert_type(checker, actual, types.child_type(expected, store))
	}
	return false
}

coerce_expr :: proc(
	checker: ^Checker,
	expr_id: hir.Expr_Id,
	expected: types.Type,
	span: source.Span,
) -> hir.Expr_Id {
	if expr_id == hir.INVALID_EXPR {
		return expr_id
	}
	if _, invalid := invalid_expr_diagnostic(checker, expr_id); invalid {
		return expr_id
	}
	actual := checker.module.exprs[expr_id].type
	if types.equal(actual, expected) {
		return expr_id
	}
	if types.can_coerce_function_pointer(actual, expected, &checker.module.types) {
		expr := checker.module.exprs[expr_id]
		if expr.kind == .Function {
			expr.type = expected
			return add_hir_expr(checker, expr)
		}
	}
	if types.can_weaken_pointer(actual, expected, &checker.module.types) {
		return add_hir_expr(checker, hir.Expr{
			kind=.Weaken_Pointer,
			span=span,
			type=expected,
			left=expr_id,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if types.can_weaken_slice(actual, expected, &checker.module.types) {
		return add_hir_expr(checker, hir.Expr{
			kind=.Weaken_Slice,
			span=span,
			type=expected,
			left=expr_id,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if types.can_decay_array_pointer(actual, expected, &checker.module.types) {
		return add_hir_expr(checker, hir.Expr{
			kind=.Decay_Array_Pointer,
			span=span,
			type=expected,
			left=expr_id,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if types.can_decay_slice_c_string(actual, expected, &checker.module.types) {
		return add_hir_expr(checker, hir.Expr{
			kind=.Slice_Ptr,
			span=span,
			type=expected,
			left=expr_id,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if types.can_sum_widen(actual, expected, &checker.module.types) {
		return add_hir_expr(checker, hir.Expr{
			kind=.Sum_Widen,
			span=span,
			type=expected,
			left=expr_id,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	if types.is_optional(expected, &checker.module.types) {
		child := types.child_type(expected, &checker.module.types)
		if types.equal(actual, child) ||
		   types.can_coerce_function_pointer(actual, child, &checker.module.types) ||
		   types.can_widen(actual, child) ||
		   types.can_coerce_c_integer(actual, child, checker.target) ||
		   types.can_coerce_c_scalar(actual, child, checker.target) ||
		   types.can_weaken_pointer(actual, child, &checker.module.types) ||
		   types.can_weaken_slice(actual, child, &checker.module.types) ||
		   types.can_decay_slice_c_string(actual, child, &checker.module.types) ||
		   types.can_decay_array_pointer(actual, child, &checker.module.types) {
			value := coerce_expr(checker, expr_id, child, span)
			return add_hir_expr(checker, hir.Expr{
				kind=.Optional_Some,
				span=span,
				type=expected,
				left=value,
				target=hir.INVALID_REF,
				right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
	}
	if types.can_widen(actual, expected) {
		return add_hir_expr(
			checker,
			hir.Expr {
				kind = .Widen,
				span = span,
				type = expected,
				left = expr_id,
				target = hir.INVALID_REF,
				right = hir.INVALID_EXPR,
				diagnostic = source.INVALID_DIAGNOSTIC,
			},
		)
	}
	if types.can_coerce_c_integer(actual, expected, checker.target) ||
	   types.can_coerce_c_scalar(actual, expected, checker.target) {
		return add_hir_expr(
			checker,
			hir.Expr {
				kind = .C_Coerce,
				span = span,
				type = expected,
				left = expr_id,
				target = hir.INVALID_REF,
				right = hir.INVALID_EXPR,
				diagnostic = source.INVALID_DIAGNOSTIC,
			},
		)
	}
	id := source.addf(
		checker.diagnostics,
		span,
		"cannot implicitly convert %s to %s",
		type_label(checker, actual),
		type_label(checker, expected),
	)
	return invalid_hir_expr(checker, span, id, expected)
}

promote_c_vararg_expr :: proc(checker: ^Checker, expr_id: hir.Expr_Id, span: source.Span) -> hir.Expr_Id {
	actual := checker.module.exprs[expr_id].type
	if !types.is_c_vararg_type(actual, &checker.module.types) {
		id := source.addf(
			checker.diagnostics,
			span,
			"C variadic argument must be a concrete scalar or pointer, got %s",
			types.name(actual),
		)
		return invalid_hir_expr(checker, span, id, types.C_INT)
	}
	promoted := types.c_vararg_promotion(actual, checker.target, &checker.module.types)
	if types.equal(actual, promoted) {
		return expr_id
	}
	return add_hir_expr(checker, hir.Expr{
		kind=.C_Vararg_Promote,
		span=span,
		type=promoted,
		left=expr_id,
		target=hir.INVALID_REF,
		right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

build_constant_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	constant: Constant,
	expected: types.Type,
) -> hir.Expr_Id {
	recovery_type := types.I64
	if types.is_concrete_integer(expected) {
		recovery_type = expected
	}
	// An integer constant in a float context (e.g. `pi float = 3`) folds to a
	// float literal, mirroring build_float_expr's bit packing.
	if constant.kind == .Value && types.is_float(expected, checker.target) {
		fval := f64(constant.value) // ponytail: silent precision loss past 2^53, like C int->double
		bits := transmute(i64)fval
		if types.bits(expected, checker.target) == 32 {
			bits = i64(transmute(u32)f32(fval))
		}
		return add_hir_expr(checker, hir.Expr{
			kind = .Float,
			span = expr.span,
			type = expected,
			integer = bits,
			target = hir.INVALID_REF,
			left = hir.INVALID_EXPR,
			right = hir.INVALID_EXPR,
			diagnostic = source.INVALID_DIAGNOSTIC,
		})
	}
	if constant.kind == .Div_By_Zero {
		id := source.add(checker.diagnostics, expr.span, "division by zero in constant expression")
		return invalid_hir_expr(checker, expr.span, id, recovery_type)
	}
	if constant.kind == .Non_Exact {
		id := source.add(checker.diagnostics, expr.span, "exact division has a remainder")
		return invalid_hir_expr(checker, expr.span, id, recovery_type)
	}
	if constant.kind == .Integer_Division {
		id := source.add(checker.diagnostics, expr.span, "integer '/' is not allowed; use divtrunc!, divfloor!, divexact!, or divceil!")
		return invalid_hir_expr(checker, expr.span, id, recovery_type)
	}
	if constant.kind == .Overflow ||
	   (!types.is_concrete_integer(expected) && expected != types.UINT && !fits_i64(constant.value)) {
		id := source.add(
			checker.diagnostics,
			expr.span,
			"integer constant expression exceeds signed i64 range",
		)
		return invalid_hir_expr(checker, expr.span, id, recovery_type)
	}

	value := i64(constant.value)
	if constant.value >= 0 && constant.value <= i128(0xffff_ffff_ffff_ffff) {
		value = transmute(i64)u64(constant.value)
	}
	result_type := constraint_integer_literal_type(expected, constant.value)
	if expected == types.UINT && !types.is_valid(result_type) {
		id := source.addf(
			checker.diagnostics,
			expr.span,
			"integer constant %d does not satisfy uint",
			constant.value,
		)
		return invalid_hir_expr(checker, expr.span, id, types.U64)
	}
	if types.is_concrete_integer(expected) {
		if !fits_integer_type(constant.value, expected, checker.target) {
			id := source.addf(
				checker.diagnostics,
				expr.span,
				"integer constant %d does not fit in %s",
				constant.value,
				types.name(expected),
			)
			return invalid_hir_expr(checker, expr.span, id, expected)
		}
		result_type = expected
	}
	return add_hir_expr(
		checker,
		hir.Expr {
			kind = .Integer,
			span = expr.span,
			type = result_type,
			integer = value,
			target = hir.INVALID_REF,
			left = hir.INVALID_EXPR,
			right = hir.INVALID_EXPR,
			diagnostic = source.INVALID_DIAGNOSTIC,
		},
	)
}

build_float_expr :: proc(checker: ^Checker, expr: ast.Expr, expected: types.Type) -> hir.Expr_Id {
	result_type := types.F64
	if types.is_float(expected, checker.target) {
		result_type = expected
	} else if types.is_valid(expected) {
		id := source.addf(
			checker.diagnostics,
			expr.span,
			"cannot implicitly convert f64 to %s",
			types.name(expected),
		)
		return invalid_hir_expr(checker, expr.span, id, expected)
	}
	value := transmute(f64)expr.integer
	bits := transmute(i64)value
	if types.bits(result_type, checker.target) == 32 {
		bits = i64(transmute(u32)f32(value))
	}
	return add_hir_expr(checker, hir.Expr{
		kind=.Float,
		span=expr.span,
		type=result_type,
		integer=bits,
		target=hir.INVALID_REF,
		left=hir.INVALID_EXPR,
		right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

Build_Expr_Frame :: struct {
	expr:       ast.Expr_Id,
	expected:   types.Type,
	target_type: types.Type,
	stage:      u8,
	left:       hir.Expr_Id,
	arg_index:  int,
	arg_mode:   Call_Argument_Mode,
	prefix:     int,
	mapping:    []int,
	built_args: []hir.Expr_Id,
	arg_types:  []types.Type,
	template:   ast.Function_Id,
	resolution: int,
}

hir_location_writable :: proc(checker: ^Checker, expr_id: hir.Expr_Id, locals: []Build_Local) -> bool {
	if expr_id == hir.INVALID_EXPR || int(expr_id) >= len(checker.module.exprs) {
		return false
	}
	expr := checker.module.exprs[expr_id]
	#partial switch expr.kind {
	case .Local:
		id := hir.as_local(expr.target)
		for local in locals {
			if local.id == id {
				return local.mutable
			}
		}
	case .Global:
		id := hir.as_global(expr.target)
		return id != hir.INVALID_GLOBAL && int(id) < len(checker.module.globals) &&
		       checker.module.globals[id].writable
	case .Deref:
		pointer_type := checker.module.exprs[expr.left].type
		return types.is_mutable(pointer_type, &checker.module.types)
	case .Index:
		container_type := checker.module.exprs[expr.left].type
		item, ok := types.container(container_type, &checker.module.types)
		if !ok || !item.mutable {
			return false
		}
		if types.is_array(container_type, &checker.module.types) {
			return hir_location_writable(checker, expr.left, locals)
		}
		return true
	case .Field:
		base_type := checker.module.exprs[expr.left].type
		if types.is_pointer(base_type, &checker.module.types) {
			return types.is_mutable(base_type, &checker.module.types)
		}
		return hir_location_writable(checker, expr.left, locals)
	case:
	}
	return false
}

hir_is_location :: proc(checker: ^Checker, expr_id: hir.Expr_Id) -> bool {
	if expr_id == hir.INVALID_EXPR || int(expr_id) >= len(checker.module.exprs) {
		return false
	}
	#partial switch checker.module.exprs[expr_id].kind {
	case .Local, .Global, .Deref, .Index, .Field:
		return true
	}
	return false
}

sequence_item :: proc(value: types.Type, store: ^types.Store) -> (types.Node, bool) {
	item, ok := types.node(store, value)
	if ok && (item.kind == .Array || item.kind == .Slice) {
		return item, true
	}
	pointer, array, pointer_ok := types.array_pointer(value, store)
	if pointer_ok {
		array.mutable = pointer.mutable && array.mutable
		return array, true
	}
	return {}, false
}

find_struct_field :: proc(checker: ^Checker, struct_type: types.Type, name: symbol.Id) -> (int, types.Field, bool) {
	for field, index in types.fields_for(&checker.module.types, struct_type) {
		if field.name == u32(name) {
			return index, field, true
		}
	}
	return 0, {}, false
}

find_struct_field_default :: proc(
	checker: ^Checker,
	struct_type: types.Type,
	name: symbol.Id,
) -> (Resolved_Field_Default, []Comptime_Value, bool) {
	resolved := types.resolve_alias(struct_type, &checker.module.types)
	for field_default in checker.ast_module.struct_field_defaults {
		if types.resolve_alias(field_default.record, &checker.module.types) == resolved &&
		   field_default.field == name {
			return Resolved_Field_Default{
				expr=field_default.expr,
				pkg=field_default.pkg,
				file=field_default.file,
				static_value=INVALID_CT_VALUE,
				span=checker.ast_module.exprs[field_default.expr].span,
			}, nil, true
		}
	}
	for entry in checker.generated_types {
		if !types.equal(entry.result, resolved) || entry.expr == ast.INVALID_EXPR ||
		   int(entry.expr) >= len(checker.ast_module.exprs) {
			continue
		}
		expr := checker.ast_module.exprs[entry.expr]
		fields := types.fields_for(&checker.module.types, resolved)
		for field, index in fields {
			if field.name != u32(name) {
				continue
			}
			if index < len(entry.defaults) && entry.defaults[index] != INVALID_CT_VALUE {
				return Resolved_Field_Default{
					expr=ast.INVALID_EXPR,
					pkg=entry.pkg,
					file=entry.file,
					static_value=entry.defaults[index],
					span=expr.span,
				}, entry.values, true
			}
			if is_intrinsic_call(checker, expr, "struct_type") {
				continue
			}
			if index < len(expr.args) && expr.args[index] != ast.INVALID_EXPR {
				return Resolved_Field_Default{
					expr=expr.args[index],
					pkg=entry.pkg,
					file=entry.file,
					static_value=INVALID_CT_VALUE,
					span=checker.ast_module.exprs[expr.args[index]].span,
				}, entry.values, true
			}
		}
	}
	return {}, nil, false
}

find_tuple_field :: proc(checker: ^Checker, tuple_type: types.Type, index: u64) -> (int, types.Field, bool) {
	item, ok := types.node(&checker.module.types, tuple_type)
	if !ok || item.kind != .Struct || !item.tuple || index >= u64(item.field_count) {
		return 0, {}, false
	}
	fields := types.fields_for(&checker.module.types, tuple_type)
	return int(index), fields[index], true
}

find_struct_field_slot :: proc(checker: ^Checker, struct_type: types.Type, name: symbol.Id) -> (int, types.Field, bool) {
	item, ok := types.node(&checker.module.types, struct_type)
	if !ok || (item.kind != .Struct && item.kind != .Union) {
		return -1, {}, false
	}
	for field, index in types.fields_for(&checker.module.types, struct_type) {
		if field.name == u32(name) {
			return int(item.field_start)+index, field, true
		}
	}
	return -1, {}, false
}

field_type_from_value :: proc(checker: ^Checker, expr: ast.Expr, base_type: types.Type) -> types.Type {
	store := &checker.module.types
	if !symbol.is_valid(expr.name) {
		value_type := types.child_type(base_type, store) if types.is_pointer(base_type, store) else base_type
		_, field, ok := find_tuple_field(checker, value_type, expr.integer)
		return field.type if ok else types.INVALID
	}
	field_name := symbol_text(checker, expr.name)
	item, has_item := types.container(base_type, store)
	if has_item && (item.kind == .Array || item.kind == .Slice) {
		if field_name == "len" {
			return types.USIZE
		}
		if field_name == "ptr" &&
		   (item.kind == .Slice || types.is_pointer(base_type, store)) {
			return container_pointer_type(store, item)
		}
	}
	value_type := base_type
	if types.is_pointer(value_type, store) {
		value_type = types.child_type(value_type, store)
	}
	_, field, ok := find_struct_field(checker, value_type, expr.name)
	return field.type if ok else types.INVALID
}

infer_qualified_value_field_type :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Infer_Local,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	demanded: ^[dynamic]Spec_Id = nil,
) -> (types.Type, bool) {
	if !symbol.is_valid(expr.qualifier) ||
	   find_import(checker, file, expr.qualifier) != ast.INVALID_IMPORT {
		return types.INVALID, false
	}
	base_type := find_infer_local(locals, expr.qualifier)
	if !types.is_valid(base_type) {
		if value, ok := current_comptime_value(checker, expr.qualifier); ok && value.kind == .Static {
			base_type = value.type
			if field_value, found := persistent_field_value(checker, value.static_value, expr.name);
			   found && field_value.kind == .Function {
				_, _, _ = function_pointer_type_for_template(
					checker, ast.Function_Id(u32(field_value.index)), demanded,
				)
			}
		}
	}
	if !types.is_valid(base_type) {
		if global := find_global(checker, expr.qualifier, pkg, file); global != ast.INVALID_GLOBAL {
			base_type = checker.global_types[global]
		}
	}
	if !types.is_valid(base_type) {
		return types.INVALID, false
	}
	return field_type_from_value(checker, expr, base_type), true
}

build_field_from_value :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	base: hir.Expr_Id,
	base_type: types.Type,
) -> (hir.Expr_Id, bool) {
	store := &checker.module.types
	if !symbol.is_valid(expr.name) {
		value_type := types.child_type(base_type, store) if types.is_pointer(base_type, store) else base_type
		index, field, ok := find_tuple_field(checker, value_type, expr.integer)
		if !ok {
			id := source.addf(checker.diagnostics, expr.span, "tuple field index %d is out of range or the value is not a tuple", expr.integer)
			return invalid_hir_expr(checker, expr.span, id), false
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Field, span=expr.span, type=field.type, integer=i64(index), left=base,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		}), true
	}
	field_name := symbol_text(checker, expr.name)
	item, has_item := types.container(base_type, store)
	if has_item && (item.kind == .Array || item.kind == .Slice) {
		if field_name == "len" {
			return add_hir_expr(checker, hir.Expr{
				kind=.Length, span=expr.span, type=types.USIZE, left=base,
				target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
			}), true
		}
		if field_name == "ptr" &&
		   (item.kind == .Slice || types.is_pointer(base_type, store)) {
			return add_hir_expr(checker, hir.Expr{
				kind=.Slice_Ptr, span=expr.span,
				type=container_pointer_type(store, item), left=base,
				target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
			}), true
		}
		if field_name == "ptr" && item.kind == .Array {
			id := source.add(checker.diagnostics, expr.span, "arrays do not expose '.ptr'; take their address first")
			return invalid_hir_expr(checker, expr.span, id), false
		}
	}
	value_type := base_type
	if types.is_pointer(value_type, store) {
		value_type = types.child_type(value_type, store)
	}
	index, field, ok := find_struct_field(checker, value_type, expr.name)
	if !ok {
		id := source.addf(checker.diagnostics, expr.span, "unknown struct field '%s'", symbol_text(checker, expr.name))
		return invalid_hir_expr(checker, expr.span, id), false
	}
	if types.is_void(field.type) {
		id := source.addf(checker.diagnostics, expr.span, "variant '%s' has no payload to read", symbol_text(checker, expr.name))
		return invalid_hir_expr(checker, expr.span, id), false
	}
	return add_hir_expr(checker, hir.Expr{
		kind=.Field, span=expr.span, type=field.type, integer=i64(index), left=base,
		target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
	}), true
}

build_qualified_value_field :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Build_Local,
	global_reads: ^[dynamic]hir.Global_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (hir.Expr_Id, bool, bool) {
	if !symbol.is_valid(expr.qualifier) ||
	   find_import(checker, file, expr.qualifier, true) != ast.INVALID_IMPORT {
		return hir.INVALID_EXPR, false, false
	}
	if local, ok := find_build_local(locals, expr.qualifier); ok {
		base := build_local_expr(checker, local, expr.span)
		value, ok := build_field_from_value(checker, expr, base, local.type)
		return value, true, ok
	}
	if value, ok := current_comptime_value(checker, expr.qualifier);
	   ok && value.kind == .Static && value.static_value != INVALID_CT_VALUE &&
	   int(value.static_value) < len(checker.static_state.values) {
		field_value, field_ok := persistent_field_value(checker, value.static_value, expr.name)
		if !field_ok {
			return hir.INVALID_EXPR, true, false
		}
		return build_static_value(checker, field_value, expr.span, types.INVALID), true, true
	}
	if global := find_global(checker, expr.qualifier, pkg, file); global != ast.INVALID_GLOBAL {
		base := build_global_reference(checker, global, expr.span, global_reads)
		value, ok := build_field_from_value(checker, expr, base, checker.global_types[global])
		return value, true, ok
	}
	return hir.INVALID_EXPR, false, false
}

find_enum_member :: proc(checker: ^Checker, enum_type: types.Type, name: symbol.Id) -> (types.Enum_Member, bool) {
	for member in types.enum_members_for(&checker.module.types, enum_type) {
		if member.name == u32(name) {
			return member, true
		}
	}
	return {}, false
}

enum_type_from_name_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (types.Type, bool) {
	if !symbol.is_valid(expr.qualifier) ||
	   find_import(checker, file, expr.qualifier) != ast.INVALID_IMPORT {
		return types.INVALID, false
	}
	enum_type := types.find_named(&checker.module.types, u32(pkg), u32(expr.qualifier), file=u32(file))
	return enum_type, types.is_enum(enum_type, &checker.module.types)
}

enum_type_from_field_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	mark_used := false,
) -> (types.Type, bool) {
	if expr.left == ast.INVALID_EXPR || int(expr.left) >= len(checker.ast_module.exprs) {
		return types.INVALID, false
	}
	base := checker.ast_module.exprs[expr.left]
	if base.kind != .Name || !symbol.is_valid(base.qualifier) {
		return types.INVALID, false
	}
	target_pkg, available := expr_package(checker, base, pkg, file, mark_used)
	if !available {
		return types.INVALID, false
	}
	enum_type := types.find_named(&checker.module.types, u32(target_pkg), u32(base.name), file=u32(expr_lookup_file(base, file)))
	return enum_type, types.is_enum(enum_type, &checker.module.types)
}

enum_member_hir :: proc(
	checker: ^Checker,
	enum_type: types.Type,
	name: symbol.Id,
	span: source.Span,
) -> hir.Expr_Id {
	member, ok := find_enum_member(checker, enum_type, name)
	if !ok {
		id := source.addf(checker.diagnostics, span, "unknown enum member '%s'", symbol_text(checker, name))
		return invalid_hir_expr(checker, span, id, enum_type)
	}
	value := i64(member.value) if member.value < 0 else transmute(i64)u64(member.value)
	return add_hir_expr(checker, hir.Expr{
		kind=.Integer,
		span=span,
		type=enum_type,
		integer=value,
		target=hir.INVALID_REF,
		left=hir.INVALID_EXPR,
		right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

build_function_value :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	span: source.Span,
	expected: types.Type,
) -> hir.Expr_Id {
	if template == ast.INVALID_FUNCTION || int(template) >= len(checker.ast_module.functions) {
		return hir.INVALID_EXPR
	}
	function := checker.ast_module.functions[template]
	if len(function.unsupported_reason) > 0 {
		id := source.addf(
			checker.diagnostics,
			span,
			"C declaration '%s' is unavailable: %s",
			symbol_text(checker, function.name),
			function.unsupported_reason,
		)
		return invalid_hir_expr(checker, span, id)
	}
	if checker.template_diagnostics[template] != source.INVALID_DIAGNOSTIC {
		return invalid_hir_expr(checker, span, checker.template_diagnostics[template])
	}
	params, result, ok := function_value_signature(checker, template)
	if !ok {
		id := source.addf(
			checker.diagnostics,
			span,
			"function '%s' cannot be used as a function value; expected a concrete non-comptime signature",
			symbol_text(checker, function.name),
		)
		return invalid_hir_expr(checker, span, id)
	}
	defer delete(params, checker.allocator)
	function_type := types.function(&checker.module.types, params, result, function.c_abi, function.variadic)
	value_type := function_type
	expected_pointer := expected
	if types.is_optional(expected_pointer, &checker.module.types) {
		expected_pointer = types.child_type(expected_pointer, &checker.module.types)
	}
	if _, _, expected_function, ok := types.function_pointer(expected_pointer, &checker.module.types); ok &&
	   types.equal(expected_function, function_type) {
		value_type = expected_pointer
	} else if types.equal(expected, function_type) {
		value_type = expected
	}
	spec := find_spec(checker, template, params)
	if spec == INVALID_SPEC {
		id := source.addf(
			checker.diagnostics,
			span,
			"could not resolve callback specialization of '%s'",
			symbol_text(checker, function.name),
		)
		return invalid_hir_expr(checker, span, id, value_type)
	}
	function_id := checker.specs[spec].hir_id
	assert(function_id != hir.INVALID_FUNCTION)
	return add_hir_expr(checker, hir.Expr{
		kind=.Function,
		span=span,
		type=value_type,
		target=hir.function_ref(function_id),
		left=hir.INVALID_EXPR,
		right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

build_nested_expr :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	locals: []Build_Local,
	global_reads: ^[dynamic]hir.Global_Id,
	calls: ^[dynamic]hir.Function_Id,
	expected: types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> hir.Expr_Id {
	outer := checker.build_stack
	checker.build_stack = nil
	checker.build_stack.allocator = checker.allocator
	result := build_expr(checker, expr_id, locals, global_reads, calls, expected, pkg, file)
	delete(checker.build_stack)
	checker.build_stack = outer
	return result
}

try_build_comptime_division :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	kind: Division_Builtin,
	expected: types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> (hir.Expr_Id, bool) {
	state := ct_state_make(checker, pkg, file, diagnose=false)
	defer ct_state_destroy(&state)
	value, flow, ok := ct_eval_division_call(&state, expr, kind, expected, 0)
	if ok && flow.kind == .Normal && value != INVALID_CT_VALUE {
		return ct_materialize_value(&state, value, expr.span, expected), true
	}
	message := ""
	#partial switch state.error {
	case .Div_By_Zero: message = "division builtin denominator is zero"
	case .Overflow:    message = "signed integer division overflow"
	case .Non_Exact:   message = "exact division has a remainder"
	}
	if len(message) == 0 {
		return hir.INVALID_EXPR, false
	}
	id := source.add(checker.diagnostics, expr.span, message)
	recovery := expected if types.is_concrete_scalar(expected) else types.I64
	return invalid_hir_expr(checker, expr.span, id, recovery), true
}

build_division_builtin :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	kind: Division_Builtin,
	locals: []Build_Local,
	global_reads: ^[dynamic]hir.Global_Id,
	calls: ^[dynamic]hir.Function_Id,
	expected: types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> hir.Expr_Id {
	if len(expr.args) != 2 {
		id := source.addf(
			checker.diagnostics, expr.span, "%s! expects 2 arguments, got %d",
			symbol_text(checker, expr.name), len(expr.args),
		)
		return invalid_hir_expr(checker, expr.span, id)
	}
	if value, handled := try_build_comptime_division(checker, expr, kind, expected, pkg, file); handled {
		return value
	}
	hint := expected if types.is_concrete_scalar(expected) && !types.is_bool(expected) else types.INVALID
	left_const := is_numeric_constant_expr(checker, expr.args[0])
	right_const := is_numeric_constant_expr(checker, expr.args[1])
	left, right := hir.INVALID_EXPR, hir.INVALID_EXPR
	if left_const && !right_const && !types.is_valid(hint) {
		right = build_nested_expr(checker, expr.args[1], locals, global_reads, calls, types.INVALID, pkg, file)
		left = build_nested_expr(checker, expr.args[0], locals, global_reads, calls, checker.module.exprs[right].type, pkg, file)
	} else {
		left = build_nested_expr(checker, expr.args[0], locals, global_reads, calls, hint, pkg, file)
		right_hint := hint if types.is_valid(hint) else checker.module.exprs[left].type
		right = build_nested_expr(checker, expr.args[1], locals, global_reads, calls, right_hint, pkg, file)
	}
	result := types.widest(checker.module.exprs[left].type, checker.module.exprs[right].type)
	if !types.is_concrete_scalar(result) || types.is_bool(result) {
		id := source.add(checker.diagnostics, expr.span, "division builtins require compatible numeric operands")
		return invalid_hir_expr(checker, expr.span, id)
	}
	left = coerce_expr(checker, left, result, checker.module.exprs[left].span)
	right = coerce_expr(checker, right, result, checker.module.exprs[right].span)
	result_kind := hir.Expr_Kind.Div_Trunc
	#partial switch kind {
	case .Floor: result_kind = .Div_Floor
	case .Exact: result_kind = .Div_Exact
	case .Ceil:  result_kind = .Div_Ceil
	case .Rem:   result_kind = .Rem
	case .Mod:   result_kind = .Mod
	case:
	}
	return add_hir_expr(checker, hir.Expr{
		kind=result_kind, span=expr.span, type=result, left=left, right=right,
		target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

fallible_aggregate :: proc(
	checker: ^Checker,
	span: source.Span,
	channel: types.Type,
	value: hir.Expr_Id,
	error_path: bool,
) -> hir.Expr_Id {
	values := make([]hir.Expr_Id, 1, checker.allocator)
	values[0] = value
	return add_hir_expr(checker, hir.Expr{
		kind=.Struct,
		span=span,
		type=channel,
		integer=1 if error_path else 0,
		args=values,
		target=hir.INVALID_REF,
		left=hir.INVALID_EXPR,
		right=hir.INVALID_EXPR,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

build_compound_expr :: proc(
	checker: ^Checker,
	expr: ast.Expr,
	locals: []Build_Local,
	global_reads: ^[dynamic]hir.Global_Id,
	calls: ^[dynamic]hir.Function_Id,
	expected: types.Type,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> hir.Expr_Id {
	store := &checker.module.types
	#partial switch expr.kind {
	case .String:
		string_type := string_literal_type(checker, expr.integer)
		return add_hir_expr(checker, hir.Expr{
			kind=.String, span=expr.span, type=string_type, integer=i64(expr.integer),
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Array:
		element_type := types.INVALID
		result_type := expected
		expected_node, has_expected := types.node(store, expected)
		if !has_expected || expected_node.kind != .Array {
			has_expected = false
			result_type = types.INVALID
		} else {
			element_type = expected_node.child
			if !expected_node.inferred_count && expected_node.count != u64(len(expr.args)) {
				id := source.addf(
					checker.diagnostics, expr.span,
					"array literal expects %d elements, got %d",
					expected_node.count, len(expr.args),
				)
				return invalid_hir_expr(checker, expr.span, id, expected)
			}
			if expected_node.inferred_count {
				result_type = types.with_array_count(store, expected, u64(len(expr.args)))
			}
		}
		if !has_expected {
			infer_locals := make([]Infer_Local, len(locals), checker.allocator)
			defer delete(infer_locals, checker.allocator)
			for local, index in locals {
				infer_locals[index] = Infer_Local{name=local.name, type=local.type, declared=local.type, statement=ast.INVALID_STMT}
			}
			for arg in expr.args {
				actual := infer_nested_expr(checker, arg, infer_locals, pkg, file, nil)
				if !types.is_valid(element_type) {
					element_type = actual
				} else {
					element_type = types.widest(element_type, actual)
				}
			}
			if !types.is_valid(element_type) {
				element_type = types.I64
			}
			result_type = types.array(store, element_type, u64(len(expr.args)), false)
		}
		args := make([]hir.Expr_Id, len(expr.args), checker.allocator)
		for arg, index in expr.args {
			args[index] = build_nested_expr(
				checker, arg, locals, global_reads, calls, element_type, pkg, file,
			)
			args[index] = coerce_expr(checker, args[index], element_type, checker.module.exprs[args[index]].span)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Array, span=expr.span, type=result_type, args=args,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .None:
		if !types.is_optional(expected, store) {
			id := source.add(checker.diagnostics, expr.span, "'none' requires an optional context")
			return invalid_hir_expr(checker, expr.span, id, expected)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.None, span=expr.span, type=expected, target=hir.INVALID_REF,
			left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Undefined:
		id := source.add(
			checker.diagnostics,
			expr.span,
			"'undefined' is only valid as a mutable local declaration initializer",
		)
		return invalid_hir_expr(checker, expr.span, id, expected)
	case .Enum_Literal:
		if types.is_tagged_union(expected, store) {
			index, field, found := find_struct_field(checker, expected, expr.name)
			if !found {
				id := source.addf(checker.diagnostics, expr.span, "unknown variant '.%s' on '%s'", symbol_text(checker, expr.name), type_label(checker, expected))
				return invalid_hir_expr(checker, expr.span, id, expected)
			}
			values := make([]hir.Expr_Id, 1, checker.allocator)
			if expr.left == ast.INVALID_EXPR {
				if !types.is_void(field.type) {
					id := source.addf(checker.diagnostics, expr.span, "variant '.%s' on '%s' needs a payload; only void variants can be built from a bare '.%s'",
						symbol_text(checker, expr.name), type_label(checker, expected), symbol_text(checker, expr.name))
					delete(values, checker.allocator)
					return invalid_hir_expr(checker, expr.span, id, expected)
				}
				values[0] = hir.INVALID_EXPR
			} else {
				if types.is_void(field.type) {
					id := source.addf(checker.diagnostics, expr.span, "void variant '%s' takes no value", symbol_text(checker, expr.name))
					delete(values, checker.allocator)
					return invalid_hir_expr(checker, expr.span, id, expected)
				}
				values[0] = build_nested_expr(checker, expr.left, locals, global_reads, calls, field.type, pkg, file)
				values[0] = coerce_expr(checker, values[0], field.type, checker.module.exprs[values[0]].span)
			}
			return add_hir_expr(checker, hir.Expr{
				kind=.Struct, span=expr.span, type=expected, args=values, integer=i64(index),
				target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		if expr.left != ast.INVALID_EXPR {
			id := source.addf(
				checker.diagnostics,
				expr.span,
				"'.%s{...}' requires a tagged-union context",
				symbol_text(checker, expr.name),
			)
			return invalid_hir_expr(checker, expr.span, id, expected)
		}
		if !types.is_enum(expected, store) {
			id := source.addf(
				checker.diagnostics,
				expr.span,
				"'.%s' requires an enum context",
				symbol_text(checker, expr.name),
			)
			return invalid_hir_expr(checker, expr.span, id, expected)
		}
		return enum_member_hir(checker, expected, expr.name, expr.span)
	case .Cast:
		target := type_from_syntax(checker, expr.type, pkg, file)
		value := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		actual := checker.module.exprs[value].type
		valid_target := types.is_concrete_scalar(target) && !types.is_bool(target)
		actual_repr := types.runtime_representation(actual, store)
		actual_item, actual_item_ok := types.node(store, actual)
		explicit_enum := actual_item_ok && actual_item.kind == .Enum && actual_item.explicit_backing
		valid_actual := (types.is_concrete_scalar(actual) || explicit_enum) &&
		                types.is_concrete_scalar(actual_repr) && !types.is_bool(actual_repr)
		if !valid_target || !valid_actual {
			id := source.addf(
				checker.diagnostics,
				expr.span,
				"scalar cast requires numeric scalar types, got %s to %s",
				types.name(actual),
				types.name(target),
			)
			return invalid_hir_expr(checker, expr.span, id, target)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Scalar_Cast,
			span=expr.span,
			type=target,
			left=value,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Address:
		// `&<array literal>` (Zig's `&.{...}`): the operand is an rvalue with no
		// address, so promote it to an anonymous global constant and take *its*
		// address. Reuses the existing non-scalar-global storage path; only the
		// stable global address enters the expression, so it never dangles. The
		// resulting `*[N]T` then decays to a slice via the usual coercion.
		if expr.left != ast.INVALID_EXPR && checker.ast_module.exprs[expr.left].kind == .Array {
			operand := checker.ast_module.exprs[expr.left]
			// Propagate an element-expected type through `&` so literal elements
			// coerce to the target slice's element type (e.g. string -> []u8).
			// Without this, `&["x"]` infers `*[1]*[N:0]u8`, which won't decay to
			// `[][]u8` because can_decay_array_pointer requires child equality.
			element := types.INVALID
			if node, ok := types.node(store, expected); ok && (node.kind == .Slice || node.kind == .Array) {
				element = node.child
			}
			synth_expected := types.INVALID
			if types.is_valid(element) {
				synth_expected = types.array(store, element, u64(len(operand.args)), false)
			}
			value := build_nested_expr(checker, expr.left, locals, global_reads, calls, synth_expected, pkg, file)
			array_type := checker.module.exprs[value].type
			hidden_id := hir.Global_Id(len(checker.ast_module.globals) + len(checker.anon_globals))
			append(&checker.anon_globals, hir.Global{
				name       = symbol.intern(checker.symbols, "__anon.array"),
				type       = array_type,
				expr       = value,
				writable   = false,
				external   = false,
				diagnostic = source.INVALID_DIAGNOSTIC,
			})
			add_unique_global(global_reads, hidden_id)
			global_ref := add_hir_expr(checker, hir.Expr{
				kind=.Global, span=expr.span, type=array_type, target=hir.global_ref(hidden_id),
				left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
			})
			return add_hir_expr(checker, hir.Expr{
				kind=.Address, span=expr.span, type=types.pointer(store, array_type, false, false),
				left=global_ref, target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		value := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, value); propagated {
			return invalid
		}
		if !hir_is_location(checker, value) {
			id := source.add(checker.diagnostics, expr.span, "'&' requires an addressable location")
			return invalid_hir_expr(checker, expr.span, id)
		}
		value_type := checker.module.exprs[value].type
		writable := hir_location_writable(checker, value, locals)
		result_type := types.pointer(store, value_type, writable, false)
		if types.is_pointer(expected, store) &&
		   types.equal(types.child_type(expected, store), value_type) &&
		   (!types.is_mutable(expected, store) || writable) {
			result_type = expected
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Address, span=expr.span, type=result_type, left=value,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Deref:
		pointer := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, pointer); propagated {
			return invalid
		}
		pointer_type := checker.module.exprs[pointer].type
		if !types.is_pointer(pointer_type, store) {
			id := source.add(checker.diagnostics, expr.span, "postfix '^' requires a pointer")
			return invalid_hir_expr(checker, expr.span, id)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Deref, span=expr.span, type=types.child_type(pointer_type, store), left=pointer,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Index:
		container := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		index := build_nested_expr(checker, expr.right, locals, global_reads, calls, types.USIZE, pkg, file)
		index = coerce_expr(checker, index, types.USIZE, expr.span)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, container, index); propagated {
			return invalid
		}
		container_type := checker.module.exprs[container].type
		item, ok := types.container(container_type, store)
		if !ok {
			id := source.add(checker.diagnostics, expr.span, "indexing requires an array, slice, pointer-to-array, or many-item pointer")
			return invalid_hir_expr(checker, expr.span, id)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Index, span=expr.span, type=item.child, left=container, right=index,
			target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Slice:
		text, comptime_ok := comptime_slice_bytes(checker, expr, pkg, file)
		if comptime_ok {
			string_id := intern_comptime_string(checker, text)
			return add_hir_expr(checker, hir.Expr{
				kind=.String, span=expr.span, type=string_literal_type(checker, string_id), integer=i64(string_id),
				target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		container := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, container); propagated {
			return invalid
		}
		container_type := checker.module.exprs[container].type
		item, ok := types.container(container_type, store)
		if !ok {
			id := source.add(checker.diagnostics, expr.span, "slicing requires an array, slice, pointer-to-array, or many-item pointer")
			return invalid_hir_expr(checker, expr.span, id)
		}
		if item.kind == .Pointer && expr.args[1] == ast.INVALID_EXPR {
			id := source.add(checker.diagnostics, expr.span, "many-item pointer slicing requires an explicit end bound")
			return invalid_hir_expr(checker, expr.span, id)
		}
		bounds := make([]hir.Expr_Id, 2, checker.allocator)
		bounds[0] = hir.INVALID_EXPR
		bounds[1] = hir.INVALID_EXPR
		for bound, index in expr.args {
			if bound != ast.INVALID_EXPR {
				bounds[index] = build_nested_expr(checker, bound, locals, global_reads, calls, types.USIZE, pkg, file)
				bounds[index] = coerce_expr(checker, bounds[index], types.USIZE, checker.ast_module.exprs[bound].span)
				if invalid, propagated := propagate_invalid_expr(checker, expr.span, bounds[index]); propagated {
					delete(bounds, checker.allocator)
					return invalid
				}
			}
		}
		preserve_sentinel := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
		result_type := types.slice(store, item.child, item.mutable, preserve_sentinel, item.sentinel)
		return add_hir_expr(checker, hir.Expr{
			kind=.Slice, span=expr.span, type=result_type, args=bounds, left=container,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Field:
		if enum_type, enum_ok := enum_type_from_field_expr(checker, expr, pkg, file, true); enum_ok {
			return enum_member_hir(checker, enum_type, expr.name, expr.span)
		}
		base := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, base); propagated {
			return invalid
		}
		base_type := checker.module.exprs[base].type
		result, _ := build_field_from_value(checker, expr, base, base_type)
		return result
	case .Unwrap:
		optional := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, optional); propagated {
			return invalid
		}
		optional_type := checker.module.exprs[optional].type
		if !types.is_optional(optional_type, store) {
			id := source.add(checker.diagnostics, expr.span, "postfix '?' requires an optional")
			return invalid_hir_expr(checker, expr.span, id)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Unwrap, span=expr.span, type=types.child_type(optional_type, store), left=optional,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Orelse:
		optional := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, optional); propagated {
			return invalid
		}
		optional_type := checker.module.exprs[optional].type
		if !types.is_optional(optional_type, store) {
			id := source.add(checker.diagnostics, expr.span, "'orelse' requires an optional left operand")
			return invalid_hir_expr(checker, expr.span, id)
		}
		child := types.child_type(optional_type, store)
		fallback := build_nested_expr(checker, expr.right, locals, global_reads, calls, child, pkg, file)
		fallback = coerce_expr(checker, fallback, child, checker.module.exprs[fallback].span)
		return add_hir_expr(checker, hir.Expr{
			kind=.Orelse, span=expr.span, type=child, left=optional, right=fallback,
			target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Try:
		if checker.current_build_ctx != nil && checker.current_build_ctx.defer_depth > 0 {
			id := source.add(checker.diagnostics, expr.span, "cannot 'try' inside a 'defer'")
			return invalid_hir_expr(checker, expr.span, id)
		}
		left_expected := expected if checker.ast_module.exprs[expr.left].kind == .Call else types.INVALID
		channel := build_nested_expr(checker, expr.left, locals, global_reads, calls, left_expected, pkg, file)
		if diagnostic, invalid := invalid_expr_diagnostic(checker, channel); invalid {
			return invalid_hir_expr(checker, expr.span, diagnostic)
		}
		channel_type := checker.module.exprs[channel].type
		success := types.fallible_success(channel_type, store)
		if !types.is_valid(success) {
			id := source.add(checker.diagnostics, expr.span, "'try' requires a fallible expression")
			return invalid_hir_expr(checker, expr.span, id)
		}
		enclosing_success := types.fallible_success(checker.current_result, store)
		enclosing_error := types.fallible_error(checker.current_result, store)
		if !types.is_valid(enclosing_success) {
			id := source.add(checker.diagnostics, expr.span, "'try' requires an enclosing fallible function")
			return invalid_hir_expr(checker, expr.span, id, success)
		}
		error_type := types.fallible_error(channel_type, store)
		if !types.equal(error_type, enclosing_error) &&
		   !types.can_sum_widen(error_type, enclosing_error, store) {
			id := source.add(checker.diagnostics, expr.span, "'try' error channel cannot be widened to the enclosing error channel")
			return invalid_hir_expr(checker, expr.span, id, success)
		}
		cleanup: []hir.Stmt_Id
		captures: []hir.Expr_Id
		if checker.current_build_ctx != nil {
			cleanup, captures = try_cleanup(checker.current_build_ctx)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Try,
			span=expr.span,
			type=success,
			left=channel,
			body=cleanup,
			args=captures,
			target=hir.INVALID_REF,
			right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Catch:
		left_expected := expected if checker.ast_module.exprs[expr.left].kind == .Call else types.INVALID
		channel := build_nested_expr(checker, expr.left, locals, global_reads, calls, left_expected, pkg, file)
		if diagnostic, invalid := invalid_expr_diagnostic(checker, channel); invalid {
			return invalid_hir_expr(checker, expr.span, diagnostic)
		}
		channel_type := checker.module.exprs[channel].type
		success := types.fallible_success(channel_type, store)
		if !types.is_valid(success) {
			id := source.add(checker.diagnostics, expr.span, "'catch' requires a fallible expression")
			return invalid_hir_expr(checker, expr.span, id)
		}
		body: []hir.Stmt_Id
		capture := hir.INVALID_LOCAL
		block_handler := false
		void_fallthrough := false
		fallback := hir.INVALID_EXPR
		if expr.right != ast.INVALID_EXPR {
			fallback = build_nested_expr(checker, expr.right, locals, global_reads, calls, success, pkg, file)
			fallback = coerce_expr(checker, fallback, success, checker.module.exprs[fallback].span)
		} else {
			block_handler = true
			ctx := checker.current_build_ctx
			if ctx == nil {
				id := source.add(checker.diagnostics, expr.span, "catch block form is only valid in a function body")
				return invalid_hir_expr(checker, expr.span, id, success)
			}
			capture_start := len(ctx.locals^)
			error_type := types.fallible_error(channel_type, store)
			if symbol.is_valid(expr.name) && expr.name != checker.sink_symbol {
				capture = append_build_local(ctx, expr.name, error_type, false, expr.span)
			}
			handler: [dynamic]hir.Stmt_Id
			handler.allocator = checker.allocator
			fallback, _ = build_value_source(ctx, &handler, expr.body, success, expr.span, allow_exit=true)
			void_fallthrough = fallback == hir.INVALID_EXPR && !all_paths_exit(&checker.module, handler[:])
			body = handler[:]
			resize(ctx.locals, capture_start)
		}
		catch_mode := hir.CATCH_EXPRESSION
		if block_handler {
			catch_mode = hir.CATCH_VOID_FALLTHROUGH if void_fallthrough else hir.CATCH_BLOCK
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Catch,
			span=expr.span,
			type=success,
			integer=catch_mode,
			left=channel,
			right=fallback,
			body=body,
			target=hir.local_ref(capture) if capture != hir.INVALID_LOCAL else hir.INVALID_REF,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Range:
		expected_child := types.INVALID
		if types.is_range(expected, store) {
			expected_child = types.child_type(expected, store)
		}
		left_const := eval_constant(checker, expr.left)
		right_const := eval_constant(checker, expr.right)
		left, right: hir.Expr_Id
		if types.is_valid(expected_child) {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, expected_child, pkg, file)
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, expected_child, pkg, file)
		} else if right_const.kind == .Value && left_const.kind != .Value {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, checker.module.exprs[left].type, pkg, file)
		} else if left_const.kind == .Value && right_const.kind != .Value {
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, checker.module.exprs[right].type, pkg, file)
		} else {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
		}
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
			return invalid
		}
		child := expected_child
		if !types.is_valid(child) {
			child = types.widest(checker.module.exprs[left].type, checker.module.exprs[right].type)
		}
		if !types.is_concrete_integer(child) {
			id := source.add(checker.diagnostics, expr.span, "range bounds must be compatible concrete integers")
			return invalid_hir_expr(checker, expr.span, id)
		}
		left = coerce_expr(checker, left, child, checker.module.exprs[left].span)
		right = coerce_expr(checker, right, child, checker.module.exprs[right].span)
		args := make([]hir.Expr_Id, 2, checker.allocator)
		args[0] = left
		args[1] = right
		return add_hir_expr(checker, hir.Expr{
			kind=.Range, span=expr.span, type=types.range(store, child),
			integer=i64(expr.integer), args=args,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Comptime:
		return build_comptime_expr(checker, expr, expected, pkg, file)
	case .Bool:
		return add_hir_expr(checker, hir.Expr{
			kind=.Bool, span=expr.span, type=types.BOOL, integer=i64(expr.integer),
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Not:
		operand := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.BOOL, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, operand); propagated {
			return invalid
		}
		operand_type := checker.module.exprs[operand].type
		if checker.module.exprs[operand].kind != .Invalid && !types.is_bool(operand_type) {
			id := source.add(checker.diagnostics, expr.span, "'!' requires a bool operand")
			return invalid_hir_expr(checker, expr.span, id, types.BOOL)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Not, span=expr.span, type=types.BOOL, left=operand,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Bit_Not:
		hint := expected if types.is_concrete_integer(expected) else types.INVALID
		operand := build_nested_expr(checker, expr.left, locals, global_reads, calls, hint, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, operand); propagated {
			return invalid
		}
		operand_type := checker.module.exprs[operand].type
		if !types.is_concrete_integer(operand_type) {
			id := source.add(checker.diagnostics, expr.span, "'~' requires a concrete integer operand")
			return invalid_hir_expr(checker, expr.span, id, operand_type)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Bit_Not, span=expr.span, type=operand_type, left=operand,
			target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Bit_And, .Bit_Or, .Bit_Xor:
		hint := expected if types.is_concrete_integer(expected) else types.INVALID
		left_const := is_numeric_constant_expr(checker, expr.left)
		right_const := is_numeric_constant_expr(checker, expr.right)
		left, right: hir.Expr_Id
		if left_const && !right_const && !types.is_valid(hint) {
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, checker.module.exprs[right].type, pkg, file)
		} else {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, hint, pkg, file)
			right_hint := hint if types.is_valid(hint) else checker.module.exprs[left].type
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, right_hint, pkg, file)
		}
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
			return invalid
		}
		result_type := types.widest(checker.module.exprs[left].type, checker.module.exprs[right].type)
		if !types.is_concrete_integer(result_type) {
			id := source.add(checker.diagnostics, expr.span, "bitwise operation requires compatible concrete integer operands")
			return invalid_hir_expr(checker, expr.span, id)
		}
		left = coerce_expr(checker, left, result_type, checker.module.exprs[left].span)
		right = coerce_expr(checker, right, result_type, checker.module.exprs[right].span)
		kind := hir.Expr_Kind.Bit_And
		#partial switch expr.kind {
		case .Bit_Or:  kind = .Bit_Or
		case .Bit_Xor: kind = .Bit_Xor
		}
		return add_hir_expr(checker, hir.Expr{
			kind=kind, span=expr.span, type=result_type, left=left, right=right,
			target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Shift_Left, .Shift_Right, .Shift_Left_Saturating:
		hint := expected if types.is_concrete_integer(expected) else types.INVALID
		left := build_nested_expr(checker, expr.left, locals, global_reads, calls, hint, pkg, file)
		right := build_nested_expr(checker, expr.right, locals, global_reads, calls, types.U64, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
			return invalid
		}
		left_type := checker.module.exprs[left].type
		right_type := checker.module.exprs[right].type
		if !types.is_concrete_integer(left_type) {
			id := source.add(checker.diagnostics, checker.module.exprs[left].span, "shifted value must be a concrete integer")
			return invalid_hir_expr(checker, expr.span, id, left_type)
		}
		if !types.is_unsigned(right_type, checker.target) {
			id := source.add(checker.diagnostics, checker.module.exprs[right].span, "shift count must be an unsigned integer")
			return invalid_hir_expr(checker, expr.span, id, left_type)
		}
		if constant := eval_integer_constant_in_context(checker, expr.right, pkg, file);
		   constant.kind == .Value && expr.kind != .Shift_Left_Saturating &&
		   constant.value >= i128(types.bits(left_type, checker.target)) {
			id := source.addf(
				checker.diagnostics, checker.module.exprs[right].span,
				"shift count %d exceeds %s width", constant.value, types.name(left_type),
			)
			return invalid_hir_expr(checker, expr.span, id, left_type)
		}
		kind := hir.Expr_Kind.Shift_Left
		#partial switch expr.kind {
		case .Shift_Right:           kind = .Shift_Right
		case .Shift_Left_Saturating: kind = .Shift_Left_Saturating
		}
		return add_hir_expr(checker, hir.Expr{
			kind=kind, span=expr.span, type=left_type, left=left, right=right,
			target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .And, .Or:
		left := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.BOOL, pkg, file)
		right := build_nested_expr(checker, expr.right, locals, global_reads, calls, types.BOOL, pkg, file)
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
			return invalid
		}
		left_type := checker.module.exprs[left].type
		right_type := checker.module.exprs[right].type
		left_ok := checker.module.exprs[left].kind == .Invalid || types.is_bool(left_type)
		right_ok := checker.module.exprs[right].kind == .Invalid || types.is_bool(right_type)
		if !left_ok || !right_ok {
			id := source.add(checker.diagnostics, expr.span, "'and'/'or' require bool operands")
			return invalid_hir_expr(checker, expr.span, id, types.BOOL)
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.And if expr.kind == .And else .Or, span=expr.span, type=types.BOOL,
			left=left, right=right, target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Eq, .Ne, .Lt, .Le, .Gt, .Ge:
		// Contextualize literals whose type comes from their peer. This covers
		// integer and enum literals as well as optional presence tests such as
		// `value == none` and `none != value`.
		left_const := eval_constant(checker, expr.left)
		right_const := eval_constant(checker, expr.right)
		left, right: hir.Expr_Id
		left_expr := checker.ast_module.exprs[expr.left]
		right_expr := checker.ast_module.exprs[expr.right]
		left_numeric_const := left_const.kind == .Value || is_float_constant_expr(checker, expr.left)
		right_numeric_const := right_const.kind == .Value || is_float_constant_expr(checker, expr.right)
		if right_expr.kind == .None && left_expr.kind != .None {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, checker.module.exprs[left].type, pkg, file)
		} else if left_expr.kind == .None && right_expr.kind != .None {
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, checker.module.exprs[right].type, pkg, file)
		} else if right_expr.kind == .Enum_Literal && left_expr.kind != .Enum_Literal {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, checker.module.exprs[left].type, pkg, file)
		} else if left_expr.kind == .Enum_Literal && right_expr.kind != .Enum_Literal {
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, checker.module.exprs[right].type, pkg, file)
		} else if right_numeric_const && !left_numeric_const {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
			hint := checker.module.exprs[left].type
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, hint, pkg, file)
		} else if left_numeric_const && !right_numeric_const {
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
			hint := checker.module.exprs[right].type
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, hint, pkg, file)
		} else {
			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
		}
		if invalid, propagated := propagate_invalid_expr(checker, expr.span, left, right); propagated {
			return invalid
		}
		left_type := checker.module.exprs[left].type
		right_type := checker.module.exprs[right].type
		operand_type := types.INVALID
		if left_expr.kind == .None || right_expr.kind == .None {
			if expr.kind != .Eq && expr.kind != .Ne ||
			   !types.is_optional(left_type, store) || !types.equal(left_type, right_type) {
				id := source.add(checker.diagnostics, expr.span, "'none' only supports '==' and '!=' with an optional value")
				return invalid_hir_expr(checker, expr.span, id, types.BOOL)
			}
			operand_type = left_type
		} else if types.is_enum(left_type, store) || types.is_enum(right_type, store) {
			if !types.equal(left_type, right_type) || (expr.kind != .Eq && expr.kind != .Ne) {
				id := source.add(checker.diagnostics, expr.span, "enum values only support '==' and '!=' with the same enum type")
				return invalid_hir_expr(checker, expr.span, id, types.BOOL)
			}
			operand_type = left_type
		} else if types.is_bool(left_type) && types.is_bool(right_type) {
			if expr.kind != .Eq && expr.kind != .Ne {
				id := source.add(checker.diagnostics, expr.span, "bool values only support '==' and '!='")
				return invalid_hir_expr(checker, expr.span, id, types.BOOL)
			}
			operand_type = types.BOOL
		} else {
			operand_type = types.widest(left_type, right_type)
			if !types.is_concrete_scalar(operand_type) || types.is_bool(operand_type) {
				id := source.add(checker.diagnostics, expr.span, "comparison requires compatible numeric operands")
				return invalid_hir_expr(checker, expr.span, id, types.BOOL)
			}
		}
		left = coerce_expr(checker, left, operand_type, checker.module.exprs[left].span)
		right = coerce_expr(checker, right, operand_type, checker.module.exprs[right].span)
		compare_kind := hir.Expr_Kind.Eq
		#partial switch expr.kind {
		case .Eq: compare_kind = .Eq
		case .Ne: compare_kind = .Ne
		case .Lt: compare_kind = .Lt
		case .Le: compare_kind = .Le
		case .Gt: compare_kind = .Gt
		case .Ge: compare_kind = .Ge
		}
		return add_hir_expr(checker, hir.Expr{
			kind=compare_kind, span=expr.span, type=types.BOOL, left=left, right=right,
			target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Struct_Literal:
		struct_type := types.INVALID
		if expr.left != ast.INVALID_EXPR {
			struct_type, _ = resolve_type_argument(checker, expr.left, pkg, file)
			struct_type = types.resolve_alias(struct_type, store)
		} else if symbol.is_valid(expr.name) {
			target_pkg, available := expr_package(checker, expr, pkg, file, true)
			struct_type = types.find_named(store, u32(target_pkg), u32(expr.name), file=u32(expr_lookup_file(expr, file))) if available else types.INVALID
			struct_type = types.resolve_alias(struct_type, store)
			if !types.is_record(struct_type, store) || types.is_opaque_struct(struct_type, store) {
				id := source.addf(checker.diagnostics, expr.span, "unknown or opaque record type '%s'", symbol_text(checker, expr.name))
				return invalid_hir_expr(checker, expr.span, id)
			}
		} else if expr.tuple {
			struct_type = types.resolve_alias(expected, store)
		} else {
			struct_type = types.resolve_alias(expected, store)
			if types.is_valid(struct_type) &&
			   (!types.is_struct(struct_type, store) || types.is_opaque_struct(struct_type, store)) {
				id := source.add(checker.diagnostics, expr.span, "keyed contextual payload requires a struct payload")
				return invalid_hir_expr(checker, expr.span, id, expected)
			}
		}
		tuple_literal := expr.tuple
		if tuple_literal && len(expr.args) == 0 && types.is_valid(struct_type) {
			item, ok := types.node(store, struct_type)
			tuple_literal = !ok || item.kind != .Struct || item.tuple
		}
		if tuple_literal {
			resolved := types.is_valid(struct_type)
			if resolved {
				item, ok := types.node(store, struct_type)
				if !ok || item.kind != .Struct || !item.tuple {
					id := source.add(checker.diagnostics, expr.span, "positional construction requires a tuple type")
					return invalid_hir_expr(checker, expr.span, id, struct_type)
				}
			}
			fields := types.fields_for(store, struct_type) if resolved else nil
			if resolved && len(expr.args) != len(fields) {
				id := source.addf(checker.diagnostics, expr.span, "tuple literal expects %d elements, got %d", len(fields), len(expr.args))
				return invalid_hir_expr(checker, expr.span, id, struct_type)
			}
			values := make([]hir.Expr_Id, len(expr.args), checker.allocator)
			inferred_fields := make([]types.Field, len(expr.args), checker.allocator)
			defer delete(inferred_fields, checker.allocator)
			for arg, index in expr.args {
				expected_field := fields[index].type if resolved else types.INVALID
				values[index] = build_nested_expr(checker, arg, locals, global_reads, calls, expected_field, pkg, file)
				if resolved {
					values[index] = coerce_expr(checker, values[index], expected_field, checker.ast_module.exprs[arg].span)
				} else {
					inferred_fields[index].type = checker.module.exprs[values[index]].type
				}
			}
			if !resolved {
				struct_type = types.struct_anonymous(store, inferred_fields, true)
			}
			return add_hir_expr(checker, hir.Expr{
				kind=.Struct, span=expr.span, type=struct_type, args=values,
				target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		if !types.is_valid(struct_type) {
			fields := make([]types.Field, len(expr.args), checker.allocator)
			defer delete(fields, checker.allocator)
			values := make([]hir.Expr_Id, len(expr.args), checker.allocator)
			for keyed, index in expr.args {
				keyed_expr := checker.ast_module.exprs[keyed]
				fields[index].name = u32(keyed_expr.name)
				for previous in fields[:index] {
					if previous.name == fields[index].name {
						source.addf(
							checker.diagnostics, keyed_expr.span,
							"duplicate initializer for struct field '%s'",
							symbol_text(checker, keyed_expr.name),
						)
					}
				}
				values[index] = build_nested_expr(
					checker, keyed_expr.left, locals, global_reads, calls, types.INVALID, pkg, file,
				)
				fields[index].type = checker.module.exprs[values[index]].type
			}
			struct_type = types.struct_anonymous(store, fields)
			return add_hir_expr(checker, hir.Expr{
				kind=.Struct, span=expr.span, type=struct_type, args=values,
				target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		fields := types.fields_for(store, struct_type)
		union_record := types.is_union(struct_type, store)
		if union_record && len(expr.args) != 1 {
			id := source.add(checker.diagnostics, expr.span, "union literal requires exactly one field initializer")
			return invalid_hir_expr(checker, expr.span, id, struct_type)
		}
		values := make([]hir.Expr_Id, 1 if union_record else len(fields), checker.allocator)
		initialized := make([]bool, len(fields), checker.allocator)
		defer delete(initialized, checker.allocator)
		for &value in values {
			value = hir.INVALID_EXPR
		}
		for keyed in expr.args {
			keyed_expr := checker.ast_module.exprs[keyed]
			index, field, ok := find_struct_field(checker, struct_type, keyed_expr.name)
			if !ok {
				source.addf(checker.diagnostics, keyed_expr.span, "unknown struct field '%s'", symbol_text(checker, keyed_expr.name))
				continue
			}
			if initialized[index] {
				source.addf(checker.diagnostics, keyed_expr.span, "duplicate initializer for struct field '%s'", symbol_text(checker, keyed_expr.name))
				continue
			}
			initialized[index] = true
			value_index := 0 if union_record else index
			is_void_field := union_record && types.is_void(field.type)
			if keyed_expr.left == ast.INVALID_EXPR {
				// Bare key `T{ variant }`: valid only to construct a void-payload
				// variant (no value); the payload slot stays INVALID_EXPR.
				if !is_void_field {
					source.addf(checker.diagnostics, keyed_expr.span, "field '%s' requires a value", symbol_text(checker, keyed_expr.name))
				}
			} else {
				if is_void_field {
					source.addf(checker.diagnostics, keyed_expr.span, "void variant '%s' takes no value", symbol_text(checker, keyed_expr.name))
				}
				values[value_index] = build_nested_expr(checker, keyed_expr.left, locals, global_reads, calls, field.type, pkg, file)
				values[value_index] = coerce_expr(checker, values[value_index], field.type, keyed_expr.span)
			}
		}
		if !union_record {
			for field, index in fields {
				if values[index] != hir.INVALID_EXPR {
					continue
				}
				if field_default, default_values, ok := find_struct_field_default(checker, struct_type, symbol.Id(field.name)); ok {
					if field_default.static_value != INVALID_CT_VALUE {
						if int(field_default.static_value) >= len(checker.static_state.values) {
							continue
						}
						values[index] = build_static_value(
							checker, checker.static_state.values[field_default.static_value], field_default.span, field.type,
						)
						values[index] = coerce_expr(checker, values[index], field.type, field_default.span)
						continue
					}
					previous := checker.current_comptime_values
					checker.current_comptime_values = default_values
					values[index] = build_nested_expr(
						checker, field_default.expr, nil, global_reads, calls,
						field.type, field_default.pkg, field_default.file,
					)
					checker.current_comptime_values = previous
					values[index] = coerce_expr(
						checker, values[index], field.type,
						field_default.span,
					)
					continue
				}
				id := source.addf(checker.diagnostics, expr.span, "missing initializer for struct field '%s'", symbol_text(checker, symbol.Id(field.name)))
				delete(values, checker.allocator)
				return invalid_hir_expr(checker, expr.span, id, struct_type)
			}
		}
		active_field: i64
		if union_record {
			// The active variant is the one initialized field. Its payload slot
			// (`values[0]`) may legitimately be INVALID_EXPR for a void variant, so
			// detect "no field" via `initialized`, not the payload value.
			found_any := false
			for value, index in initialized {
				if value {
					active_field = i64(index)
					found_any = true
					break
				}
			}
			if !found_any {
				delete(values, checker.allocator)
				return invalid_hir_expr(checker, expr.span, source.add(checker.diagnostics, expr.span, "union literal requires a known field"), struct_type)
			}
		}
		return add_hir_expr(checker, hir.Expr{
			kind=.Struct, span=expr.span, type=struct_type, args=values, integer=active_field,
			target=hir.INVALID_REF, left=hir.INVALID_EXPR, right=hir.INVALID_EXPR,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Keyed:
		id := source.add(checker.diagnostics, expr.span, "keyed initializer is only valid inside a struct literal")
		return invalid_hir_expr(checker, expr.span, id)
	case:
		return invalid_hir_expr(checker, expr.span, expr.diagnostic)
	}
}

// build_binary_arith constructs the HIR node for `left op right`, where `op` is
// an arithmetic AST kind (`Add`/`Sub`/`Mul`/`Div`). It models many-pointer `+`
// as `Pointer_Add`, coerces both operands to their common type, and emits the
// "arithmetic requires compatible numeric operands" diagnostic when they have no
// shared numeric type.
build_binary_arith :: proc(
	checker: ^Checker,
	op: ast.Expr_Kind,
	left, right: hir.Expr_Id,
	span: source.Span,
) -> hir.Expr_Id {
	if invalid, propagated := propagate_invalid_expr(checker, span, left, right); propagated {
		return invalid
	}
	// Pointer arithmetic is only defined for `+` (many-pointer + usize).
	if op == .Add &&
	   types.is_many_pointer(checker.module.exprs[left].type, &checker.module.types) &&
	   types.equal(checker.module.exprs[right].type, types.USIZE) {
		return add_hir_expr(checker, hir.Expr{
			kind=.Pointer_Add, span=span, type=checker.module.exprs[left].type,
			left=left, right=right, target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	result := types.widest(checker.module.exprs[left].type, checker.module.exprs[right].type)
	if !types.is_concrete_scalar(result) {
		id := source.add(checker.diagnostics, span, "arithmetic requires compatible numeric operands")
		return invalid_hir_expr(checker, span, id)
	}
	if op == .Div && !types.is_float(result, checker.target) {
		id := source.add(
			checker.diagnostics, span,
			"integer '/' is not allowed; use divtrunc!, divfloor!, divexact!, or divceil!",
		)
		return invalid_hir_expr(checker, span, id, result)
	}
	result_kind := hir.Expr_Kind.Add
	#partial switch op {
	case .Sub: result_kind = .Sub
	case .Mul: result_kind = .Mul
	case .Div: result_kind = .Div
	}
	coerced_left := coerce_expr(checker, left, result, checker.module.exprs[left].span)
	coerced_right := coerce_expr(checker, right, result, checker.module.exprs[right].span)
	return add_hir_expr(checker, hir.Expr{
		kind=result_kind, span=span, type=result, left=coerced_left, right=coerced_right,
		target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

build_expr :: proc(
	checker: ^Checker,
	expr_id: ast.Expr_Id,
	locals: []Build_Local,
	global_reads: ^[dynamic]hir.Global_Id,
	calls: ^[dynamic]hir.Function_Id,
	expected := types.INVALID,
	pkg := ast.Package_Id(0),
	file := ast.File_Id(0),
) -> hir.Expr_Id {
	stack := checker.build_stack
	checker.build_stack = nil
	clear_dynamic_array(&stack)
	defer {
		for frame in stack {
			delete(frame.built_args, checker.allocator)
			delete(frame.arg_types, checker.allocator)
			delete(frame.mapping, checker.allocator)
		}
		clear_dynamic_array(&stack)
		if checker.build_stack == nil {
			checker.build_stack = stack
		} else {
			delete(stack)
		}
	}
	append(&stack, Build_Expr_Frame{expr=expr_id, expected=expected, template=ast.INVALID_FUNCTION})
	last := hir.INVALID_EXPR

	for len(stack) > 0 {
		frame_index := len(stack)-1
		frame := stack[frame_index]
		if frame.expr == ast.INVALID_EXPR || int(frame.expr) >= len(checker.ast_module.exprs) {
			id := source.add(checker.diagnostics, source.Span{}, "missing expression")
			last = invalid_hir_expr(checker, source.Span{}, id)
			_ = pop(&stack)
			continue
		}
		expr := checker.ast_module.exprs[frame.expr]
		if frame.stage == 0 {
			if expr.kind == .Name || expr.kind == .Field || expr.kind == .Index || expr.kind == .Unwrap {
				if specialized, ok := try_build_specialization_expr(
					checker, frame.expr, frame.expected, pkg, file,
				); ok {
					last = specialized
					_ = pop(&stack)
					continue
				}
			}
			if expr.kind == .Bit_Not || expr.kind == .Bit_And || expr.kind == .Bit_Or || expr.kind == .Bit_Xor ||
			   expr.kind == .Shift_Left || expr.kind == .Shift_Right || expr.kind == .Shift_Left_Saturating {
				if folded, ok := try_fold_typed_integer_expr(checker, frame.expr, frame.expected, pkg, file); ok {
					last = folded
					_ = pop(&stack)
					continue
				}
			}
			constant := Constant{}
			_, static_name := current_static_binding(checker, expr.name)
			if expr.kind != .Name || symbol.is_valid(expr.qualifier) || !static_name {
				constant = eval_constant(checker, frame.expr)
			}
			if constant.kind == .Value || constant.kind == .Overflow || constant.kind == .Div_By_Zero || constant.kind == .Non_Exact {
				last = build_constant_expr(checker, expr, constant, frame.expected)
				_ = pop(&stack)
				continue
			}
			switch expr.kind {
			case .String, .Array, .None, .Undefined, .Address, .Deref, .Index, .Slice,
			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed,
			     .Bool, .Cast, .Comptime, .Not, .Bit_Not, .Bit_And, .Bit_Or, .Bit_Xor, .Shift_Left,
			     .Shift_Right, .Shift_Left_Saturating, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range,
			     .Enum_Literal:
				last = build_compound_expr(
					checker, expr, locals, global_reads, calls, frame.expected, pkg, file,
				)
				_ = pop(&stack)
			case .Function_Literal:
				template := ast.Function_Id(u32(expr.integer))
				last = build_function_value(checker, template, expr.span, frame.expected)
				_ = pop(&stack)
			case .Type, .Anonymous_Struct_Type:
				id := source.add(checker.diagnostics, expr.span, "type is not a runtime value")
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
			case .Invalid, .Inference_Hole, .Integer:
				last = invalid_hir_expr(checker, expr.span, expr.diagnostic)
				_ = pop(&stack)
			case .Float:
				last = build_float_expr(checker, expr, frame.expected)
				_ = pop(&stack)
			case .Name:
				last = hir.INVALID_EXPR
				if !symbol.is_valid(expr.qualifier) {
					if local, ok := find_build_local(locals, expr.name); ok {
						last = build_local_expr(checker, local, expr.span)
					}
				} else if local, ok := find_build_local(locals, expr.qualifier); ok {
					base := build_local_expr(checker, local, expr.span)
					base_type := local.type
					item, has_item := types.container(base_type, &checker.module.types)
					field_name := symbol_text(checker, expr.name)
					if has_item && (item.kind == .Array || item.kind == .Slice) {
						if field_name == "len" {
							last = add_hir_expr(checker, hir.Expr{
								kind=.Length, span=expr.span, type=types.USIZE, left=base,
								target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
							})
						} else if field_name == "ptr" &&
						          (item.kind == .Slice || types.is_pointer(base_type, &checker.module.types)) {
							last = add_hir_expr(checker, hir.Expr{
								kind=.Slice_Ptr, span=expr.span,
								type=container_pointer_type(&checker.module.types, item), left=base,
								target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
							})
						} else if field_name == "ptr" && item.kind == .Array {
							id := source.add(checker.diagnostics, expr.span, "arrays do not expose '.ptr'; take their address first")
							last = invalid_hir_expr(checker, expr.span, id)
						}
					}
					if types.is_pointer(base_type, &checker.module.types) {
						base_type = types.child_type(base_type, &checker.module.types)
					}
					index, field, found := find_struct_field(checker, base_type, expr.name)
					if last == hir.INVALID_EXPR && found {
						last = add_hir_expr(checker, hir.Expr{
							kind=.Field, span=expr.span, type=field.type, integer=i64(index), left=base,
							target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
						})
					}
				}
				if last == hir.INVALID_EXPR && symbol.is_valid(expr.qualifier) {
					if value, ok := current_comptime_value(checker, expr.qualifier);
					   ok && value.kind == .Static && value.static_value != INVALID_CT_VALUE &&
					   int(value.static_value) < len(checker.static_state.values) {
						if field_value, found := persistent_field_value(checker, value.static_value, expr.name); found {
							last = build_static_value(checker, field_value, expr.span, frame.expected)
						}
					}
				}
				if last == hir.INVALID_EXPR && symbol.is_valid(expr.qualifier) {
					if value, ok := static_field_value(checker, expr.qualifier, expr.name); ok {
						last = build_static_value(checker, value, expr.span, frame.expected)
					}
				}
				if last == hir.INVALID_EXPR && symbol.is_valid(expr.qualifier) &&
				   find_import(checker, file, expr.qualifier) == ast.INVALID_IMPORT {
					if global := find_global(checker, expr.qualifier, pkg, file); global != ast.INVALID_GLOBAL {
						base := build_global_reference(checker, global, expr.span, global_reads)
						base_type := checker.global_types[global]
						item, has_item := types.container(base_type, &checker.module.types)
						field_name := symbol_text(checker, expr.name)
						if has_item && (item.kind == .Array || item.kind == .Slice) {
							if field_name == "len" {
								last = add_hir_expr(checker, hir.Expr{
									kind=.Length, span=expr.span, type=types.USIZE, left=base,
									target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
								})
							} else if field_name == "ptr" &&
							          (item.kind == .Slice || types.is_pointer(base_type, &checker.module.types)) {
								last = add_hir_expr(checker, hir.Expr{
									kind=.Slice_Ptr, span=expr.span,
									type=container_pointer_type(&checker.module.types, item), left=base,
									target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
								})
							} else if field_name == "ptr" && item.kind == .Array {
								id := source.add(checker.diagnostics, expr.span, "arrays do not expose '.ptr'; take their address first")
								last = invalid_hir_expr(checker, expr.span, id)
							}
						}
						if types.is_pointer(base_type, &checker.module.types) {
							base_type = types.child_type(base_type, &checker.module.types)
						}
						index, field, found := find_struct_field(checker, base_type, expr.name)
						if last == hir.INVALID_EXPR && found {
							last = add_hir_expr(checker, hir.Expr{
								kind=.Field, span=expr.span, type=field.type, integer=i64(index), left=base,
								target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
							})
						}
					}
				}
				if last == hir.INVALID_EXPR {
					if !symbol.is_valid(expr.qualifier) {
						if binding, ok := current_static_binding(checker, expr.name); ok &&
						   binding.value != INVALID_CT_VALUE && int(binding.value) < len(checker.static_state.values) {
							last = build_static_value(checker, checker.static_state.values[binding.value], expr.span, frame.expected)
						}
						if last == hir.INVALID_EXPR {
							if value, ok := current_comptime_value(checker, expr.name); ok {
								if value.kind == .Integer {
									expected_type := value.type
									if types.is_concrete_integer(frame.expected) || types.is_float(frame.expected, checker.target) {
										expected_type = frame.expected
									}
									last = build_constant_expr(
										checker,
										expr,
										Constant{kind=.Value, value=value.value},
										expected_type,
									)
								} else if value.kind == .String {
									string_id := u64(0)
									for text, index in checker.ast_module.strings {
										if text == value.text {
											string_id = u64(index)
											break
										}
									}
									constant_expr := expr
									constant_expr.kind = .String
									constant_expr.integer = string_id
									last = build_compound_expr(
										checker, constant_expr, locals, global_reads, calls,
										frame.expected, pkg, file,
									)
								} else if value.kind == .Static && value.static_value != INVALID_CT_VALUE &&
								          int(value.static_value) < len(checker.static_state.values) {
									last = build_static_value(
										checker, checker.static_state.values[value.static_value], expr.span, frame.expected,
									)
								} else {
									id := source.addf(
										checker.diagnostics,
										expr.span,
										"type parameter '%s' is not a runtime value",
										symbol_text(checker, expr.name),
									)
									last = invalid_hir_expr(checker, expr.span, id)
								}
							}
						}
					}
				}
				if last == hir.INVALID_EXPR {
					if enum_type, enum_ok := enum_type_from_name_expr(checker, expr, pkg, file); enum_ok {
						last = enum_member_hir(checker, enum_type, expr.name, expr.span)
					}
				}
				if last == hir.INVALID_EXPR {
					target_pkg, available := expr_package(checker, expr, pkg, file, true)
					if !available {
						id := add_package_resolution_diagnostic(checker, expr, file)
						last = invalid_hir_expr(checker, expr.span, id)
					} else if global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file)); global != ast.INVALID_GLOBAL {
						last = build_global_reference(checker, global, expr.span, global_reads)
					} else {
						template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
						if template != ast.INVALID_FUNCTION {
							last = build_function_value(checker, template, expr.span, frame.expected)
						} else {
							id := add_unsupported_diagnostic(checker, expr.span, target_pkg, expr.name)
							if id == source.INVALID_DIAGNOSTIC {
								id = add_name_resolution_diagnostic(checker, expr, target_pkg, file)
							}
							last = invalid_hir_expr(checker, expr.span, id)
						}
					}
				}
				_ = pop(&stack)
			case .Negate:
				stack[frame_index].stage = 5
				append(&stack, Build_Expr_Frame{expr=expr.left, expected=types.INVALID, template=ast.INVALID_FUNCTION})
			case .Add, .Sub, .Mul, .Div:
				stack[frame_index].stage = 1
				// Preserve assignment/return context for literal operands, e.g.
				// assigning `i + 1` back into a `u32` local.
				left_expected := types.INVALID
				if types.is_concrete_scalar(frame.expected) && !types.is_bool(frame.expected) {
					left_expected = frame.expected
				}
				append(&stack, Build_Expr_Frame{expr=expr.left, expected=left_expected, template=ast.INVALID_FUNCTION})
			case .Call:
				if expr.left != ast.INVALID_EXPR {
					stack[frame_index].stage = 6
					append(&stack, Build_Expr_Frame{expr=expr.left, expected=types.INVALID, template=ast.INVALID_FUNCTION})
					continue
				}
				if builtin := type_builtin_call(checker, expr); builtin != .None {
					last = build_type_builtin(checker, expr, builtin, pkg, file)
					_ = pop(&stack)
					continue
				}
				if builtin := division_builtin_call(checker, expr); builtin != .None {
					last = build_division_builtin(
						checker, expr, builtin, locals, global_reads, calls, frame.expected, pkg, file,
					)
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "some") {
					if len(expr.args) != 1 {
						id := source.addf(checker.diagnostics, expr.span, "some! expects 1 argument, got %d", len(expr.args))
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					if !types.is_optional(frame.expected, &checker.module.types) {
						id := source.add(checker.diagnostics, expr.span, "some! requires an optional context")
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					child_type := types.child_type(frame.expected, &checker.module.types)
					child := build_nested_expr(checker, expr.args[0], locals, global_reads, calls, child_type, pkg, file)
					child = coerce_expr(checker, child, child_type, checker.ast_module.exprs[expr.args[0]].span)
					last = add_hir_expr(checker, hir.Expr{
						kind=.Optional_Some, span=expr.span, type=frame.expected, left=child,
						target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
					})
					_ = pop(&stack)
					continue
				}
				if field_expr, handled := field_intrinsic_expr(checker, expr, pkg, file); handled {
					if enum_type, ok := resolve_type_argument(checker, field_expr.left, pkg, file);
					   ok && types.is_enum(enum_type, &checker.module.types) {
						last = enum_member_hir(checker, enum_type, field_expr.name, field_expr.span)
						_ = pop(&stack)
						continue
					}
					base := build_nested_expr(
						checker, field_expr.left, locals, global_reads, calls,
						types.INVALID, pkg, file,
					)
					last, _ = build_field_from_value(
						checker, field_expr, base, checker.module.exprs[base].type,
					)
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "typeinfo") {
					id := source.add(
						checker.diagnostics, expr.span,
						"typeinfo! produces compile-time-only metadata and cannot be stored or passed at runtime",
					)
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "tag") {
					last = build_tag_intrinsic(checker, expr, locals, global_reads, calls, pkg, file)
					_ = pop(&stack)
					continue
				}
				if is_intrinsic_call(checker, expr, "tagname") {
					last = build_tagname_intrinsic(checker, expr, pkg, file)
					_ = pop(&stack)
					continue
				}
				if is_ptrcast_call(checker, expr) {
					if len(expr.args) != 2 {
						id := source.addf(checker.diagnostics, expr.span, "ptrcast! expects 2 arguments, got %d", len(expr.args))
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					target, target_ok := resolve_type_argument(checker, expr.args[0], pkg, file)
					if !target_ok {
						id := source.add(checker.diagnostics, checker.ast_module.exprs[expr.args[0]].span, "ptrcast! target must be a type")
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					if !valid_ptrcast_child(checker, target) {
						id := source.addf(checker.diagnostics, checker.ast_module.exprs[expr.args[0]].span, "ptrcast! target must be a sized runtime object type, got %s", type_label(checker, target))
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					stack[frame_index].target_type = target
					stack[frame_index].stage = 9
					append(&stack, Build_Expr_Frame{expr=expr.args[1], expected=types.INVALID, template=ast.INVALID_FUNCTION})
					continue
				}
				if is_intrinsic_call(checker, expr, "compile_error") {
					message := "compile_error! requires one comptime string argument"
					if len(expr.args) == 1 {
						if text, ok := comptime_string_argument(checker, expr.args[0], pkg, file); ok {
							message = text
						}
					}
					id := source.add(checker.diagnostics, expr.span, message)
					last = invalid_hir_expr(checker, expr.span, id, types.VOID)
					_ = pop(&stack)
					continue
				}
				if expr.intrinsic {
					id := source.INVALID_DIAGNOSTIC
					if symbol.is_valid(expr.qualifier) {
						id = source.add(checker.diagnostics, expr.span, "intrinsic calls must be unqualified")
					} else {
						id = source.addf(checker.diagnostics, expr.span, "unknown intrinsic '%s!'", symbol_text(checker, expr.name))
					}
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				if callee, handled, ok := build_qualified_value_field(checker, expr, locals, global_reads, pkg, file); handled {
					if !ok {
						last = callee
						_ = pop(&stack)
						continue
					}
					stack[frame_index].stage = 6
					last = callee
					continue
				}
				target_pkg, available := expr_package(checker, expr, pkg, file, true)
				if !available {
					id := add_package_resolution_diagnostic(checker, expr, file)
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				template := find_template(checker, expr.name, target_pkg, expr_lookup_file(expr, file))
				if template == ast.INVALID_FUNCTION {
					callee := hir.INVALID_EXPR
					non_callable := false
					non_callable_global := false
					if !symbol.is_valid(expr.qualifier) {
						if local, ok := find_build_local(locals, expr.name); ok {
							if _, _, _, callable := types.callable_function(local.type, &checker.module.types); callable {
								callee = build_local_expr(checker, local, expr.span)
							} else {
								non_callable = true
							}
						}
						if callee == hir.INVALID_EXPR && !non_callable {
							if binding, ok := current_static_binding(checker, expr.name); ok &&
							   binding.value != INVALID_CT_VALUE && int(binding.value) < len(checker.static_state.values) {
								callee = build_static_value(
									checker, checker.static_state.values[binding.value], expr.span, binding.type,
								)
							}
						}
						if callee == hir.INVALID_EXPR && !non_callable {
							if value, ok := current_comptime_value(checker, expr.name);
							   ok && value.kind == .Static && value.static_value != INVALID_CT_VALUE &&
							   int(value.static_value) < len(checker.static_state.values) {
								callee = build_static_value(
									checker, checker.static_state.values[value.static_value], expr.span, types.INVALID,
								)
							}
						}
					}
					if callee == hir.INVALID_EXPR && !non_callable {
						if global := find_global(checker, expr.name, target_pkg, expr_lookup_file(expr, file)); global != ast.INVALID_GLOBAL {
							if _, _, _, callable := types.callable_function(checker.global_types[global], &checker.module.types); callable {
								callee = build_global_reference(checker, global, expr.span, global_reads)
							} else {
								non_callable = true
								non_callable_global = true
							}
						}
					}
					if callee == hir.INVALID_EXPR {
						distinct_type := types.find_named(&checker.module.types, u32(target_pkg), u32(expr.name), file=u32(expr_lookup_file(expr, file)))
						distinct_item, distinct_ok := types.node(&checker.module.types, distinct_type)
						if distinct_ok && distinct_item.kind == .Distinct {
							if !is_runtime_type(checker, distinct_type) {
								id := source.addf(
									checker.diagnostics,
									expr.span,
									"distinct type '%s' has no concrete runtime backing type",
									symbol_text(checker, expr.name),
								)
								last = invalid_hir_expr(checker, expr.span, id)
								_ = pop(&stack)
								continue
							}
							if len(expr.args) != 1 {
								id := source.addf(
									checker.diagnostics,
									expr.span,
									"distinct type '%s' expects 1 argument, got %d",
									symbol_text(checker, expr.name),
									len(expr.args),
								)
								last = invalid_hir_expr(checker, expr.span, id, distinct_type)
								_ = pop(&stack)
								continue
							}
							stack[frame_index].target_type = distinct_type
							stack[frame_index].stage = 8
							append(&stack, Build_Expr_Frame{
								expr=expr.args[0],
								expected=distinct_item.child,
								template=ast.INVALID_FUNCTION,
							})
							continue
						}
						id := source.INVALID_DIAGNOSTIC
						if non_callable {
							id = add_call_resolution_diagnostic(checker, expr, target_pkg, file) if non_callable_global else
							     source.add(checker.diagnostics, expr.span, "call target is not callable")
						} else {
							id = add_unsupported_diagnostic(checker, expr.span, target_pkg, expr.name)
							if id == source.INVALID_DIAGNOSTIC {
								id = add_call_resolution_diagnostic(checker, expr, target_pkg, file)
							}
						}
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					_, function_item, function_type, _ := types.callable_function(checker.module.exprs[callee].type, &checker.module.types)
					if !valid_callable_arity(function_item, len(expr.args)) {
						message := "function expects at least %d arguments, got %d" if function_item.variadic else
						           "function expects %d arguments, got %d"
						id := source.addf(checker.diagnostics, expr.span, message, function_item.field_count, len(expr.args))
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					stack[frame_index].left = callee
					stack[frame_index].built_args = make([]hir.Expr_Id, len(expr.args), checker.allocator)
					stack[frame_index].stage = 7
					if len(expr.args) > 0 {
						arg_expected := callable_arg_expected(function_type, function_item, &checker.module.types, 0)
						if !is_runtime_type(checker, arg_expected) {
							arg_expected = types.INVALID
						}
						append(&stack, Build_Expr_Frame{expr=expr.args[0], expected=arg_expected, template=ast.INVALID_FUNCTION})
					}
					continue
				}
				if len(checker.ast_module.functions[template].unsupported_reason) > 0 {
					id := source.addf(
						checker.diagnostics,
						expr.span,
						"C declaration '%s' is unavailable: %s",
						symbol_text(checker, expr.name),
						checker.ast_module.functions[template].unsupported_reason,
					)
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				if checker.template_diagnostics[template] != source.INVALID_DIAGNOSTIC {
					last = invalid_hir_expr(checker, expr.span, checker.template_diagnostics[template])
					_ = pop(&stack)
					continue
				}
				function := checker.ast_module.functions[template]
				if is_type_metatype_syntax(checker, function.result) {
					id := source.addf(checker.diagnostics, expr.span, "type factory '%s' is only valid in type position", symbol_text(checker, expr.name))
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				resolution_index, resolved := find_call_resolution(checker, frame.expr)
				mapping: []int
				mode := Call_Argument_Mode.Invalid
				prefix := comptime_param_count(function)
				mapping_failure := ""
				if resolved {
					mapping = slice.clone(checker.call_resolutions[resolution_index].mapping, checker.allocator)
					mode = call_mapping_mode(function, mapping)
				} else {
					infer_locals := make([]Infer_Local, len(locals), checker.allocator)
					for local, index in locals {
						infer_locals[index] = Infer_Local{name=local.name, type=local.type, declared=local.type}
					}
					mapping, mode, prefix, mapping_failure = call_argument_mapping(
						checker, &function, expr.args, pkg, file, frame.expected, infer_locals,
					)
					delete(infer_locals, checker.allocator)
				}
				if mode == .Invalid {
					id := source.INVALID_DIAGNOSTIC
					if !function_has_comptime_params(function) {
						message := "function '%s' expects at least %d arguments, got %d" if function.variadic else
						           "function '%s' expects %d arguments, got %d"
						id = source.addf(
							checker.diagnostics, expr.span, message,
							symbol_text(checker, expr.name), len(function.params), len(expr.args),
						)
						if !function.imported {
							source.add_secondary_label(checker.diagnostics, id, function.span, "function declared here")
						}
					} else if len(mapping_failure) > 0 {
						id = source.addf(
							checker.diagnostics, expr.span,
							"call to '%s' has no unique complete argument mapping: %s",
							symbol_text(checker, expr.name), mapping_failure,
						)
					} else {
						id = source.addf(
							checker.diagnostics, expr.span,
							"call to '%s' has no unique complete argument mapping",
							symbol_text(checker, expr.name),
						)
					}
					delete(mapping_failure, checker.allocator)
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				stack[frame_index].template = template
				stack[frame_index].arg_mode = mode
				stack[frame_index].prefix = prefix
				stack[frame_index].resolution = resolution_index if resolved else -1
				stack[frame_index].mapping = mapping
				stack[frame_index].built_args = make([]hir.Expr_Id, len(expr.args), checker.allocator)
				stack[frame_index].arg_types = make([]types.Type, len(function.params), checker.allocator)
				for &arg in stack[frame_index].built_args {
					arg = hir.INVALID_EXPR
				}
				stack[frame_index].arg_index = next_runtime_call_arg(function, mapping, 0, len(expr.args))
				stack[frame_index].stage = 3
				if stack[frame_index].arg_index < len(expr.args) {
					param_index := call_param_index(mapping, stack[frame_index].arg_index)
					arg_expected := resolved_call_arg_expected(
						checker, function, param_index, stack[frame_index].resolution,
					)
					if !is_runtime_type(checker, arg_expected) {
						arg_expected = types.INVALID
					}
					append(&stack, Build_Expr_Frame{
						expr=expr.args[stack[frame_index].arg_index],
						expected=arg_expected,
						template=ast.INVALID_FUNCTION,
					})
				}
			}
			continue
		}
		if frame.stage == 5 {
			operand := last
			if invalid, propagated := propagate_invalid_expr(checker, expr.span, operand); propagated {
				last = invalid
				_ = pop(&stack)
				continue
			}
			operand_type := checker.module.exprs[operand].type
			if !types.is_signed(operand_type, checker.target) && !types.is_float(operand_type, checker.target) {
				id := source.add(checker.diagnostics, expr.span, "negation requires a signed integer or float")
				last = invalid_hir_expr(checker, expr.span, id)
			} else {
				last = add_hir_expr(checker, hir.Expr{
					kind=.Negate, span=expr.span, type=operand_type, left=operand,
					target=hir.INVALID_REF, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
				})
				if types.is_signed(frame.expected, checker.target) || types.is_float(frame.expected, checker.target) {
					last = coerce_expr(checker, last, frame.expected, expr.span)
				}
			}
			_ = pop(&stack)
			continue
		}
		if frame.stage == 1 {
			stack[frame_index].left = last
			stack[frame_index].stage = 2
			right_expected := types.INVALID
			if types.is_many_pointer(checker.module.exprs[last].type, &checker.module.types) {
				right_expected = types.USIZE
			} else if eval_constant(checker, expr.right).kind == .Value {
				// A constant RHS adopts the concrete LHS type before numeric
				// compatibility is checked.
				right_expected = checker.module.exprs[last].type
			} else if types.is_concrete_scalar(frame.expected) && !types.is_bool(frame.expected) {
				right_expected = frame.expected
			}
			append(&stack, Build_Expr_Frame{expr=expr.right, expected=right_expected, template=ast.INVALID_FUNCTION})
			continue
		}
		if frame.stage == 2 {
			last = build_binary_arith(checker, expr.kind, frame.left, last, expr.span)
			_ = pop(&stack)
			continue
		}
		if frame.stage == 3 {
			if frame.arg_index < len(expr.args) {
				stack[frame_index].built_args[frame.arg_index] = last
				param_index := call_param_index(frame.mapping, frame.arg_index)
				if param_index < len(stack[frame_index].arg_types) {
					stack[frame_index].arg_types[param_index] = checker.module.exprs[last].type
				}
				next := next_runtime_call_arg(
					checker.ast_module.functions[frame.template], frame.mapping,
					frame.arg_index+1, len(expr.args),
				)
				stack[frame_index].arg_index = next
				if next < len(expr.args) {
					next_param := call_param_index(frame.mapping, next)
					arg_expected := resolved_call_arg_expected(
						checker, checker.ast_module.functions[frame.template], next_param,
						frame.resolution,
					)
					if !is_runtime_type(checker, arg_expected) {
						arg_expected = types.INVALID
					}
					append(&stack, Build_Expr_Frame{expr=expr.args[next], expected=arg_expected, template=ast.INVALID_FUNCTION})
					continue
				}
			}
			if invalid, propagated := propagate_invalid_expr(checker, expr.span, ..stack[frame_index].built_args); propagated {
				delete(stack[frame_index].arg_types, checker.allocator)
				stack[frame_index].arg_types = nil
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				delete(stack[frame_index].mapping, checker.allocator)
				stack[frame_index].mapping = nil
				last = invalid
				_ = pop(&stack)
				continue
			}
			function := checker.ast_module.functions[frame.template]
			comptime_values: []Comptime_Value
			comptime_ok := false
			if frame.resolution >= 0 && frame.resolution < len(checker.call_resolutions) {
				comptime_values = clone_comptime_values(
					checker.call_resolutions[frame.resolution].comptime_values,
					checker.allocator,
				)
				comptime_ok = true
			} else {
				comptime_values, comptime_ok = infer_call_comptime_values(
					checker, function, frame.prefix, frame.mapping, expr.args, stack[frame_index].arg_types,
					frame.expected, pkg, file, diagnose=true,
				)
			}
			defer delete(comptime_values, checker.allocator)
			if comptime_ok {
				previous_comptime := checker.current_comptime_values
				checker.current_comptime_values = comptime_values
				for source_index in 0..<len(expr.args) {
					param_index := call_param_index(frame.mapping, source_index)
					if param_index >= len(function.params) || function.params[param_index].comptime_value ||
					   !is_numeric_constant_expr(checker, expr.args[source_index]) {
						continue
					}
					expected_arg := call_arg_expected(checker, function, param_index)
					if !is_runtime_type(checker, expected_arg) {
						continue
					}
					rebuilt := build_nested_expr(
						checker, expr.args[source_index], locals, global_reads, calls,
						expected_arg, pkg, file,
					)
					stack[frame_index].built_args[source_index] = rebuilt
					stack[frame_index].arg_types[param_index] = checker.module.exprs[rebuilt].type
				}
				checker.current_comptime_values = previous_comptime
			}
			arg_violation := source.INVALID_DIAGNOSTIC
			if comptime_ok {
				previous_comptime := checker.current_comptime_values
				checker.current_comptime_values = comptime_values
				params := function.params
				for index in 0..<len(params) {
					if index >= len(stack[frame_index].arg_types) {
						break
					}
					if params[index].comptime_value {
						continue
					}
					declared := type_from_syntax(checker, params[index].type, function.pkg, function.file)
					actual := stack[frame_index].arg_types[index]
					if types.is_constraint(declared) && types.is_valid(actual) &&
					   !types.is_valid(types.constraint_target(declared, actual, &checker.module.types)) {
						arg_violation = source.addf(
							checker.diagnostics, expr.span,
							"cannot pass %s to '%s' parameter '%s'",
							types.name(actual), types.name(declared),
							symbol_text(checker, params[index].name),
						)
						break
					}
				}
				checker.current_comptime_values = previous_comptime
			}
			spec := INVALID_SPEC
			if comptime_ok && frame.resolution >= 0 && frame.resolution < len(checker.call_resolutions) {
				spec = find_spec(
					checker, frame.template,
					checker.call_resolutions[frame.resolution].runtime_types,
					comptime_values,
				)
			}
			delete(stack[frame_index].arg_types, checker.allocator)
			stack[frame_index].arg_types = nil
			if !comptime_ok || arg_violation != source.INVALID_DIAGNOSTIC {
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				delete(stack[frame_index].mapping, checker.allocator)
				stack[frame_index].mapping = nil
				diagnostic := arg_violation
				if diagnostic == source.INVALID_DIAGNOSTIC {
					diagnostic = source.add(checker.diagnostics, expr.span, "invalid comptime argument")
				}
				last = invalid_hir_expr(checker, expr.span, diagnostic)
				_ = pop(&stack)
				continue
			}
			if spec == INVALID_SPEC {
				id := source.addf(
					checker.diagnostics,
					expr.span,
					"could not resolve specialization of '%s'",
					symbol_text(checker, expr.name),
				)
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				delete(stack[frame_index].mapping, checker.allocator)
				stack[frame_index].mapping = nil
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
				continue
			}
			source_args := stack[frame_index].built_args
			runtime_count := runtime_param_count(function)
			runtime_arg_count := runtime_count + max(0, len(source_args)-len(function.params))
			runtime_args := make([]hir.Expr_Id, runtime_arg_count, checker.allocator)
			runtime_index := 0
			for param, param_index in function.params {
				if param.comptime_value {
					continue
				}
				source_index := -1
				for mapped_param, mapped_source in frame.mapping {
					if mapped_param == param_index {
						source_index = mapped_source
						break
					}
				}
				assert(source_index >= 0)
				arg := source_args[source_index]
				runtime_args[runtime_index] = coerce_expr(
					checker,
					arg,
					checker.specs[spec].args[runtime_index],
					checker.module.exprs[arg].span,
				)
				runtime_index += 1
			}
			for source_index in len(function.params)..<len(source_args) {
				arg := source_args[source_index]
				runtime_args[runtime_index] = promote_c_vararg_expr(
					checker,
					arg,
					checker.module.exprs[arg].span,
				)
				runtime_index += 1
			}
			delete(stack[frame_index].built_args, checker.allocator)
			stack[frame_index].built_args = nil
			delete(stack[frame_index].mapping, checker.allocator)
			stack[frame_index].mapping = nil
			function_id := checker.specs[spec].hir_id
			assert(function_id != hir.INVALID_FUNCTION)
			add_unique_function(calls, function_id)
			result := checker.specs[spec].result
			if !types.is_valid(result) {
				id := source.addf(
					checker.diagnostics,
					expr.span,
					"could not resolve result type for specialization of '%s'",
					symbol_text(checker, expr.name),
				)
				delete(runtime_args, checker.allocator)
				last = invalid_hir_expr(checker, expr.span, id)
			} else {
				last = add_hir_expr(checker, hir.Expr{
					kind=.Call, span=expr.span, type=result, target=hir.function_ref(function_id),
					left = hir.INVALID_EXPR, right = hir.INVALID_EXPR, args=runtime_args, diagnostic = source.INVALID_DIAGNOSTIC,
				})
			}
			_ = pop(&stack)
		}
		if frame.stage == 6 {
			callee := last
			if invalid, propagated := propagate_invalid_expr(checker, expr.span, callee); propagated {
				last = invalid
				_ = pop(&stack)
				continue
			}
			_, function_item, function_type, ok := types.callable_function(checker.module.exprs[callee].type, &checker.module.types)
			if !ok {
				id := source.add(checker.diagnostics, expr.span, "call target is not callable")
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
				continue
			}
			if !valid_callable_arity(function_item, len(expr.args)) {
				message := "function expects at least %d arguments, got %d" if function_item.variadic else
				           "function expects %d arguments, got %d"
				id := source.addf(checker.diagnostics, expr.span, message, function_item.field_count, len(expr.args))
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
				continue
			}
			stack[frame_index].left = callee
			stack[frame_index].built_args = make([]hir.Expr_Id, len(expr.args), checker.allocator)
			stack[frame_index].stage = 7
			if len(expr.args) > 0 {
				arg_expected := callable_arg_expected(function_type, function_item, &checker.module.types, 0)
				if !is_runtime_type(checker, arg_expected) {
					arg_expected = types.INVALID
				}
				append(&stack, Build_Expr_Frame{expr=expr.args[0], expected=arg_expected, template=ast.INVALID_FUNCTION})
			}
			continue
		}
		if frame.stage == 7 {
			if frame.arg_index < len(expr.args) {
				stack[frame_index].built_args[frame.arg_index] = last
				stack[frame_index].arg_index += 1
				if frame.arg_index+1 < len(expr.args) {
					next := frame.arg_index+1
					callee_type := checker.module.exprs[frame.left].type
					_, function_item, function_type, _ := types.callable_function(callee_type, &checker.module.types)
					arg_expected := callable_arg_expected(function_type, function_item, &checker.module.types, next)
					if !is_runtime_type(checker, arg_expected) {
						arg_expected = types.INVALID
					}
					append(&stack, Build_Expr_Frame{expr=expr.args[next], expected=arg_expected, template=ast.INVALID_FUNCTION})
					continue
				}
			}
			invalid, propagated := propagate_invalid_expr(checker, expr.span, frame.left)
			if !propagated {
				invalid, propagated = propagate_invalid_expr(checker, expr.span, ..stack[frame_index].built_args)
			}
			if propagated {
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				last = invalid
				_ = pop(&stack)
				continue
			}
			callee_type := checker.module.exprs[frame.left].type
			_, function_item, function_type, ok := types.callable_function(callee_type, &checker.module.types)
			if !ok {
				id := source.add(checker.diagnostics, expr.span, "call target is not callable")
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
				continue
			}
			fixed_count := int(function_item.field_count)
			for index in 0..<min(fixed_count, len(stack[frame_index].built_args)) {
				expected_arg := callable_arg_expected(function_type, function_item, &checker.module.types, index)
				stack[frame_index].built_args[index] = coerce_expr(
					checker,
					stack[frame_index].built_args[index],
					expected_arg,
					checker.module.exprs[stack[frame_index].built_args[index]].span,
				)
			}
			for index in fixed_count..<len(stack[frame_index].built_args) {
				arg := stack[frame_index].built_args[index]
				stack[frame_index].built_args[index] = promote_c_vararg_expr(
					checker,
					arg,
					checker.module.exprs[arg].span,
				)
			}
			result := function_item.child
			if !types.is_valid(result) {
				id := source.add(checker.diagnostics, expr.span, "could not resolve callable result type")
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				last = invalid_hir_expr(checker, expr.span, id)
			} else {
				target := hir.INVALID_REF
				callee := frame.left
				callee_expr := checker.module.exprs[frame.left]
				if callee_expr.kind == .Function {
					function := hir.as_function(callee_expr.target)
					if function != hir.INVALID_FUNCTION {
						target = callee_expr.target
						callee = hir.INVALID_EXPR
						add_unique_function(calls, function)
					}
				}
				last = add_hir_expr(checker, hir.Expr{
					kind=.Call, span=expr.span, type=result, target=target,
					left=callee, right=hir.INVALID_EXPR, args=stack[frame_index].built_args,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
				stack[frame_index].built_args = nil
			}
			_ = pop(&stack)
		}
		if frame.stage == 8 {
			distinct_item, ok := types.node(&checker.module.types, frame.target_type)
			actual := checker.module.exprs[last].type
			if !ok || distinct_item.kind != .Distinct || !types.equal(actual, distinct_item.child) {
				id := source.addf(
					checker.diagnostics,
					expr.span,
					"distinct type '%s' requires an exact %s value, got %s",
					symbol_text(checker, expr.name),
					types.name(distinct_item.child),
					types.name(actual),
				)
				last = invalid_hir_expr(checker, expr.span, id, frame.target_type)
			} else {
				last = add_hir_expr(checker, hir.Expr{
					kind=.Retype,
					span=expr.span,
					type=frame.target_type,
					left=last,
					target=hir.INVALID_REF,
					right=hir.INVALID_EXPR,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			}
			_ = pop(&stack)
		}
		if frame.stage == 9 {
			result, ok := types.replace_pointer_child(&checker.module.types, checker.module.exprs[last].type, frame.target_type)
			if !ok {
				id := source.add(checker.diagnostics, expr.span, "ptrcast! operand must be a pointer or optional pointer")
				last = invalid_hir_expr(checker, expr.span, id)
			} else {
				last = add_hir_expr(checker, hir.Expr{
					kind=.Pointer_Cast,
					span=expr.span,
					type=result,
					left=last,
					target=hir.INVALID_REF,
					right=hir.INVALID_EXPR,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			}
			_ = pop(&stack)
		}
	}
	return last
}

make_link_name :: proc(checker: ^Checker, id: Spec_Id) -> string {
	spec := checker.specs[id]
	function := checker.ast_module.functions[spec.template]
	if function.pkg == 0 && function.name == checker.main_symbol && checker.entry_point == .Plain {
		return fmt.aprintf("main", allocator = checker.allocator)
	}
	if function.generated {
		return fmt.aprintf("bro__p%d__anon%d", function.pkg, spec.template, allocator = checker.allocator)
	}
	if !function.has_body && function.c_abi {
		if len(function.link_name) > 0 {
			return strings.clone(function.link_name, checker.allocator)
		}
		return fmt.aprintf("%s", symbol_text(checker, function.name), allocator = checker.allocator)
	}
	builder := strings.builder_make(checker.allocator)
	defer strings.builder_destroy(&builder)
	strings.write_string(&builder, "bro_c__" if function.c_abi else "bro__")
	fmt.sbprintf(&builder, "p%d__", function.pkg)
	if function.file_hidden {
		fmt.sbprintf(&builder, "f%d__", function.file)
	}
	strings.write_string(&builder, symbol_text(checker, function.name))
	for arg in spec.args {
		strings.write_string(&builder, "__")
		if arg >= types.DYNAMIC_START {
			fmt.sbprintf(&builder, "t%d", arg)
		} else {
			strings.write_string(&builder, types.name(arg))
		}
	}
	for value in spec.comptime_values {
		if value.kind == .Type {
			strings.write_string(&builder, "__ct")
			if value.type >= types.DYNAMIC_START {
				fmt.sbprintf(&builder, "t%d", value.type)
			} else {
				strings.write_string(&builder, types.name(value.type))
			}
		} else if value.kind == .String {
			fmt.sbprintf(&builder, "__cs%d_", len(value.text))
			hex := "0123456789abcdef"
			for byte in transmute([]byte)value.text {
				strings.write_byte(&builder, hex[byte>>4])
				strings.write_byte(&builder, hex[byte&0xf])
			}
		} else if value.kind == .Static {
			fmt.sbprintf(&builder, "__ca%d_%016x", len(value.key), value.fingerprint)
		} else {
			strings.write_string(&builder, "__cv")
			if value.value < 0 {
				strings.write_string(&builder, "n")
				fmt.sbprintf(&builder, "%d", -value.value)
			} else {
				fmt.sbprintf(&builder, "%d", value.value)
			}
		}
	}
	return fmt.aprintf("%s", strings.to_string(builder), allocator = checker.allocator)
}

// Replay eligible cleanup in `[lo, len(defers))` in LIFO order. Error exits run
// both defer forms; other exits skip errdefer. A direct error return supplies its
// preserved payload so captured errors can be initialized before each cleanup.
flush_defers :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	lo: int,
	error_exit := false,
	error_value := hir.INVALID_EXPR,
) {
	for i := len(ctx.defers^) - 1; i >= lo; i -= 1 {
		entry := ctx.defers^[i]
		if entry.error_only && !error_exit {
			continue
		}
		if error_exit && entry.capture != hir.INVALID_LOCAL && error_value != hir.INVALID_EXPR {
			append(body, hir.stmt_id(len(ctx.checker.module.statements)))
			append(&ctx.checker.module.statements, hir.Stmt{
				kind=.Declaration, local=entry.capture, expr=error_value,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		for stmt_id in entry.body {
			append(body, stmt_id)
		}
	}
}

// Copy the active error-exit cleanup onto a Try expression. Capture locals are
// initialized by lowering once the propagated error has been extracted/widened.
try_cleanup :: proc(ctx: ^Build_Ctx) -> ([]hir.Stmt_Id, []hir.Expr_Id) {
	body: [dynamic]hir.Stmt_Id
	body.allocator = ctx.checker.allocator
	captures: [dynamic]hir.Expr_Id
	captures.allocator = ctx.checker.allocator
	for i := len(ctx.defers^) - 1; i >= 0; i -= 1 {
		entry := ctx.defers^[i]
		if entry.error_only && entry.capture != hir.INVALID_LOCAL {
			append(&captures, hir.Expr_Id(entry.capture))
		}
		append(&body, ..entry.body)
	}
	return body[:], captures[:]
}

specialization_bool :: proc(checker: ^Checker, expr: ast.Expr_Id, pkg: ast.Package_Id, file: ast.File_Id) -> (bool, bool) {
	if len(checker.current_comptime_values) == 0 && len(checker.static_bindings) == 0 {
		return false, false
	}
	state := ct_state_make(checker, pkg, file, values=checker.current_comptime_values, diagnose=false)
	defer ct_state_destroy(&state)
	value, flow, ok := ct_eval_expr(&state, expr, types.BOOL, 0)
	if !ok || flow.kind != .Normal {
		return false, false
	}
	return ct_bool_value(&state, value)
}

specialization_match_body :: proc(
	checker: ^Checker,
	statement: ast.Stmt,
	pkg: ast.Package_Id,
	file: ast.File_Id,
) -> ([]ast.Stmt_Id, Static_Binding, bool, bool) {
	if len(checker.current_comptime_values) == 0 && len(checker.static_bindings) == 0 {
		return nil, {}, false, false
	}
	state := ct_state_make(checker, pkg, file, values=checker.current_comptime_values, diagnose=false)
	defer ct_state_destroy(&state)
	subject, flow, ok := ct_eval_expr(&state, statement.expr, types.INVALID, 0)
	if !ok || flow.kind != .Normal || subject == INVALID_CT_VALUE || int(subject) >= len(state.values) {
		return nil, {}, false, false
	}
	selection, selected := ct_select_match_arm(&state, statement, subject, 0)
	if !selected {
		return nil, {}, false, false
	}
	arm := checker.ast_module.statements[selection.arm]
	if arm.expand {
		return nil, {}, false, false
	}
	if arm.pointer_capture {
		return nil, {}, false, false
	}
	if len(arm.captures) > 0 {
		if selection.payload == INVALID_CT_VALUE || types.is_void(selection.payload_type) {
			return nil, {}, false, false
		}
		if arm.captures[0] != checker.sink_symbol {
			return arm.body, store_static_binding(checker, &state, selection.payload, arm.captures[0]), true, true
		}
	}
	return arm.body, {}, false, true
}

Expand_Binding_Error :: enum u8 {
	None,
	Invalid,
	Quota,
	Diagnosed,
}

expand_field_bindings :: proc(
	checker: ^Checker,
	expr: ast.Expr_Id,
	capture: symbol.Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	diagnose := false,
) -> ([]Static_Binding, Expand_Binding_Error) {
	state := ct_state_make(checker, pkg, file, values=checker.current_comptime_values, diagnose=diagnose)
	defer ct_state_destroy(&state)
	value_id, flow, ok := ct_eval_expr(&state, expr, types.INVALID, 0)
	if !ok || flow.kind != .Normal || value_id == INVALID_CT_VALUE || int(value_id) >= len(state.values) {
		return nil, .Diagnosed if state.diagnostic != source.INVALID_DIAGNOSTIC else .Invalid
	}
	value := state.values[value_id]
	if ct_value_contains_undefined(&state, value_id) {
		if diagnose {
			_ = ct_fail(&state, .Not_Comptime, checker.ast_module.exprs[expr].span, "expand for cannot expand an undefined comptime value")
		}
		return nil, .Diagnosed if state.diagnostic != source.INVALID_DIAGNOSTIC else .Invalid
	}
	if value.kind == .Range {
		parts := ct_child_slice(&state, value)
		if len(parts) != 2 {
			return nil, .Invalid
		}
		start, start_ok := ct_integer_value(&state, parts[0])
		end, end_ok := ct_integer_value(&state, parts[1])
		if !start_ok || !end_ok || end < start {
			return nil, .Invalid
		}
		count := int(end-start)
		if value.active != 0 {
			count += 1
		}
		if count < 0 || count > COMPTIME_EVAL_QUOTA {
			return nil, .Quota
		}
		bindings := make([]Static_Binding, count, checker.allocator)
		for index in 0..<count {
			id := ct_add_value(&state, Ct_Value{
				kind=.Integer,
				type=types.child_type(value.type, &checker.module.types),
				integer=start+i128(index),
			})
			bindings[index] = store_static_binding(checker, &state, id, capture)
		}
		return bindings, .None
	}
	owned_children: [dynamic]Ct_Value_Id
	owned_children.allocator = checker.allocator
	defer delete(owned_children)
	children: []Ct_Value_Id
	if value.kind == .Array {
		children = ct_child_slice(&state, value)
	} else if value.kind == .Slice {
		for index in 0..<int(value.count) {
			place, _, _ := ct_slice_element_place(&state, value, index)
			child, child_ok := ct_place_get(&state, place)
			if !child_ok {
				return nil, .Invalid
			}
			append(&owned_children, child)
		}
		children = owned_children[:]
	} else if value.kind == .Struct {
		item, tuple_ok := types.node(&checker.module.types, value.type)
		if !tuple_ok || item.kind != .Struct || !item.tuple {
			return nil, .Invalid
		}
		children = ct_child_slice(&state, value)
	} else {
		return nil, .Invalid
	}
	bindings := make([]Static_Binding, len(children), checker.allocator)
	for child, index in children {
		if child == INVALID_CT_VALUE || int(child) >= len(state.values) {
			delete(bindings, checker.allocator)
			return nil, .Invalid
		}
		bindings[index] = store_static_binding(checker, &state, child, capture)
	}
	return bindings, .None
}

push_expand_binding :: proc(
	checker: ^Checker,
	binding: Static_Binding,
	index_name: symbol.Id,
	index: int,
	statement: ast.Stmt_Id,
) -> int {
	start := len(checker.static_bindings)
	append(&checker.static_bindings, binding)
	append(&checker.expand_context, Expand_Expansion{statement=statement, index=u32(index)})
	if symbol.is_valid(index_name) {
		value := ct_add_value(&checker.static_state, Ct_Value{kind=.Integer, type=types.USIZE, integer=i128(index)})
		append(&checker.static_bindings, Static_Binding{name=index_name, type=types.USIZE, value=value})
	}
	return start
}

pop_expand_binding :: proc(checker: ^Checker, start: int) {
	resize(&checker.static_bindings, start)
	_ = pop(&checker.expand_context)
}

Expand_Control :: enum u8 {
	Normal,
	Break,
	Continue,
	Invalid,
}

expand_control_target :: proc(statement: ast.Stmt, target_label: symbol.Id, allow_unlabeled: bool) -> Expand_Control {
	if statement.kind != .Break && statement.kind != .Continue {
		return .Normal
	}
	if symbol.is_valid(statement.label) {
		if !symbol.is_valid(target_label) || statement.label != target_label {
			return .Normal
		}
	} else if !allow_unlabeled {
		return .Normal
	}
	return .Break if statement.kind == .Break else .Continue
}

contains_expand_control :: proc(
	checker: ^Checker,
	statements: []ast.Stmt_Id,
	target_label: symbol.Id,
	allow_unlabeled: bool,
) -> bool {
	for statement_id in statements {
		statement := checker.ast_module.statements[statement_id]
		if expand_control_target(statement, target_label, allow_unlabeled) != .Normal {
			return true
		}
		#partial switch statement.kind {
		case .Block:
			if contains_expand_control(checker, statement.body, target_label, allow_unlabeled) {
				return true
			}
		case .If:
			if contains_expand_control(checker, statement.body, target_label, allow_unlabeled) ||
			   contains_expand_control(checker, statement.else_body, target_label, allow_unlabeled) {
				return true
			}
		case .Match, .Match_Arm:
			if contains_expand_control(checker, statement.body, target_label, allow_unlabeled) {
				return true
			}
		case .For, .While:
			// Unlabelled control belongs to the nested loop. A labelled jump can still
			// name the surrounding expand loop and is therefore relevant here.
			if contains_expand_control(checker, statement.body, target_label, false) {
				return true
			}
		case .Defer:
			if statement.update != ast.INVALID_STMT &&
			   contains_expand_control(checker, []ast.Stmt_Id{statement.update}, target_label, false) {
				return true
			}
		}
	}
	return false
}

copy_stmt_ids :: proc(checker: ^Checker, source_ids: []ast.Stmt_Id) -> []ast.Stmt_Id {
	result := make([]ast.Stmt_Id, len(source_ids), checker.ast_module.allocator)
	copy(result, source_ids)
	return result
}

copy_symbols :: proc(checker: ^Checker, source_ids: []symbol.Id) -> []symbol.Id {
	result := make([]symbol.Id, len(source_ids), checker.ast_module.allocator)
	copy(result, source_ids)
	return result
}

copy_expr_ids :: proc(checker: ^Checker, source_ids: []ast.Expr_Id) -> []ast.Expr_Id {
	result := make([]ast.Expr_Id, len(source_ids), checker.ast_module.allocator)
	copy(result, source_ids)
	return result
}

clone_statement_body :: proc(checker: ^Checker, statement: ast.Stmt, body: []ast.Stmt_Id) -> ast.Stmt_Id {
	clone := statement
	clone.captures = copy_symbols(checker, statement.captures)
	clone.patterns = copy_expr_ids(checker, statement.patterns)
	clone.body = copy_stmt_ids(checker, body)
	clone.else_body = copy_stmt_ids(checker, statement.else_body)
	id := ast.stmt_id(len(checker.ast_module.statements))
	append(&checker.ast_module.statements, clone)
	return id
}

append_expand_block :: proc(checker: ^Checker, span: source.Span, body: []ast.Stmt_Id, out: ^[dynamic]ast.Stmt_Id) {
	if len(body) == 0 {
		return
	}
	id := ast.stmt_id(len(checker.ast_module.statements))
	append(&checker.ast_module.statements, ast.Stmt{
		kind=.Block,
		span=span,
		body=copy_stmt_ids(checker, body),
		guard=ast.INVALID_EXPR,
		target=ast.INVALID_EXPR,
		expr=ast.INVALID_EXPR,
		update=ast.INVALID_STMT,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
	append(out, id)
}

same_stmt_ids :: proc(left, right: []ast.Stmt_Id) -> bool {
	if len(left) != len(right) {
		return false
	}
	for item, index in left {
		if item != right[index] {
			return false
		}
	}
	return true
}

flatten_expand_iteration :: proc(
	checker: ^Checker,
	statements: []ast.Stmt_Id,
	pkg: ast.Package_Id,
	file: ast.File_Id,
	out: ^[dynamic]ast.Stmt_Id,
	target_label: symbol.Id,
	diagnostic: ^source.Diagnostic_Id,
) -> Expand_Control {
	for statement_id in statements {
		statement := checker.ast_module.statements[statement_id]
		if control := expand_control_target(statement, target_label, true); control != .Normal {
			return control
		}
		if statement.kind == .If && len(statement.captures) == 0 {
			if selected, comptime_ok := specialization_bool(checker, statement.expr, pkg, file); comptime_ok {
				selected_body := statement.body if selected else statement.else_body
				branch: [dynamic]ast.Stmt_Id
				branch.allocator = checker.allocator
				flow := flatten_expand_iteration(checker, selected_body, pkg, file, &branch, target_label, diagnostic)
				append_expand_block(checker, statement.span, branch[:], out)
				delete(branch)
				if flow != .Normal {
					return flow
				}
				continue
			}
			if contains_expand_control(checker, statement.body, target_label, true) ||
			   contains_expand_control(checker, statement.else_body, target_label, true) {
				if diagnostic != nil {
					diagnostic^ = source.add(
						checker.diagnostics, statement.span,
						"break or continue targeting an expand loop must be compile-time-resolvable",
					)
				}
				return .Invalid
			}
		}
		if statement.kind == .Match {
			if selected_body, _, _, comptime_ok := specialization_match_body(checker, statement, pkg, file); comptime_ok {
				selected: [dynamic]ast.Stmt_Id
				selected.allocator = checker.allocator
				flow := flatten_expand_iteration(checker, selected_body, pkg, file, &selected, target_label, diagnostic)
				if flow != .Normal {
					arm_index := -1
					for arm_id, index in statement.body {
						if same_stmt_ids(checker.ast_module.statements[arm_id].body, selected_body) {
							arm_index = index
							break
						}
					}
					if arm_index >= 0 && len(selected) > 0 {
						arm_id := statement.body[arm_index]
						cloned_arm := clone_statement_body(checker, checker.ast_module.statements[arm_id], selected[:])
						arms := copy_stmt_ids(checker, statement.body)
						arms[arm_index] = cloned_arm
						cloned_match := clone_statement_body(checker, statement, arms)
						delete(arms, checker.ast_module.allocator)
						append(out, cloned_match)
					}
					delete(selected)
					return flow
				}
				delete(selected)
				append(out, statement_id)
				continue
			}
			if contains_expand_control(checker, statement.body, target_label, true) {
				if diagnostic != nil {
					diagnostic^ = source.add(
						checker.diagnostics, statement.span,
						"break or continue targeting an expand loop must be compile-time-resolvable",
					)
				}
				return .Invalid
			}
		}
		if statement.kind == .Block {
			block: [dynamic]ast.Stmt_Id
			block.allocator = checker.allocator
			flow := flatten_expand_iteration(checker, statement.body, pkg, file, &block, target_label, diagnostic)
			if flow == .Normal {
				append(out, statement_id)
			} else {
				append_expand_block(checker, statement.span, block[:], out)
			}
			delete(block)
			if flow != .Normal {
				return flow
			}
			continue
		}
		if (statement.kind == .For || statement.kind == .While || statement.kind == .Defer) &&
		   contains_expand_control(checker, []ast.Stmt_Id{statement_id}, target_label, false) {
			if diagnostic != nil {
				diagnostic^ = source.add(
					checker.diagnostics, statement.span,
					"break or continue targeting an expand loop must be compile-time-resolvable",
				)
			}
			return .Invalid
		}
		append(out, statement_id)
	}
	return .Normal
}

build_block :: proc(
	ctx: ^Build_Ctx,
	statements: []ast.Stmt_Id,
	duplicate_scope_start := -1,
	close := true,
) -> []hir.Stmt_Id {
	checker := ctx.checker
	store := &checker.module.types
	body: [dynamic]hir.Stmt_Id
	body.allocator = checker.allocator
	scope_start := len(ctx.locals^)
	defer_start := len(ctx.defers^)
	duplicate_start := scope_start if duplicate_scope_start < 0 else duplicate_scope_start
	for statement_id in statements {
		statement := checker.ast_module.statements[statement_id]
		if statement.diagnostic != source.INVALID_DIAGNOSTIC {
			if statement.kind == .Declaration && symbol.is_valid(statement.name) {
				if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); !found {
					_ = append_build_local(
						ctx, statement.name, types.I64, !statement.immutable, statement.span,
					)
				}
			}
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL, diagnostic = statement.diagnostic,
			})
			ctx.problematic^ = true
			continue
		}
		switch statement.kind {
		case .Declaration:
			// A value block (`x :: { ... yield v }` / `x T = { ... }`): the parser
			// leaves `expr` invalid and stashes the block in `body`. Build it, then
			// declare the local from the yielded value (its type for an untyped `::`).
			if statement.expr == ast.INVALID_EXPR {
				expected := types.INVALID
				typed := is_runtime_type(checker, type_from_syntax(checker, statement.type, ctx.pkg, ctx.file))
				if typed {
					expected = type_from_syntax(checker, statement.type, ctx.pkg, ctx.file)
				}
				value, value_type := build_value_source(ctx, &body, statement.body, expected, statement.span, statement.label, statement.value_control_flow)
				if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); found {
					id := source.addf(
						checker.diagnostics, statement.span,
						"duplicate local '%s'", symbol_text(checker, statement.name),
					)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				if id := add_shadow_diagnostic(
					checker, statement.span, statement.name, "local",
					ctx.pkg, ctx.file, ctx.locals^[:], ctx.loop_labels^[:], ctx.yield_targets^[:],
				); id != source.INVALID_DIAGNOSTIC {
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				local_id := append_build_local(ctx, statement.name, value_type, !statement.immutable, statement.span)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Declaration, span = statement.span, local = local_id, expr = value,
					diagnostic = source.INVALID_DIAGNOSTIC,
				})
				continue
			}
			declared := resolve_inferred_array(checker, type_from_syntax(checker, statement.type, ctx.pkg, ctx.file), statement.expr)
			// Adopt the type inference resolved for this local when the declaration has no
			// concrete annotation and inference carried useful numeric context: constraints,
			// `undefined`, open numeric constants, or arithmetic expressions.
			open_const_decl := !is_runtime_type(checker, declared) && !is_undefined_expr(checker, statement.expr)
			if open_const_decl {
				constant := eval_integer_constant_in_context(checker, statement.expr, ctx.pkg, ctx.file)
				open_const_decl = constant.kind == .Value && fits_i64(constant.value) ||
				                  is_float_constant_expr(checker, statement.expr)
			}
			numeric_arithmetic_decl := !is_runtime_type(checker, declared) &&
			                           is_numeric_arithmetic_expr(checker, statement.expr)
			if statement_id != ast.INVALID_STMT && int(statement_id) < len(ctx.local_types) &&
			   is_runtime_type(checker, ctx.local_types[statement_id]) &&
			   (types.is_constraint(declared) || is_undefined_expr(checker, statement.expr) ||
			    open_const_decl || numeric_arithmetic_decl || has_inferred_array_count(checker, declared)) {
				declared = ctx.local_types[statement_id]
			}
			// A still-unresolved constraint means the initializer's numeric
			// family did not satisfy `int`/`float` (`undefined` reports its own).
			if types.is_constraint(declared) && !is_undefined_expr(checker, statement.expr) {
				id := source.addf(
					checker.diagnostics,
					statement.span,
					"could not resolve the '%s' constraint for local '%s'",
					types.name(declared),
					symbol_text(checker, statement.name),
				)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			expected := types.INVALID
			value := hir.INVALID_EXPR
			value_type := types.INVALID
			if is_undefined_expr(checker, statement.expr) {
				if statement.immutable {
					id := source.add(
						checker.diagnostics,
						statement.span,
						"'undefined' requires a mutable local declaration",
					)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				if !is_runtime_type(checker, declared) || has_inferred_array_count(checker, declared) {
					id := source.addf(
						checker.diagnostics,
						statement.span,
						"could not infer a concrete type for local '%s'",
						symbol_text(checker, statement.name),
					)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				value_type = declared
			} else {
				if is_runtime_type(checker, declared) {
					expected = declared
				}
				value = build_expr(
					checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
					expected, ctx.pkg, ctx.file,
				)
				value_type = checker.module.exprs[value].type
				if is_runtime_type(checker, declared) {
					value = coerce_expr(checker, value, declared, statement.span)
					value_type = checker.module.exprs[value].type
				} else if types.is_void(declared) {
					id := source.add(checker.diagnostics, statement.span, "locals cannot have type void")
					value = invalid_hir_expr(checker, statement.span, id)
					value_type = types.INVALID
				}
			}
			if types.is_comptime_only(value_type, &checker.module.types) {
				id := source.addf(
					checker.diagnostics,
					statement.span,
					"local '%s' has a comptime-only type and cannot be stored at runtime",
					symbol_text(checker, statement.name),
				)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); found {
				id := source.addf(
					checker.diagnostics, statement.span,
					"duplicate local '%s'", symbol_text(checker, statement.name),
				)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			if id := add_shadow_diagnostic(
				checker, statement.span, statement.name, "local",
				ctx.pkg, ctx.file, ctx.locals^[:], ctx.loop_labels^[:], ctx.yield_targets^[:],
			); id != source.INVALID_DIAGNOSTIC {
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			local_id := append_build_local(ctx, statement.name, value_type, !statement.immutable, statement.span)
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Declaration, span = statement.span, local = local_id, expr = value,
				diagnostic = source.INVALID_DIAGNOSTIC,
			})
			if value != hir.INVALID_EXPR {
				ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
			}
		case .Assignment:
			if statement.target != ast.INVALID_EXPR {
				target_expr := build_expr(
					checker, statement.target, ctx.locals^[:], ctx.global_reads, ctx.calls,
					types.INVALID, ctx.pkg, ctx.file,
				)
				if diagnostic, invalid := invalid_expr_diagnostic(checker, target_expr); invalid {
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
						target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=diagnostic,
					})
					ctx.problematic^ = true
					continue
				}
				target_type := checker.module.exprs[target_expr].type
				if !hir_location_writable(checker, target_expr, ctx.locals^[:]) {
					id := source.add(checker.diagnostics, statement.span, "assignment target is not writable")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
						target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=id,
					})
					ctx.problematic^ = true
					continue
				}
				value: hir.Expr_Id
				assignment_op := hir.Assignment_Op.Set
				if statement.assignment_op != .Set {
					rhs_expected := target_type
					if types.is_many_pointer(target_type, &checker.module.types) {
						rhs_expected = types.USIZE if statement.assignment_op == .Add else types.INVALID
					} else if statement.assignment_op == .Shift_Left ||
					          statement.assignment_op == .Shift_Right ||
					          statement.assignment_op == .Shift_Left_Saturating {
						rhs_expected = types.U64
					}
					value = build_expr(
						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
						rhs_expected, ctx.pkg, ctx.file,
					)
					if diagnostic, invalid := invalid_expr_diagnostic(checker, value); invalid {
						append(&body, hir.stmt_id(len(checker.module.statements)))
						append(&checker.module.statements, hir.Stmt{
							kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
							target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=diagnostic,
						})
						ctx.problematic^ = true
						continue
					}
					if types.is_many_pointer(target_type, &checker.module.types) {
						assignment_op = .Pointer_Add
						if statement.assignment_op != .Add {
							id := source.add(
								checker.diagnostics,
								statement.span,
								"many-item pointers only support '+=' compound assignment",
							)
							value = invalid_hir_expr(checker, statement.span, id, types.USIZE)
						} else {
							value = coerce_expr(checker, value, types.USIZE, statement.span)
						}
					} else {
						#partial switch statement.assignment_op {
						case .Add: assignment_op = .Add
						case .Sub: assignment_op = .Sub
						case .Mul: assignment_op = .Mul
						case .Div: assignment_op = .Div
						case .Bit_And: assignment_op = .Bit_And
						case .Bit_Or: assignment_op = .Bit_Or
						case .Bit_Xor: assignment_op = .Bit_Xor
						case .Shift_Left: assignment_op = .Shift_Left
						case .Shift_Right: assignment_op = .Shift_Right
						case .Shift_Left_Saturating: assignment_op = .Shift_Left_Saturating
						}
						rhs_type := checker.module.exprs[value].type
						is_shift := statement.assignment_op == .Shift_Left ||
						            statement.assignment_op == .Shift_Right ||
						            statement.assignment_op == .Shift_Left_Saturating
						is_bitwise := statement.assignment_op == .Bit_And ||
						              statement.assignment_op == .Bit_Or ||
						              statement.assignment_op == .Bit_Xor
						result_type := types.widest(target_type, rhs_type)
						if is_shift {
							if !types.is_concrete_integer(target_type) || !types.is_unsigned(rhs_type, checker.target) {
								id := source.add(checker.diagnostics, statement.span, "shift assignment requires an integer target and unsigned integer count")
								value = invalid_hir_expr(checker, statement.span, id, target_type)
							} else if constant := eval_integer_constant_in_context(checker, statement.expr, ctx.pkg, ctx.file);
							          constant.kind == .Value && statement.assignment_op != .Shift_Left_Saturating &&
							          constant.value >= i128(types.bits(target_type, checker.target)) {
								id := source.addf(
									checker.diagnostics, statement.span,
									"shift count %d exceeds %s width", constant.value, types.name(target_type),
								)
								value = invalid_hir_expr(checker, statement.span, id, target_type)
							}
						} else if is_bitwise {
							if !types.is_concrete_integer(result_type) {
								id := source.add(checker.diagnostics, statement.span, "bitwise assignment requires compatible concrete integer operands")
								value = invalid_hir_expr(checker, statement.span, id, target_type)
							} else {
								value = coerce_expr(checker, value, target_type, statement.span)
							}
						} else if statement.assignment_op == .Div && types.is_concrete_integer(result_type) {
							id := source.add(
								checker.diagnostics,
								statement.span,
								"integer '/=' is not allowed; assign through an explicit division builtin",
							)
							value = invalid_hir_expr(checker, statement.span, id, target_type)
						} else if !types.is_concrete_scalar(result_type) ||
						   types.is_bool(result_type) {
							id := source.add(
								checker.diagnostics,
								statement.span,
								"arithmetic requires compatible numeric operands",
							)
							value = invalid_hir_expr(checker, statement.span, id, target_type)
						} else {
							// Compound assignment stores back into the original
							// target type, so only an equal or widening RHS
							// conversion is permitted.
							value = coerce_expr(checker, value, target_type, statement.span)
						}
					}
				} else if statement.expr == ast.INVALID_EXPR {
					// `target = { ... yield v }`: build the value block against the
					// target's type (build_value_block coerces internally).
					value, _ = build_value_source(ctx, &body, statement.body, target_type, statement.span, statement.label, statement.value_control_flow)
				} else {
					value = build_expr(
						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
						target_type, ctx.pkg, ctx.file,
					)
					value = coerce_expr(checker, value, target_type, statement.span)
				}
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind=.Assignment, span=statement.span, local=hir.INVALID_LOCAL,
					assignment_op=assignment_op,
					target=target_expr, expr=value, diagnostic=source.INVALID_DIAGNOSTIC,
				})
				ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
				continue
			}
			if statement.name == checker.sink_symbol {
				value: hir.Expr_Id
				if statement.expr == ast.INVALID_EXPR {
					value, _ = build_value_source(ctx, &body, statement.body, types.INVALID, statement.span, statement.label, statement.value_control_flow)
				} else {
					value = build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
				}
				if types.is_void(checker.module.exprs[value].type) {
					id := source.add(checker.diagnostics, statement.span, "cannot assign a void expression to '_'")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
				} else {
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Sink, span = statement.span, expr = value,
						local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
					})
				}
				continue
			}
			if _, is_comptime := current_comptime_value(checker, statement.name); is_comptime {
				id := source.addf(
					checker.diagnostics,
					statement.span,
					"cannot assign comptime parameter '%s'",
					symbol_text(checker, statement.name),
				)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			local, found := find_build_local(ctx.locals^[:], statement.name)
			if !found {
				global := find_global(checker, statement.name, ctx.pkg, ctx.file)
				if global == ast.INVALID_GLOBAL {
					id := source.addf(checker.diagnostics, statement.span, "cannot assign unresolved local '%s'", symbol_text(checker, statement.name))
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				target_expr := build_global_reference(checker, global, statement.span, ctx.global_reads)
				if diagnostic, invalid := invalid_expr_diagnostic(checker, target_expr); invalid {
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
						target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=diagnostic,
					})
					ctx.problematic^ = true
					continue
				}
				target_type := checker.module.exprs[target_expr].type
				if !hir_location_writable(checker, target_expr, ctx.locals^[:]) {
					id := source.add(checker.diagnostics, statement.span, "assignment target is not writable")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind=.Trap, span=statement.span, local=hir.INVALID_LOCAL,
						target=hir.INVALID_EXPR, expr=hir.INVALID_EXPR, diagnostic=id,
					})
					ctx.problematic^ = true
					continue
				}
				value: hir.Expr_Id
				if statement.expr == ast.INVALID_EXPR {
					value, _ = build_value_source(ctx, &body, statement.body, target_type, statement.span, statement.label, statement.value_control_flow)
				} else {
					value = build_expr(
						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
						target_type, ctx.pkg, ctx.file,
					)
					value = coerce_expr(checker, value, target_type, statement.span)
				}
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind=.Assignment, span=statement.span, local=hir.INVALID_LOCAL,
					assignment_op=.Set,
					target=target_expr, expr=value, diagnostic=source.INVALID_DIAGNOSTIC,
				})
				ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
				continue
			}
			if !local.mutable {
				id := source.addf(checker.diagnostics, statement.span, "cannot assign immutable local '%s'", symbol_text(checker, statement.name))
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			value: hir.Expr_Id
			if statement.expr == ast.INVALID_EXPR {
				value, _ = build_value_source(ctx, &body, statement.body, local.type, statement.span, statement.label, statement.value_control_flow)
			} else {
				value = build_expr(
					checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
					local.type, ctx.pkg, ctx.file,
				)
				value = coerce_expr(checker, value, local.type, statement.span)
			}
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Assignment, span = statement.span, expr = value, local = local.id,
				target = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
			})
			ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
		case .Return:
			if ctx.defer_depth > 0 {
				id := source.add(checker.diagnostics, statement.span, "cannot 'return' inside a 'defer'")
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			if statement.expr == ast.INVALID_EXPR && !statement.value_control_flow {
				if types.kind(ctx.result, store) == .Fallible &&
				   types.is_void(types.fallible_success(ctx.result, store)) {
					flush_defers(ctx, &body, 0)
					value := fallible_aggregate(checker, statement.span, ctx.result, hir.INVALID_EXPR, false)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Return, span = statement.span, expr = value,
						local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
					})
				} else if !types.is_void(ctx.result) {
					id := source.add(checker.diagnostics, statement.span, "non-void function must return a value")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
				} else {
					flush_defers(ctx, &body, 0)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Return, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
					})
				}
				continue
			}
			if types.is_void(ctx.result) {
				id := source.add(checker.diagnostics, statement.span, "void function cannot return a value")
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			value := hir.INVALID_EXPR
			error_exit := false
			error_value := hir.INVALID_EXPR
			if statement.value_control_flow {
				if types.kind(ctx.result, store) == .Fallible {
					success := types.fallible_success(ctx.result, store)
					value, _ = build_value_source(ctx, &body, statement.body, success, statement.span, symbol.INVALID, statement.value_control_flow)
					value = coerce_expr(checker, value, success, statement.span)
					value = fallible_aggregate(checker, statement.span, ctx.result, value, false)
				} else {
					value, _ = build_value_source(ctx, &body, statement.body, ctx.result, statement.span, symbol.INVALID, statement.value_control_flow)
					value = coerce_expr(checker, value, ctx.result, statement.span)
				}
			} else if types.kind(ctx.result, store) == .Fallible {
				success := types.fallible_success(ctx.result, store)
				error_type := types.fallible_error(ctx.result, store)
				expr_ast := checker.ast_module.exprs[statement.expr]
				if expr_ast.kind == .Enum_Literal {
					success_has := types.sum_has_name(store, success, u32(expr_ast.name))
					error_has := types.sum_has_name(store, error_type, u32(expr_ast.name))
					if error_has && !success_has {
						error_exit = true
					} else if error_has && success_has {
						id := source.add(checker.diagnostics, expr_ast.span, "ambiguous fallible return member")
						value = invalid_hir_expr(checker, expr_ast.span, id, ctx.result)
					}
				} else if expr_ast.kind == .Struct_Literal {
					target_pkg, available := expr_package(checker, expr_ast, ctx.pkg, ctx.file, true)
					named := types.find_named(store, u32(target_pkg), u32(expr_ast.name), file=u32(expr_lookup_file(expr_ast, ctx.file))) if available else types.INVALID
					named = types.resolve_alias(named, store)
					error_exit = can_implicitly_convert_type(checker, named, error_type)
				} else {
					probe := build_expr(
						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
						types.INVALID, ctx.pkg, ctx.file,
					)
					probe_type := checker.module.exprs[probe].type
					if can_implicitly_convert_type(checker, probe_type, error_type) &&
					   !can_implicitly_convert_type(checker, probe_type, success) {
						error_exit = true
						value = probe
					} else if can_implicitly_convert_type(checker, probe_type, success) {
						value = probe
					}
				}
				if value == hir.INVALID_EXPR {
					expected := error_type if error_exit else success
					value = build_expr(
						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
						expected, ctx.pkg, ctx.file,
					)
				}
				expected := error_type if error_exit else success
				value = coerce_expr(checker, value, expected, statement.span)
				if error_exit && len(ctx.defers^) > 0 && checker.module.exprs[value].kind != .Invalid {
					tmp := append_tracked_local(
						ctx.hir_locals, ctx.local_spans, ctx.local_used, ctx.local_warnable,
						hir.Local{name=checker.sink_symbol, type=error_type, mutable=false}, source.Span{},
					)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind=.Declaration, span=statement.span, local=tmp, expr=value,
						diagnostic=source.INVALID_DIAGNOSTIC,
					})
					error_value = hir.expr_id(len(checker.module.exprs))
					append(&checker.module.exprs, hir.Expr{
						kind=.Local, span=statement.span, type=error_type, target=hir.local_ref(tmp),
					})
					value = error_value
				}
				value = fallible_aggregate(checker, statement.span, ctx.result, value, error_exit)
			} else {
				value = build_expr(
					checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
					ctx.result, ctx.pkg, ctx.file,
				)
				value = coerce_expr(checker, value, ctx.result, statement.span)
			}
			ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
			// Run deferred statements before returning, but capture the return value
			// first (spill it to a temp) so a defer that mutates the returned local
			// can't change what is returned — Zig evaluates the return value, then
			// runs defers.
			if len(ctx.defers^) > 0 {
				if !error_exit && checker.module.exprs[value].kind != .Invalid {
					tmp := append_tracked_local(
						ctx.hir_locals,
						ctx.local_spans,
						ctx.local_used,
						ctx.local_warnable,
						hir.Local{name = checker.sink_symbol, type = ctx.result, mutable = false},
						source.Span{},
					)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Declaration, span = statement.span, local = tmp, expr = value,
						diagnostic = source.INVALID_DIAGNOSTIC,
					})
					value = hir.expr_id(len(checker.module.exprs))
					append(&checker.module.exprs, hir.Expr{
						kind = .Local, span = statement.span, type = ctx.result, target = hir.local_ref(tmp),
					})
				}
				flush_defers(ctx, &body, 0, error_exit, error_value)
			}
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Return, span = statement.span, expr = value,
				local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
			})
		case .Expression:
			value := build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
			if diagnostic, invalid := invalid_expr_diagnostic(checker, value); invalid {
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = diagnostic,
				})
				ctx.problematic^ = true
			} else if !types.is_void(checker.module.exprs[value].type) {
				id := source.add(checker.diagnostics, statement.span, "non-void expression result must be consumed or assigned to '_'")
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
			} else {
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Expression, span = statement.span, expr = value,
					local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
				})
			}
		case .If:
			if len(statement.captures) == 0 {
				if selected, comptime_ok := specialization_bool(checker, statement.expr, ctx.pkg, ctx.file); comptime_ok {
					selected_body := statement.body if selected else statement.else_body
					branch := build_block(ctx, selected_body)
					append(&body, ..branch)
					delete(branch, checker.allocator)
					continue
				}
			}
			if len(statement.captures) > 0 {
				ast_operands: [dynamic]ast.Expr_Id
				ast_operands.allocator = checker.allocator
				flatten_conditional_unwrap_operands(checker.ast_module, statement.expr, &ast_operands)

				valid_unwrap := true
				diagnostic := source.INVALID_DIAGNOSTIC
				if len(ast_operands) != len(statement.captures) {
					diagnostic = source.addf(
						checker.diagnostics,
						statement.span,
						"'if' unwrap has %d operands but %d captures",
						len(ast_operands),
						len(statement.captures),
					)
					valid_unwrap = false
				}

				values := make([]hir.Expr_Id, len(ast_operands), checker.allocator)
				child_types := make([]types.Type, len(ast_operands), checker.allocator)
				for operand, index in ast_operands {
					child_types[index] = types.INVALID
					value := build_expr(
						checker, operand, ctx.locals^[:], ctx.global_reads, ctx.calls,
						types.INVALID, ctx.pkg, ctx.file,
					)
					values[index] = value
					value_type := checker.module.exprs[value].type
					if checker.module.exprs[value].kind == .Invalid {
						valid_unwrap = false
						if diagnostic == source.INVALID_DIAGNOSTIC {
							diagnostic = checker.module.exprs[value].diagnostic
						}
					} else if !types.is_optional(value_type, &checker.module.types) {
						diagnostic = source.addf(
							checker.diagnostics,
							checker.ast_module.exprs[operand].span,
							"'if' unwrap requires an optional value (operand %d)",
							index + 1,
						)
						valid_unwrap = false
					} else {
						child_types[index] = types.child_type(value_type, &checker.module.types)
					}
				}

				capture_start := len(ctx.locals^)
				unwraps: [dynamic]hir.Conditional_Unwrap
				unwraps.allocator = checker.allocator
				for capture, index in statement.captures {
					child := child_types[index] if index < len(child_types) else types.INVALID
					local := hir.INVALID_LOCAL
					if capture != checker.sink_symbol {
						if _, duplicate := find_build_local(ctx.locals^[capture_start:], capture); duplicate {
							diagnostic = source.add(
								checker.diagnostics,
								statement.span,
								"'if' unwrap captures must have distinct names",
							)
							valid_unwrap = false
						} else if id := add_shadow_diagnostic(
							checker, statement.span, capture, "capture",
							ctx.pkg, ctx.file, ctx.locals^[:capture_start], ctx.loop_labels^[:], ctx.yield_targets^[:],
						); id != source.INVALID_DIAGNOSTIC {
							diagnostic = id
							valid_unwrap = false
						}
						local = append_build_local(ctx, capture, child, false, statement.span)
					}
					if index < len(values) {
						append(&unwraps, hir.Conditional_Unwrap{expr=values[index], local=local})
					}
				}

				guard := hir.INVALID_EXPR
				if statement.guard != ast.INVALID_EXPR {
					guard = build_expr(
						checker, statement.guard, ctx.locals^[:], ctx.global_reads, ctx.calls,
						types.BOOL, ctx.pkg, ctx.file,
					)
					if checker.module.exprs[guard].kind == .Invalid {
						valid_unwrap = false
						if diagnostic == source.INVALID_DIAGNOSTIC {
							diagnostic = checker.module.exprs[guard].diagnostic
						}
					} else if !types.is_bool(checker.module.exprs[guard].type) {
						diagnostic = source.add(
							checker.diagnostics,
							checker.ast_module.exprs[statement.guard].span,
							"'if' unwrap guard must be a bool",
						)
						valid_unwrap = false
					}
				}

				then_body := build_block(ctx, statement.body, capture_start)
				resize(ctx.locals, capture_start)
				else_body: []hir.Stmt_Id = nil
				if statement.else_body != nil {
					else_body = build_block(ctx, statement.else_body)
				}
				append(&body, hir.stmt_id(len(checker.module.statements)))
				if valid_unwrap {
					append(&checker.module.statements, hir.Stmt{
						kind=.If,
						span=statement.span,
						expr=hir.INVALID_EXPR,
						unwraps=unwraps[:],
						guard=guard,
						then_body=then_body,
						else_body=else_body,
						local=hir.INVALID_LOCAL,
						target=hir.INVALID_EXPR,
						diagnostic=source.INVALID_DIAGNOSTIC,
					})
				} else {
					delete(unwraps)
					delete(then_body, checker.allocator)
					delete(else_body, checker.allocator)
					append(&checker.module.statements, hir.Stmt{
						kind=.Trap,
						span=statement.span,
						expr=hir.INVALID_EXPR,
						guard=hir.INVALID_EXPR,
						local=hir.INVALID_LOCAL,
						target=hir.INVALID_EXPR,
						diagnostic=diagnostic,
					})
					ctx.problematic^ = true
				}
				delete(values, checker.allocator)
				delete(child_types, checker.allocator)
				delete(ast_operands)
				continue
			}
			condition := build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.BOOL, ctx.pkg, ctx.file)
			if checker.module.exprs[condition].kind != .Invalid && !types.is_bool(checker.module.exprs[condition].type) {
				id := source.add(checker.diagnostics, statement.span, "'if' condition must be a bool")
				condition = invalid_hir_expr(checker, statement.span, id, types.BOOL)
				ctx.problematic^ = true
			}
			then_body := build_block(ctx, statement.body)
			else_body: []hir.Stmt_Id = nil
			if statement.else_body != nil {
				else_body = build_block(ctx, statement.else_body)
			}
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .If, span = statement.span, expr = condition,
				guard = hir.INVALID_EXPR,
				then_body = then_body, else_body = else_body,
				local = hir.INVALID_LOCAL, target = hir.INVALID_EXPR,
				diagnostic = source.INVALID_DIAGNOSTIC,
			})
			ctx.problematic^ = ctx.problematic^ || checker.module.exprs[condition].kind == .Invalid
		case .While:
			condition := build_expr(
				checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
				types.BOOL, ctx.pkg, ctx.file,
			)
			if checker.module.exprs[condition].kind != .Invalid &&
			   !types.is_bool(checker.module.exprs[condition].type) {
				id := source.add(checker.diagnostics, statement.span, "'while' condition must be a bool")
				condition = invalid_hir_expr(checker, statement.span, id, types.BOOL)
				ctx.problematic^ = true
			}
			if id := add_label_shadow_diagnostic(ctx, statement.span, statement.label);
			   id != source.INVALID_DIAGNOSTIC {
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
			}
			append(ctx.loop_defer_starts, len(ctx.defers^))
			append(ctx.loop_labels, statement.label)
			append(ctx.loop_is_loop, true)
			loop_body := build_block(ctx, statement.body)
			pop(ctx.loop_is_loop)
			pop(ctx.loop_labels)
			pop(ctx.loop_defer_starts)
			update := hir.INVALID_STMT
			if statement.update != ast.INVALID_STMT {
				update_ast := [1]ast.Stmt_Id{statement.update}
				update_body := build_block(ctx, update_ast[:])
				if len(update_body) > 0 {
					update = update_body[0]
				}
				delete(update_body, checker.allocator)
			}
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind=.While,
				span=statement.span,
				label=statement.label,
				expr=condition,
				then_body=loop_body,
				update=update,
				local=hir.INVALID_LOCAL,
				target=hir.INVALID_EXPR,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
			ctx.problematic^ = ctx.problematic^ || checker.module.exprs[condition].kind == .Invalid
		case .For:
			if statement.expand {
				if statement.pointer_capture {
					id := source.add(checker.diagnostics, statement.span, "expand for does not support pointer captures")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{kind=.Trap, span=statement.span, diagnostic=id})
					ctx.problematic^ = true
					continue
				}
				bindings, expand_error := expand_field_bindings(checker, statement.expr, statement.name, ctx.pkg, ctx.file, true)
				if expand_error != .None && expand_error != .Diagnosed {
					message := "expand for requires a comptime tuple, fixed array, range, slice, or reflection value"
					if expand_error == .Quota {
						message = "expand for expansion exceeds the compile-time evaluation quota"
					}
					id := source.add(
						checker.diagnostics, statement.span,
						message,
					)
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{kind=.Trap, span=statement.span, diagnostic=id})
					ctx.problematic^ = true
				} else if expand_error == .None {
					for binding, expand_index in bindings {
						binding_start := push_expand_binding(checker, binding, statement.index_name, expand_index, statement_id)
						iteration: [dynamic]ast.Stmt_Id
						iteration.allocator = checker.allocator
						diagnostic := source.INVALID_DIAGNOSTIC
						control := flatten_expand_iteration(
							checker, statement.body, ctx.pkg, ctx.file, &iteration, statement.label, &diagnostic,
						)
						if control == .Invalid {
							append(&body, hir.stmt_id(len(checker.module.statements)))
							append(&checker.module.statements, hir.Stmt{
								kind=.Trap, span=statement.span, diagnostic=diagnostic,
							})
							ctx.problematic^ = true
						} else {
							expanded := build_block(ctx, iteration[:])
							append(&body, ..expanded)
							delete(expanded, checker.allocator)
						}
						delete(iteration)
						pop_expand_binding(checker, binding_start)
						if control == .Break || control == .Invalid {
							break
						}
					}
				}
				delete(bindings, checker.allocator)
				continue
			}
			iterable := build_expr(
				checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
				types.INVALID, ctx.pkg, ctx.file,
			)
			iterable_type := checker.module.exprs[iterable].type
			capture_type := types.I64
			iterator_type := types.INVALID
			valid_loop := checker.module.exprs[iterable].kind != .Invalid
			diagnostic := source.INVALID_DIAGNOSTIC
			is_range := types.is_range(iterable_type, &checker.module.types)
			if is_range {
				capture_type = types.child_type(iterable_type, &checker.module.types)
				if statement.pointer_capture {
					diagnostic = source.add(checker.diagnostics, statement.span, "range loops do not support pointer captures")
					valid_loop = false
				}
				if symbol.is_valid(statement.index_name) {
					diagnostic = source.add(checker.diagnostics, statement.span, "range loops do not support index captures")
					valid_loop = false
				}
			} else {
				item, ok := sequence_item(iterable_type, &checker.module.types)
				if !ok {
					diagnostic = source.add(
						checker.diagnostics,
						statement.span,
						"for-loop iterable must be a range, array, slice, or pointer-to-array",
					)
					valid_loop = false
				} else {
					iterator_type = types.pointer(
						&checker.module.types,
						item.child,
						item.mutable,
						true,
						item.has_sentinel,
						item.sentinel,
					)
					capture_type = item.child
					if statement.pointer_capture {
						_, _, array_pointer_ok := types.array_pointer(iterable_type, &checker.module.types)
						if !types.is_slice(iterable_type, &checker.module.types) && !array_pointer_ok {
							diagnostic = source.add(
								checker.diagnostics,
								statement.span,
								"pointer capture over an array requires a pointer-to-array such as '&items'",
							)
							valid_loop = false
						}
						capture_type = types.pointer(&checker.module.types, item.child, item.mutable, false)
					}
				}
			}

			capture_start := len(ctx.locals^)
			if statement.name != checker.sink_symbol {
				if id := add_shadow_diagnostic(
					checker, statement.span, statement.name, "capture",
					ctx.pkg, ctx.file, ctx.locals^[:], ctx.loop_labels^[:], ctx.yield_targets^[:],
				); id != source.INVALID_DIAGNOSTIC {
					diagnostic = id
					valid_loop = false
				}
			}
			item_local := append_build_local(ctx, statement.name, capture_type, false, statement.span)
			index_local := hir.INVALID_LOCAL
			if symbol.is_valid(statement.index_name) {
				if statement.index_name == statement.name {
					diagnostic = source.add(checker.diagnostics, statement.span, "for-loop captures must have distinct names")
					valid_loop = false
				} else if id := add_shadow_diagnostic(
					checker, statement.span, statement.index_name, "capture",
					ctx.pkg, ctx.file, ctx.locals^[:capture_start], ctx.loop_labels^[:], ctx.yield_targets^[:],
				); id != source.INVALID_DIAGNOSTIC {
					diagnostic = id
					valid_loop = false
				} else {
					index_local = append_build_local(ctx, statement.index_name, types.USIZE, false, statement.span)
				}
			}
			if id := add_label_shadow_diagnostic(ctx, statement.span, statement.label);
			   id != source.INVALID_DIAGNOSTIC {
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
			}
			append(ctx.loop_defer_starts, len(ctx.defers^))
			append(ctx.loop_labels, statement.label)
			append(ctx.loop_is_loop, true)
			loop_body := build_block(ctx, statement.body, capture_start)
			pop(ctx.loop_is_loop)
			pop(ctx.loop_labels)
			pop(ctx.loop_defer_starts)
			resize(ctx.locals, capture_start)

			append(&body, hir.stmt_id(len(checker.module.statements)))
			if valid_loop {
				append(&checker.module.statements, hir.Stmt{
					kind=.For,
					span=statement.span,
					label=statement.label,
					local=item_local,
					index_local=index_local,
					expr=iterable,
					iterator_type=iterator_type,
					pointer_capture=statement.pointer_capture,
					then_body=loop_body,
					update=hir.INVALID_STMT,
					target=hir.INVALID_EXPR,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			} else {
				delete(loop_body, checker.allocator)
				append(&checker.module.statements, hir.Stmt{
					kind=.Trap,
					span=statement.span,
					local=hir.INVALID_LOCAL,
					index_local=hir.INVALID_LOCAL,
					expr=hir.INVALID_EXPR,
					target=hir.INVALID_EXPR,
					diagnostic=diagnostic,
				})
				ctx.problematic^ = true
			}
		case .Break, .Continue:
			// `break :L` / `continue :L` targets the innermost enclosing loop labeled `L`;
			// an unlabeled one targets the innermost loop. Inside a `defer`, `loop_floor`
			// hides the loops opened outside the defer.
			target_index := -1
			if symbol.is_valid(statement.label) {
				// `break :L` targets a labeled loop or block; `continue :L` only a loop.
				for i := len(ctx.loop_labels^) - 1; i >= ctx.loop_floor; i -= 1 {
					if ctx.loop_labels^[i] == statement.label &&
					   (ctx.loop_is_loop^[i] || statement.kind == .Break) {
						target_index = i
						break
					}
				}
			} else {
				// Unlabeled `break`/`continue` targets the innermost loop, skipping blocks.
				for i := len(ctx.loop_defer_starts^) - 1; i >= ctx.loop_floor; i -= 1 {
					if ctx.loop_is_loop^[i] {
						target_index = i
						break
					}
				}
			}
			if target_index < 0 {
				keyword := "break" if statement.kind == .Break else "continue"
				id: source.Diagnostic_Id
				if symbol.is_valid(statement.label) {
					id = source.addf(checker.diagnostics, statement.span,
						"no enclosing loop is labeled '%s'", symbol_text(checker, statement.label))
				} else {
					id = source.addf(checker.diagnostics, statement.span, "'%s' outside of a loop", keyword)
				}
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			// Exit the loop body and any blocks between here and the target loop: run their
			// deferred statements down to and including the target loop body.
			flush_defers(ctx, &body, ctx.loop_defer_starts^[target_index])
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Break if statement.kind == .Break else .Continue,
				span = statement.span, label = statement.label, expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
			})
		case .Block:
			if symbol.is_valid(statement.label) {
				// A labeled block statement (`blk: { … break :blk … }`): a break target
				// with an exit-label boundary, built as a HIR `.Block`. Not a loop, so
				// unlabeled `break`/`continue` and `continue :blk` skip it.
				if id := add_label_shadow_diagnostic(ctx, statement.span, statement.label);
				   id != source.INVALID_DIAGNOSTIC {
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
				}
				append(ctx.loop_defer_starts, len(ctx.defers^))
				append(ctx.loop_labels, statement.label)
				append(ctx.loop_is_loop, false)
				built := build_block(ctx, statement.body)
				pop(ctx.loop_is_loop)
				pop(ctx.loop_labels)
				pop(ctx.loop_defer_starts)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Block, span = statement.span, label = statement.label,
					then_body = built, local = hir.INVALID_LOCAL, target = hir.INVALID_EXPR,
					diagnostic = source.INVALID_DIAGNOSTIC,
				})
				continue
			}
			// A bare `{ ... }` scope: build it (its own locals/defers are scoped by
			// the recursive call) and splice its statements in.
			block := build_block(ctx, statement.body)
			for stmt in block {
				append(&body, stmt)
			}
			delete(block, checker.allocator)
		case .Yield:
			// A labeled `yield :blk x` exits the value-loop labeled `blk`: assign the
			// result slot, then `break` (which flushes defers down to the loop body and
			// branches to its exit). HIR holds no `.Yield` — it becomes Assignment + Break.
			if symbol.is_valid(statement.label) {
				target_index := -1
				for i := len(ctx.yield_targets^) - 1; i >= 0; i -= 1 {
					if ctx.yield_targets^[i].label == statement.label {
						target_index = i
						break
					}
				}
				if target_index < 0 {
					id := source.addf(checker.diagnostics, statement.span,
						"no enclosing value loop or block is labeled '%s'", symbol_text(checker, statement.label))
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				target := &ctx.yield_targets^[target_index]
				yielded := hir.INVALID_EXPR
				if statement.value_control_flow {
					yielded, _ = build_value_source(ctx, &body, statement.body, target.slot_type, statement.span, symbol.INVALID, statement.value_control_flow)
				} else {
					yielded = build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, target.slot_type, ctx.pkg, ctx.file)
				}
				if yielded != hir.INVALID_EXPR && types.is_void(checker.module.exprs[yielded].type) {
					id := source.add(checker.diagnostics, statement.span, "'yield' expression must produce a non-void value")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				yielded = resolve_loop_slot(ctx, target, yielded, checker.module.exprs[yielded].type if yielded != hir.INVALID_EXPR else types.INVALID, statement.span)
				if target.slot == hir.INVALID_LOCAL || yielded == hir.INVALID_EXPR {
					id := source.add(checker.diagnostics, statement.span,
						"could not determine the value loop's yield type; annotate the binding or yield a concrete value first")
					append(&body, hir.stmt_id(len(checker.module.statements)))
					append(&checker.module.statements, hir.Stmt{
						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
						local = hir.INVALID_LOCAL, diagnostic = id,
					})
					ctx.problematic^ = true
					continue
				}
				// slot = value (the slot is un-nameable, so no defer can mutate it; no spill).
				emit_slot_assign(checker, &body, target.slot, yielded, statement.span)
				// Exit the target: flush defers down to its body, then a labeled break.
				flush_defers(ctx, &body, target.defer_floor)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Break, span = statement.span, label = target.label, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = source.INVALID_DIAGNOSTIC,
				})
				continue
			}
			// An unlabeled yield reaching here is misplaced: a legitimate trailing yield
			// is peeled by the value builders (value block / if branch / loop fall-through).
			id := source.add(
				checker.diagnostics, statement.span,
				"'yield' is only valid as the final statement of a value block, or as 'yield :label' inside a labeled value loop or block",
			)
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL, diagnostic = id,
			})
			ctx.problematic^ = true
		case .Defer:
			deferred := checker.ast_module.statements[statement.update]
			if deferred.kind == .Return || deferred.kind == .Break ||
			   deferred.kind == .Continue || deferred.kind == .Defer {
				keyword := "return"
				if deferred.kind == .Break    { keyword = "break" }
				if deferred.kind == .Continue { keyword = "continue" }
				if deferred.kind == .Defer    { keyword = "defer" }
				id := source.addf(checker.diagnostics, statement.span, "cannot defer a '%s' statement", keyword)
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				ctx.problematic^ = true
				continue
			}
			if statement.error_only && types.kind(ctx.result, store) != .Fallible {
				id := source.add(checker.diagnostics, statement.span, "'errdefer' requires an enclosing fallible function")
				append(&body, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind=.Trap, span=statement.span, expr=hir.INVALID_EXPR,
					local=hir.INVALID_LOCAL, diagnostic=id,
				})
				ctx.problematic^ = true
				continue
			}
			// Build the deferred statement once, guarded so a `return` inside it is
			// rejected and `break`/`continue` only target loops opened within the
			// defer; its hir is replayed at each scope exit, not emitted here.
			capture_start := len(ctx.locals^)
			capture := hir.INVALID_LOCAL
			if statement.error_only && len(statement.captures) > 0 &&
			   statement.captures[0] != checker.sink_symbol {
				capture = append_build_local(
					ctx, statement.captures[0], types.fallible_error(ctx.result, store), false, statement.span,
				)
			}
			saved_floor := ctx.loop_floor
			ctx.defer_depth += 1
			ctx.loop_floor = len(ctx.loop_defer_starts^)
			entry := build_block(ctx, []ast.Stmt_Id{statement.update})
			ctx.loop_floor = saved_floor
			ctx.defer_depth -= 1
			resize(ctx.locals, capture_start)
			append(ctx.defers, Defer_Entry{
				body=entry, error_only=statement.error_only, capture=capture,
			})
		case .Match:
			if selected_body, capture, has_capture, comptime_ok := specialization_match_body(checker, statement, ctx.pkg, ctx.file); comptime_ok {
				if has_capture {
					append(&checker.static_bindings, capture)
				}
				expanded := build_block(ctx, selected_body)
				if has_capture {
					_ = pop(&checker.static_bindings)
				}
				append(&body, ..expanded)
				delete(expanded, checker.allocator)
			} else {
				build_match(ctx, &body, statement)
			}
		case .Match_Arm:
			// Arms are only reachable through their enclosing `.Match`; one on its own
			// is a parser bug.
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL,
				diagnostic = source.add(checker.diagnostics, statement.span, "unexpected match arm outside 'match'"),
			})
			ctx.problematic^ = true
		case .Invalid:
			append(&body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL, diagnostic = statement.diagnostic,
			})
			ctx.problematic^ = true
		}
	}
	// Normal fall-through exit: run this block's own deferred statements, unless
	// every path already exited early (return/break/continue) — that would only
	// emit unreachable duplicates. A value block (`close=false`) skips this so its
	// caller can capture the yielded value before flushing the block's defers.
	if close {
		if !all_paths_exit(&checker.module, body[:]) {
			flush_defers(ctx, &body, defer_start)
		}
		// Free this block's deferred-statement entry slices (their stmt ids were
		// already replayed at every path that can leave this block) and pop the frame.
		for i := defer_start; i < len(ctx.defers^); i += 1 {
			delete(ctx.defers^[i].body, checker.allocator)
		}
		resize(ctx.defers, defer_start)
		resize(ctx.locals, scope_start)
	}
	return body[:]
}

// build_value_block builds a `{ ... yield v }` value block whose final statement
// must be a `yield`. Catch handlers may instead exit on every path or complete with
// void, in which case `allow_exit` leaves the fallback expression invalid. Otherwise
// it builds the leading statements inline, evaluates the yield in their scope, then
// captures the value before running defers. `expected` is the binding's type (INVALID
// for an untyped `::`).
build_value_block :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	body_stmts: []ast.Stmt_Id,
	expected: types.Type,
	span: source.Span,
	allow_exit := false,
) -> (value: hir.Expr_Id, value_type: types.Type) {
	checker := ctx.checker
	n := len(body_stmts)
	if n == 0 || checker.ast_module.statements[body_stmts[n - 1]].kind != .Yield {
		// Build whatever is there so inner errors (and misplaced yields) surface, then
		// report the missing trailing yield.
		inner := build_block(ctx, body_stmts)
		for s in inner {
			append(body, s)
		}
		if allow_exit && (types.is_void(expected) || all_paths_exit(&checker.module, inner)) {
			delete(inner, checker.allocator)
			return hir.INVALID_EXPR, expected
		}
		delete(inner, checker.allocator)
		id := source.add(checker.diagnostics, span, "a value block must end with an explicit 'yield'")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}
	scope_start := len(ctx.locals^)
	defer_start := len(ctx.defers^)
	// Leading statements keep the scope open (close=false) so the yield can still see
	// the block's locals; any nested `yield` hits the erroring `.Yield` switch case.
	leading := build_block(ctx, body_stmts[:n - 1], close = false)
	for s in leading {
		append(body, s)
	}
	delete(leading, checker.allocator)

	yield_stmt := checker.ast_module.statements[body_stmts[n - 1]]
	if yield_stmt.value_control_flow {
		value, value_type = build_value_source(ctx, body, yield_stmt.body, expected, yield_stmt.span, symbol.INVALID, yield_stmt.value_control_flow)
	} else {
		value = build_expr(
			checker, yield_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
			expected, ctx.pkg, ctx.file,
		)
		value_type = checker.module.exprs[value].type
	}
	if types.is_void(value_type) {
		id := source.add(checker.diagnostics, yield_stmt.span, "'yield' expression must produce a non-void value")
		value = invalid_hir_expr(checker, yield_stmt.span, id)
		value_type = types.INVALID
		ctx.problematic^ = true
	} else if types.is_void(expected) {
		id := source.add(checker.diagnostics, yield_stmt.span, "void value context must fall through instead of yielding")
		value = invalid_hir_expr(checker, yield_stmt.span, id)
		value_type = types.INVALID
		ctx.problematic^ = true
	} else if is_runtime_type(checker, expected) {
		value = coerce_expr(checker, value, expected, yield_stmt.span)
		value_type = checker.module.exprs[value].type
	}
	ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid

	// Run the block's deferred statements before the value escapes, but capture the
	// value first (spill to a temp) so a defer can't change what is yielded — the same
	// rule as `return`.
	if len(ctx.defers^) > defer_start {
		if checker.module.exprs[value].kind != .Invalid {
			tmp := append_tracked_local(
				ctx.hir_locals,
				ctx.local_spans,
				ctx.local_used,
				ctx.local_warnable,
				hir.Local{name = checker.sink_symbol, type = value_type, mutable = false},
				source.Span{},
			)
			append(body, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Declaration, span = yield_stmt.span, local = tmp, expr = value,
				diagnostic = source.INVALID_DIAGNOSTIC,
			})
			value = hir.expr_id(len(checker.module.exprs))
			append(&checker.module.exprs, hir.Expr{
				kind = .Local, span = yield_stmt.span, type = value_type, target = hir.local_ref(tmp),
			})
		}
		flush_defers(ctx, body, defer_start)
	}
	// Close the scope (build_block left it open for us).
	for i := defer_start; i < len(ctx.defers^); i += 1 {
		delete(ctx.defers^[i].body, checker.allocator)
	}
	resize(ctx.defers, defer_start)
	resize(ctx.locals, scope_start)
	return value, value_type
}

// build_value_source feeds a declaration/assignment RHS into the right value builder.
// Braced blocks are plain value blocks and must end in their own `yield`; bare RHS
// control flow (`x :: match ...`) sets `value_control_flow` and can produce directly.
// `label` is the labeled-block label (INVALID otherwise).
build_value_source :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	body_stmts: []ast.Stmt_Id,
	expected: types.Type,
	span: source.Span,
	label := symbol.INVALID,
	value_control_flow := false,
	allow_exit := false,
) -> (value: hir.Expr_Id, value_type: types.Type) {
	checker := ctx.checker
	if symbol.is_valid(label) {
		return build_value_labeled_block(ctx, body, body_stmts, label, expected, span)
	}
	if value_control_flow && len(body_stmts) == 1 {
		#partial switch checker.ast_module.statements[body_stmts[0]].kind {
		case .If:
			return build_value_if(ctx, body, body_stmts[0], expected, span)
		case .For, .While:
			return build_value_loop(ctx, body, body_stmts[0], expected, span)
		case .Match:
			return build_value_match(ctx, body, body_stmts[0], expected, span)
		}
	}
	return build_value_block(ctx, body, body_stmts, expected, span, allow_exit)
}

// new_value_slot allocates a fresh, un-nameable mutable local to hold a value-if/loop
// result. Branches/iterations assign it; the construct's value is a read of it.
new_value_slot :: proc(ctx: ^Build_Ctx, slot_type: types.Type) -> hir.Local_Id {
	return append_tracked_local(
		ctx.hir_locals,
		ctx.local_spans,
		ctx.local_used,
		ctx.local_warnable,
		hir.Local{name = ctx.checker.sink_symbol, type = slot_type, mutable = true},
		source.Span{},
	)
}

// slot_read builds a `.Local` read of a result slot.
slot_read :: proc(checker: ^Checker, slot: hir.Local_Id, slot_type: types.Type, span: source.Span) -> hir.Expr_Id {
	id := hir.expr_id(len(checker.module.exprs))
	append(&checker.module.exprs, hir.Expr{
		kind = .Local, span = span, type = slot_type, target = hir.local_ref(slot),
	})
	return id
}

// emit_slot_assign appends a bare-local `slot = value` assignment to `out`.
emit_slot_assign :: proc(checker: ^Checker, out: ^[dynamic]hir.Stmt_Id, slot: hir.Local_Id, value: hir.Expr_Id, span: source.Span) {
	append(out, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Assignment, span = span, expr = value, local = slot,
		target = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
	})
}

string_peer_slice_type :: proc(checker: ^Checker, value: types.Type) -> types.Type {
	pointer, array, ok := types.array_pointer(value, &checker.module.types)
	if ok && !pointer.mutable && !array.mutable &&
	   array.child == types.U8 && array.has_sentinel && array.sentinel == 0 {
		return types.slice(&checker.module.types, types.U8, false, true, 0)
	}
	return types.INVALID
}

adopt_value_slot :: proc(
	ctx: ^Build_Ctx,
	slot: ^hir.Local_Id,
	slot_type: ^types.Type,
	value: hir.Expr_Id,
	vtype: types.Type,
	span: source.Span,
) -> hir.Expr_Id {
	checker := ctx.checker
	if slot^ == hir.INVALID_LOCAL {
		peer := string_peer_slice_type(checker, vtype)
		if types.is_valid(peer) {
			slot_type^ = peer
			slot^ = new_value_slot(ctx, slot_type^)
			return coerce_expr(checker, value, slot_type^, span)
		}
		slot_type^ = vtype
		slot^ = new_value_slot(ctx, slot_type^)
		return value
	}
	return coerce_expr(checker, value, slot_type^, span)
}

// build_value_if turns `if c { … yield A } else { … yield B }` into a result slot
// each branch assigns, read after the if. Every path must yield: a mandatory `else`,
// each branch ends in `yield`, and all branches share a type (the first establishes it
// when untyped; later branches coerce). HIR holds an ordinary `.If` + a `.Local` read.
build_value_if :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	if_id: ast.Stmt_Id,
	expected: types.Type,
	span: source.Span,
) -> (value: hir.Expr_Id, value_type: types.Type) {
	checker := ctx.checker
	slot := hir.INVALID_LOCAL
	slot_type := types.INVALID
	if is_runtime_type(checker, expected) {
		slot_type = expected
		slot = new_value_slot(ctx, slot_type)
	}
	subtree: [dynamic]hir.Stmt_Id
	subtree.allocator = checker.allocator
	ok := emit_value_if(ctx, &subtree, if_id, &slot, &slot_type, span)
	if !ok || slot == hir.INVALID_LOCAL {
		for s in subtree {
			append(body, s)
		}
		delete(subtree)
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, source.INVALID_DIAGNOSTIC), types.INVALID
	}
	// The slot's poison declaration precedes the if; every path assigns it.
	append(body, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Declaration, span = span, local = slot, expr = hir.INVALID_EXPR,
		diagnostic = source.INVALID_DIAGNOSTIC,
	})
	for s in subtree {
		append(body, s)
	}
	delete(subtree)
	value = slot_read(checker, slot, slot_type, span)
	return value, slot_type
}

// emit_value_if builds one `if`/`else if`/`else` level of a value-if, appending the
// assembled `.If` to `out`. `slot`/`slot_type` thread through so the first branch can
// fix an untyped slot and `else if` chains share it.
emit_value_if :: proc(
	ctx: ^Build_Ctx,
	out: ^[dynamic]hir.Stmt_Id,
	if_id: ast.Stmt_Id,
	slot: ^hir.Local_Id,
	slot_type: ^types.Type,
	span: source.Span,
) -> bool {
	checker := ctx.checker
	if_stmt := checker.ast_module.statements[if_id]

	condition := hir.INVALID_EXPR
	guard := hir.INVALID_EXPR
	unwraps: []hir.Conditional_Unwrap = nil
	capture_start := len(ctx.locals^)

	if len(if_stmt.captures) > 0 {
		// Unwrap value-if (`name :: if opt |v| { yield v } else { yield 0 }`): mirror the
		// build-pass unwrap arm to bind captures + guard; each branch then assigns the slot
		// like any other branch, and the HIR `.If` carries the unwraps (lowering handles it).
		ast_operands: [dynamic]ast.Expr_Id
		ast_operands.allocator = checker.allocator
		flatten_conditional_unwrap_operands(checker.ast_module, if_stmt.expr, &ast_operands)
		ok := true
		if len(ast_operands) != len(if_stmt.captures) {
			source.addf(checker.diagnostics, if_stmt.span,
				"'if' unwrap has %d operands but %d captures", len(ast_operands), len(if_stmt.captures))
			ok = false
		}
		values := make([]hir.Expr_Id, len(ast_operands), checker.allocator)
		child_types := make([]types.Type, len(ast_operands), checker.allocator)
		for operand, index in ast_operands {
			child_types[index] = types.INVALID
			v := build_expr(checker, operand, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
			values[index] = v
			vt := checker.module.exprs[v].type
			if checker.module.exprs[v].kind == .Invalid {
				ok = false
			} else if !types.is_optional(vt, &checker.module.types) {
				source.addf(checker.diagnostics, checker.ast_module.exprs[operand].span,
					"'if' unwrap requires an optional value (operand %d)", index + 1)
				ok = false
			} else {
				child_types[index] = types.child_type(vt, &checker.module.types)
			}
		}
		unwrap_list: [dynamic]hir.Conditional_Unwrap
		unwrap_list.allocator = checker.allocator
		for capture, index in if_stmt.captures {
			child := child_types[index] if index < len(child_types) else types.INVALID
			local := hir.INVALID_LOCAL
			if capture != checker.sink_symbol {
				if _, dup := find_build_local(ctx.locals^[capture_start:], capture); dup {
					source.add(checker.diagnostics, if_stmt.span, "'if' unwrap captures must have distinct names")
					ok = false
				} else if add_shadow_diagnostic(
					checker, if_stmt.span, capture, "capture",
					ctx.pkg, ctx.file, ctx.locals^[:capture_start], ctx.loop_labels^[:], ctx.yield_targets^[:],
				) != source.INVALID_DIAGNOSTIC {
					ok = false
				}
				local = append_build_local(ctx, capture, child, false, if_stmt.span)
			}
			if index < len(values) {
				append(&unwrap_list, hir.Conditional_Unwrap{expr=values[index], local=local})
			}
		}
		if if_stmt.guard != ast.INVALID_EXPR {
			guard = build_expr(checker, if_stmt.guard, ctx.locals^[:], ctx.global_reads, ctx.calls, types.BOOL, ctx.pkg, ctx.file)
			if checker.module.exprs[guard].kind == .Invalid {
				ok = false
			} else if !types.is_bool(checker.module.exprs[guard].type) {
				source.add(checker.diagnostics, checker.ast_module.exprs[if_stmt.guard].span, "'if' unwrap guard must be a bool")
				ok = false
			}
		}
		delete(values, checker.allocator)
		delete(child_types, checker.allocator)
		delete(ast_operands)
		if !ok {
			resize(ctx.locals, capture_start)
			delete(unwrap_list)
			ctx.problematic^ = true
			return false
		}
		unwraps = unwrap_list[:]
	} else {
		condition = build_expr(checker, if_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.BOOL, ctx.pkg, ctx.file)
		if checker.module.exprs[condition].kind != .Invalid && !types.is_bool(checker.module.exprs[condition].type) {
			id := source.add(checker.diagnostics, if_stmt.span, "'if' condition must be a bool")
			condition = invalid_hir_expr(checker, if_stmt.span, id, types.BOOL)
			ctx.problematic^ = true
		}
	}

	// then-branch (unwrap captures, if any, are in scope here, then dropped before else).
	then_body: [dynamic]hir.Stmt_Id
	then_body.allocator = checker.allocator
	then_ok := emit_value_branch(ctx, &then_body, if_stmt.body, slot, slot_type, span)
	resize(ctx.locals, capture_start)
	if !then_ok {
		delete(then_body)
		delete(unwraps)
		return false
	}
	if if_stmt.else_body == nil {
		source.add(checker.diagnostics, if_stmt.span, "an 'if' used as a value must have an 'else' so every path yields")
		delete(then_body)
		delete(unwraps)
		ctx.problematic^ = true
		return false
	}
	else_body: [dynamic]hir.Stmt_Id
	else_body.allocator = checker.allocator
	branch_ok := true
	if len(if_stmt.else_body) == 1 && checker.ast_module.statements[if_stmt.else_body[0]].kind == .If {
		branch_ok = emit_value_if(ctx, &else_body, if_stmt.else_body[0], slot, slot_type, span)
	} else {
		branch_ok = emit_value_branch(ctx, &else_body, if_stmt.else_body, slot, slot_type, span)
	}
	if !branch_ok {
		delete(then_body)
		delete(else_body)
		delete(unwraps)
		return false
	}
	append(out, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .If, span = if_stmt.span, expr = condition, guard = guard, unwraps = unwraps,
		then_body = then_body[:], else_body = else_body[:],
		local = hir.INVALID_LOCAL, target = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
	})
	return true
}

// emit_value_branch builds one branch of a value-if as a value block (leading stmts +
// trailing yield) and appends `slot = <value>`. The first branch of an untyped value-if
// fixes the slot type; later branches coerce to it (a mismatch is the "same type" error).
// A branch that does not yield is valid only if it exits on every path (return/break/
// continue) — it then produces no value and never reaches the slot read.
emit_value_branch :: proc(
	ctx: ^Build_Ctx,
	out: ^[dynamic]hir.Stmt_Id,
	branch_stmts: []ast.Stmt_Id,
	slot: ^hir.Local_Id,
	slot_type: ^types.Type,
	span: source.Span,
) -> bool {
	checker := ctx.checker
	n := len(branch_stmts)
	if n == 1 && checker.ast_module.statements[branch_stmts[0]].kind == .Expression {
		expr_stmt := checker.ast_module.statements[branch_stmts[0]]
		expected := slot_type^ if slot^ != hir.INVALID_LOCAL else types.INVALID
		value := build_expr(checker, expr_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, expected, ctx.pkg, ctx.file)
		if checker.module.exprs[value].kind == .Invalid {
			ctx.problematic^ = true
			return false
		}
		value = adopt_value_slot(ctx, slot, slot_type, value, checker.module.exprs[value].type, span)
		emit_slot_assign(checker, out, slot^, value, span)
		return true
	}
	ends_in_yield := n > 0 &&
		checker.ast_module.statements[branch_stmts[n - 1]].kind == .Yield &&
		!symbol.is_valid(checker.ast_module.statements[branch_stmts[n - 1]].label)
	if !ends_in_yield {
		// Not a value block: only allowed if every path exits (e.g. `else { return -1 }`),
		// in which case it contributes no value to the slot.
		built := build_block(ctx, branch_stmts)
		for s in built {
			append(out, s)
		}
		terminates := all_paths_exit(&checker.module, built)
		delete(built, checker.allocator)
		if terminates {
			return true
		}
		source.add(checker.diagnostics, span,
			"a value branch must end with 'yield' or exit on every path (return/break/continue)")
		ctx.problematic^ = true
		return false
	}
	value, vtype := build_value_block(ctx, out, branch_stmts, slot_type^, span)
	if checker.module.exprs[value].kind == .Invalid {
		return false
	}
	value = adopt_value_slot(ctx, slot, slot_type, value, vtype, span)
	emit_slot_assign(checker, out, slot^, value, span)
	return true
}

// Match_Built_Arm holds one already-built arm: its dispatch condition (`INVALID_EXPR`
// for the terminal `else`/exhaustive arm) and its body statements. The chain is
// assembled backward from these so diagnostics stay in source order.
Match_Built_Arm :: struct {
	condition: hir.Expr_Id,
	body:      []hir.Stmt_Id,
	terminal:  bool,
}

// match_subject_location yields a fresh location expr for the `match` subject: a direct
// read of the value temp, or a deref of the pointer temp when an arm pointer-captures (so
// captures alias the original storage). Either way its lowered address is the subject's.
match_subject_location :: proc(checker: ^Checker, subj_local: hir.Local_Id, is_pointer: bool, subject_type, ptr_type: types.Type, span: source.Span) -> hir.Expr_Id {
	if !is_pointer {
		return slot_read(checker, subj_local, subject_type, span)
	}
	return add_hir_expr(checker, hir.Expr{
		kind = .Deref, span = span, type = subject_type,
		left = slot_read(checker, subj_local, ptr_type, span),
		target = hir.INVALID_REF, right = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
	})
}

// match_or ORs a fresh dispatch comparison into an arm's accumulating condition (for a
// multi-pattern arm), or returns it directly for the first pattern.
match_or :: proc(checker: ^Checker, condition, cmp: hir.Expr_Id, span: source.Span) -> hir.Expr_Id {
	if condition == hir.INVALID_EXPR {
		return cmp
	}
	return add_hir_expr(checker, hir.Expr{
		kind = .Or, span = span, type = types.BOOL,
		left = condition, right = cmp, target = hir.INVALID_REF, diagnostic = source.INVALID_DIAGNOSTIC,
	})
}

// emit_match desugars a `match` into a single subject spill, one dispatch key read, and
// an `if`/`else if` chain. `as_value` (with `slot`/`slot_type`) routes each arm body
// through the value-branch machinery so the construct produces a value; otherwise arm
// bodies are ordinary statement blocks. Returns false (and emits a `.Trap`) on any error.
emit_match :: proc(
	ctx: ^Build_Ctx,
	out: ^[dynamic]hir.Stmt_Id,
	statement: ast.Stmt,
	as_value: bool,
	slot: ^hir.Local_Id,
	slot_type: ^types.Type,
) -> bool {
	checker := ctx.checker
	store := &checker.module.types
	span := statement.span

	fail :: proc(ctx: ^Build_Ctx, out: ^[dynamic]hir.Stmt_Id, span: source.Span, diagnostic: source.Diagnostic_Id) -> bool {
		checker := ctx.checker
		append(out, hir.stmt_id(len(checker.module.statements)))
		append(&checker.module.statements, hir.Stmt{
			kind = .Trap, span = span, expr = hir.INVALID_EXPR,
			local = hir.INVALID_LOCAL, diagnostic = diagnostic,
		})
		ctx.problematic^ = true
		return false
	}

	// 1. Subject. A pointer capture (`|@cap|`) must alias the original storage, so when
	//    any arm requests one we spill the subject's *address* (it must be an addressable
	//    lvalue) and route reads through a deref; otherwise we spill the value as a copy.
	//    Either spill evaluates the subject exactly once.
	subject := build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
	subject_type := checker.module.exprs[subject].type
	if checker.module.exprs[subject].kind == .Invalid {
		return fail(ctx, out, span, checker.module.exprs[subject].diagnostic)
	}

	is_tagged := types.is_tagged_union(subject_type, store)
	is_enum_subject := types.is_enum(subject_type, store)
	if types.is_union(subject_type, store) && !is_tagged {
		return fail(ctx, out, span, source.add(checker.diagnostics, span, "cannot 'match' on an untagged union; it has no tag to dispatch on"))
	}
	if !is_tagged && !is_enum_subject && !types.is_concrete_scalar(subject_type) {
		return fail(ctx, out, span, source.addf(checker.diagnostics, span,
			"'match' subject must be a tagged union, enum, or scalar value, not '%s'", type_label(checker, subject_type)))
	}

	ok := true

	wants_pointer := false
	for arm_id in statement.body {
		arm := checker.ast_module.statements[arm_id]
		if arm.kind == .Match_Arm && arm.pointer_capture {
			wants_pointer = true
			break
		}
	}
	subj_is_pointer := false
	subj_writable := false
	ptr_type := types.INVALID
	if wants_pointer && is_tagged {
		if hir_is_location(checker, subject) {
			subj_is_pointer = true
			subj_writable = hir_location_writable(checker, subject, ctx.locals^[:])
			ptr_type = types.pointer(store, subject_type, subj_writable, false)
		} else {
			source.add(checker.diagnostics, span, "a pointer capture requires an addressable 'match' subject (bind it to a variable first)")
			ok = false
		}
	}

	spill_type := ptr_type if subj_is_pointer else subject_type
	spill_value := subject
	if subj_is_pointer {
		spill_value = add_hir_expr(checker, hir.Expr{
			kind = .Address, span = span, type = ptr_type, left = subject,
			target = hir.INVALID_REF, right = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
		})
	}
	subj_local := append_tracked_local(
		ctx.hir_locals,
		ctx.local_spans,
		ctx.local_used,
		ctx.local_warnable,
		hir.Local{name = checker.sink_symbol, type = spill_type, mutable = false},
		source.Span{},
	)
	append(out, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Declaration, span = span, local = subj_local, expr = spill_value,
		diagnostic = source.INVALID_DIAGNOSTIC,
	})

	// 2. Dispatch key: a tagged union reads its discriminant into its own temp; an enum
	//    or scalar compares the subject directly.
	key_local := subj_local
	key_type := spill_type
	tag_enum := types.INVALID
	if is_tagged {
		tag_enum = types.union_tag_enum(subject_type, store)
		tag_read := add_hir_expr(checker, hir.Expr{
			kind = .Union_Tag, span = span, type = tag_enum,
			left = match_subject_location(checker, subj_local, subj_is_pointer, subject_type, ptr_type, span),
			target = hir.INVALID_REF, right = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
		})
		tag_local := append_tracked_local(
			ctx.hir_locals,
			ctx.local_spans,
			ctx.local_used,
			ctx.local_warnable,
			hir.Local{name = checker.sink_symbol, type = tag_enum, mutable = false},
			source.Span{},
		)
		append(out, hir.stmt_id(len(checker.module.statements)))
		append(&checker.module.statements, hir.Stmt{
			kind = .Declaration, span = span, local = tag_local, expr = tag_read,
			diagnostic = source.INVALID_DIAGNOSTIC,
		})
		key_local = tag_local
		key_type = tag_enum
	}

	// 3. Build each arm (forward, for source-order diagnostics). An arm's `patterns` may
	//    list several alternatives (`.a, .b:` / `0, 1:`); their conditions are OR'd.
	built: [dynamic]Match_Built_Arm
	built.allocator = checker.allocator
	defer delete(built)
	covered: [dynamic]symbol.Id
	covered.allocator = checker.allocator
	defer delete(covered)
	has_else := false
	has_expand := false

	for arm_id in statement.body {
		arm := checker.ast_module.statements[arm_id]
		if arm.kind != .Match_Arm {
			ok = false
			continue
		}
		if has_else || has_expand {
			message := "arms after 'else' are unreachable" if has_else else "arms after 'expand' are unreachable"
			source.add(checker.diagnostics, arm.span, message)
			ok = false
		}
		if arm.expand {
			if !is_tagged && !is_enum_subject {
				source.add(checker.diagnostics, arm.span, "'expand' requires an enum or tagged-union match subject")
				ok = false
				continue
			}
			expected_captures := 1 if is_enum_subject else 2
			if len(arm.captures) == 0 || len(arm.captures) > expected_captures {
				description := "exactly one capture" if is_enum_subject else "one or two captures"
				source.addf(checker.diagnostics, arm.span, "expanded match on '%s' requires %s", type_label(checker, subject_type), description)
				ok = false
			}
			if is_enum_subject && arm.pointer_capture {
				source.add(checker.diagnostics, arm.span, "enum expansion does not support pointer captures")
				ok = false
			}
			if len(arm.captures) > 1 && arm.captures[0] != checker.sink_symbol && arm.captures[0] == arm.captures[1] {
				source.add(checker.diagnostics, arm.span, "expand captures must have distinct names")
				ok = false
			}
			remaining := 0
			member_enum := tag_enum if is_tagged else subject_type
			if is_tagged {
				for field, field_index in types.fields_for(store, subject_type) {
					name := symbol.Id(field.name)
					if contains_name(covered[:], name) {
						continue
					}
					member, found := find_enum_member(checker, member_enum, name)
					if !found {
						ok = false
						continue
					}
					remaining += 1
					append(&covered, name)
					member_expr := enum_member_hir(checker, member_enum, name, arm.span)
					condition := add_hir_expr(checker, hir.Expr{
						kind=.Eq, span=arm.span, type=types.BOOL,
						left=slot_read(checker, key_local, key_type, arm.span), right=member_expr,
						target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
					})
					static_start := len(checker.static_bindings)
					if len(arm.captures) > 1 {
						_ = push_static_integer_binding(checker, arm.captures[1], member_enum, member.value)
					}
					body_arm := arm
					if types.is_void(field.type) && !arm.pointer_capture {
						if len(arm.captures) > 0 {
							_ = push_static_void_binding(checker, arm.captures[0])
						}
						body_arm.captures = nil
					}
					arm_body, body_ok := build_match_arm_body(
						ctx, body_arm, subject_type, subj_local, subj_is_pointer, ptr_type, subj_writable,
						field_index, field.type, as_value, slot, slot_type, span,
					)
					pop_static_bindings(checker, static_start)
					ok = body_ok && ok
					append(&built, Match_Built_Arm{condition=condition, body=arm_body})
				}
			} else {
				for member in types.enum_members_for(store, subject_type) {
					name := symbol.Id(member.name)
					if contains_name(covered[:], name) {
						continue
					}
					remaining += 1
					append(&covered, name)
					member_expr := enum_member_hir(checker, subject_type, name, arm.span)
					condition := add_hir_expr(checker, hir.Expr{
						kind=.Eq, span=arm.span, type=types.BOOL,
						left=slot_read(checker, key_local, key_type, arm.span), right=member_expr,
						target=hir.INVALID_REF, diagnostic=source.INVALID_DIAGNOSTIC,
					})
					static_start := push_static_integer_binding(
						checker, arm.captures[0] if len(arm.captures) > 0 else symbol.INVALID,
						subject_type, member.value,
					)
					body_arm := arm
					body_arm.captures = nil
					arm_body, body_ok := build_match_arm_body(
						ctx, body_arm, subject_type, subj_local, subj_is_pointer, ptr_type, subj_writable,
						-1, types.INVALID, as_value, slot, slot_type, span,
					)
					pop_static_bindings(checker, static_start)
					ok = body_ok && ok
					append(&built, Match_Built_Arm{condition=condition, body=arm_body})
				}
			}
			if remaining == 0 {
				source.add(checker.diagnostics, arm.span, "redundant 'expand': the 'match' already covers every variant")
				ok = false
			}
			has_expand = true
			continue
		}
		is_else := len(arm.patterns) == 0
		condition := hir.INVALID_EXPR
		field_index := -1
		payload_type := types.INVALID
		has_capture := len(arm.captures) > 0

		if is_else {
			if has_capture {
				source.add(checker.diagnostics, arm.span, "the 'else' arm cannot capture a payload")
				ok = false
			}
			has_else = true
		} else if is_tagged || is_enum_subject {
			// The capture payload (if any) must be one type across every listed variant.
			capture_field := -1
			capture_payload := types.INVALID
			capture_conflict := types.INVALID
			member_enum := tag_enum if is_tagged else subject_type
			for pat_id in arm.patterns {
				pattern := checker.ast_module.exprs[pat_id]
				if pattern.kind == .Range {
					source.add(checker.diagnostics, arm.span, "range patterns only apply to scalar 'match' subjects")
					ok = false
					continue
				}
				if pattern.kind != .Enum_Literal {
					source.add(checker.diagnostics, arm.span, "an enum or tagged-union 'match' arm must be a '.variant' pattern")
					ok = false
					continue
				}
				if contains_name(covered[:], pattern.name) {
					source.addf(checker.diagnostics, arm.span, "duplicate 'match' arm for '.%s'", symbol_text(checker, pattern.name))
					ok = false
				} else {
					append(&covered, pattern.name)
				}
				if is_tagged {
					index, field, found := find_struct_field(checker, subject_type, pattern.name)
					if !found {
						source.addf(checker.diagnostics, arm.span, "unknown variant '.%s' on '%s'", symbol_text(checker, pattern.name), type_label(checker, subject_type))
						ok = false
						continue
					}
					if capture_field < 0 {
						capture_field = index
						capture_payload = field.type
					} else if !types.equal(capture_payload, field.type) {
						capture_conflict = field.type
					}
				} else {
					if _, found := find_enum_member(checker, subject_type, pattern.name); !found {
						source.addf(checker.diagnostics, arm.span, "unknown member '.%s' on '%s'", symbol_text(checker, pattern.name), type_label(checker, subject_type))
						ok = false
						continue
					}
				}
				member := enum_member_hir(checker, member_enum, pattern.name, arm.span)
				cmp := add_hir_expr(checker, hir.Expr{
					kind = .Eq, span = arm.span, type = types.BOOL,
					left = slot_read(checker, key_local, key_type, arm.span), right = member,
					target = hir.INVALID_REF, diagnostic = source.INVALID_DIAGNOSTIC,
				})
				condition = match_or(checker, condition, cmp, arm.span)
			}
			if has_capture {
				if !is_tagged {
					source.add(checker.diagnostics, arm.span, "only tagged-union variants can capture a payload")
					ok = false
				} else if types.is_valid(capture_conflict) {
					source.addf(checker.diagnostics, arm.span, "capture group with incompatible types '%s' and '%s'",
						type_label(checker, capture_payload), type_label(checker, capture_conflict))
					ok = false
				} else if types.is_void(capture_payload) {
					source.add(checker.diagnostics, arm.span, "this variant has a void payload; there is nothing to capture")
					ok = false
				} else {
					field_index = capture_field
					payload_type = capture_payload
				}
			}
		} else {
			// Scalar subject: each pattern is a literal or a range, compared to the subject.
			if has_capture {
				source.add(checker.diagnostics, arm.span, "only tagged-union variants can capture a payload")
				ok = false
			}
			for pat_id in arm.patterns {
				pat_ast := checker.ast_module.exprs[pat_id]
				cmp := hir.INVALID_EXPR
				if pat_ast.kind == .Range {
					lo := build_expr(checker, pat_ast.left, ctx.locals^[:], ctx.global_reads, ctx.calls, subject_type, ctx.pkg, ctx.file)
					lo = coerce_expr(checker, lo, subject_type, arm.span)
					hi := build_expr(checker, pat_ast.right, ctx.locals^[:], ctx.global_reads, ctx.calls, subject_type, ctx.pkg, ctx.file)
					hi = coerce_expr(checker, hi, subject_type, arm.span)
					if checker.module.exprs[lo].kind == .Invalid || checker.module.exprs[hi].kind == .Invalid {
						ok = false
						continue
					}
					ge := add_hir_expr(checker, hir.Expr{
						kind = .Ge, span = arm.span, type = types.BOOL,
						left = slot_read(checker, key_local, key_type, arm.span), right = lo,
						target = hir.INVALID_REF, diagnostic = source.INVALID_DIAGNOSTIC,
					})
					hi_cmp := add_hir_expr(checker, hir.Expr{
						kind = .Le if pat_ast.integer == 1 else .Lt, span = arm.span, type = types.BOOL,
						left = slot_read(checker, key_local, key_type, arm.span), right = hi,
						target = hir.INVALID_REF, diagnostic = source.INVALID_DIAGNOSTIC,
					})
					cmp = add_hir_expr(checker, hir.Expr{
						kind = .And, span = arm.span, type = types.BOOL,
						left = ge, right = hi_cmp, target = hir.INVALID_REF, diagnostic = source.INVALID_DIAGNOSTIC,
					})
				} else {
					pattern := build_expr(checker, pat_id, ctx.locals^[:], ctx.global_reads, ctx.calls, subject_type, ctx.pkg, ctx.file)
					pattern = coerce_expr(checker, pattern, subject_type, arm.span)
					if checker.module.exprs[pattern].kind == .Invalid {
						ok = false
						continue
					}
					cmp = add_hir_expr(checker, hir.Expr{
						kind = .Eq, span = arm.span, type = types.BOOL,
						left = slot_read(checker, key_local, key_type, arm.span), right = pattern,
						target = hir.INVALID_REF, diagnostic = source.INVALID_DIAGNOSTIC,
					})
				}
				condition = match_or(checker, condition, cmp, arm.span)
			}
		}

		arm_body, body_ok := build_match_arm_body(ctx, arm, subject_type, subj_local, subj_is_pointer, ptr_type, subj_writable, field_index, payload_type, as_value, slot, slot_type, span)
		if !body_ok {
			ok = false
		}
		append(&built, Match_Built_Arm{condition = condition, body = arm_body, terminal = is_else})
	}

	// 4. Exhaustiveness. Enum/union matches must cover every variant or supply `else`;
	//    an already-exhaustive match must not carry a redundant `else`. The last
	//    covered arm is promoted to the unconditional `else` so the chain terminates.
	if is_tagged || is_enum_subject {
		all_names := types.enum_members_for(store, tag_enum) if is_tagged else types.enum_members_for(store, subject_type)
		names: [dynamic]symbol.Id
		names.allocator = checker.allocator
		defer delete(names)
		if is_tagged {
			for field in types.fields_for(store, subject_type) {
				append(&names, symbol.Id(field.name))
			}
		} else {
			for member in all_names {
				append(&names, symbol.Id(member.name))
			}
		}
		missing: [dynamic]symbol.Id
		missing.allocator = checker.allocator
		defer delete(missing)
		for name in names {
			if !contains_name(covered[:], name) {
				append(&missing, name)
			}
		}
		if has_else {
			if len(missing) == 0 {
				source.add(checker.diagnostics, span, "redundant 'else': the 'match' already covers every variant")
				ok = false
			}
		} else if len(missing) > 0 {
			builder: strings.Builder
			strings.builder_init(&builder, checker.allocator)
			defer strings.builder_destroy(&builder)
			for name, index in missing {
				if index > 0 {
					strings.write_string(&builder, ", ")
				}
				strings.write_string(&builder, ".")
				strings.write_string(&builder, symbol_text(checker, name))
			}
			source.addf(checker.diagnostics, span, "'match' on '%s' is not exhaustive; missing variants: %s (add the arms or an 'else')",
				type_label(checker, subject_type), strings.to_string(builder))
			ok = false
		} else if len(built) > 0 {
			built[len(built) - 1].terminal = true
		}
	} else if !has_else {
		source.addf(checker.diagnostics, span, "a 'match' on '%s' requires an 'else' arm", type_label(checker, subject_type))
		ok = false
	}

	if !ok {
		// The arm bodies never get wired into the (un-assembled) chain, so free them here.
		for arm in built {
			delete(arm.body, checker.allocator)
		}
		return fail(ctx, out, span, source.INVALID_DIAGNOSTIC)
	}

	// 5. Assemble the if/else chain backward from the built arms. The terminal arm is the
	//    final (else / promoted) one; the rest nest as `if cond { body } else { … }`.
	else_chain: []hir.Stmt_Id = nil
	start := len(built)
	if len(built) > 0 && built[len(built) - 1].terminal {
		else_chain = built[len(built) - 1].body
		start = len(built) - 1
	}
	for i := start - 1; i >= 0; i -= 1 {
		arm := built[i]
		wrapper := make([]hir.Stmt_Id, 1, checker.allocator)
		wrapper[0] = hir.stmt_id(len(checker.module.statements))
		append(&checker.module.statements, hir.Stmt{
			kind = .If, span = span, expr = arm.condition, guard = hir.INVALID_EXPR,
			then_body = arm.body, else_body = else_chain,
			local = hir.INVALID_LOCAL, target = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
		})
		else_chain = wrapper
	}
	for s in else_chain {
		append(out, s)
	}
	// The outermost chain slice's ids are now copied into `out`; the inner slices are
	// owned by their enclosing `.If` (freed with the HIR module).
	delete(else_chain, checker.allocator)
	return true
}

// build_match_arm_body builds one arm's body, prefixed with the optional payload capture
// (`cap := subject.variant`, an unchecked reinterpret like a Zig union field read). For a
// statement match it is a plain block; for a value match each path assigns the result slot
// (a single-expression arm yields implicitly).
build_match_arm_body :: proc(
	ctx: ^Build_Ctx,
	arm: ast.Stmt,
	subject_type: types.Type,
	subj_local: hir.Local_Id,
	subj_is_pointer: bool,
	ptr_type: types.Type,
	subj_writable: bool,
	field_index: int,
	payload_type: types.Type,
	as_value: bool,
	slot: ^hir.Local_Id,
	slot_type: ^types.Type,
	span: source.Span,
) -> ([]hir.Stmt_Id, bool) {
	checker := ctx.checker
	result: [dynamic]hir.Stmt_Id
	result.allocator = checker.allocator
	capture_start := len(ctx.locals^)
	capture_ok := true

	if len(arm.captures) > 0 && field_index >= 0 {
		capture := arm.captures[0]
		if capture != checker.sink_symbol {
			if id := add_shadow_diagnostic(
				checker, span, capture, "capture",
				ctx.pkg, ctx.file, ctx.locals^[:], ctx.loop_labels^[:], ctx.yield_targets^[:],
			); id != source.INVALID_DIAGNOSTIC {
				append(&result, hir.stmt_id(len(checker.module.statements)))
				append(&checker.module.statements, hir.Stmt{
					kind = .Trap, span = span, expr = hir.INVALID_EXPR,
					local = hir.INVALID_LOCAL, diagnostic = id,
				})
				capture_ok = false
			}
			// The payload sits at the subject's shared carrier offset. A value capture
			// loads it; a `|@cap|` capture binds a pointer to it (mutability follows the
			// subject), aliasing the original storage via the subject location.
			field_read := add_hir_expr(checker, hir.Expr{
				kind = .Field, span = span, type = payload_type, integer = i64(field_index),
				left = match_subject_location(checker, subj_local, subj_is_pointer, subject_type, ptr_type, span),
				target = hir.INVALID_REF, right = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
			})
			cap_type := payload_type
			cap_value := field_read
			if arm.pointer_capture {
				cap_type = types.pointer(&checker.module.types, payload_type, subj_writable, false)
				cap_value = add_hir_expr(checker, hir.Expr{
					kind = .Address, span = span, type = cap_type, left = field_read,
					target = hir.INVALID_REF, right = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
				})
			}
			cap_local := append_build_local(ctx, capture, cap_type, false, span)
			append(&result, hir.stmt_id(len(checker.module.statements)))
			append(&checker.module.statements, hir.Stmt{
				kind = .Declaration, span = span, local = cap_local, expr = cap_value,
				diagnostic = source.INVALID_DIAGNOSTIC,
			})
		}
	}

	body_ok := capture_ok
	if !as_value {
		built := build_block(ctx, arm.body)
		for s in built {
			append(&result, s)
		}
		delete(built, checker.allocator)
	} else {
		body_ok = build_value_arm(ctx, &result, arm.body, slot, slot_type, span) && body_ok
	}
	resize(ctx.locals, capture_start)
	return result[:], body_ok
}

// build_value_arm appends a value-match arm's slot assignment(s) to `out`.
build_value_arm :: proc(
	ctx: ^Build_Ctx,
	out: ^[dynamic]hir.Stmt_Id,
	arm_body: []ast.Stmt_Id,
	slot: ^hir.Local_Id,
	slot_type: ^types.Type,
	span: source.Span,
) -> bool {
	return emit_value_branch(ctx, out, arm_body, slot, slot_type, span)
}

// build_match desugars a statement-position `match` into its if/else chain.
build_match :: proc(ctx: ^Build_Ctx, body: ^[dynamic]hir.Stmt_Id, statement: ast.Stmt) {
	slot := hir.INVALID_LOCAL
	slot_type := types.INVALID
	emit_match(ctx, body, statement, false, &slot, &slot_type)
}

// build_value_match desugars a `match` used as a declaration/assignment RHS: a result slot
// each arm assigns, read after the chain. Mirrors build_value_if.
build_value_match :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	match_id: ast.Stmt_Id,
	expected: types.Type,
	span: source.Span,
) -> (value: hir.Expr_Id, value_type: types.Type) {
	checker := ctx.checker
	statement := checker.ast_module.statements[match_id]
	slot := hir.INVALID_LOCAL
	slot_type := types.INVALID
	if is_runtime_type(checker, expected) {
		slot_type = expected
		slot = new_value_slot(ctx, slot_type)
	}
	subtree: [dynamic]hir.Stmt_Id
	subtree.allocator = checker.allocator
	ok := false
	if selected_body, capture, has_capture, comptime_ok := specialization_match_body(
		checker, statement, ctx.pkg, ctx.file,
	); comptime_ok {
		if has_capture {
			append(&checker.static_bindings, capture)
		}
		ok = build_value_arm(ctx, &subtree, selected_body, &slot, &slot_type, span)
		if has_capture {
			_ = pop(&checker.static_bindings)
		}
	} else {
		ok = emit_match(ctx, &subtree, statement, true, &slot, &slot_type)
	}
	if !ok || slot == hir.INVALID_LOCAL {
		for s in subtree {
			append(body, s)
		}
		delete(subtree)
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, source.INVALID_DIAGNOSTIC), types.INVALID
	}
	append(body, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Declaration, span = span, local = slot, expr = hir.INVALID_EXPR,
		diagnostic = source.INVALID_DIAGNOSTIC,
	})
	for s in subtree {
		append(body, s)
	}
	delete(subtree)
	return slot_read(checker, slot, slot_type, span), slot_type
}

// loop_yields_none reports whether any `yield` that targets this loop (a labeled
// `yield :blk` inside `if`/block branches, or the trailing fall-through) yields the
// literal `none` — making the loop's result optional. Pure AST walk; does not descend
// into nested loops or value sources, whose yields belong to them.
loop_yields_none :: proc(checker: ^Checker, stmts: []ast.Stmt_Id) -> bool {
	for id in stmts {
		s := checker.ast_module.statements[id]
		#partial switch s.kind {
		case .Yield:
			if s.expr != ast.INVALID_EXPR && checker.ast_module.exprs[s.expr].kind == .None {
				return true
			}
		case .If:
			if loop_yields_none(checker, s.body) || loop_yields_none(checker, s.else_body) {
				return true
			}
		case .Block:
			if loop_yields_none(checker, s.body) {
				return true
			}
		}
	}
	return false
}

// resolve_loop_slot fixes a value-loop's result slot from its first concrete yield (an
// optional element type when the loop also yields `none`) and coerces `value` into it.
// Returns INVALID when the type can't be fixed yet (a `none`/invalid first yield).
resolve_loop_slot :: proc(ctx: ^Build_Ctx, target: ^Yield_Target, value: hir.Expr_Id, vtype: types.Type, span: source.Span) -> hir.Expr_Id {
	checker := ctx.checker
	if target.slot == hir.INVALID_LOCAL {
		if !is_runtime_type(checker, vtype) {
			return hir.INVALID_EXPR
		}
		target.slot_type = types.optional(&checker.module.types, vtype) if target.result_optional else vtype
		target.slot = new_value_slot(ctx, target.slot_type)
	}
	return coerce_expr(checker, value, target.slot_type, span)
}

// first_concrete_yield_expr returns the AST expr of the first yield (source order) that is
// not the literal `none`, descending into `if`/block branches but not nested loops or value
// sources (whose yields belong to them). INVALID when the loop yields only `none`.
first_concrete_yield_expr :: proc(checker: ^Checker, stmts: []ast.Stmt_Id) -> ast.Expr_Id {
	for id in stmts {
		s := checker.ast_module.statements[id]
		#partial switch s.kind {
		case .Yield:
			if s.expr != ast.INVALID_EXPR && checker.ast_module.exprs[s.expr].kind != .None {
				return s.expr
			}
		case .If:
			if e := first_concrete_yield_expr(checker, s.body); e != ast.INVALID_EXPR {
				return e
			}
			if e := first_concrete_yield_expr(checker, s.else_body); e != ast.INVALID_EXPR {
				return e
			}
		case .Block:
			if e := first_concrete_yield_expr(checker, s.body); e != ast.INVALID_EXPR {
				return e
			}
		}
	}
	return ast.INVALID_EXPR
}

// value_loop_element_type pre-types the element of an untyped value loop from its first
// concrete (non-`none`) yield, so a `none` yielded before any concrete value still resolves
// the result to `?T`. The loop's captures are bound temporarily for the probe and the probe
// expr is discarded; returns INVALID when the loop yields only `none`.
value_loop_element_type :: proc(ctx: ^Build_Ctx, loop_stmt: ast.Stmt) -> types.Type {
	checker := ctx.checker
	yield_expr := first_concrete_yield_expr(checker, loop_stmt.body)
	if yield_expr == ast.INVALID_EXPR {
		return types.INVALID
	}
	capture_start := len(ctx.locals^)
	local_start := len(ctx.hir_locals^)
	if loop_stmt.kind == .For {
		// Mirror the `.For` arm's capture-type computation just enough to type the probe.
		iterable := build_expr(checker, loop_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
		iterable_type := checker.module.exprs[iterable].type
		capture_type := types.INVALID
		if types.is_range(iterable_type, &checker.module.types) {
			capture_type = types.child_type(iterable_type, &checker.module.types)
		} else if item, ok := sequence_item(iterable_type, &checker.module.types); ok {
			capture_type = item.child
			if loop_stmt.pointer_capture {
				capture_type = types.pointer(&checker.module.types, item.child, item.mutable, false)
			}
		}
		if symbol.is_valid(loop_stmt.name) {
			append_build_local(ctx, loop_stmt.name, capture_type, false, loop_stmt.span)
		}
		if symbol.is_valid(loop_stmt.index_name) {
			append_build_local(ctx, loop_stmt.index_name, types.USIZE, false, loop_stmt.span)
		}
	}
	probe := build_expr(checker, yield_expr, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
	result := checker.module.exprs[probe].type if checker.module.exprs[probe].kind != .Invalid else types.INVALID
	resize(ctx.locals, capture_start)
	ignore_tracked_locals(ctx, local_start)
	return result
}

// stmt_contains_yield reports whether a statement contains a `yield` anywhere within it
// (recursing through if/block/loop bodies). Used to stop the leading probe build before any
// statement that yields (the block's yield target is not pushed during the probe).
stmt_contains_yield :: proc(checker: ^Checker, id: ast.Stmt_Id) -> bool {
	s := checker.ast_module.statements[id]
	#partial switch s.kind {
	case .Yield:
		return true
	case .If, .For, .While, .Block:
		for sub in s.body {
			if stmt_contains_yield(checker, sub) {
				return true
			}
		}
		for sub in s.else_body {
			if stmt_contains_yield(checker, sub) {
				return true
			}
		}
	}
	return false
}

// block_element_type pre-types an untyped value block's element from its first concrete
// `yield :blk`, building the block's leading (yield-free) statements first so the probe can
// reference block locals declared before the first yield. The leading build is a throwaway
// (its scope is restored). INVALID when there is no concrete yield, or the concrete yield
// references a local only in scope past the first yield (annotate the binding instead).
block_element_type :: proc(ctx: ^Build_Ctx, block_stmts: []ast.Stmt_Id) -> types.Type {
	checker := ctx.checker
	concrete := first_concrete_yield_expr(checker, block_stmts)
	if concrete == ast.INVALID_EXPR {
		return types.INVALID
	}
	lead_end := len(block_stmts)
	for id, i in block_stmts {
		if stmt_contains_yield(checker, id) {
			lead_end = i
			break
		}
	}
	scope_start := len(ctx.locals^)
	local_start := len(ctx.hir_locals^)
	defer_start := len(ctx.defers^)
	lead := build_block(ctx, block_stmts[:lead_end], close = false)
	delete(lead, checker.allocator)
	probe := build_expr(checker, concrete, ctx.locals^[:], ctx.global_reads, ctx.calls, types.INVALID, ctx.pkg, ctx.file)
	result := checker.module.exprs[probe].type if checker.module.exprs[probe].kind != .Invalid else types.INVALID
	// Discard the throwaway leading build's scope (its hir stmts/locals are dead but stable).
	for i := defer_start; i < len(ctx.defers^); i += 1 {
		delete(ctx.defers^[i].body, checker.allocator)
	}
	resize(ctx.defers, defer_start)
	resize(ctx.locals, scope_start)
	ignore_tracked_locals(ctx, local_start)
	return result
}

// build_value_labeled_block turns `x :: blk: { …; yield :blk v }` into a result slot each
// `yield :blk` assigns (via the build_block `.Yield` desugar → `slot = v; break :blk`), then
// reads it after the block. Every path must yield (or otherwise exit); HIR holds a `.Block`
// that emits the body and the exit label the labeled breaks branch to. No iteration / no
// fall-through (unlike a value loop). The type is the annotation when typed, else the first
// concrete `yield :blk`'s type (optional when any yield is `none`).
build_value_labeled_block :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	block_stmts: []ast.Stmt_Id,
	label: symbol.Id,
	expected: types.Type,
	span: source.Span,
) -> (value: hir.Expr_Id, value_type: types.Type) {
	checker := ctx.checker
	result_optional := loop_yields_none(checker, block_stmts)
	slot := hir.INVALID_LOCAL
	slot_type := types.INVALID
	if is_runtime_type(checker, expected) {
		slot_type = expected
		slot = new_value_slot(ctx, slot_type)
		result_optional = types.is_optional(slot_type, &checker.module.types)
	} else if result_optional {
		// Untyped block that also yields `none`: pre-type the element from the first
		// concrete yield (regardless of source order) so a `none` yielded first still
		// resolves the result to `?T`.
		elem := block_element_type(ctx, block_stmts)
		if is_runtime_type(checker, elem) {
			slot_type = types.optional(&checker.module.types, elem)
			slot = new_value_slot(ctx, slot_type)
		}
	}
	if id := add_label_shadow_diagnostic(ctx, span, label); id != source.INVALID_DIAGNOSTIC {
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}
	append(ctx.yield_targets, Yield_Target{
		label = label, slot = slot, slot_type = slot_type, result_optional = result_optional,
		defer_floor = len(ctx.defers^),
	})
	built := build_block(ctx, block_stmts)
	target := pop(ctx.yield_targets)

	if target.slot == hir.INVALID_LOCAL {
		for s in built {
			append(body, s)
		}
		delete(built, checker.allocator)
		id := source.add(checker.diagnostics, span,
			"could not determine the value block's yield type; annotate the binding")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}
	// Every path must `yield :blk` (or return/break out); otherwise a path falls through
	// to the slot read with a poison value.
	if !all_paths_exit(&checker.module, built) {
		for s in built {
			append(body, s)
		}
		delete(built, checker.allocator)
		id := source.add(checker.diagnostics, span, "a labeled value block must 'yield' on every path")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}

	append(body, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Declaration, span = span, local = target.slot, expr = hir.INVALID_EXPR,
		diagnostic = source.INVALID_DIAGNOSTIC,
	})
	append(body, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Block, span = span, label = label, then_body = built,
		local = hir.INVALID_LOCAL, target = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
	})
	value = slot_read(checker, target.slot, target.slot_type, span)
	return value, target.slot_type
}

// build_value_loop turns a labeled `for/while ... blk: { … }` whose body ends in a
// fall-through `yield` (and may early-exit via `yield :blk x`) into a result slot:
// the fall-through value initializes the slot before the loop, each `yield :blk x`
// desugars (in build_block) to `slot = x; break`, and the construct's value is a read
// of the slot after the loop. Reuses the ordinary `.For`/`.While` build via a peeled
// copy; no new HIR. The yielded type is the annotation when typed, else the first
// concrete yield's type (optional when any yield is `none`).
build_value_loop :: proc(
	ctx: ^Build_Ctx,
	body: ^[dynamic]hir.Stmt_Id,
	loop_id: ast.Stmt_Id,
	expected: types.Type,
	span: source.Span,
) -> (value: hir.Expr_Id, value_type: types.Type) {
	checker := ctx.checker
	loop_stmt := checker.ast_module.statements[loop_id]
	n := len(loop_stmt.body)
	last_is_fallthrough := n > 0 &&
		checker.ast_module.statements[loop_stmt.body[n - 1]].kind == .Yield &&
		!symbol.is_valid(checker.ast_module.statements[loop_stmt.body[n - 1]].label)
	if !symbol.is_valid(loop_stmt.label) {
		id := source.add(checker.diagnostics, span,
			"a value loop must label its body (e.g. 'blk:') so a 'yield :blk' can exit it")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}
	if !last_is_fallthrough {
		id := source.add(checker.diagnostics, span,
			"a value loop's body must end with a 'yield' for when the loop completes")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}
	fall_stmt := checker.ast_module.statements[loop_stmt.body[n - 1]]

	result_optional := loop_yields_none(checker, loop_stmt.body)
	slot := hir.INVALID_LOCAL
	slot_type := types.INVALID
	if is_runtime_type(checker, expected) {
		slot_type = expected
		slot = new_value_slot(ctx, slot_type)
		result_optional = types.is_optional(slot_type, &checker.module.types)
	} else if result_optional {
		// Untyped loop that also yields `none`: pre-type the element from the first
		// concrete yield (regardless of source order) so a `none` built before any
		// concrete yield still resolves the result to `?T`.
		elem := value_loop_element_type(ctx, loop_stmt)
		if is_runtime_type(checker, elem) {
			slot_type = types.optional(&checker.module.types, elem)
			slot = new_value_slot(ctx, slot_type)
		}
	}
	append(ctx.yield_targets, Yield_Target{
		label = loop_stmt.label, slot = slot, slot_type = slot_type, result_optional = result_optional,
		defer_floor = len(ctx.defers^),
	})

	// Build the loop with the fall-through peeled off, reusing the normal For/While arm.
	// The peeled body is a fresh copy so destroy_module won't double-free the original.
	peeled := loop_stmt
	peeled_body := make([]ast.Stmt_Id, n - 1, checker.ast_module.allocator)
	copy(peeled_body, loop_stmt.body[:n - 1])
	peeled.body = peeled_body
	peeled_id := ast.stmt_id(len(checker.ast_module.statements))
	append(&checker.ast_module.statements, peeled)
	loop_block := build_block(ctx, []ast.Stmt_Id{peeled_id})

	target := pop(ctx.yield_targets)

	// The fall-through value initializes the slot before the loop (loop captures are
	// out of scope here), so the loop completing leaves it as the result.
	fall_value := build_expr(checker, fall_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, target.slot_type, ctx.pkg, ctx.file)
	if fall_value != hir.INVALID_EXPR && types.is_void(checker.module.exprs[fall_value].type) {
		for s in loop_block {
			append(body, s)
		}
		delete(loop_block, checker.allocator)
		id := source.add(checker.diagnostics, fall_stmt.span, "'yield' expression must produce a non-void value")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, fall_stmt.span, id), types.INVALID
	}
	fall_value = resolve_loop_slot(ctx, &target, fall_value, checker.module.exprs[fall_value].type if fall_value != hir.INVALID_EXPR else types.INVALID, fall_stmt.span)
	if target.slot == hir.INVALID_LOCAL || fall_value == hir.INVALID_EXPR {
		for s in loop_block {
			append(body, s)
		}
		delete(loop_block, checker.allocator)
		id := source.add(checker.diagnostics, span,
			"could not determine the value loop's yield type; annotate the binding")
		ctx.problematic^ = true
		return invalid_hir_expr(checker, span, id), types.INVALID
	}

	append(body, hir.stmt_id(len(checker.module.statements)))
	append(&checker.module.statements, hir.Stmt{
		kind = .Declaration, span = span, local = target.slot, expr = fall_value,
		diagnostic = source.INVALID_DIAGNOSTIC,
	})
	for s in loop_block {
		append(body, s)
	}
	delete(loop_block, checker.allocator)

	value = slot_read(checker, target.slot, target.slot_type, span)
	return value, target.slot_type
}

Return_Enum_Guard :: struct {
	local: hir.Local_Id,
	type:  types.Type,
	value: i64,
}

return_enum_guard :: proc(
	module: ^hir.Module,
	statement: hir.Stmt,
	locals: []hir.Local,
) -> (Return_Enum_Guard, bool) {
	if statement.kind != .If || statement.else_body != nil ||
	   statement.expr == hir.INVALID_EXPR || int(statement.expr) >= len(module.exprs) {
		return {}, false
	}
	condition := module.exprs[statement.expr]
	if condition.kind != .Eq || condition.left == hir.INVALID_EXPR || condition.right == hir.INVALID_EXPR ||
	   int(condition.left) >= len(module.exprs) || int(condition.right) >= len(module.exprs) {
		return {}, false
	}
	left := module.exprs[condition.left]
	right := module.exprs[condition.right]
	if left.kind == .Integer {
		left, right = right, left
	}
	if left.kind != .Local || right.kind != .Integer ||
	   !types.equal(left.type, right.type) || !types.is_enum(left.type, &module.types) {
		return {}, false
	}
	local := hir.as_local(left.target)
	if local == hir.INVALID_LOCAL || int(local) >= len(locals) || locals[local].mutable {
		return {}, false
	}
	if !all_paths_return(module, statement.then_body, locals) {
		return {}, false
	}
	return Return_Enum_Guard{local=local, type=left.type, value=right.integer}, true
}

enum_guards_return :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id, locals: []hir.Local) -> bool {
	guards: [dynamic]Return_Enum_Guard
	guards.allocator = module.allocator
	defer delete(guards)
	for id in stmts {
		if guard, ok := return_enum_guard(module, module.statements[id], locals); ok {
			append(&guards, guard)
		}
	}
	for guard, index in guards {
		seen := false
		for previous in guards[:index] {
			seen = seen || previous.local == guard.local
		}
		if seen {
			continue
		}
		members := types.enum_members_for(&module.types, guard.type)
		if len(members) == 0 {
			continue
		}
		complete := true
		for member in members {
			value := i64(member.value) if member.value < 0 else transmute(i64)u64(member.value)
			covered := false
			for candidate in guards {
				covered = covered || candidate.local == guard.local && candidate.value == value
			}
			complete = complete && covered
		}
		if complete {
			return true
		}
	}
	return false
}

// Reports whether every control-flow path through `stmts` terminates (returns or traps),
// so the end of the block is unreachable. A `.Return` or `.Trap` terminates outright; an
// `.If` terminates only when it has an `else` and both arms terminate. A literal
// `while true` cannot fall through because the language has no `break` statement.
// Exhaustive equality guards over one immutable enum local also terminate collectively.
all_paths_return :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id, locals: []hir.Local = nil) -> bool {
	for id in stmts {
		statement := module.statements[id]
		#partial switch statement.kind {
		case .Return, .Trap:
			return true
		case .If:
			if statement.else_body != nil &&
			   all_paths_return(module, statement.then_body, locals) &&
			   all_paths_return(module, statement.else_body, locals) {
				return true
			}
		case .While:
			// A literal `while true` makes the end of the block unreachable —
			// unless its body can `break` out of this loop.
			if statement.expr != hir.INVALID_EXPR && int(statement.expr) < len(module.exprs) {
				condition := module.exprs[statement.expr]
				if condition.kind == .Bool && condition.integer != 0 &&
				   !loop_body_breaks(module, statement.then_body) {
					return true
				}
			}
		}
	}
	return enum_guards_return(module, stmts, locals)
}

// Reports whether `stmts` contains a `break` that targets the enclosing loop:
// a `.Break` at this level or inside `if`/`else` branches counts, but a `break`
// inside a nested `.While`/`.For` targets that inner loop, so we do not descend.
loop_body_breaks :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id) -> bool {
	for id in stmts {
		statement := module.statements[id]
		#partial switch statement.kind {
		case .Break:
			return true
		case .If:
			if loop_body_breaks(module, statement.then_body) ||
			   loop_body_breaks(module, statement.else_body) {
				return true
			}
		}
	}
	return false
}

// Like `all_paths_return`, but also treats `break`/`continue` as terminating the
// block. Used only to decide whether `build_block` may skip the fall-through defer
// flush (a block that always exits early would otherwise emit unreachable copies).
all_paths_exit :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id) -> bool {
	for id in stmts {
		statement := module.statements[id]
		#partial switch statement.kind {
		case .Return, .Trap, .Break, .Continue:
			return true
		case .If:
			if statement.else_body != nil &&
			   all_paths_exit(module, statement.then_body) &&
			   all_paths_exit(module, statement.else_body) {
				return true
			}
		case .While:
			if statement.expr != hir.INVALID_EXPR && int(statement.expr) < len(module.exprs) {
				condition := module.exprs[statement.expr]
				if condition.kind == .Bool && condition.integer != 0 &&
				   !loop_body_breaks(module, statement.then_body) {
					return true
				}
			}
		}
	}
	return false
}

build_function :: proc(checker: ^Checker, id: Spec_Id) {
	spec := checker.specs[id]
	function := checker.ast_module.functions[spec.template]
	previous_comptime := checker.current_comptime_values
	checker.current_comptime_values = spec.comptime_values
	defer checker.current_comptime_values = previous_comptime
	signature_diagnostic := source.INVALID_DIAGNOSTIC
	unresolved_result := !types.is_void(spec.result) && !is_runtime_type(checker, spec.result)
	if unresolved_result {
		if types.is_comptime_only(spec.result, &checker.module.types) {
			signature_diagnostic = source.addf(
				checker.diagnostics,
				function.span,
				"function '%s' has a comptime-only result and cannot return it through the runtime ABI",
				symbol_text(checker, function.name),
			)
		}
		checker.specs[id].result = types.I64
		spec.result = types.I64
	}
	for arg in spec.args {
		if !is_runtime_type(checker, arg) {
			signature_diagnostic = source.addf(
				checker.diagnostics,
				function.span,
				"could not resolve a concrete parameter type for '%s'",
				symbol_text(checker, function.name),
			)
			break
		}
	}
	assert(spec.hir_id == hir.function_id(len(checker.module.functions)))

	locals: [dynamic]Build_Local
	locals.allocator = checker.allocator
	hir_locals: [dynamic]hir.Local
	hir_locals.allocator = checker.allocator
	local_spans: [dynamic]source.Span
	local_spans.allocator = checker.allocator
	local_used: [dynamic]bool
	local_used.allocator = checker.allocator
	local_warnable: [dynamic]bool
	local_warnable.allocator = checker.allocator
	params: [dynamic]hir.Local_Id
	params.allocator = checker.allocator
	body: [dynamic]hir.Stmt_Id
	body.allocator = checker.allocator
	global_reads: [dynamic]hir.Global_Id
	global_reads.allocator = checker.allocator
	calls: [dynamic]hir.Function_Id
	calls.allocator = checker.allocator
	demanded: [dynamic]Spec_Id
	demanded.allocator = checker.allocator
	local_types, _ := infer_spec_locals_and_result(checker, id, &demanded)
	defer {
		delete(local_types, checker.allocator)
		delete(demanded)
	}

	runtime_index := 0
	for param in function.params {
		if param.comptime_value {
			continue
		}
		param_type := types.INVALID
		if runtime_index < len(spec.args) {
			param_type = spec.args[runtime_index]
		}
		local_id := append_tracked_local(
			&hir_locals,
			&local_spans,
			&local_used,
			&local_warnable,
			hir.Local{name = param.name, type = param_type, parameter = true},
			param.span,
		)
		if param.name != checker.sink_symbol && block_reads_name(checker, function.body, param.name) {
			local_used[int(local_id)] = true
		}
		append(&locals, Build_Local{name = param.name, type = param_type, id = local_id})
		append(&params, local_id)
		runtime_index += 1
	}

	problematic := signature_diagnostic != source.INVALID_DIAGNOSTIC ||
	               checker.template_diagnostics[spec.template] != source.INVALID_DIAGNOSTIC
	native_main := function.pkg == 0 && function.name == checker.main_symbol && checker.entry_point == .Plain
	if !function.has_body {
		if unresolved_result && signature_diagnostic == source.INVALID_DIAGNOSTIC &&
		   checker.template_diagnostics[spec.template] == source.INVALID_DIAGNOSTIC {
			signature_diagnostic = source.addf(
				checker.diagnostics,
				function.span,
				"could not resolve a concrete result type for '%s'",
				symbol_text(checker, function.name),
			)
			problematic = true
		}
		assert(spec.hir_id == hir.function_id(len(checker.module.functions)))
		append(
			&checker.module.functions,
			hir.Function {
				name = function.name,
				link_name = make_link_name(checker, id),
				calling_convention = .C if function.c_abi else .Brolang,
				implementation = .Declaration,
				linkage = .External if function.c_abi else .Internal,
				is_main = native_main,
				variadic = function.variadic,
				params = params[:],
				result = spec.result,
				locals = hir_locals[:],
				body = body[:],
				direct_global_reads = global_reads,
				calls = calls[:],
				problematic = problematic,
				diagnostic = checker.template_diagnostics[spec.template],
			},
		)
		delete(locals)
		delete(local_spans)
		delete(local_used)
		delete(local_warnable)
		return
	}

	if signature_diagnostic != source.INVALID_DIAGNOSTIC {
		append(&body, hir.stmt_id(len(checker.module.statements)))
		append(
			&checker.module.statements,
			hir.Stmt {
				kind = .Trap,
				span = function.span,
				expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL,
				diagnostic = signature_diagnostic,
			},
		)
	}
	defers: [dynamic]Defer_Entry
	defers.allocator = checker.allocator
	loop_defer_starts: [dynamic]int
	loop_defer_starts.allocator = checker.allocator
	loop_labels: [dynamic]symbol.Id
	loop_labels.allocator = checker.allocator
	loop_is_loop: [dynamic]bool
	loop_is_loop.allocator = checker.allocator
	yield_targets: [dynamic]Yield_Target
	yield_targets.allocator = checker.allocator
	ctx := Build_Ctx{
		checker = checker,
		pkg = function.pkg,
		file = function.file,
		result = spec.result,
		local_types = local_types,
		locals = &locals,
		hir_locals = &hir_locals,
		local_spans = &local_spans,
		local_used = &local_used,
		local_warnable = &local_warnable,
		global_reads = &global_reads,
		calls = &calls,
		problematic = &problematic,
		defers = &defers,
		loop_defer_starts = &loop_defer_starts,
		loop_labels = &loop_labels,
		loop_is_loop = &loop_is_loop,
		yield_targets = &yield_targets,
	}
	previous_result := checker.current_result
	previous_ctx := checker.current_build_ctx
	checker.current_result = spec.result
	checker.current_build_ctx = &ctx
	block := build_block(&ctx, function.body)
	checker.current_result = previous_result
	checker.current_build_ctx = previous_ctx
	returns := all_paths_return(&checker.module, block, hir_locals[:])
	for block_stmt in block {
		append(&body, block_stmt)
	}
	delete(block, checker.allocator)
	if unresolved_result && signature_diagnostic == source.INVALID_DIAGNOSTIC &&
	   checker.template_diagnostics[spec.template] == source.INVALID_DIAGNOSTIC && !problematic {
		signature_diagnostic = source.addf(
			checker.diagnostics,
			function.span,
			"could not resolve a concrete result type for '%s'",
			symbol_text(checker, function.name),
		)
		append(&body, hir.stmt_id(len(checker.module.statements)))
		append(&checker.module.statements, hir.Stmt{
			kind=.Trap, span=function.span, expr=hir.INVALID_EXPR,
			local=hir.INVALID_LOCAL, diagnostic=signature_diagnostic,
		})
		problematic = true
		returns = true
	}

	fallible_void := types.kind(spec.result, &checker.module.types) == .Fallible &&
	                 types.is_void(types.fallible_success(spec.result, &checker.module.types))
	if !returns && fallible_void {
		value := fallible_aggregate(checker, function.span, spec.result, hir.INVALID_EXPR, false)
		append(&body, hir.stmt_id(len(checker.module.statements)))
		append(&checker.module.statements, hir.Stmt{
			kind=.Return, span=function.span, expr=value, local=hir.INVALID_LOCAL,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	} else if !types.is_void(spec.result) && !returns {
		id := source.addf(
			checker.diagnostics,
			function.span,
			"function '%s' does not return a value",
			symbol_text(checker, function.name),
		)
		append(&body, hir.stmt_id(len(checker.module.statements)))
		append(
			&checker.module.statements,
			hir.Stmt{kind = .Trap, span = function.span, expr = hir.INVALID_EXPR, local = hir.INVALID_LOCAL, diagnostic = id},
		)
		problematic = true
	}

	record_unused_locals(checker, hir_locals[:], local_spans[:], local_used[:], local_warnable[:])

	assert(spec.hir_id == hir.function_id(len(checker.module.functions)))
	append(
		&checker.module.functions,
		hir.Function {
			name = function.name,
			link_name = make_link_name(checker, id),
			calling_convention = .C if function.c_abi || native_main else .Brolang,
			implementation = .Definition,
			linkage = .External if function.c_abi || native_main else .Internal,
			is_main = native_main,
			variadic = function.variadic,
			params = params[:],
			result = spec.result,
			locals = hir_locals[:],
			body = body[:],
			direct_global_reads = global_reads,
			calls = calls[:],
			problematic = problematic,
			diagnostic = source.INVALID_DIAGNOSTIC,
		},
	)
	for entry in defers {
		delete(entry.body, checker.allocator)
	}
	delete(defers)
	delete(loop_defer_starts)
	delete(loop_labels)
	delete(loop_is_loop)
	delete(yield_targets)
	delete(locals)
	delete(local_spans)
	delete(local_used)
	delete(local_warnable)
}

expr_problematic :: proc(checker: ^Checker, expr_id: hir.Expr_Id) -> bool {
	module := &checker.module
	stack := checker.hir_expr_stack
	clear_dynamic_array(&stack)
	defer {
		clear_dynamic_array(&stack)
		checker.hir_expr_stack = stack
	}
	append(&stack, expr_id)
	for len(stack) > 0 {
		id := pop(&stack)
		if id == hir.INVALID_EXPR || int(id) >= len(module.exprs) {
			return true
		}
		expr := module.exprs[id]
		if expr.kind == .Invalid {
			return true
		}
		if expr.left != hir.INVALID_EXPR {
			append(&stack, expr.left)
		}
		if expr.right != hir.INVALID_EXPR {
			append(&stack, expr.right)
		}
		append(&stack, ..expr.args)
	}
	return false
}

static_integer_value :: proc(module: ^hir.Module, expr_id: hir.Expr_Id) -> (i64, bool) {
	current := expr_id
	for current != hir.INVALID_EXPR && int(current) < len(module.exprs) {
		expr := module.exprs[current]
		if expr.kind == .Integer {
			return expr.integer, true
		}
		if expr.kind != .Retype {
			break
		}
		current = expr.left
	}
	return 0, false
}

build_globals :: proc(checker: ^Checker) {
	for global, global_index in checker.ast_module.globals {
		if global.diagnostic != source.INVALID_DIAGNOSTIC {
			global_type := checker.global_types[global_index]
			if !is_runtime_type(checker, global_type) {
				global_type = types.I64
			}
			expr := hir.INVALID_EXPR
			if !global.external {
				expr = invalid_hir_expr(checker, global.span, global.diagnostic, global_type)
			}
			append(&checker.module.globals, hir.Global{
				name=global.name,
				link_name=strings.clone(global.link_name, checker.allocator),
				type=global_type,
				expr=expr,
				external=global.external,
				writable=global.writable || !global.immutable,
				direct_problem=true,
				problematic=true,
				diagnostic=global.diagnostic,
			})
			continue
		}
		if global.external {
			global_type := checker.global_types[global_index]
			writable := global.writable
			canonical := checker.external_global_canonical[global_index]
			if canonical != ast.INVALID_GLOBAL && int(canonical) < len(checker.ast_module.globals) {
				canonical_global := checker.ast_module.globals[canonical]
				writable = canonical_global.writable
				global_type = checker.global_types[canonical]
			}
			diagnostic := checker.external_global_diagnostics[global_index]
			if !is_runtime_type(checker, global_type) {
				if diagnostic == source.INVALID_DIAGNOSTIC {
					diagnostic = source.addf(
						checker.diagnostics,
						global.span,
						"could not resolve a concrete type for external global '%s'",
						symbol_text(checker, global.name),
					)
				}
				global_type = types.I64
			}
			_ = hir.global_id(len(checker.module.globals))
			append(&checker.module.globals, hir.Global{
				name=global.name,
				link_name=strings.clone(global.link_name, checker.allocator),
				type=global_type,
				expr=hir.INVALID_EXPR,
				external=true,
				writable=writable,
				direct_problem=diagnostic != source.INVALID_DIAGNOSTIC,
				problematic=diagnostic != source.INVALID_DIAGNOSTIC,
				diagnostic=diagnostic,
			})
			continue
		}
		dependencies: [dynamic]hir.Global_Id
		dependencies.allocator = checker.allocator
		calls: [dynamic]hir.Function_Id
		calls.allocator = checker.allocator
		declared := resolve_inferred_array(checker, type_from_syntax(checker, global.type, global.pkg, global.file), global.expr)
		if has_inferred_array_count(checker, declared) &&
		   is_runtime_type(checker, checker.global_types[global_index]) {
			declared = checker.global_types[global_index]
		}
		expected := types.INVALID
		if is_runtime_type(checker, declared) {
			expected = declared
		} else if constant := eval_integer_constant_in_context(checker, global.expr, global.pkg, global.file);
		   constant.kind == .Value &&
		   (fits_i64(constant.value) || declared == types.UINT && fits_u64(constant.value)) &&
		   is_runtime_type(checker, checker.global_types[global_index]) {
			// Open integer constant: build against its demanded/defaulted type. Gated
			// to the infer-side open-constant condition so out-of-range constants keep
			// their original range diagnostic.
			expected = checker.global_types[global_index]
		} else if (is_float_constant_expr(checker, global.expr) ||
		           is_numeric_arithmetic_expr(checker, global.expr)) &&
		          is_runtime_type(checker, checker.global_types[global_index]) {
			expected = checker.global_types[global_index]
		}
		expr := build_expr(checker, global.expr, nil, &dependencies, &calls, expected, global.pkg, global.file)
		global_type := checker.global_types[global_index]
		if is_runtime_type(checker, checker.global_types[global_index]) {
			expr = coerce_expr(checker, expr, checker.global_types[global_index], global.span)
			global_type = checker.module.exprs[expr].type
		} else if is_runtime_type(checker, checker.module.exprs[expr].type) {
			global_type = checker.module.exprs[expr].type
		}
		diagnostic := source.INVALID_DIAGNOSTIC
		if root, invalid := invalid_expr_diagnostic(checker, expr); invalid {
			diagnostic = root
			if !is_runtime_type(checker, global_type) {
				global_type = types.I64
			}
		}
		if diagnostic == source.INVALID_DIAGNOSTIC && types.is_constraint(declared) &&
		   !types.constraint_accepts(declared, global_type, &checker.module.types) {
			diagnostic = source.addf(
				checker.diagnostics,
				global.span,
				"could not resolve the '%s' constraint for global '%s'",
				types.name(declared),
				symbol_text(checker, global.name),
			)
			global_type = constraint_recovery_type(checker, declared)
			expr = invalid_hir_expr(checker, global.span, diagnostic, global_type)
		}
		if !global.immutable && is_undefined_expr(checker, global.expr) {
			diagnostic = source.add(
				checker.diagnostics,
				global.span,
				"'undefined' is only valid as a mutable local declaration initializer",
			)
			if !is_runtime_type(checker, global_type) {
				global_type = types.I64
			}
			expr = invalid_hir_expr(checker, global.span, diagnostic, global_type)
		}
		if diagnostic == source.INVALID_DIAGNOSTIC && !is_runtime_type(checker, global_type) {
			if types.is_comptime_only(global_type, &checker.module.types) ||
			   types.is_comptime_only(declared, &checker.module.types) {
				diagnostic = source.addf(
					checker.diagnostics,
					global.span,
					"global '%s' has a comptime-only type and cannot be stored at runtime",
					symbol_text(checker, global.name),
				)
			} else {
				diagnostic = source.addf(
					checker.diagnostics,
					global.span,
					"could not resolve a concrete type for global '%s'",
					symbol_text(checker, global.name),
				)
			}
			global_type = types.I64
			expr = invalid_hir_expr(checker, global.span, diagnostic, global_type)
		}
		if global.type == types.VOID {
			diagnostic = source.add(
				checker.diagnostics,
				global.span,
				"void is only valid as a function result type",
			)
			expr = invalid_hir_expr(checker, global.span, diagnostic)
		}
		if !global.immutable {
			if !types.is_valid(global.type) {
				diagnostic = source.addf(
					checker.diagnostics,
					global.span,
					"mutable global '%s' requires a type annotation",
					symbol_text(checker, global.name),
				)
				global_type = types.I64
				expr = invalid_hir_expr(checker, global.span, diagnostic, global_type)
			}
		}
		static_value, is_static := static_integer_value(&checker.module, expr)
		is_static = is_static && diagnostic == source.INVALID_DIAGNOSTIC && global.immutable
		_ = hir.global_id(len(checker.module.globals))
		append(
			&checker.module.globals,
			hir.Global {
				name = global.name,
				link_name = strings.clone(global.link_name, checker.allocator),
				type = global_type,
				expr = expr,
				static_value = static_value,
				is_static = is_static,
				external = false,
				writable = !global.immutable,
				dependencies = dependencies,
				calls = calls[:],
				direct_problem = expr_problematic(checker, expr),
				problematic = expr_problematic(checker, expr),
				diagnostic = diagnostic,
			},
		)
	}
}

propagate_problems :: proc(checker: ^Checker) {
	changed := true
	for changed {
		changed = false
		for &function in checker.module.functions {
			if function.problematic {
				continue
			}
			for call in function.calls {
				if call != hir.INVALID_FUNCTION && int(call) < len(checker.module.functions) &&
				   checker.module.functions[call].problematic {
					function.problematic = true
					changed = true
					break
				}
			}
		}
		for &global in checker.module.globals {
			if global.problematic {
				continue
			}
			for dependency in global.dependencies {
				if dependency != hir.INVALID_GLOBAL &&
				   int(dependency) < len(checker.module.globals) &&
				   checker.module.globals[dependency].problematic {
					global.problematic = true
					changed = true
					break
				}
			}
			if global.problematic {
				continue
			}
			for call in global.calls {
				if call != hir.INVALID_FUNCTION && int(call) < len(checker.module.functions) &&
				   checker.module.functions[call].problematic {
					global.problematic = true
					changed = true
					break
				}
			}
		}
	}
}

append_unique_global :: proc(values: ^[dynamic]hir.Global_Id, value: hir.Global_Id) -> bool {
	for existing in values^ {
		if existing == value {
			return false
		}
	}
	append(values, value)
	return true
}

propagate_global_reads :: proc(checker: ^Checker) {
	changed := true
	for changed {
		changed = false
		for &function in checker.module.functions {
			for call in function.calls {
				if call == hir.INVALID_FUNCTION || int(call) >= len(checker.module.functions) {
					continue
				}
				for global_id in checker.module.functions[call].direct_global_reads {
					if append_unique_global(&function.direct_global_reads, global_id) {
						changed = true
					}
				}
			}
		}
	}
	for &global in checker.module.globals {
		for call in global.calls {
			if call == hir.INVALID_FUNCTION || int(call) >= len(checker.module.functions) {
				continue
			}
			for dependency in checker.module.functions[call].direct_global_reads {
				_ = append_unique_global(&global.dependencies, dependency)
			}
		}
	}
}

Cycle_Frame :: struct {
	global:          hir.Global_Id,
	next_dependency: int,
}

cycle_global_location :: proc(checker: ^Checker, global_id: hir.Global_Id) -> (name, path: string, line: int) {
	name = "<unknown>"
	path = "<unknown>"
	line = 1
	index := int(global_id)
	if global_id == hir.INVALID_GLOBAL || index >= len(checker.module.globals) {
		return
	}
	name = symbol_text(checker, checker.module.globals[global_id].name)
	if index >= len(checker.ast_module.globals) {
		return
	}
	span := checker.ast_module.globals[global_id].span
	source_file := source.source_for_span(checker.diagnostics, span)
	if source_file == nil {
		return
	}
	path = source_file.path
	line, _ = source.line_and_column(source_file, span.start)
	return
}

detect_global_cycles_visit :: proc(checker: ^Checker, global_id: hir.Global_Id, states: []u8) {
	if states[global_id] == 2 {
		return
	}
	stack := checker.cycle_stack
	clear_dynamic_array(&stack)
	defer {
		clear_dynamic_array(&stack)
		checker.cycle_stack = stack
	}
	append(&stack, Cycle_Frame{global=global_id})
	for len(stack) > 0 {
		frame_index := len(stack)-1
		frame := &stack[frame_index]
		if states[frame.global] == 0 {
			states[frame.global] = 1
		}
		dependencies := checker.module.globals[frame.global].dependencies
		if frame.next_dependency >= len(dependencies) {
			states[frame.global] = 2
			if checker.module.globals[frame.global].problematic && frame_index > 0 {
				checker.module.globals[stack[frame_index-1].global].problematic = true
			}
			_ = pop(&stack)
			continue
		}
		dependency := dependencies[frame.next_dependency]
		frame.next_dependency += 1
		if dependency == hir.INVALID_GLOBAL || int(dependency) >= len(states) {
			continue
		}
		if states[dependency] == 1 {
			start_name, start_path, start_line := cycle_global_location(checker, dependency)
			end_name, end_path, end_line := cycle_global_location(checker, frame.global)
			id := source.addf(
				checker.diagnostics,
				checker.ast_module.globals[dependency].span,
				"global initialization cycle from '%s' at %s:%d to '%s' at %s:%d",
				start_name,
				start_path,
				start_line,
				end_name,
				end_path,
				end_line,
			)
			checker.module.globals[dependency].diagnostic = id
			checker.module.globals[dependency].problematic = true
			checker.module.globals[frame.global].problematic = true
			continue
		}
		if states[dependency] == 2 {
			if checker.module.globals[dependency].problematic {
				checker.module.globals[frame.global].problematic = true
			}
			continue
		}
		append(&stack, Cycle_Frame{global=dependency})
	}
}

synthesize_trap_main :: proc(checker: ^Checker) {
	id := source.add(checker.diagnostics, source.Span{}, "missing or unusable main function")
	statement_id := hir.stmt_id(len(checker.module.statements))
	_ = hir.function_id(len(checker.module.functions))
	append(
		&checker.module.statements,
		hir.Stmt{kind = .Trap, span = source.Span{}, expr = hir.INVALID_EXPR, local = hir.INVALID_LOCAL, diagnostic = id},
	)
	body := make([]hir.Stmt_Id, 1, checker.allocator)
	body[0] = statement_id
	append(
		&checker.module.functions,
		hir.Function {
			name = checker.main_symbol,
			link_name = fmt.aprintf("main", allocator = checker.allocator),
			calling_convention = .C,
			implementation = .Definition,
			linkage = .External,
			is_main = true,
			result = types.VOID,
			body = body,
			problematic = true,
			diagnostic = id,
		},
	)
}

replace_main_with_trap :: proc(checker: ^Checker, diagnostic: source.Diagnostic_Id) {
	for &function in checker.module.functions {
		if !function.is_main {
			continue
		}
		delete(function.params, checker.allocator)
		delete(function.body, checker.allocator)
		function.params = nil
		function.result = types.VOID
		function.calling_convention = .C
		function.implementation = .Definition
		function.linkage = .External
		function.problematic = true
		function.diagnostic = diagnostic
		statement_id := hir.stmt_id(len(checker.module.statements))
		append(
			&checker.module.statements,
			hir.Stmt {
				kind = .Trap,
				span = source.Span{},
				expr = hir.INVALID_EXPR,
				local = hir.INVALID_LOCAL,
				diagnostic = diagnostic,
			},
		)
		function.body = make([]hir.Stmt_Id, 1, checker.allocator)
		function.body[0] = statement_id
		return
	}
	synthesize_trap_main(checker)
}

check :: proc(
	ast_module: ^ast.Module,
	diagnostics: ^source.Diagnostics,
	symbols: ^symbol.Table,
	selected := target.DEFAULT,
	allocator := context.allocator,
) -> hir.Module {
	checker := Checker {
		ast_module  = ast_module,
		diagnostics = diagnostics,
		symbols     = symbols,
		module      = hir.init_module(selected, allocator),
		main_symbol = symbol.intern(symbols, "main"),
		io_provider_template = ast.INVALID_FUNCTION,
		sink_symbol = symbol.intern(symbols, "_"),
		type_symbol = symbol.intern(symbols, "type"),
		target      = selected,
		allocator   = allocator,
	}
	checker.specs.allocator = allocator
	types.destroy_store(&checker.module.types)
	checker.module.types = types.clone_store(&ast_module.type_store, allocator)
	checker.module.types.selected = selected
	for value in ast_module.strings {
		append(&checker.module.strings, strings.clone(value, allocator))
	}
	checker.constant_stack.allocator = allocator
	checker.ast_expr_stack.allocator = allocator
	checker.hir_expr_stack.allocator = allocator
	checker.infer_stack.allocator = allocator
	checker.build_stack.allocator = allocator
	checker.cycle_stack.allocator = allocator
	checker.anon_globals.allocator = allocator
	checker.type_factories.allocator = allocator
	checker.generated_types.allocator = allocator
	checker.type_factory_origins.allocator = allocator
	checker.call_resolutions.allocator = allocator
	checker.static_bindings.allocator = allocator
	checker.comptime_keys.allocator = allocator
	checker.comptime_static_values.allocator = allocator
	checker.expand_context.allocator = allocator
	checker.static_state = ct_state_make(&checker, 0, ast.INVALID_FILE)
	build_symbol_indexes(&checker)
	checker.global_types = make([]types.Type, len(ast_module.globals), allocator)
	checker.global_demands = make([]types.Type, len(ast_module.globals), allocator)
	checker.global_open_const = make([]bool, len(ast_module.globals), allocator)
	checker.global_open_float = make([]bool, len(ast_module.globals), allocator)
	checker.global_const_value = make([]i128, len(ast_module.globals), allocator)
	checker.record_field_constraints = make([]types.Type, len(checker.module.types.fields), allocator)
	checker.record_field_defaults = make([]types.Type, len(checker.module.types.fields), allocator)
	checker.record_field_conflicts = make([]types.Type, len(checker.module.types.fields), allocator)
	checker.record_field_conflict_spans = make([]source.Span, len(checker.module.types.fields), allocator)
	checker.poisoned_packages = make([]bool, max(len(ast_module.packages), 1), allocator)
	init_record_field_inference(&checker)
	checker.external_global_canonical = make([]ast.Global_Id, len(ast_module.globals), allocator)
	checker.external_global_diagnostics = make([]source.Diagnostic_Id, len(ast_module.globals), allocator)
	for &canonical in checker.external_global_canonical {
		canonical = ast.INVALID_GLOBAL
	}
	for &diagnostic in checker.external_global_diagnostics {
		diagnostic = source.INVALID_DIAGNOSTIC
	}
	checker.constants = make([]Constant, len(ast_module.exprs), allocator)
	checker.template_diagnostics = make([]source.Diagnostic_Id, len(ast_module.functions), allocator)
	for &diagnostic, index in checker.template_diagnostics {
		diagnostic = ast_module.functions[index].diagnostic
		pkg := ast_module.functions[index].pkg
		if diagnostic != source.INVALID_DIAGNOSTIC && int(pkg) < len(checker.poisoned_packages) {
			checker.poisoned_packages[pkg] = true
		}
	}
	for global in ast_module.globals {
		if global.diagnostic != source.INVALID_DIAGNOSTIC && int(global.pkg) < len(checker.poisoned_packages) {
			checker.poisoned_packages[global.pkg] = true
		}
	}
	defer {
		for spec in checker.specs {
			delete(spec.args, allocator)
			delete(spec.comptime_values, allocator)
		}
		delete(checker.specs)
		delete(checker.function_index, allocator)
		delete(checker.global_index, allocator)
		delete(checker.import_index, allocator)
		delete(checker.global_types, allocator)
		delete(checker.global_demands, allocator)
		delete(checker.global_open_const, allocator)
		delete(checker.global_open_float, allocator)
		delete(checker.global_const_value, allocator)
		delete(checker.record_field_constraints, allocator)
		delete(checker.record_field_defaults, allocator)
		delete(checker.record_field_conflicts, allocator)
		delete(checker.record_field_conflict_spans, allocator)
		delete(checker.poisoned_packages, allocator)
		delete(checker.external_global_canonical, allocator)
		delete(checker.external_global_diagnostics, allocator)
		delete(checker.constants, allocator)
		delete(checker.template_diagnostics, allocator)
		delete(checker.constant_stack)
		delete(checker.ast_expr_stack)
		delete(checker.hir_expr_stack)
		delete(checker.infer_stack)
		delete(checker.build_stack)
		delete(checker.cycle_stack)
		for entry in checker.type_factories {
			delete(entry.values, allocator)
		}
		for entry in checker.generated_types {
			delete(entry.values, allocator)
			delete(entry.defaults, allocator)
		}
		for origin in checker.type_factory_origins {
			delete(origin.values, allocator)
		}
		for resolution in checker.call_resolutions {
			delete(resolution.ctx, allocator)
			delete(resolution.expand_ctx, allocator)
			delete(resolution.mapping, allocator)
			delete(resolution.comptime_values, allocator)
			delete(resolution.runtime_types, allocator)
		}
		delete(checker.type_factories)
		delete(checker.generated_types)
		delete(checker.type_factory_origins)
		delete(checker.call_resolutions)
		ct_state_destroy(&checker.static_state)
		delete(checker.static_bindings)
		for key in checker.comptime_keys {
			delete(key, allocator)
		}
		delete(checker.comptime_keys)
		delete(checker.comptime_static_values)
		delete(checker.expand_context)
	}

	for function, index in ast_module.functions {
		if function.generated {
			continue
		}
		for previous in ast_module.functions[:index] {
			if !previous.generated && previous.pkg == function.pkg && previous.name == function.name &&
			   declarations_conflict(function.file, function.file_hidden, previous.file, previous.file_hidden) {
				source.addf(diagnostics, function.span, "duplicate function '%s'", symbol_text(&checker, function.name))
			}
		}
		for global in ast_module.globals {
			if global.pkg == function.pkg && global.name == function.name &&
			   declarations_conflict(function.file, function.file_hidden, global.file, global.file_hidden) {
				source.addf(diagnostics, function.span, "package declaration '%s' conflicts with a global", symbol_text(&checker, function.name))
			}
		}
		if type_declaration_conflicts(&checker, function.pkg, function.name, function.file, function.file_hidden) {
			source.addf(diagnostics, function.span, "function '%s' shadows visible type", symbol_text(&checker, function.name))
		}
	}
	for global, index in ast_module.globals {
		for previous in ast_module.globals[:index] {
			if previous.pkg == global.pkg && previous.name == global.name &&
			   declarations_conflict(global.file, global.file_hidden, previous.file, previous.file_hidden) {
				source.addf(diagnostics, global.span, "duplicate global '%s'", symbol_text(&checker, global.name))
			}
		}
		if type_declaration_conflicts(&checker, global.pkg, global.name, global.file, global.file_hidden) {
			source.addf(diagnostics, global.span, "global '%s' shadows visible type", symbol_text(&checker, global.name))
		}
	}

	validate_type_nodes(&checker)
	validate_declarations(&checker)
	configure_entry_point(&checker)
	infer_all(&checker)
	finalize_record_field_inference(&checker)
	validate_external_globals(&checker)
	prune_specs(&checker)
	build_globals(&checker)
	for index := 0; index < len(checker.specs); index += 1 {
		build_function(&checker, spec_id(index))
	}
	// Flush anonymous globals synthesized for `&<array literal>`. Appended only now
	// (after every ast global was built at its identity-mapped index) so their ids,
	// pre-assigned as len(ast.globals)+stage_index, land exactly.
	for anon in checker.anon_globals {
		append(&checker.module.globals, anon)
	}
	propagate_global_reads(&checker)

	main_template := find_template(&checker, checker.main_symbol, 0)
	main_declarations := 0
	for function in ast_module.functions {
		if !function.generated && function.pkg == 0 && function.name == checker.main_symbol {
			main_declarations += 1
		}
	}
	if main_declarations == 0 {
		synthesize_trap_main(&checker)
	} else {
		template := ast_module.functions[main_template]
		valid_params := checker.entry_point == .Plain && len(template.params) == 0 ||
		                checker.entry_point == .Process && len(template.params) == 1
		if main_declarations != 1 ||
		   !template.has_body ||
		   !valid_params ||
		   !(template.result == types.VOID || template.result == types.I32 || template.result == types.INT) {
			id := checker.template_diagnostics[main_template]
			if id == source.INVALID_DIAGNOSTIC {
				id = source.add(
					diagnostics,
					template.span,
					"main must be unique, have a body, take no parameters or one @std/process Init, and return void, i32, or int",
				)
			}
			checker.module.injected_main = hir.INVALID_FUNCTION
			checker.module.io_provider = hir.INVALID_FUNCTION
			replace_main_with_trap(&checker, id)
		} else if checker.entry_point == .Process {
			main_spec := find_spec(&checker, main_template, nil)
			provider_spec := find_spec(&checker, checker.io_provider_template, nil)
			if main_spec != INVALID_SPEC && provider_spec != INVALID_SPEC {
				checker.module.injected_main = checker.specs[main_spec].hir_id
				checker.module.io_provider = checker.specs[provider_spec].hir_id
			}
		}
	}

	propagate_problems(&checker)
	states := make([]u8, len(checker.module.globals), allocator)
	for index in 0 ..< len(checker.module.globals) {
		detect_global_cycles_visit(&checker, hir.global_id(index), states)
	}
	delete(states, allocator)
	propagate_problems(&checker)
	for import_item in ast_module.imports {
		if !import_item.test_only && import_item.valid && !import_item.used {
			source.addf_warning(diagnostics, import_item.span, "unused import '%s'", symbol_text(&checker, import_item.alias))
		}
	}
	return checker.module
}