package checker

import "../ast"
import "../hir"
import "../source"
import "../symbol"
import "../target"
import "../types"
import "base:intrinsics"
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,
	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 marks a local whose initializer is a compile-time integer with no
	// concrete annotation: like an open-constant global, it is sign-agnostic until a
	// backward demand from a use picks its family/width (see merge_local_demand).
	open_const:  bool,
	const_value: i128,
	demanded:    bool,
}

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

// 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.
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,
	global_reads: ^[dynamic]hir.Global_Id,
	calls:        ^[dynamic]hir.Function_Id,
	problematic:  ^bool,
}

Constant_Kind :: enum {
	Unknown,
	Not_Constant,
	Value,
	Overflow,
	Div_By_Zero,
}

Constant :: struct {
	kind:  Constant_Kind,
	value: i128,
}

Function_Index_Entry :: struct {
	scope: ast.Package_Id,
	name:  symbol.Id,
	id:    ast.Function_Id,
}

Global_Index_Entry :: struct {
	scope: ast.Package_Id,
	name:  symbol.Id,
	id:    ast.Global_Id,
}

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

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_const_value:   []i128,
	global_demands_dirty: 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,
	main_symbol:          symbol.Id,
	sink_symbol:          symbol.Id,
	target:               target.Target,
	allocator:            mem.Allocator,
}

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

Constant_Frame :: struct {
	expr:  ast.Expr_Id,
	stage: u8,
}

eval_constant :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> Constant {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return Constant{kind = .Not_Constant}
	}
	stack := checker.constant_stack
	clear_dynamic_array(&stack)
	defer {
		clear_dynamic_array(&stack)
		checker.constant_stack = stack
	}
	append(&stack, Constant_Frame{expr=expr_id})

	for len(stack) > 0 {
		frame_index := len(stack)-1
		frame := stack[frame_index]
		if checker.constants[frame.expr].kind != .Unknown {
			_ = pop(&stack)
			continue
		}
		expr := checker.ast_module.exprs[frame.expr]
		if expr.kind != .Add && expr.kind != .Sub && expr.kind != .Mul &&
		   expr.kind != .Div && expr.kind != .Negate {
			result := Constant{kind = .Not_Constant}
			if expr.kind == .Integer {
				result = Constant{kind = .Value, value = i128(expr.integer)}
			}
			checker.constants[frame.expr] = result
			_ = pop(&stack)
			continue
		}
		if frame.stage == 0 {
			stack[frame_index].stage = 1
			if expr.left != ast.INVALID_EXPR && int(expr.left) < len(checker.ast_module.exprs) &&
			   checker.constants[expr.left].kind == .Unknown {
				append(&stack, Constant_Frame{expr=expr.left})
			}
			continue
		}
		if frame.stage == 1 && expr.kind == .Negate {
			operand := Constant{kind = .Not_Constant}
			if expr.left != ast.INVALID_EXPR && int(expr.left) < len(checker.constants) {
				operand = checker.constants[expr.left]
			}
			result := Constant{kind = .Not_Constant}
			if operand.kind == .Div_By_Zero {
				result = Constant{kind = .Div_By_Zero}
			} else if operand.kind == .Overflow {
				result = Constant{kind = .Overflow}
			} else if operand.kind == .Value {
				value, overflow := intrinsics.overflow_sub(i128(0), operand.value)
				result = Constant{kind = .Overflow} if overflow else Constant{kind = .Value, value = value}
			}
			checker.constants[frame.expr] = result
			_ = pop(&stack)
			continue
		}
		if frame.stage == 1 {
			stack[frame_index].stage = 2
			if expr.right != ast.INVALID_EXPR && int(expr.right) < len(checker.ast_module.exprs) &&
			   checker.constants[expr.right].kind == .Unknown {
				append(&stack, Constant_Frame{expr=expr.right})
			}
			continue
		}
		left := Constant{kind = .Not_Constant}
		right := Constant{kind = .Not_Constant}
		if expr.left != ast.INVALID_EXPR && int(expr.left) < len(checker.constants) {
			left = checker.constants[expr.left]
		}
		if expr.right != ast.INVALID_EXPR && int(expr.right) < len(checker.constants) {
			right = checker.constants[expr.right]
		}
		result := Constant{kind = .Not_Constant}
		if left.kind == .Div_By_Zero || right.kind == .Div_By_Zero {
			result = Constant{kind = .Div_By_Zero}
		} else if left.kind == .Overflow || right.kind == .Overflow {
			result = Constant{kind = .Overflow}
		} else if left.kind == .Value && right.kind == .Value {
			value: i128
			overflow: bool
			div_by_zero: bool
			#partial switch expr.kind {
			case .Sub: value, overflow = intrinsics.overflow_sub(left.value, right.value)
			case .Mul: value, overflow = intrinsics.overflow_mul(left.value, right.value)
			case .Div:
				if right.value == 0 {
					div_by_zero = true
				} else {
					value = left.value / right.value
				}
			case: value, overflow = intrinsics.overflow_add(left.value, right.value)
			}
			switch {
			case div_by_zero: result = Constant{kind = .Div_By_Zero}
			case overflow:    result = Constant{kind = .Overflow}
			case:             result = Constant{kind = .Value, value = value}
			}
		}
		checker.constants[frame.expr] = result
		_ = pop(&stack)
	}
	return checker.constants[expr_id]
}

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)
}

type_from_syntax :: proc(value: ast.Type_Syntax) -> types.Type {
	return value
}

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

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
}

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)))
}

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) -> 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
		}
	}
	if low < len(index) && index[low].scope == scope && index[low].name == name {
		return index[low].id
	}
	return ast.INVALID_FUNCTION
}

find_global_symbol :: proc(index: []Global_Index_Entry, scope: ast.Package_Id, name: symbol.Id) -> 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
		}
	}
	if low < len(index) && index[low].scope == scope && index[low].name == name {
		return index[low].id
	}
	return ast.INVALID_GLOBAL
}

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) {
	checker.function_index = make([]Function_Index_Entry, len(checker.ast_module.functions), checker.allocator)
	for function, id in checker.ast_module.functions {
		checker.function_index[id] = Function_Index_Entry{scope=function.pkg, name=function.name, id=ast.function_id(id)}
	}
	slice.sort_by(checker.function_index, function_index_less)

	checker.global_index = make([]Global_Index_Entry, len(checker.ast_module.globals), checker.allocator)
	for global, id in checker.ast_module.globals {
		checker.global_index[id] = Global_Index_Entry{scope=global.pkg, name=global.name, id=ast.global_id(id)}
	}
	slice.sort_by(checker.global_index, global_index_less)

	checker.import_index = make([]Import_Index_Entry, len(checker.ast_module.imports), checker.allocator)
	for import_item, id in checker.ast_module.imports {
		checker.import_index[id] = Import_Index_Entry{scope=import_item.file, name=import_item.alias, id=ast.import_id(id)}
	}
	slice.sort_by(checker.import_index, import_index_less)
}

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

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

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
}

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
}

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 {
		return source.addf(checker.diagnostics, expr.span, "unknown package alias '%s'", symbol_text(checker, expr.qualifier))
	}
	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) -> source.Diagnostic_Id {
	if find_template(checker, expr.name, target_pkg) != ast.INVALID_FUNCTION {
		return source.addf(checker.diagnostics, expr.span, "'%s' is a function, not a global value", symbol_text(checker, expr.name))
	}
	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),
		)
	}
	return source.addf(checker.diagnostics, expr.span, "unresolved global '%s'", symbol_text(checker, expr.name))
}

add_call_resolution_diagnostic :: proc(checker: ^Checker, expr: ast.Expr, target_pkg: ast.Package_Id) -> source.Diagnostic_Id {
	if find_global(checker, expr.name, target_pkg) != ast.INVALID_GLOBAL {
		return source.addf(checker.diagnostics, expr.span, "'%s' is a global, not a function", symbol_text(checker, expr.name))
	}
	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),
		)
	}
	return source.addf(checker.diagnostics, expr.span, "unresolved function '%s'", symbol_text(checker, expr.name))
}

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)
	}
	return source.INVALID_DIAGNOSTIC
}

function_signatures_equal :: proc(left, right: ast.Function) -> bool {
	if left.result != right.result || 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 {
			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_arg_expected :: proc(function: ast.Function, index: int) -> types.Type {
	if index < 0 || index >= len(function.params) {
		return types.INVALID
	}
	declared := type_from_syntax(function.params[index].type)
	// 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.
	// `int`/`range` constraints have no single default and keep building naturally.
	if declared == types.FLOAT {
		return types.F64
	}
	return declared
}

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.c_abi {
		return nil, types.INVALID, false
	}
	result = type_from_syntax(function.result)
	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(param.type)
		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_pointer_type_for_template :: proc(
	checker: ^Checker,
	template: ast.Function_Id,
	demanded: ^[dynamic]Spec_Id = nil,
) -> (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, true, function.variadic)
	pointer_type := types.pointer(&checker.module.types, function_type, false, true)
	spec := INVALID_SPEC
	if demanded == nil {
		spec = ensure_spec(checker, template, params)
	} else {
		spec = find_spec(checker, template, params)
		mark_spec_demanded(checker, spec, demanded)
	}
	return pointer_type, spec, spec != INVALID_SPEC
}

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 (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, .Address, .Deref, .Field, .Unwrap, .Keyed:
			append(&stack, expr.left)
		case .Add, .Sub, .Mul, .Div, .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, .Name, .Enum_Literal:
		}
	}
}

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]
		switch statement.kind {
		case .Declaration, .Assignment, .Return, .Expression:
			mark_expr_imports_used(checker, statement.expr, file)
			if statement.target != ast.INVALID_EXPR {
				mark_expr_imports_used(checker, statement.target, 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 .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
		}
	}
}

validate_declarations :: proc(checker: ^Checker) {
	for function, function_id in checker.ast_module.functions {
		if len(function.unsupported_reason) > 0 {
			continue
		}
		locals: [dynamic]symbol.Id
		locals.allocator = checker.allocator
		for param in function.params {
			if diagnostic := add_unsupported_type_diagnostic(checker, param.span, type_from_syntax(param.type));
			   diagnostic != source.INVALID_DIAGNOSTIC {
				checker.template_diagnostics[function_id] = diagnostic
				continue
			}
			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),
				)
			}
			append(&locals, param.name)
			if types.contains_c_struct_by_value(type_from_syntax(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 diagnostic := add_unsupported_type_diagnostic(checker, function.span, type_from_syntax(function.result));
		   diagnostic != source.INVALID_DIAGNOSTIC {
			checker.template_diagnostics[function_id] = diagnostic
		}
		if types.contains_c_struct_by_value(type_from_syntax(function.result), &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 !function.has_body && !function.c_abi {
			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) {
			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 {
			for param in function.params {
				param_type := type_from_syntax(param.type)
				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(function.result)
			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",
				)
			}
		}
		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
		}
	}
}

validate_type_nodes :: proc(checker: ^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 {
			if item.c_layout && !item.opaque && item.field_count == 0 {
				source.add(
					checker.diagnostics,
					source.Span{},
					"c_struct definitions require at least one field",
				)
			}
			for field in types.fields_for(&checker.module.types, id) {
				if !types.is_runtime_value(field.type, &checker.module.types) {
					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",
					)
				}
			}
		}
		if item.kind == .Function {
			if !item.c_abi {
				source.add(checker.diagnostics, source.Span{}, "only c_func function pointer types are supported")
			}
			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{}, "function 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{}, "function pointer results must be concrete C signature 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) -> Spec_Id {
	function := checker.ast_module.functions[template]
	for spec, index in checker.specs {
		if spec.template != template || len(spec.args) != len(function.params) {
			continue
		}
		matches := true
		for param, param_index in function.params {
			actual := types.INVALID
			if param_index < len(actual_args) {
				actual = actual_args[param_index]
			}
			if !types.equal(spec.args[param_index], specialized_param_type(checker, param.type, actual)) {
				matches = false
				break
			}
		}
		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`/`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) -> types.Type {
	declared := type_from_syntax(syntax)
	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) -> bool {
	for param, index in function.params {
		actual := types.INVALID
		if index < len(actual_args) {
			actual = actual_args[index]
		}
		if !is_runtime_type(checker, specialized_param_type(checker, param.type, actual)) {
			return false
		}
	}
	return true
}

ensure_spec :: proc(checker: ^Checker, template: ast.Function_Id, actual_args: []types.Type) -> Spec_Id {
	if existing := find_spec(checker, template, actual_args); existing != INVALID_SPEC {
		return existing
	}
	function := checker.ast_module.functions[template]
	signature: [dynamic]types.Type
	signature.allocator = checker.allocator
	for param, index in function.params {
		actual := types.INVALID
		if index < len(actual_args) {
			actual = actual_args[index]
		}
		append(&signature, specialized_param_type(checker, param.type, actual))
	}
	result := type_from_syntax(function.result)
	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT {
		result = types.I32
	}
	index := spec_id(len(checker.specs))
	append(
		&checker.specs,
		Spec{template = template, args = 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,
	stage:     u8,
	left:      types.Type,
	arg_index: int,
	args:      []types.Type,
	template:  ast.Function_Id,
}

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,
) -> 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)
	delete(checker.infer_stack)
	checker.infer_stack = outer
	return result
}

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,
) -> types.Type {
	store := &checker.module.types
	#partial switch expr.kind {
	case .Bool:
		return types.BOOL
	case .Not:
		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		return types.BOOL
	case .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or:
		_ = 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 types.INVALID
	case .Undefined:
		return types.INVALID
	case .Enum_Literal:
		return types.INVALID
	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:
		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
		item, ok := types.container(value, store)
		if !ok || item.kind == .Pointer {
			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)
		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)
		}
		_, field, ok := find_struct_field(checker, value, expr.name)
		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 .Struct_Literal:
		for keyed in expr.args {
			_ = infer_nested_expr(checker, checker.ast_module.exprs[keyed].left, locals, pkg, file, demanded, local_types)
		}
		target_pkg, available := expr_package(checker, expr, pkg, file)
		value := types.find_named(store, u32(target_pkg), u32(expr.name)) if available else types.INVALID
		return types.resolve_alias(value, store)
	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,
) -> types.Type {
	stack := checker.infer_stack
	clear_dynamic_array(&stack)
	defer {
		for frame in stack {
			delete(frame.args, checker.allocator)
		}
		clear_dynamic_array(&stack)
		checker.infer_stack = stack
	}
	append(&stack, Infer_Frame{expr=expr_id, 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 := eval_constant(checker, frame.expr)
			if constant.kind == .Overflow || constant.kind == .Div_By_Zero ||
			   (constant.kind == .Value && !fits_i64(constant.value)) {
				last = types.I64
				_ = pop(&stack)
				continue
			}
			if constant.kind == .Value {
				last = types.smallest_signed_for_literal(i64(constant.value))
				_ = pop(&stack)
				continue
			}
			switch expr.kind {
			case .Invalid:
				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, .Struct_Literal, .Keyed, .Enum_Literal,
			     .Bool, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
				last = infer_compound_expr(checker, expr, locals, pkg, file, demanded, local_types)
				_ = 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) {
						_, field, ok := find_struct_field(checker, base_type, expr.name)
						if ok {
							last = field.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)
						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)
						if template != ast.INVALID_FUNCTION &&
						   len(checker.ast_module.functions[template].unsupported_reason) == 0 &&
						   checker.template_diagnostics[template] == source.INVALID_DIAGNOSTIC {
							pointer_type, _, ok := function_pointer_type_for_template(checker, template, demanded)
							if ok {
								last = pointer_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.function_pointer(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)
				}
				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) && available {
						global := find_global(checker, expr.name, target_pkg)
						if global != ast.INVALID_GLOBAL {
							callee_type = checker.global_types[global]
						}
					}
					_, function_item, function_type, ok := types.function_pointer(callee_type, &checker.module.types)
					if !ok {
						distinct_type := types.find_named(&checker.module.types, u32(target_pkg), u32(expr.name))
						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 := type_from_syntax(checker.ast_module.functions[template].result)
					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
					_ = pop(&stack)
					continue
				}
				stack[frame_index].template = template
				stack[frame_index].args = make([]types.Type, len(expr.args), checker.allocator)
				stack[frame_index].stage = 3
				if len(expr.args) > 0 {
					append(&stack, Infer_Frame{expr=expr.args[0], 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 {
			if expr.kind == .Add && types.is_many_pointer(frame.left, &checker.module.types) && types.is_concrete_integer(last) {
				last = frame.left
			} else {
				last = types.widest(frame.left, last)
			}
			_ = pop(&stack)
			continue
		}
		if frame.stage == 3 {
			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 := checker.ast_module.functions[frame.template]
			// A bare-name argument passed to a concrete (non-constraint) parameter pushes
			// that parameter type back onto the argument's slot, so an open constant adopts
			// it (e.g. `take_u16(a)` resolves `a` to u16). Constraint params have no single
			// type to demand; the callee's result flowing back is milestone 14.5.
			for arg_index in 0..<len(expr.args) {
				record_demand(checker, expr.args[arg_index], call_arg_expected(function, arg_index), locals, local_types, pkg, file)
			}
			if valid_call_arity(function, len(expr.args)) &&
			   can_specialize(checker, function, stack[frame_index].args) {
				spec := INVALID_SPEC
				if demanded == nil {
					spec = ensure_spec(checker, frame.template, stack[frame_index].args)
				} else {
					spec = find_spec(checker, frame.template, stack[frame_index].args)
					mark_spec_demanded(checker, spec, demanded)
				}
				if spec != INVALID_SPEC {
					last = checker.specs[spec].result
				} else {
					declared := type_from_syntax(function.result)
					last = declared if is_runtime_type(checker, declared) || types.is_void(declared) else types.INVALID
				}
			} else {
				declared := type_from_syntax(function.result)
				if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT {
					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
			_ = 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:
			declared_local := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
			value_type := types.INVALID
			if !is_undefined_expr(checker, statement.expr) {
				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
			}
			if is_runtime_type(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
			const_val := i128(0)
			if !is_runtime_type(checker, declared_local) && !is_undefined_expr(checker, statement.expr) {
				constant := eval_constant(checker, statement.expr)
				if constant.kind == .Value && fits_i64(constant.value) {
					open = true
					const_val = constant.value
				}
			}
			local := Infer_Local{
				name=statement.name,
				type=value_type,
				declared=declared_local,
				statement=statement_id,
				mutable=!statement.immutable,
				open_const=open,
				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).
			record_demand(checker, statement.expr, value_type, locals^[:], local_types, pkg, file)
		case .Assignment:
			value_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
			if statement.target != ast.INVALID_EXPR {
				_ = 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].mutable {
						_ = merge_infer_local_type(checker, &locals^[local_index], value_type, local_types)
					}
				}
			} 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)
				}
			}
		case .Expression:
			_ = 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)
				if is_runtime_type(checker, result_hint) {
					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(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)
				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:
			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)
		}
	}
	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]
	declared := type_from_syntax(function.result)
	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) 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)
	for param, index in function.params {
		param_type := types.INVALID
		if index < len(spec.args) {
			param_type = spec.args[index]
		}
		append(&locals, Infer_Local{name=param.name, type=param_type, declared=param_type, statement=ast.INVALID_STMT})
	}

	result := types.INVALID
	infer_statements(checker, function.body, &locals, local_types, function.pkg, function.file, demanded, &result, result_hint)
	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
}

// root_demand_target returns the global that an initializer pushes a backward type
// demand onto: when the initializer's root expression is a bare name referencing a
// global (e.g. `Z i32 :: Y`). Returns INVALID_GLOBAL for any other shape — demands
// deliberately do not flow through arithmetic, calls, or other operators (that is L3).
root_demand_target :: proc(checker: ^Checker, expr_id: ast.Expr_Id, pkg: ast.Package_Id, file: ast.File_Id) -> ast.Global_Id {
	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return ast.INVALID_GLOBAL
	}
	expr := checker.ast_module.exprs[expr_id]
	if expr.kind != .Name {
		return ast.INVALID_GLOBAL
	}
	target_pkg, available := expr_package(checker, expr, pkg, file)
	if !available {
		return ast.INVALID_GLOBAL
	}
	return find_global(checker, expr.name, target_pkg)
}

// merge_open_const_demand records a concrete integer demand onto an open-constant
// global's slot. The constant is sign-agnostic until used, so it may adopt any
// integer family/width whose range holds its value (first demand wins; later demands
// may only widen within the chosen family). Non-integer or non-fitting demands are
// ignored, leaving the constant to default and the genuine mismatch to surface at the
// use's boundary coercion.
merge_open_const_demand :: proc(checker: ^Checker, slot: ^types.Type, demand: types.Type, value: i128) -> bool {
	if !types.is_concrete_integer(demand) || !fits_integer_type(value, demand, checker.target) {
		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
	}
	changed: bool
	if checker.global_open_const[index] {
		changed = merge_open_const_demand(checker, &checker.global_demands[index], demand, 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 integer demand onto an open-constant local.
// Like an open-constant global it adopts any integer family/width whose range holds its
// value (gated by its constraint family if it has one); the first demand replaces the
// literal's signed default, 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 || !types.is_concrete_integer(demand) {
		return false
	}
	if types.is_constraint(local.declared) &&
	   !types.constraint_accepts(local.declared, demand, &checker.module.types) {
		return false
	}
	if !fits_integer_type(local.const_value, demand, checker.target) {
		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
}

// record_demand pushes a concrete type demand onto the slot of a bare-name expression
// (a typed declaration's initializer, a call argument, a return value). When the name
// resolves to an open-constant local or global, that slot adopts the demand; any other
// shape is ignored — demands flow only through bare names, never through arithmetic or
// across a call's result (the latter is 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,
) {
	if !is_runtime_type(checker, demand) ||
	   expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
		return
	}
	expr := checker.ast_module.exprs[expr_id]
	if expr.kind != .Name {
		return
	}
	if !symbol.is_valid(expr.qualifier) {
		if index, ok := find_infer_local_index(locals, expr.name); ok {
			_ = merge_local_demand(checker, &locals[index], demand, local_types)
			return
		}
	}
	target_pkg, available := expr_package(checker, expr, pkg, file)
	if !available {
		return
	}
	global := find_global(checker, expr.name, target_pkg)
	if global != ast.INVALID_GLOBAL {
		_ = merge_global_demand(checker, global, demand)
	}
}

infer_all :: proc(checker: ^Checker) {
	// An "open constant" global has no concrete declared type and a compile-time
	// integer initializer. Its slot stays sign-agnostic so a backward demand from any
	// reachable use can pick its family/width; absent a demand it defaults to the
	// smallest signed type (legacy behaviour). Demands accumulate in global_demands so
	// the default never blocks a later cross-family (e.g. unsigned) demand.
	for global, index in checker.ast_module.globals {
		declared := type_from_syntax(global.type)
		if is_runtime_type(checker, declared) {
			checker.global_types[index] = declared
			continue
		}
		if global.external {
			continue
		}
		constant := eval_constant(checker, global.expr)
		if constant.kind == .Value && fits_i64(constant.value) {
			checker.global_open_const[index] = true
			checker.global_const_value[index] = constant.value
		}
	}

	main_template := find_template(checker, checker.main_symbol, 0)
	if main_template != ast.INVALID_FUNCTION {
		ensure_spec(checker, main_template, nil)
	}

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

		// Backward demands: a global whose initializer's root is a bare name referencing
		// another global pushes its own (declared or already-resolved) type onto that
		// referent. Open constants adopt any fitting family; other referents widen only.
		for global, index in checker.ast_module.globals {
			if global.external {
				continue
			}
			demand := checker.global_types[index]
			if !is_runtime_type(checker, demand) {
				continue
			}
			target := root_demand_target(checker, global.expr, global.pkg, global.file)
			if target != ast.INVALID_GLOBAL {
				merge_global_demand(checker, target, demand)
			}
		}

		// 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 {
				continue
			}
			inferred := infer_expr(checker, global.expr, nil, global.pkg, global.file)
			if is_runtime_type(checker, type_from_syntax(global.type)) {
				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] {
				resolved := types.smallest_signed_for_literal(i64(checker.global_const_value[index]))
				if !types.equal(checker.global_types[index], resolved) {
					checker.global_types[index] = resolved
					changed = true
				}
			} 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)
			inferred := infer_spec_result(checker, id)
			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 !changed {
			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)
	}
	for global in checker.ast_module.globals {
		if global.external {
			continue
		}
		_ = infer_expr(checker, global.expr, nil, global.pkg, global.file, &stack)
	}
	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)
			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,
		},
	)
}

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
}

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
	}
	actual := checker.module.exprs[expr_id].type
	if types.equal(actual, expected) {
		return expr_id
	}
	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.is_optional(expected, &checker.module.types) {
		child := types.child_type(expected, &checker.module.types)
		if types.equal(actual, child) ||
		   types.can_widen(actual, child) ||
		   types.can_weaken_pointer(actual, child, &checker.module.types) ||
		   types.can_weaken_slice(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) {
		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",
		types.name(actual),
		types.name(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 == .Overflow ||
	   (!types.is_concrete_integer(expected) && !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 := types.smallest_signed_for_literal(value)
	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,
	built_args: []hir.Expr_Id,
	arg_types:  []types.Type,
	template:   ast.Function_Id,
}

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_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))
	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))
	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 C callback; expected a concrete c_func",
			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, true, function.variadic)
	pointer_type := types.pointer(&checker.module.types, function_type, false, true)
	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, pointer_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=pointer_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
}

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_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 .Address:
		value := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		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)
		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)
		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:
		container := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		container_type := checker.module.exprs[container].type
		item, ok := types.container(container_type, store)
		if !ok || item.kind == .Pointer {
			id := source.add(checker.diagnostics, expr.span, "slicing requires an array, slice, or pointer-to-array")
			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)
			}
		}
		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)
		base_type := checker.module.exprs[base].type
		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,
				})
			}
			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,
				})
			}
			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)
			}
		}
		if types.is_pointer(base_type, store) {
			base_type = types.child_type(base_type, store)
		}
		index, field, ok := find_struct_field(checker, base_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)
		}
		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,
		})
	case .Unwrap:
		optional := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
		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)
		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 .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)
		}
		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 .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)
		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 .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)
		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 a bare integer-literal operand to the other operand's type
		// so comparisons like `count > 0` or `0 < count` type-check.
		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]
		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_const.kind == .Value && left_const.kind != .Value {
			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_const.kind == .Value && right_const.kind != .Value {
			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)
		}
		left_type := checker.module.exprs[left].type
		right_type := checker.module.exprs[right].type
		if checker.module.exprs[left].kind == .Invalid || checker.module.exprs[right].kind == .Invalid {
			return invalid_hir_expr(checker, expr.span, expr.diagnostic, types.BOOL)
		}
		operand_type := types.INVALID
		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:
		target_pkg, available := expr_package(checker, expr, pkg, file, true)
		struct_type := types.find_named(store, u32(target_pkg), u32(expr.name)) 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)
		}
		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
			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
				}
				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 {
			if values[0] == hir.INVALID_EXPR {
				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)
			}
			for value, index in initialized {
				if value {
					active_field = i64(index)
					break
				}
			}
		}
		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 {
	// 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)
	}
	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
	clear_dynamic_array(&stack)
	defer {
		for frame in stack {
			delete(frame.built_args, checker.allocator)
			delete(frame.arg_types, checker.allocator)
		}
		clear_dynamic_array(&stack)
		checker.build_stack = 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 {
			constant := eval_constant(checker, frame.expr)
			if constant.kind == .Value || constant.kind == .Overflow || constant.kind == .Div_By_Zero {
				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, .Struct_Literal, .Keyed,
			     .Bool, .Not, .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 .Invalid, .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 = add_hir_expr(checker, hir.Expr{
							kind=.Local, span=expr.span, type=local.type, target=hir.local_ref(local.id),
							left = hir.INVALID_EXPR, right = hir.INVALID_EXPR, diagnostic = source.INVALID_DIAGNOSTIC,
						})
					}
				} else if local, ok := find_build_local(locals, expr.qualifier); ok {
					base := add_hir_expr(checker, hir.Expr{
						kind=.Local, span=expr.span, type=local.type, target=hir.local_ref(local.id),
						left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
					})
					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 {
					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); global != ast.INVALID_GLOBAL {
						last = build_global_reference(checker, global, expr.span, global_reads)
					} else {
						template := find_template(checker, expr.name, target_pkg)
						if template != ast.INVALID_FUNCTION && checker.ast_module.functions[template].c_abi {
							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)
							}
							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
				}
				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)
				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.function_pointer(local.type, &checker.module.types); callable {
								callee = add_hir_expr(checker, hir.Expr{
									kind=.Local, span=expr.span, type=local.type, target=hir.local_ref(local.id),
									left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
								})
							} else {
								non_callable = true
							}
						}
					}
					if callee == hir.INVALID_EXPR && !non_callable {
						if global := find_global(checker, expr.name, target_pkg); global != ast.INVALID_GLOBAL {
							if _, _, _, callable := types.function_pointer(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))
						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) if non_callable_global else
							     source.add(checker.diagnostics, expr.span, "call target is not a function pointer")
						} 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)
							}
						}
						last = invalid_hir_expr(checker, expr.span, id)
						_ = pop(&stack)
						continue
					}
					_, function_item, function_type, _ := types.function_pointer(checker.module.exprs[callee].type, &checker.module.types)
					if !valid_callable_arity(function_item, len(expr.args)) {
						message := "function pointer expects at least %d arguments, got %d" if function_item.variadic else
						           "function pointer 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 !valid_call_arity(function, len(expr.args)) {
					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),
					)
					last = invalid_hir_expr(checker, expr.span, id)
					_ = pop(&stack)
					continue
				}
				stack[frame_index].template = template
				stack[frame_index].built_args = make([]hir.Expr_Id, len(expr.args), checker.allocator)
				stack[frame_index].arg_types = make([]types.Type, len(expr.args), checker.allocator)
				stack[frame_index].stage = 3
				if len(expr.args) > 0 {
					arg_expected := call_arg_expected(function, 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 == 5 {
			operand := last
			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
				stack[frame_index].arg_types[frame.arg_index] = checker.module.exprs[last].type
				stack[frame_index].arg_index += 1
				if frame.arg_index+1 < len(expr.args) {
					next := frame.arg_index+1
					arg_expected := call_arg_expected(checker.ast_module.functions[frame.template], 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
				}
			}
			arg_violation := source.INVALID_DIAGNOSTIC
			{
				params := checker.ast_module.functions[frame.template].params
				for index in 0..<len(params) {
					if index >= len(stack[frame_index].arg_types) {
						break
					}
					declared := type_from_syntax(params[index].type)
					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
					}
				}
			}
			spec := find_spec(checker, frame.template, stack[frame_index].arg_types)
			delete(stack[frame_index].arg_types, checker.allocator)
			stack[frame_index].arg_types = nil
			if arg_violation != source.INVALID_DIAGNOSTIC {
				delete(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				last = invalid_hir_expr(checker, expr.span, arg_violation)
				_ = 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
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
				continue
			}
			fixed_count := len(checker.ast_module.functions[frame.template].params)
			for index in 0..<fixed_count {
				stack[frame_index].built_args[index] = coerce_expr(
					checker,
					stack[frame_index].built_args[index],
					checker.specs[spec].args[index],
					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,
				)
			}
			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(stack[frame_index].built_args, checker.allocator)
				stack[frame_index].built_args = nil
				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=stack[frame_index].built_args, diagnostic = source.INVALID_DIAGNOSTIC,
				})
				stack[frame_index].built_args = nil
			}
			_ = pop(&stack)
		}
		if frame.stage == 6 {
			callee := last
			_, function_item, function_type, ok := types.function_pointer(checker.module.exprs[callee].type, &checker.module.types)
			if !ok {
				id := source.add(checker.diagnostics, expr.span, "call target is not a function pointer")
				last = invalid_hir_expr(checker, expr.span, id)
				_ = pop(&stack)
				continue
			}
			if !valid_callable_arity(function_item, len(expr.args)) {
				message := "function pointer expects at least %d arguments, got %d" if function_item.variadic else
				           "function pointer 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.function_pointer(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
				}
			}
			callee_type := checker.module.exprs[frame.left].type
			_, function_item, function_type, ok := types.function_pointer(callee_type, &checker.module.types)
			if !ok {
				id := source.add(checker.diagnostics, expr.span, "call target is not a function pointer")
				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 function pointer 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 {
				last = add_hir_expr(checker, hir.Expr{
					kind=.Call, span=expr.span, type=result, target=hir.INVALID_REF,
					left=frame.left, 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)
		}
	}
	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 {
		return fmt.aprintf("main", 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)
	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))
		}
	}
	return fmt.aprintf("%s", strings.to_string(builder), allocator = checker.allocator)
}

build_block :: proc(
	ctx: ^Build_Ctx,
	statements: []ast.Stmt_Id,
	duplicate_scope_start := -1,
) -> []hir.Stmt_Id {
	checker := ctx.checker
	body: [dynamic]hir.Stmt_Id
	body.allocator = checker.allocator
	scope_start := len(ctx.locals^)
	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]
		switch statement.kind {
		case .Declaration:
			declared := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
			// Adopt the type inference resolved for this local when the declaration has no
			// concrete annotation (a constraint, `undefined`, or an un-annotated open
			// integer constant): the slot may have absorbed a backward demand (e.g. `a :: 10`
			// built as u16 after `take_u16(a)`). Gated to compile-time integer constants so
			// strings/arrays/pointers keep their own initializer type.
			open_const_decl := !is_runtime_type(checker, declared) && !is_undefined_expr(checker, statement.expr)
			if open_const_decl {
				constant := eval_constant(checker, statement.expr)
				open_const_decl = constant.kind == .Value && fits_i64(constant.value)
			}
			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) {
				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) {
					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 _, 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
			}
			local_id := hir.local_id(len(ctx.hir_locals^))
			append(ctx.hir_locals, hir.Local{
				name = statement.name, type = value_type, mutable = !statement.immutable,
			})
			append(ctx.locals, Build_Local{
				name = statement.name, type = value_type, mutable = !statement.immutable, id = local_id,
			})
			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,
				)
				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
					}
					value = build_expr(
						checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
						rhs_expected, ctx.pkg, ctx.file,
					)
					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
						}
						rhs_type := checker.module.exprs[value].type
						result_type := types.widest(target_type, rhs_type)
						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 {
					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 := 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
			}
			local, found := find_build_local(ctx.locals^[:], statement.name)
			if !found {
				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
			}
			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 := 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 statement.expr == ast.INVALID_EXPR {
				if !types.is_void(ctx.result) {
					id := source.add(checker.diagnostics, statement.span, "'return _' is only valid in a void 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
				} else {
					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 := 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)
			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,
			})
			ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
		case .Expression:
			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, "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 {
				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
						}
						local = hir.local_id(len(ctx.hir_locals^))
						append(ctx.hir_locals, hir.Local{name=capture, type=child, mutable=false})
						append(ctx.locals, Build_Local{name=capture, type=child, mutable=false, id=local})
					}
					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
			}
			loop_body := build_block(ctx, statement.body)
			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,
				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:
			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^)
			item_local := hir.local_id(len(ctx.hir_locals^))
			append(ctx.hir_locals, hir.Local{name=statement.name, type=capture_type, mutable=false})
			append(ctx.locals, Build_Local{name=statement.name, type=capture_type, mutable=false, id=item_local})
			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 {
					index_local = hir.local_id(len(ctx.hir_locals^))
					append(ctx.hir_locals, hir.Local{name=statement.index_name, type=types.USIZE, mutable=false})
					append(ctx.locals, Build_Local{name=statement.index_name, type=types.USIZE, mutable=false, id=index_local})
				}
			}
			loop_body := build_block(ctx, statement.body, capture_start)
			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,
					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 .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
		}
	}
	resize(ctx.locals, scope_start)
	return body[:]
}

// 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.
// Recursion into the branch slices handles nested ifs and `else if` chains.
all_paths_return :: proc(module: ^hir.Module, stmts: []hir.Stmt_Id) -> 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) &&
			   all_paths_return(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 {
					return true
				}
			}
		}
	}
	return false
}

build_function :: proc(checker: ^Checker, id: Spec_Id) {
	spec := checker.specs[id]
	function := checker.ast_module.functions[spec.template]
	signature_diagnostic := source.INVALID_DIAGNOSTIC
	if !types.is_void(spec.result) && !is_runtime_type(checker, spec.result) {
		checker.specs[id].result = types.I64
		spec.result = types.I64
		signature_diagnostic = source.addf(
			checker.diagnostics,
			function.span,
			"could not resolve a concrete result type for '%s'",
			symbol_text(checker, function.name),
		)
	}
	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
	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)
	}

	for param, index in function.params {
		local_id := hir.local_id(len(hir_locals))
		param_type := types.INVALID
		if index < len(spec.args) {
			param_type = spec.args[index]
		}
		append(&hir_locals, hir.Local{name = param.name, type = param_type, parameter = true})
		append(&locals, Build_Local{name = param.name, type = param_type, id = local_id})
		append(&params, local_id)
	}

	problematic := signature_diagnostic != source.INVALID_DIAGNOSTIC ||
	               checker.template_diagnostics[spec.template] != source.INVALID_DIAGNOSTIC
	if !function.has_body {
		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 = function.pkg == 0 && function.name == checker.main_symbol,
				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)
		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,
			},
		)
	}
	ctx := Build_Ctx{
		checker = checker,
		pkg = function.pkg,
		file = function.file,
		result = spec.result,
		local_types = local_types,
		locals = &locals,
		hir_locals = &hir_locals,
		global_reads = &global_reads,
		calls = &calls,
		problematic = &problematic,
	}
	block := build_block(&ctx, function.body)
	returns := all_paths_return(&checker.module, block)
	for block_stmt in block {
		append(&body, block_stmt)
	}
	delete(block, checker.allocator)

	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
	}

	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 || (function.pkg == 0 && function.name == checker.main_symbol) else .Brolang,
			implementation = .Definition,
			linkage = .External if function.c_abi || (function.pkg == 0 && function.name == checker.main_symbol) else .Internal,
			is_main = function.pkg == 0 && function.name == checker.main_symbol,
			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,
		},
	)
	delete(locals)
}

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.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(global.type), global.expr)
		expected := types.INVALID
		if is_runtime_type(checker, declared) {
			expected = declared
		} else if constant := eval_constant(checker, global.expr);
		   constant.kind == .Value && fits_i64(constant.value) &&
		   is_runtime_type(checker, checker.global_types[global_index]) {
			// Open constant: build the initializer against the type inference resolved
			// for this slot, so it adopts its demanded/defaulted type (e.g. `A :: 10`
			// built as u16 when a use demanded u16). Gated to compile-time values fitting
			// i64 — exactly the infer-side open-constant condition — so out-of-range
			// constants keep their original "exceeds signed i64 range" diagnostic.
			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, declared) {
			expr = coerce_expr(checker, expr, declared, 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 !is_runtime_type(checker, global_type) {
			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 {
			diagnostic = source.add(
				checker.diagnostics,
				global.span,
				"mutable declarations are only valid inside functions",
			)
			expr = invalid_hir_expr(checker, global.span, diagnostic)
		}
		static_value, is_static := static_integer_value(&checker.module, expr)
		is_static = is_static && diagnostic == source.INVALID_DIAGNOSTIC
		_ = 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 = false,
				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,
}

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 {
			id := source.addf(
				checker.diagnostics,
				checker.ast_module.globals[dependency].span,
				"global initialization cycle involving '%s'",
				symbol_text(checker, checker.module.globals[dependency].name),
			)
			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"),
		sink_symbol = symbol.intern(symbols, "_"),
		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
	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_const_value = make([]i128, len(ast_module.globals), allocator)
	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 in checker.template_diagnostics {
		diagnostic = source.INVALID_DIAGNOSTIC
	}
	defer {
		for spec in checker.specs {
			delete(spec.args, 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_const_value, 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 function, index in ast_module.functions {
		for previous in ast_module.functions[:index] {
			if previous.pkg == function.pkg && previous.name == function.name {
				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 {
				source.addf(diagnostics, function.span, "package declaration '%s' conflicts with a global", 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 {
				source.addf(diagnostics, global.span, "duplicate global '%s'", symbol_text(&checker, global.name))
			}
		}
	}

	validate_type_nodes(&checker)
	validate_declarations(&checker)
	infer_all(&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))
	}
	propagate_global_reads(&checker)

	main_template := find_template(&checker, checker.main_symbol, 0)
	main_declarations := 0
	for function in ast_module.functions {
		if 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]
		if main_declarations != 1 ||
		   !template.has_body ||
		   len(template.params) != 0 ||
		   !(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, and return void, i32, or int",
				)
			}
			replace_main_with_trap(&checker, 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.valid && !import_item.used {
			source.addf(diagnostics, import_item.span, "unused import '%s'", symbol_text(&checker, import_item.alias))
		}
	}
	return checker.module
}