import "@std/mem"
import "@std/arraylist"
import "@std/hashmap"
import "@source/ast"
import "@source/diagnostics"
import "@source/strpool"
import "@source/lower"
import "@source/interpreter"
import "@source/ir"

sema :: import "@source/sema"
typecheck :: import "@source/sema/check"

@hide:file no_node :: maxval!(ast.NodeId)
@hide:file no_index :: maxval!(usize)

@hide:file
Error :: alias mem.AllocError | diagnostics.CapacityError

@hide:file
ValueKind :: enum {
    invalid
    type
    integer
    boolean
    string
    function
    null
    real
    aggregate
}

@hide:file
Value :: struct {
    kind ValueKind = .invalid
    node ast.NodeId = no_node
    magnitude u64 = 0
    negative bool = false
    boolean bool = false
    annotation ast.NodeId = no_node
}

@hide:file
Instance :: struct {
    function ast.NodeId
    arguments []mut Value
    declaration ast.NodeId = no_node
    type ast.NodeId = no_node
    next usize = no_index
    diagnostic diagnostics.DiagnosticId = diagnostics.no_diagnostic
    signature ast.NodeId = no_node
}

@hide:file
Rewrite :: struct { target ast.NodeId, saved ast.NodeId }

#! owns an undo log and retained checking obligations; deinit releases them with their retained allocator.
#! borrows the caller's AST builder; call deinit before finishing or deinitializing that builder.
#! generated nodes remain arena-owned and are released with the AST.
Result :: struct {
    builder @mut ast.Builder
    changes arraylist.ArrayList(Rewrite)
    constraints arraylist.ArrayList(ast.NodeId)
}

#! restores rewritten source nodes in the borrowed builder and releases the undo log and checking obligations.
#! does not remove generated arena nodes or restore resolution bindings.
deinit proc(result @mut Result) void {
    index := result.changes.items.len
    while (index > 0) {
        index -= 1
        change :: result.changes.items[index]
        ast.replace_node_from_source(result.builder, change.target, change.saved)
    }
    arraylist.deinit(&result.changes)
    arraylist.deinit(&result.constraints)
}

@hide:file
State :: struct {
    allocator mem.Allocator
    builder @mut ast.Builder
    tree ast.Tree
    bindings arraylist.ArrayList(sema.Binding)
    declarations arraylist.ArrayList(ast.NodeId)
    instances arraylist.ArrayList(Instance)
    cache hashmap.AutoHashMap(usize, usize)
    expanded arraylist.ArrayList(u8)
    diagnostics @mut diagnostics.Store
    depth usize = 0
    original_count usize
    rewrites @mut Result
    mapping []mut ast.NodeId
    resolution @mut sema.Resolution
    evaluation_diagnostic diagnostics.DiagnosticId = diagnostics.no_diagnostic
    type_execution_depth usize = 0
    type_execution_remaining usize = 0
}

#! elaborates templates into cached concrete declarations in the caller-owned `builder` arena.
#! extends `resolution` to cover generated nodes; no template reaches checking or lowering.
#! `allocator` supplies returned logs, temporary storage released on every exit, and replacement resolution bindings.
#! use the allocator retained by `resolution`; the returned Result borrows `builder` until the caller calls deinit.
#! on failure, restores rewritten source nodes and releases logs; generated nodes remain arena-owned.
elaborate proc(
    allocator mem.Allocator,
    builder @mut ast.Builder,
    root ast.NodeId,
    resolution @mut sema.Resolution,
    diagnostic_store @mut diagnostics.Store,
) Result ! Error {
    result := Result{ builder = builder, changes = arraylist.init(allocator), constraints = arraylist.init(allocator) }
    errdefer deinit(&result)
    # explicit evaluation is useful even in packages without generic declarations.
    original :: ast.view(builder, root)
    needed := false
    for (0..ast.node_count(&original)) |i| {
        id :: ast.NodeId(i)
        class :: ast.node_class(&original, id)
        if (class == .expression and ast.expr_kind(&original, id) == .comptime_expr) needed = true
        if (class == .function and ast.function_generic(&original, id)) needed = true
        if (class == .function) {
            result_type :: sema.binding(resolution, ast.function_decl(&original, id).return_type)
            if (result_type.kind == .builtin and result_type.data.builtin == .type) needed = true
        }
        if (needed) break
    }
    if (!needed) return result
    mapping :: try mem.alloc(ast.NodeId, allocator, ast.node_count(&original))
    defer mem.free(allocator, mapping)
    state := State{
        allocator = allocator, builder = builder, tree = ast.view(builder, root),
        bindings = arraylist.init(allocator), declarations = arraylist.init(allocator),
        instances = arraylist.init(allocator), cache = hashmap.init(allocator),
        expanded = arraylist.init(allocator), diagnostics = diagnostic_store,
        original_count = ast.node_count(&original),
        rewrites = &result,
        mapping = mapping,
        resolution = resolution,
    }
    defer arraylist.deinit(&state.bindings)
    defer arraylist.deinit(&state.declarations)
    defer arraylist.deinit(&state.expanded)
    defer hashmap.deinit(&state.cache)
    defer {
        for (state.instances.items) |instance| mem.free(allocator, instance.arguments)
        arraylist.deinit(&state.instances)
    }
    count :: ast.node_count(&state.tree)
    for (0..count) |i| try arraylist.append(&state.bindings, sema.binding(resolution, ast.NodeId(i)))
    for (0..ast.package_decl_count(&state.tree)) |i| {
        id :: ast.package_decl(&state.tree, i)
        if (ast.decl_kind(&state.tree, id) == .function and template(&state, id)) continue
        try arraylist.append(&state.declarations, id)
    }
    index := 0
    while (index < state.declarations.items.len) {
        try expand(&state, state.declarations.items[index])
        index += 1
    }
    try remember(&state, root)
    try ast.set_package_declarations(builder, root, state.declarations.items)
    try refresh(&state)
    owned :: try mem.alloc(sema.Binding, allocator, state.bindings.items.len)
    memcopy!(owned, state.bindings.items)
    sema.replace_bindings(resolution, owned)
    return result
}

@hide:file
refresh proc(state @mut State) void ! mem.AllocError {
    state.tree = ast.view(state.builder, state.tree.root)
    count :: ast.node_count(&state.tree)
    while (state.bindings.items.len < count) try arraylist.append(&state.bindings, sema.Binding{})
    while (state.expanded.items.len < count) try arraylist.append(&state.expanded, u8(0))
}

@hide:file
binding proc(state @State, id ast.NodeId) sema.Binding {
    if (usize(id) >= state.bindings.items.len) return sema.Binding{}
    return state.bindings.items[usize(id)]
}

@hide:file
reject proc(state @mut State, id ast.NodeId, code diagnostics.Code) void ! Error {
    span :: ast.node_span(&state.tree, id)
    diagnostic :: try diagnostics.add(state.diagnostics, code, span)
    invalid :: try ast.add_invalid_expr(state.builder, span, diagnostic)
    try replace(state, id, invalid)
    try refresh(state)
    state.bindings.items[usize(id)] = sema.Binding{ kind = .invalid, data = sema.BindingData{ diagnostic = diagnostic } }
}

@hide:file
expand proc(state @mut State, id ast.NodeId) void ! Error {
    try refresh(state)
    if (state.expanded.items[usize(id)] != 0) return
    state.expanded.items[usize(id)] = 1
    class :: ast.node_class(&state.tree, id)
    if (class == .statement and ast.stmt_kind(&state.tree, id) == .expression) {
        expression :: ast.statement_expr(&state.tree, id)
        if (
            ast.expr_kind(&state.tree, expression) == .match_expr
            and try specialize_match(state, expression, id)
        ) return
    }
    if (
        class == .expression and ast.expr_kind(&state.tree, id) == .match_expr
        and try specialize_match(state, id, no_node)
    ) return
    if (class == .statement and ast.stmt_kind(&state.tree, id) == .if_stmt) {
        conditional :: ast.if_stmt(&state.tree, id)
        explicit :: ast.expr_kind(&state.tree, conditional.condition) == .comptime_expr
        if (conditional.capture == null and (explicit or usize(id) >= state.original_count)) {
            condition :: if (explicit) ast.comptime_operand(&state.tree, conditional.condition) else conditional.condition
            try expand(state, condition)
            if (explicit or branch_constant(state, condition, 0)) {
                state.evaluation_diagnostic = diagnostics.no_diagnostic
                boolean :: try builtin(state, condition, .bool)
                value :: try evaluate_ir(state, condition, boolean)
                if (explicit and value.kind == .invalid and state.evaluation_diagnostic == diagnostics.no_diagnostic)
                    state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                        .comptime_argument_required, ast.node_span(&state.tree, condition))
                if (state.evaluation_diagnostic != diagnostics.no_diagnostic) {
                    invalid :: try ast.add_invalid_stmt(state.builder, ast.node_span(&state.tree, id),
                        state.evaluation_diagnostic)
                    try replace(state, id, invalid)
                    try refresh(state)
                    return
                }
                if (value.kind == .boolean) {
                    branch := conditional.then_block
                    if (!value.boolean) {
                        if (conditional.else_block) |other| branch = other
                        else branch = try ast.add_block(state.builder, ast.node_span(&state.tree, id), &[])
                    }
                    replacement :: try ast.add_block_stmt(state.builder, branch)
                    try replace(state, id, replacement)
                    try refresh(state)
                    try expand(state, branch)
                    return
                }
            }
        }
    }
    if (class == .expression and ast.expr_kind(&state.tree, id) == .comptime_expr) {
        operand :: ast.comptime_operand(&state.tree, id)
        try arraylist.append(&state.rewrites.constraints, operand)
        state.evaluation_diagnostic = diagnostics.no_diagnostic
        value :: try evaluate(state, operand)
        if (value.kind == .invalid) {
            try reject_evaluation(state, id)
            return
        }
        concrete :: try materialize(state, value)
        replacement := concrete.node
        if (value.annotation != no_node and value.kind != .type) {
            replacement = try ast.add_typed_constant(state.builder, ast.node_span(&state.tree, id),
                value.annotation, replacement)
            try refresh(state)
        }
        try replace(state, id, replacement)
        try refresh(state)
        return
    }
    if (class == .expression and ast.expr_kind(&state.tree, id) == .call) {
        callee :: ast.call_expr(&state.tree, id).callee
        try expand(state, callee)
        resolved :: binding(state, callee)
        if (
            resolved.kind == .declaration
            and ast.node_class(&state.tree, resolved.data.declaration) == .function
            and template(state, resolved.data.declaration)
        ) {
            try specialize_call(state, id, resolved.data.declaration)
            return
        }
    }
    count :: ast.child_count(&state.tree, id)
    for (0..count) |i| {
        child :: ast.child(&state.tree, id, i)
        try expand(state, child)
    }
    if (!try type_array_projection(state, id)) {
        try reject_evaluation(state, id)
        return
    }
    if (class == .expression and ast.expr_kind(&state.tree, id) == .binary) {
        view :: ast.binary_expr(&state.tree, id)
        if (
            (view.operator == .equal or view.operator == .not_equal)
            and branch_constant(state, view.left, 0) and branch_constant(state, view.right, 0)
        ) {
            left :: try evaluate(state, view.left)
            right :: try evaluate(state, view.right)
            if (left.kind == .type and right.kind == .type) _ = try evaluate_binary(state, id)
        }
    }
    if (usize(id) >= state.original_count and class == .expression and ast.expr_kind(&state.tree, id) == .array_type) {
        view :: ast.array_type(&state.tree, id)
        if (view.count) |count_node| {
            try arraylist.append(&state.rewrites.constraints, count_node)
            value :: try evaluate(state, count_node)
            if (value.kind == .integer) {
                folded :: try materialize(state, value)
                replacement :: try ast.add_array_type(state.builder, ast.node_span(&state.tree, id),
                    folded.node, view.element, view.mutable)
                try replace(state, id, replacement)
                try refresh(state)
            }
        }
    }
}

# runtime bodies share the type-producer selector, but retain ordinary runtime
# parameters and control transfers in the selected arm.
@hide:file
specialize_match proc(state @mut State, id, statement ast.NodeId) bool ! Error {
    subject :: ast.match_expr(&state.tree, id).subject
    explicit :: ast.expr_kind(&state.tree, subject) == .comptime_expr
    if (!explicit and usize(id) < state.original_count) return false
    try expand(state, subject)
    if (!explicit and !branch_constant(state, subject, 0)) return false
    state.evaluation_diagnostic = diagnostics.no_diagnostic
    index :: try type_match_arm(state, id)
    if (index == no_index) {
        try reject_evaluation(state, id)
        return true
    }
    arm :: ast.match_arm_view(&state.tree, ast.match_arm(&state.tree, id, index))
    selected :: try type_match_body(state, id, arm)
    if (selected == no_node or (statement == no_node and !arm.expression)) {
        try reject_evaluation(state, id)
        return true
    }
    try expand(state, selected)
    if (statement != no_node) {
        replacement := no_node
        if (arm.expression) replacement = try ast.add_expression_stmt(state.builder, selected)
        else replacement = try ast.add_block_stmt(state.builder, selected)
        try replace(state, statement, replacement)
    } else {
        resolved :: binding(state, selected)
        try remember(state, id)
        ast.replace_node_from_source(state.builder, id, selected)
        try refresh(state)
        state.bindings.items[usize(id)] = resolved
    }
    try refresh(state)
    return true
}

# only probe side-effect-free expressions. ordinary calls stay runtime unless the
# source requests evaluation with $(); probing must not execute FFI or emit traps.
@hide:file
branch_constant proc(state @State, id ast.NodeId, depth usize) bool {
    if (depth >= 128) return false
    resolved :: binding(state, id)
    if (resolved.kind == .builtin) return true
    if (resolved.kind == .declaration) {
        target :: resolved.data.declaration
        if (ast.node_class(&state.tree, target) == .function) return true
        if (ast.node_class(&state.tree, target) != .statement) return false
        declaration :: ast.statement_decl(&state.tree, target)
        initializer :: ast.expr_kind(&state.tree, declaration.initializer)
        # contextual aggregates may omit defaults with runtime calls.
        if (declaration.type != null and (initializer == .record_literal or initializer == .array_literal)) return false
        return (
            declaration.mutability == .constant
            and branch_constant(state, declaration.initializer, depth + 1)
        )
    }
    match ast.expr_kind(&state.tree, id) {
        .literal_int, .literal_float, .literal_bool, .literal_string, .literal_null, .literal_void: return true
        .typed_constant: return branch_constant(state, ast.typed_constant(&state.tree, id).value, depth + 1)
        .enum_literal: return true
        .record_literal: {
            # typed literals may omit defaults; materialized constants are complete
            # contextual literals inside a typed_constant wrapper.
            for (0..ast.record_field_count(&state.tree, id)) |i| {
                field :: ast.field(&state.tree, ast.record_field(&state.tree, id, i))
                if (field.value == null or !branch_constant(state, field.value?, depth + 1)) return false
            }
            return ast.record_literal(&state.tree, id).type == null
        }
        .array_literal: {
            for (0..ast.array_element_count(&state.tree, id)) |i| {
                if (!branch_constant(state, ast.array_element(&state.tree, id, i), depth + 1)) return false
            }
            return true
        }
        .selection: return branch_constant(state, ast.selection_expr(&state.tree, id).base, depth + 1)
        .index: {
            view :: ast.index_expr(&state.tree, id)
            return branch_constant(state, view.base, depth + 1) and branch_constant(state, view.index, depth + 1)
        }
        .unary: {
            view :: ast.unary_expr(&state.tree, id)
            return (
                (view.operator == .negate or view.operator == .logical_not)
                and branch_constant(state, view.operand, depth + 1)
            )
        }
        .binary: {
            view :: ast.binary_expr(&state.tree, id)
            return (
                branch_constant(state, view.left, depth + 1)
                and branch_constant(state, view.right, depth + 1)
            )
        }
        .pointer_type: return branch_constant(state, ast.pointer_type(&state.tree, id).element, depth + 1)
        .optional_type, .alias_expr: return branch_constant(state, ast.type_operand(&state.tree, id), depth + 1)
        .record_type, .union_type, .enum_type, .distinct_type, .array_type, .slice_type, .function_type,
        .fallible_type, .sum_type: return true
        else: return false
    }
}

@hide:file
builtin proc(state @mut State, at ast.NodeId, kind sema.BuiltinType) ast.NodeId ! Error {
    id :: try ast.add_identifier(state.builder, ast.node_span(&state.tree, at), strpool.no_str)
    try refresh(state)
    state.bindings.items[usize(id)] = sema.Binding{ kind = .builtin, data = sema.BindingData{ builtin = kind } }
    return id
}

@hide:file
reference proc(state @mut State, at, target ast.NodeId) ast.NodeId ! Error {
    id :: try ast.add_identifier(state.builder, ast.node_span(&state.tree, at), strpool.no_str)
    try refresh(state)
    state.bindings.items[usize(id)] = sema.Binding{ kind = .declaration, data = sema.BindingData{ declaration = target } }
    return id
}

@hide:file
is_type proc(state @State, id ast.NodeId) bool {
    value :: binding(state, id)
    return value.kind == .builtin and value.data.builtin == .type
}

@hide:file
specialize_call proc(state @mut State, call, function ast.NodeId) void ! Error {
    if (state.depth >= 128 or state.instances.items.len >= 4096) {
        try reject(state, call, .specialization_limit)
        return
    }
    state.depth += 1
    defer state.depth -= 1
    count :: ast.function_param_count(&state.tree, function)
    # a type result has no runtime call: every input participates in specialization.
    type_result :: is_type(state, ast.function_decl(&state.tree, function).return_type)
    runtime := usize(0)
    for (0..count) |i| {
        if (!type_result and !ast.param(&state.tree, ast.function_param(&state.tree, function, i)).comptime) runtime += 1
    }
    supplied :: ast.call_arg_count(&state.tree, call)
    explicit :: supplied == count
    if (!explicit and (type_result or supplied != runtime)) {
        try reject(state, call, .argument_count_mismatch)
        return
    }
    arguments :: try mem.alloc(Value, state.allocator, count)
    defer mem.free(state.allocator, arguments)
    mem.set(arguments, Value{})
    runtime_arguments := arraylist.init(ast.NodeId, state.allocator)
    defer arraylist.deinit(&runtime_arguments)
    position := 0
    for (0..count) |i| {
        parameter :: ast.function_param(&state.tree, function, i)
        view :: ast.param(&state.tree, parameter)
        if (view.comptime and !explicit) continue
        argument :: ast.call_arg(&state.tree, call, position)
        position += 1
        try expand(state, argument)
        if (type_result or view.comptime) {
            state.evaluation_diagnostic = diagnostics.no_diagnostic
            annotation := view.type
            contextual := contextual_literal(state, argument)
            if (!contextual and !is_type(state, annotation)) {
                type :: try canonical_type(state, annotation)
                contextual = type != no_node and ast.expr_kind(&state.tree, type) == .fallible_type
            }
            if (contextual) annotation = try argument_annotation(state, function, view.type, arguments)
            arguments[i] = try evaluate_expected(state, argument, annotation)
            if (arguments[i].kind == .invalid) {
                try reject_evaluation(state, call)
                return
            }
            if (is_type(state, view.type) != (arguments[i].kind == .type)) {
                try reject(state, call, .type_mismatch)
                return
            }
        } else {
            try arraylist.append(&runtime_arguments, argument)
        }
    }
    if (!explicit) {
        for (0..usize(2)) |phase| {
            position = 0
            for (0..count) |i| {
                parameter :: ast.function_param(&state.tree, function, i)
                view :: ast.param(&state.tree, parameter)
                if (view.comptime) continue
                argument :: runtime_arguments.items[position]
                position += 1
                contextual :: flexible(state, argument)
                if (contextual != (phase == 1)) continue
                actual :: try expression_type(state, argument, 0)
                if (actual == no_node or !try infer(state, function, view.type, actual, arguments, contextual)) {
                    try reject(state, call, .cannot_infer_generic)
                    return
                }
            }
        }
    }
    for (0..count) |i| {
        parameter :: ast.function_param(&state.tree, function, i)
        if (ast.param(&state.tree, parameter).comptime and arguments[i].kind == .invalid) {
            try reject(state, call, .cannot_infer_generic)
            return
        }
    }
    key := usize(function)
    for arguments |argument| {
        if (argument.kind != .invalid) key = mix(key, try value_hash(state, argument))
    }
    head := no_index
    if (hashmap.get(&state.cache, key)) |found| head = found
    current := head
    instance := no_index
    while (current != no_index) {
        entry :: state.instances.items[current]
        equal := entry.function == function and entry.arguments.len == count
        if (equal) {
            for (0..count) |i| {
                if (!try value_equal(state, entry.arguments[i], arguments[i])) {
                    equal = false
                    break
                }
            }
        }
        if (equal) {
            instance = current
            break
        }
        current = entry.next
    }
    if (instance == no_index) {
        for (0..count) |i| {
            if (arguments[i].kind != .invalid) arguments[i] = try materialize(state, arguments[i])
        }
        values :: try mem.alloc(Value, state.allocator, count)
        memcopy!(values, arguments)
        instance = state.instances.items.len
        arraylist.append(&state.instances, Instance{ function = function, arguments = values, next = head }) catch |err| {
            mem.free(state.allocator, values)
            return err
        }
        # cache heads are replaced without reallocating a second entry for the same hash.
        if (head != no_index) {
            for (state.cache.entries) |@entry| {
                if (entry.hash != 0 and entry.key == key) {
                    entry.value = instance
                    break
                }
            }
        } else {
            hashmap.put(&state.cache, key, instance) catch |err| {
                match err {
                    .key_exists: unreachable
                    else: return err
                }
            }
        }
        try instantiate(state, instance, call)
        if (binding(state, call).kind == .invalid) {
            state.instances.items[instance].diagnostic = binding(state, call).data.diagnostic
            return
        }
    }
    diagnostic :: state.instances.items[instance].diagnostic
    if (diagnostic != diagnostics.no_diagnostic) {
        invalid :: try ast.add_invalid_expr(state.builder, ast.node_span(&state.tree, call), diagnostic)
        try replace(state, call, invalid)
        try refresh(state)
        state.bindings.items[usize(call)] = sema.Binding{ kind = .invalid, data = sema.BindingData{ diagnostic = diagnostic } }
        return
    }
    target :: state.instances.items[instance].declaration
    if (target == no_node) {
        try reject(state, call, .specialization_cycle)
        return
    }
    if (explicit) {
        signature :: state.instances.items[instance].signature
        for (0..count) |i| {
            parameter :: ast.function_param(&state.tree, signature, i)
            view :: ast.param(&state.tree, parameter)
            if ((!type_result and !view.comptime) or is_type(state, view.type)) continue
            original :: ast.call_arg(&state.tree, call, i)
            constraint :: try ast.add_typed_constant(state.builder, ast.node_span(&state.tree, original), view.type, original)
            try refresh(state)
            try arraylist.append(&state.rewrites.constraints, constraint)
        }
    }
    callee :: try reference(state, call, target)
    replacement := no_node
    if (state.instances.items[instance].type != no_node) replacement = try ast.add_type_operand(state.builder, ast.node_span(&state.tree, call), callee, true)
    else replacement = try ast.add_call(state.builder, ast.node_span(&state.tree, call), callee, runtime_arguments.items)
    try replace(state, call, replacement)
    try refresh(state)
}

@hide:file
instantiate proc(state @mut State, index usize, call ast.NodeId) void ! Error {
    function :: state.instances.items[index].function
    type_result :: is_type(state, ast.function_decl(&state.tree, function).return_type)
    count :: state.original_count
    clone_start :: ast.node_count(&state.tree)
    mapping :: state.mapping
    mem.set(mapping, no_node)
    parameters :: ast.function_param_count(&state.tree, function)
    for (0..parameters) |i| {
        parameter :: ast.function_param(&state.tree, function, i)
        if (!type_result and !ast.param(&state.tree, parameter).comptime) continue
        value :: state.instances.items[index].arguments[i]
        replacement := value.node
        annotation :: ast.param(&state.tree, parameter).type
        if (!is_type(state, annotation)) {
            concrete_type :: try ast.clone_subtree(state.builder, annotation, mapping)
            replacement = try ast.add_typed_constant(state.builder, ast.node_span(&state.tree, parameter), concrete_type, value.node)
            try refresh(state)
        }
        for (0..count) |n| {
            resolved :: binding(state, ast.NodeId(n))
            if (
                resolved.kind == .declaration and resolved.data.declaration == parameter
                and ast.node_class(&state.tree, ast.NodeId(n)) == .expression
            ) mapping[n] = replacement
        }
    }
    clone :: try ast.clone_subtree(state.builder, function, mapping)
    try refresh(state)
    # copy original resolution, remapping references to cloned locals and runtime parameters.
    for (0..count) |i| {
        target :: mapping[i]
        if (target == no_node or usize(target) < clone_start) continue
        if (ast.node_class(&state.tree, target) == .expression and ast.expr_kind(&state.tree, target) == .typed_constant) continue
        value := state.bindings.items[i]
        if (value.kind == .declaration) {
            original :: value.data.declaration
            if (original != function and usize(original) < count and mapping[usize(original)] != no_node)
                value.data.declaration = mapping[usize(original)]
        }
        state.bindings.items[usize(target)] = value
    }
    state.instances.items[index].signature = try ast.copy_node(state.builder, clone)
    try refresh(state)
    view :: ast.function_decl(&state.tree, clone)
    for (0..parameters) |i| {
        parameter :: ast.function_param(&state.tree, clone, i)
        annotation :: ast.param(&state.tree, parameter).type
        if ((type_result or ast.param(&state.tree, parameter).comptime) and !is_type(state, annotation)) {
            if (!try valid_value(state, annotation, state.instances.items[index].arguments[i])) {
                value :: state.instances.items[index].arguments[i]
                code :: if (value.kind == .integer and try integer_type(state, annotation))
                    diagnostics.Code.literal_out_of_range else diagnostics.Code.type_mismatch
                try reject(state, call, code)
                return
            }
        }
    }
    if (is_type(state, view.return_type)) {
        if (state.type_execution_depth == 0) state.type_execution_remaining = 4096
        state.type_execution_depth += 1
        defer state.type_execution_depth -= 1
        state.evaluation_diagnostic = diagnostics.no_diagnostic
        constraints_start :: state.rewrites.constraints.items.len
        returned :: try return_type_value(state, view.body, ast.NodeId(clone_start))
        if (returned == no_node) {
            try reject_type_result(state, call, returned, constraints_start)
            return
        }
        result := returned
        kind :: ast.expr_kind(&state.tree, returned)
        if (kind != .record_type and kind != .union_type and kind != .enum_type and kind != .distinct_type) {
            result = try ast.add_type_operand(state.builder, ast.node_span(&state.tree, returned), returned, true)
            try refresh(state)
        }
        declaration :: try ast.add_statement_decl(state.builder, view.span, view.name,
            ast.name_id(&state.tree, function), null, .constant, result)
        try refresh(state)
        state.instances.items[index].declaration = declaration
        state.instances.items[index].type = returned
        state.bindings.items[usize(declaration)] = sema.Binding{ kind = .declaration, data = sema.BindingData{ declaration = declaration } }
        try arraylist.append(&state.declarations, declaration)
        if (!try expand_type_matches(state, declaration)) {
            try reject_type_result(state, call, result, constraints_start)
            return
        }
        try expand(state, declaration)
        constraint_count :: state.rewrites.constraints.items.len
        for (constraints_start..constraint_count) |i| {
            constraint :: state.rewrites.constraints.items[i]
            if (!try expand_type_matches(state, constraint)) {
                try reject_type_result(state, call, result, constraints_start)
                return
            }
            try expand(state, constraint)
        }
    } else {
        runtime := arraylist.init(ast.NodeId, state.allocator)
        defer arraylist.deinit(&runtime)
        for (0..parameters) |i| {
            parameter :: ast.function_param(&state.tree, clone, i)
            if (!ast.param(&state.tree, parameter).comptime) try arraylist.append(&runtime, parameter)
        }
        concrete :: try ast.add_function_decl(state.builder, view.span, view.name,
            ast.name_id(&state.tree, function), view.return_type, runtime.items, view.body)
        ast.replace_node(state.builder, clone, concrete)
        try refresh(state)
        state.instances.items[index].declaration = clone
        state.bindings.items[usize(clone)] = sema.Binding{ kind = .declaration, data = sema.BindingData{ declaration = clone } }
        try arraylist.append(&state.declarations, clone)
        try expand(state, clone)
    }
}

@hide:file
reject_evaluation proc(state @mut State, call ast.NodeId) void ! Error {
    diagnostic :: state.evaluation_diagnostic
    if (diagnostic == diagnostics.no_diagnostic) {
        try reject(state, call, .comptime_argument_required)
        return
    }
    invalid :: try ast.add_invalid_expr(state.builder, ast.node_span(&state.tree, call), diagnostic)
    try replace(state, call, invalid)
    try refresh(state)
    state.bindings.items[usize(call)] = sema.Binding{ kind = .invalid, data = sema.BindingData{ diagnostic = diagnostic } }
}

# failed deferred elaboration must not leave unchecked type expressions in the package.
@hide:file
reject_type_result proc(state @mut State, call, result ast.NodeId, constraints_start usize) void ! Error {
    try reject_evaluation(state, call)
    state.evaluation_diagnostic = binding(state, call).data.diagnostic
    if (result != no_node) try reject_evaluation(state, result)
    for (constraints_start..(state.rewrites.constraints.items.len)) |i|
        try reject_evaluation(state, state.rewrites.constraints.items[i])
}

@hide:file
evaluation_dependencies proc(
    state @mut State, id ast.NodeId, flags []mut u8, declarations @mut arraylist.ArrayList(ast.NodeId),
) bool ! Error {
    if ((flags[usize(id)] & 4) != 0) return true
    flags[usize(id)] = flags[usize(id)] | 4
    resolved :: binding(state, id)
    if (resolved.kind == .declaration) {
        target :: resolved.data.declaration
        if ((flags[usize(target)] & 8) == 0 and (flags[usize(target)] & 2) == 0) {
            class :: ast.node_class(&state.tree, target)
            if (class == .parameter) return false
            if (class == .statement and ast.statement_decl(&state.tree, target).mutability == .variable) return false
            flags[usize(target)] = flags[usize(target)] | 2
            try arraylist.append(declarations, target)
        }
    }
    for (0..ast.child_count(&state.tree, id)) |i| {
        if (!try evaluation_dependencies(state, ast.child(&state.tree, id, i), flags, declarations)) return false
    }
    return true
}

@hide:file
evaluation_owned proc(state @State, id ast.NodeId, flags []mut u8) void {
    if ((flags[usize(id)] & 8) != 0) return
    flags[usize(id)] = flags[usize(id)] | 8
    for (0..ast.child_count(&state.tree, id)) |i| evaluation_owned(state, ast.child(&state.tree, id, i), flags)
}

@hide:file
contextual_literal proc(state @State, id ast.NodeId) bool {
    kind :: ast.expr_kind(&state.tree, id)
    return kind == .record_literal or kind == .array_literal or kind == .enum_literal
}

@hide:file
argument_annotation proc(
    state @mut State, function, annotation ast.NodeId, arguments []Value,
) ast.NodeId ! Error {
    mapping :: try mem.alloc(ast.NodeId, state.allocator, state.original_count)
    defer mem.free(state.allocator, mapping)
    mem.set(mapping, no_node)
    for (0..ast.function_param_count(&state.tree, function)) |i| {
        if (arguments[i].kind == .invalid) continue
        parameter :: ast.function_param(&state.tree, function, i)
        value :: try materialize(state, arguments[i])
        for (0..mapping.len) |n| {
            resolved :: binding(state, ast.NodeId(n))
            if (
                resolved.kind == .declaration and resolved.data.declaration == parameter
                and ast.node_class(&state.tree, ast.NodeId(n)) == .expression
            ) mapping[n] = value.node
        }
    }
    start :: ast.node_count(&state.tree)
    result :: try ast.clone_subtree(state.builder, annotation, mapping)
    try refresh(state)
    for (0..mapping.len) |i| {
        target :: mapping[i]
        if (target != no_node and usize(target) >= start) state.bindings.items[usize(target)] = state.bindings.items[i]
    }
    return result
}

@hide:file
evaluate_expected proc(state @mut State, id, annotation ast.NodeId) Value ! Error {
    if (!is_type(state, annotation)) {
        type :: try canonical_type(state, annotation)
        if (type != no_node and (contextual_literal(state, id) or ast.expr_kind(&state.tree, type) == .fallible_type))
            return try evaluate_ir(state, id, type)
    }
    return try evaluate(state, id)
}

# values leaving the temporary interpreter must own no interpreter addresses or module IDs.
# rebuild contextual source literals while the module and its escaped flat cells are alive.
@hide:file
serialize_value proc(
    state @mut State, module @ir.Module, checked @typecheck.Result,
    type typecheck.Type, value ir.Value, span ast.Span,
) ast.NodeId ! Error {
    result := no_node
    if (type.kind == .array) {
        array :: typecheck.array_type(checked, type.array_type)
        elements := arraylist.init(ast.NodeId, state.allocator)
        defer arraylist.deinit(&elements)
        for (0..array.len) |i| {
            child :: ir.aggregate_item(module, value, i)
            node :: try serialize_value(state, module, checked, array.element, child, span)
            if (node == no_node) return no_node
            try arraylist.append(&elements, node)
        }
        result = try ast.add_array_literal(state.builder, span, elements.items)
    } else if (type.kind == .record) {
        record :: typecheck.record_type(checked, type.record_type)
        # untagged storage has no active member; choosing one could discard bytes.
        if (record.is_union and record.tag_type.kind != .enumeration) return no_node
        fields := arraylist.init(ast.NodeId, state.allocator)
        defer arraylist.deinit(&fields)
        for (record.fields) |field, i| {
            if (record.is_union and !ir.union_active(module, value, i)) continue
            if (record.is_union and field.type.kind == .void) {
                result = try ast.add_enum_literal(state.builder, span, field.name?)
                try refresh(state)
                return result
            }
            child :: ir.aggregate_item(module, value, i)
            node :: try serialize_value(state, module, checked, field.type, child, span)
            if (node == no_node) return no_node
            generated :: try ast.add_field(state.builder, span, field.name, null, node)
            try refresh(state)
            try arraylist.append(&fields, generated)
        }
        result = try ast.add_record_literal(state.builder, span, null, fields.items)
    } else if (type.kind == .fallible) {
        container :: typecheck.array_type(checked, type.array_type)
        failed :: ir.union_tag(module, value).data.integer == 1
        index :: if (failed) usize(1) else usize(0)
        channel :: if (failed) container.error_type else container.element
        child :: ir.aggregate_item(module, value, index)
        result = try serialize_value(state, module, checked, channel, child, span)
        if (result == no_node) return no_node
        # keep named success/error types distinct even when their members overlap.
        annotation := no_node
        if (channel.nominal != no_node) annotation = channel.nominal
        else if (channel.kind == .enumeration) annotation = typecheck.enum_type(checked, channel.enum_type).declaration
        else if (channel.kind == .record) annotation = typecheck.record_type(checked, channel.record_type).declaration
        if (annotation != no_node) {
            concrete :: try materialize(state, Value{ kind = .type, node = annotation })
            result = try ast.add_typed_constant(state.builder, span, concrete.node, result)
        }
    } else if (type.kind == .enumeration) {
        enumeration :: typecheck.enum_type(checked, type.enum_type)
        negative :: !ir.unsigned_integer(value.type) and value.data.integer < 0
        magnitude :: if (negative) u64(-(value.data.integer + 1)) + 1 else value.data.unsigned
        for (enumeration.members) |member| {
            if (member.magnitude == magnitude and member.negative == negative) {
                result = try ast.add_enum_literal(state.builder, span, member.name)
                break
            }
        }
    } else if (type.kind == .optional) {
        container :: typecheck.array_type(checked, type.array_type)
        if (container.element.kind == .pointer or container.element.kind == .function) {
            absent :: if (container.element.kind == .function)
                value.data.function == maxval!(ir.FunctionId)
                else value.data.unsigned == maxval!(u64)
            if (!absent) return no_node
            result = try ast.add_literal(state.builder, .null, span)
        } else if (!ir.aggregate_item(module, value, 0).data.boolean) {
            result = try ast.add_literal(state.builder, .null, span)
        } else {
            child :: ir.aggregate_item(module, value, 1)
            return try serialize_value(state, module, checked, container.element, child, span)
        }
    } else if (ir.integer_bits(value.type) != 0 and type.kind != .pointer) {
        negative :: !ir.unsigned_integer(value.type) and value.data.integer < 0
        magnitude :: if (negative) u64(-(value.data.integer + 1)) + 1 else value.data.unsigned
        result = try ast.add_integer(state.builder, span, magnitude)
        if (negative) result = try ast.add_unary(state.builder, span, .negate, result)
    } else if (type.kind == .float) {
        result = try ast.add_float(state.builder, span, value.data.real)
    } else if (type.kind == .bool) {
        result = try ast.add_boolean(state.builder, span, value.data.boolean)
    } else if (type.kind == .string) {
        result = try ast.add_string(state.builder, span, ir.string_bytes(module, value.data.string))
    } else if (type.kind == .void) {
        result = try ast.add_literal(state.builder, .void, span)
    }
    try refresh(state)
    return result
}

@hide:file
constant_equal proc(state @mut State, left, right ast.NodeId) bool ! Error {
    kind :: ast.expr_kind(&state.tree, left)
    if (kind != ast.expr_kind(&state.tree, right)) return false
    if (kind == .literal_void) return true
    if (kind == .typed_constant) {
        a :: ast.typed_constant(&state.tree, left)
        b :: ast.typed_constant(&state.tree, right)
        return try same_type(state, a.type, b.type) and try constant_equal(state, a.value, b.value)
    }
    if (kind == .array_literal) {
        count :: ast.array_element_count(&state.tree, left)
        if (count != ast.array_element_count(&state.tree, right)) return false
        for (0..count) |i| {
            if (!try constant_equal(state, ast.array_element(&state.tree, left, i), ast.array_element(&state.tree, right, i))) return false
        }
        return true
    }
    if (kind == .record_literal) {
        count :: ast.record_field_count(&state.tree, left)
        if (count != ast.record_field_count(&state.tree, right)) return false
        for (0..count) |i| {
            a :: ast.field(&state.tree, ast.record_field(&state.tree, left, i))
            b :: ast.field(&state.tree, ast.record_field(&state.tree, right, i))
            if ((a.name == null) != (b.name == null)) return false
            if (a.name != null and ast.name_id(&state.tree, a.name?) != ast.name_id(&state.tree, b.name?)) return false
            if (!try constant_equal(state, a.value?, b.value?)) return false
        }
        return true
    }
    if (kind == .enum_literal)
        return ast.name_id(&state.tree, ast.enum_literal(&state.tree, left)) == ast.name_id(&state.tree, ast.enum_literal(&state.tree, right))
    return try value_equal(state, try evaluate(state, left), try evaluate(state, right))
}

@hide:file
constant_hash proc(state @mut State, id ast.NodeId) usize ! Error {
    kind :: ast.expr_kind(&state.tree, id)
    hash :: enum_index(kind)
    if (kind == .literal_void) return hash
    if (kind == .typed_constant) {
        value :: ast.typed_constant(&state.tree, id)
        return mix(mix(hash, try type_hash(state, value.type)), try constant_hash(state, value.value))
    }
    if (kind == .array_literal) {
        result := hash
        for (0..ast.array_element_count(&state.tree, id)) |i|
            result = mix(result, try constant_hash(state, ast.array_element(&state.tree, id, i)))
        return result
    }
    if (kind == .record_literal) {
        result := hash
        for (0..ast.record_field_count(&state.tree, id)) |i| {
            field :: ast.field(&state.tree, ast.record_field(&state.tree, id, i))
            if (field.name) |name| result = mix(result, usize(ast.name_id(&state.tree, name)))
            result = mix(result, try constant_hash(state, field.value?))
        }
        return result
    }
    if (kind == .enum_literal) return mix(hash, usize(ast.name_id(&state.tree, ast.enum_literal(&state.tree, id))))
    return try value_hash(state, try evaluate(state, id))
}

@hide:file
evaluate_call proc(state @mut State, call ast.NodeId) Value ! Error {
    callee :: try evaluate(state, ast.call_expr(&state.tree, call).callee)
    if (callee.kind != .function and callee.kind != .type) return Value{}
    result_type := callee.node
    function := no_node
    if (callee.kind == .function) {
        resolved :: binding(state, callee.node)
        if (resolved.kind != .declaration) return Value{}
        function = resolved.data.declaration
        result_type = ast.function_decl(&state.tree, resolved.data.declaration).return_type
    }
    arguments := arraylist.init(ast.NodeId, state.allocator)
    defer arraylist.deinit(&arguments)
    for (0..ast.call_arg_count(&state.tree, call)) |i| {
        argument :: ast.call_arg(&state.tree, call, i)
        value := Value{}
        if (function != no_node and i < ast.function_param_count(&state.tree, function)) {
            annotation :: ast.param(&state.tree, ast.function_param(&state.tree, function, i)).type
            value = try evaluate_expected(state, argument, annotation)
        } else value = try evaluate(state, argument)
        if (value.kind == .invalid) return Value{}
        concrete := try materialize(state, value)
        if (value.annotation != no_node) {
            concrete.node = try ast.add_typed_constant(state.builder, ast.node_span(&state.tree, value.node),
                value.annotation, concrete.node)
            try refresh(state)
        }
        try arraylist.append(&arguments, concrete.node)
    }
    # reuse checking, typed IR and the interpreter: compile-time arithmetic and local
    # inference must have exactly the same semantics as an ordinary runtime call.
    span :: ast.node_span(&state.tree, call)
    invocation :: try ast.add_call(state.builder, span, callee.node, arguments.items)
    try refresh(state)
    return try evaluate_ir(state, invocation, result_type)
}

@hide:file
evaluate_ir proc(state @mut State, expression, result_type ast.NodeId) Value ! Error {
    if (result_type == no_node) return Value{}
    span :: ast.node_span(&state.tree, expression)
    returned :: try ast.add_return_stmt(state.builder, span, expression)
    statements [1]ast.NodeId :: [returned]
    body :: try ast.add_block(state.builder, span, statements[..])
    try refresh(state)
    return try evaluate_ir_body(state, expression, body, result_type)
}

@hide:file
evaluate_ir_body proc(state @mut State, at, body, result_type ast.NodeId) Value ! Error {
    span :: ast.node_span(&state.tree, at)
    wrapper :: try ast.add_function_decl(state.builder, span, span, strpool.no_str, result_type, &[], body)
    try refresh(state)
    declarations := arraylist.init(ast.NodeId, state.allocator)
    defer arraylist.deinit(&declarations)
    try arraylist.append(&declarations, wrapper)
    flags := arraylist.init(u8, state.allocator)
    defer arraylist.deinit(&flags)
    index := usize(0)
    while (index < declarations.items.len) {
        declaration :: declarations.items[index]
        try expand(state, declaration)
        while (flags.items.len < ast.node_count(&state.tree)) try arraylist.append(&flags, u8(0))
        flags.items[usize(declaration)] = flags.items[usize(declaration)] | 2
        evaluation_owned(state, declaration, flags.items)
        if (!try evaluation_dependencies(state, declaration, flags.items, &declarations)) return Value{}
        index += 1
    }
    root :: try ast.add_package(state.builder, span, declarations.items)
    try refresh(state)
    bindings :: try mem.alloc(sema.Binding, state.allocator, state.bindings.items.len)
    memcopy!(bindings, state.bindings.items)
    sema.replace_bindings(state.resolution, bindings)
    tree :: ast.view(state.builder, root)
    diagnostic_store := diagnostics.init(state.allocator)
    defer diagnostics.deinit(&diagnostic_store)
    # Resolution's existing diagnostic IDs remain valid in this temporary store.
    for (state.diagnostics.list.items) |diagnostic| try arraylist.append(&diagnostic_store.list, diagnostic)
    original_diagnostics :: diagnostics.count(&diagnostic_store)
    checked := try typecheck.check(state.allocator, &tree, state.resolution, &diagnostic_store)
    defer typecheck.deinit(&checked)
    if (diagnostics.count(&diagnostic_store) != original_diagnostics) {
        diagnostic :: diagnostic_store.list.items[original_diagnostics]
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics, diagnostic.code, diagnostic.span)
        return Value{}
    }
    module := try lower.lower_evaluation(state.allocator, &tree, state.resolution, &checked, wrapper, &diagnostic_store)
    defer ir.deinit(&module)
    outcome := try interpreter.run_evaluation(state.allocator, &module, &diagnostic_store)
    defer interpreter.deinit(&outcome)
    if (outcome.trap) |trap| {
        if (usize(trap) < original_diagnostics) state.evaluation_diagnostic = trap
        else {
            diagnostic :: diagnostics.get(&diagnostic_store, trap)?
            state.evaluation_diagnostic = try diagnostics.add(state.diagnostics, diagnostic.code, diagnostic.span)
        }
        return Value{}
    }
    value :: outcome.value
    source_type :: typecheck.type_of(&checked, result_type)
    node :: try serialize_value(state, &module, &checked, source_type, value, span)
    if (node == no_node) {
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics, .comptime_value_escape, span)
        return Value{}
    }
    if (ir.aggregate(value.type) or source_type.kind == .enumeration or source_type.kind == .void)
        return Value{ kind = .aggregate, node = node, annotation = result_type }
    result := try evaluate(state, node)
    result.annotation = result_type
    return result
}

@hide:file
TypeLocal :: struct {
    declaration ast.NodeId
    value ast.NodeId
    annotation ast.NodeId
}

@hide:file
TypeExecution :: struct {
    locals arraylist.ArrayList(TypeLocal)
    mapping []mut ast.NodeId
    first_owned ast.NodeId
    returned ast.NodeId = no_node
    loop_depth usize = 0
}

@hide:file
TypeFlow :: enum { next, returned, break, continue, invalid }

# type values cannot enter runtime IR. execute their control flow here, substituting
# owned constants into snapshots; ordinary value operations still use checked IR.
@hide:file
return_type_value proc(state @mut State, block, first_owned ast.NodeId) ast.NodeId ! Error {
    # ponytail: snapshots recheck scalar IR; cap type-control work at 4096 steps
    # until type values can share one persistent checked execution frame.
    execution := TypeExecution{
        locals = arraylist.init(state.allocator),
        mapping = try mem.alloc(ast.NodeId, state.allocator, ast.node_count(&state.tree)),
        first_owned = first_owned,
    }
    defer arraylist.deinit(&execution.locals)
    defer mem.free(state.allocator, execution.mapping)
    mem.set(execution.mapping, no_node)
    if (try execute_type_block(state, &execution, block) != .returned) return no_node
    return execution.returned
}

@hide:file
type_step proc(state @mut State, at ast.NodeId) bool ! Error {
    if (state.type_execution_remaining == 0) {
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
            .comptime_execution_limit, ast.node_span(&state.tree, at))
        return false
    }
    state.type_execution_remaining -= 1
    return true
}

@hide:file
prepare_type_snapshot proc(state @mut State, execution @mut TypeExecution, id ast.NodeId) void {
    if (usize(id) >= execution.mapping.len) return
    # substituted arguments belong to the caller, including nominal type identity.
    if (id < execution.first_owned) {
        execution.mapping[usize(id)] = id
        return
    }
    execution.mapping[usize(id)] = no_node
    resolved :: binding(state, id)
    if (resolved.kind == .declaration) {
        for (execution.locals.items) |local| {
            if (local.declaration == resolved.data.declaration) {
                execution.mapping[usize(id)] = local.value
                return
            }
        }
    }
    for (0..ast.child_count(&state.tree, id)) |i|
        prepare_type_snapshot(state, execution, ast.child(&state.tree, id, i))
}

@hide:file
bind_type_snapshot proc(state @mut State, execution @TypeExecution, id, start ast.NodeId) void {
    if (usize(id) >= execution.mapping.len) return
    target :: execution.mapping[usize(id)]
    if (target == no_node or target < start) return
    resolved := binding(state, id)
    if (resolved.kind == .declaration) {
        declaration :: resolved.data.declaration
        if (usize(declaration) < execution.mapping.len and execution.mapping[usize(declaration)] != no_node)
            resolved.data.declaration = execution.mapping[usize(declaration)]
    }
    state.bindings.items[usize(target)] = resolved
    for (0..ast.child_count(&state.tree, id)) |i|
        bind_type_snapshot(state, execution, ast.child(&state.tree, id, i), start)
}

@hide:file
type_snapshot proc(state @mut State, execution @mut TypeExecution, id ast.NodeId) ast.NodeId ! Error {
    prepare_type_snapshot(state, execution, id)
    start :: ast.NodeId(ast.node_count(&state.tree))
    result :: try ast.clone_subtree(state.builder, id, execution.mapping)
    try refresh(state)
    bind_type_snapshot(state, execution, id, start)
    return result
}

@hide:file
nominal_type_value proc(state @State, id ast.NodeId) bool {
    kind :: ast.expr_kind(&state.tree, id)
    return kind == .record_type or kind == .union_type or kind == .enum_type or kind == .distinct_type
}

# select with ordinary checked IR so patterns, overlap and exhaustiveness have
# the same rules as runtime matching, without checking unexecuted type bodies.
@hide:file
type_match_arm proc(state @mut State, id ast.NodeId) usize ! Error {
    if (!try expand_type_matches(state, ast.match_expr(&state.tree, id).subject)) return no_index
    try expand(state, ast.match_expr(&state.tree, id).subject)
    arms := arraylist.init(ast.NodeId, state.allocator)
    defer arraylist.deinit(&arms)
    patterns := arraylist.init(ast.NodeId, state.allocator)
    defer arraylist.deinit(&patterns)
    for (0..ast.match_arm_count(&state.tree, id)) |i| {
        arm_id :: ast.match_arm(&state.tree, id, i)
        arm :: ast.match_arm_view(&state.tree, arm_id)
        # selector bodies omit source capture uses, but must retain their resolver failures.
        if (arm.capture) |capture| {
            resolved :: binding(state, capture)
            if (resolved.kind == .invalid) {
                state.evaluation_diagnostic = resolved.data.diagnostic
                return no_index
            }
        }
        if (
            i > 0
            and arm.expression != ast.match_arm_view(
                &state.tree,
                ast.match_arm(&state.tree, id, 0),
            ).expression
        ) {
            state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                .type_mismatch, ast.node_span(&state.tree, arm_id))
            return no_index
        }
        patterns.items = patterns.items[..0]
        for (0..ast.match_pattern_count(&state.tree, arm_id)) |j| {
            pattern :: ast.match_pattern(&state.tree, arm_id, j)
            if (!try expand_type_matches(state, pattern)) return no_index
            try expand(state, pattern)
            try arraylist.append(&patterns, pattern)
        }
        span :: ast.node_span(&state.tree, arm_id)
        index :: try ast.add_integer(state.builder, span, u64(i))
        selector :: try ast.add_match_arm(state.builder, span, patterns.items,
            arm.capture, index, true, arm.catch_all)
        try refresh(state)
        try arraylist.append(&arms, selector)
    }
    selector :: try ast.add_match(state.builder, ast.node_span(&state.tree, id),
        ast.match_expr(&state.tree, id).subject, arms.items)
    try refresh(state)
    # selector arms contain only indices. do not specialize this synthetic match
    # again while its checked evaluation expands the wrapper.
    state.expanded.items[usize(selector)] = 1
    selected :: try evaluate_ir(state, selector, try builtin(state, id, .usize))
    if (selected.kind != .integer) return no_index
    return usize(selected.magnitude)
}

# match checking has already validated every capture. copy the active payload
# out of owned constant storage, never an address in the temporary interpreter.
@hide:file
type_match_capture proc(state @mut State, id ast.NodeId) ast.NodeId ! Error {
    subject :: ast.match_expr(&state.tree, id).subject
    annotation :: try expression_type(state, subject, 0)
    value :: try evaluate_ir(state, subject, annotation)
    if (value.kind != .aggregate or ast.expr_kind(&state.tree, value.node) != .record_literal) return no_node
    payload :: ast.field(&state.tree, ast.record_field(&state.tree, value.node, 0))
    for (0..ast.record_field_count(&state.tree, annotation)) |i| {
        field :: ast.field(&state.tree, ast.record_field(&state.tree, annotation, i))
        if (
            field.name != null and payload.name != null
            and ast.name_id(&state.tree, field.name?) == ast.name_id(&state.tree, payload.name?)
        ) {
            result :: try ast.add_typed_constant(state.builder,
                ast.node_span(&state.tree, subject), field.type?, payload.value?)
            try refresh(state)
            return result
        }
    }
    return no_node
}

@hide:file
type_match_body proc(state @mut State, id ast.NodeId, arm ast.MatchArmView) ast.NodeId ! Error {
    if (arm.capture == null) return arm.body
    value :: try type_match_capture(state, id)
    if (value == no_node) return no_node
    execution := TypeExecution{
        locals = arraylist.init(state.allocator),
        mapping = try mem.alloc(ast.NodeId, state.allocator, ast.node_count(&state.tree)),
        first_owned = ast.NodeId(0),
    }
    defer arraylist.deinit(&execution.locals)
    defer mem.free(state.allocator, execution.mapping)
    mem.set(execution.mapping, no_node)
    try arraylist.append(&execution.locals, TypeLocal{
        declaration = arm.capture?, value = value, annotation = ast.typed_constant(&state.tree, value).type,
    })
    return try type_snapshot(state, &execution, arm.body)
}

@hide:file
type_value_expression proc(state @mut State, input ast.NodeId) ast.NodeId ! Error {
    expression := input
    while (ast.expr_kind(&state.tree, expression) == .match_expr) {
        if (!try type_step(state, expression)) return no_node
        index :: try type_match_arm(state, expression)
        if (index == no_index) return no_node
        arm :: ast.match_arm_view(&state.tree, ast.match_arm(&state.tree, expression, index))
        if (!arm.expression) return no_node
        expression = try type_match_body(state, expression, arm)
        if (expression == no_node) return no_node
    }
    return expression
}

# normalize only selected expression arms before ordinary expansion can visit them.
# nominal results enter here after their instance declaration is cached.
@hide:file
expand_type_matches proc(state @mut State, id ast.NodeId) bool ! Error {
    if (ast.node_class(&state.tree, id) == .expression and ast.expr_kind(&state.tree, id) == .match_expr) {
        selected :: try type_value_expression(state, id)
        if (selected == no_node) return false
        resolved :: binding(state, selected)
        try remember(state, id)
        ast.replace_node_from_source(state.builder, id, selected)
        try refresh(state)
        state.bindings.items[usize(id)] = resolved
    }
    for (0..ast.child_count(&state.tree, id)) |i| {
        if (!try expand_type_matches(state, ast.child(&state.tree, id, i))) return false
    }
    return try type_array_projection(state, id)
}

@hide:file
type_array_projection proc(state @mut State, id ast.NodeId) bool ! Error {
    if (ast.node_class(&state.tree, id) != .expression) return true
    kind :: ast.expr_kind(&state.tree, id)
    if (kind == .index or kind == .selection) {
        base :: if (kind == .index) ast.index_expr(&state.tree, id).base else ast.selection_expr(&state.tree, id).base
        if (ast.expr_kind(&state.tree, base) != .typed_constant) return true
        array :: ast.typed_constant(&state.tree, base)
        if (!try type_array_storage(state, array.type)) return true
        selected := no_node
        if (kind == .index) {
            index :: try type_array_index(state, ast.index_expr(&state.tree, id).index,
                ast.array_element_count(&state.tree, array.value))
            if (index == no_index) return false
            selected = ast.array_element(&state.tree, array.value, index)
        } else {
            member :: ast.selection_expr(&state.tree, id).member
            if (!mem.eql(ast.source_text(&state.tree, ast.node_span(&state.tree, member)), "len")) {
                state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                    .unknown_field, ast.node_span(&state.tree, member))
                return false
            }
            count :: try ast.add_integer(state.builder, ast.node_span(&state.tree, id),
                u64(ast.array_element_count(&state.tree, array.value)))
            selected = try ast.add_typed_constant(state.builder, ast.node_span(&state.tree, id),
                try builtin(state, id, .usize), count)
            try refresh(state)
        }
        # copying a nominal definition into the projection would create a new
        # identity on every read. retain its original node behind a type alias.
        if (nominal_type_value(state, selected)) {
            selected = try ast.add_type_operand(state.builder, ast.node_span(&state.tree, id), selected, true)
            try refresh(state)
        }
        resolved :: binding(state, selected)
        try remember(state, id)
        ast.replace_node_from_source(state.builder, id, selected)
        try refresh(state)
        state.bindings.items[usize(id)] = resolved
    }
    return true
}

# type arrays are persistent AST values, never runtime storage. keep the same
# typed-constant snapshots as ordinary aggregates, but validate their leaves here.
@hide:file
type_array_storage proc(state @mut State, annotation ast.NodeId) bool ! Error {
    current := annotation
    for (0..128) |_| {
        if (is_type(state, current)) return current != annotation
        current = try canonical_type(state, current)
        if (current == no_node or ast.expr_kind(&state.tree, current) != .array_type) return false
        current = ast.array_type(&state.tree, current).element
    }
    return false
}

@hide:file
type_array_index proc(state @mut State, expression ast.NodeId, count usize) usize ! Error {
    usize_type :: try builtin(state, expression, .usize)
    actual :: try expression_type(state, expression, 0)
    if (!flexible(state, expression) and (actual == no_node or !try same_type(state, actual, usize_type))) {
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
            .invalid_index_type, ast.node_span(&state.tree, expression))
        return no_index
    }
    value :: try evaluate_ir(state, expression, usize_type)
    if (value.kind != .integer) return no_index
    if (value.magnitude >= u64(count)) {
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
            .index_out_of_bounds, ast.node_span(&state.tree, expression))
        return no_index
    }
    return usize(value.magnitude)
}

@hide:file
type_array_compatible proc(state @mut State, expected, actual ast.NodeId) bool ! Error {
    if (is_type(state, expected) or is_type(state, actual))
        return is_type(state, expected) and is_type(state, actual)
    left :: try canonical_type(state, expected)
    right :: try canonical_type(state, actual)
    if (left == no_node or right == no_node) return false
    if (ast.expr_kind(&state.tree, left) != .array_type or ast.expr_kind(&state.tree, right) != .array_type) return false
    a :: ast.array_type(&state.tree, left)
    b :: ast.array_type(&state.tree, right)
    if (a.mutable != b.mutable or b.count == null) return false
    if (a.count) |count| {
        if (!try value_equal(state, try evaluate(state, count), try evaluate(state, b.count?))) return false
    }
    return try type_array_compatible(state, a.element, b.element)
}

@hide:file
type_array_value proc(state @mut State, expression, annotation ast.NodeId) ast.NodeId ! Error {
    type :: try canonical_type(state, annotation)
    array :: ast.array_type(&state.tree, type)
    if (ast.expr_kind(&state.tree, expression) == .typed_constant) {
        actual :: ast.typed_constant(&state.tree, expression)
        if (try type_array_compatible(state, type, actual.type)) return expression
    } else if (ast.expr_kind(&state.tree, expression) == .array_literal) {
        count :: ast.array_element_count(&state.tree, expression)
        if (array.count) |length| {
            value :: try evaluate(state, length)
            if (value.kind != .integer or value.negative) {
                state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                    .invalid_array_length, ast.node_span(&state.tree, length))
                return no_node
            }
            if (value.magnitude != u64(count)) {
                state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                    .array_length_mismatch, ast.node_span(&state.tree, expression))
                return no_node
            }
        }
        elements := arraylist.init(ast.NodeId, state.allocator)
        defer arraylist.deinit(&elements)
        element_type := array.element
        for (0..count) |i| {
            element :: try type_execution_value(state,
                ast.array_element(&state.tree, expression, i), element_type)
            if (element == no_node) return no_node
            if (ast.expr_kind(&state.tree, element) == .typed_constant)
                element_type = ast.typed_constant(&state.tree, element).type
            try arraylist.append(&elements, element)
        }
        # empty outer arrays still need concrete, valid child dimensions.
        if (count == 0) {
            child := element_type
            while (!is_type(state, child)) {
                child = try canonical_type(state, child)
                descriptor :: ast.array_type(&state.tree, child)
                value := Value{}
                if (descriptor.count) |length| value = try evaluate(state, length)
                if (value.kind != .integer or value.negative) {
                    state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                        .invalid_array_length, ast.node_span(&state.tree, child))
                    return no_node
                }
                child = descriptor.element
            }
        }
        span :: ast.node_span(&state.tree, expression)
        length :: try ast.add_integer(state.builder, span, u64(count))
        concrete :: try ast.add_array_type(state.builder, ast.node_span(&state.tree, annotation),
            length, element_type, array.mutable)
        literal :: try ast.add_array_literal(state.builder, span, elements.items)
        result :: try ast.add_typed_constant(state.builder, span, concrete, literal)
        try refresh(state)
        return result
    }
    state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
        .type_mismatch, ast.node_span(&state.tree, expression))
    return no_node
}

# rebuild only the indexed path so previous snapshots retain their owned values.
@hide:file
type_array_update proc(
    state @mut State, execution @mut TypeExecution, target, root, current, input ast.NodeId,
) ast.NodeId ! Error {
    # find the outermost index; rebuild its child before copying this parent.
    first := target
    while (true) {
        if (ast.expr_kind(&state.tree, first) != .index) {
            state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                .invalid_assignment_target, ast.node_span(&state.tree, target))
            return no_node
        }
        base :: ast.index_expr(&state.tree, first).base
        if (base == root) break
        first = base
    }
    array :: ast.typed_constant(&state.tree, current)
    descriptor :: ast.array_type(&state.tree, array.type)
    if (!descriptor.mutable) {
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
            .invalid_index_assignment, ast.node_span(&state.tree, target))
        return no_node
    }
    expression :: try type_snapshot(state, execution, ast.index_expr(&state.tree, first).index)
    if (!try expand_type_matches(state, expression)) return no_node
    count :: ast.array_element_count(&state.tree, array.value)
    index :: try type_array_index(state, expression, count)
    if (index == no_index) return no_node
    previous :: ast.array_element(&state.tree, array.value, index)
    updated := no_node
    if (first == target) updated = try type_execution_value(state, input, descriptor.element)
    else updated = try type_array_update(state, execution, target, first, previous, input)
    if (updated == no_node) return no_node
    elements := arraylist.init(ast.NodeId, state.allocator)
    defer arraylist.deinit(&elements)
    for (0..count) |i| {
        element :: if (i == index) updated else ast.array_element(&state.tree, array.value, i)
        try arraylist.append(&elements, element)
    }
    span :: ast.node_span(&state.tree, target)
    literal :: try ast.add_array_literal(state.builder, span, elements.items)
    result :: try ast.add_typed_constant(state.builder, span, array.type, literal)
    try refresh(state)
    return result
}

@hide:file
type_execution_value proc(state @mut State, input, annotation ast.NodeId) ast.NodeId ! Error {
    if (annotation == no_node) return no_node
    expression :: try type_value_expression(state, input)
    if (expression == no_node) return no_node
    if (!try expand_type_matches(state, annotation)) return no_node
    # a recursive record needs its cached declaration before expanding its fields.
    if (is_type(state, annotation) and nominal_type_value(state, expression)) {
        concrete :: try materialize(state, Value{ kind = .type, node = expression })
        constraint :: try ast.add_type_operand(state.builder, ast.node_span(&state.tree, expression), concrete.node, true)
        try refresh(state)
        try arraylist.append(&state.rewrites.constraints, constraint)
        return concrete.node
    }
    if (!try expand_type_matches(state, expression)) return no_node
    if (try type_array_storage(state, annotation)) return try type_array_value(state, expression, annotation)
    try expand(state, expression)
    resolved :: binding(state, expression)
    if (resolved.kind == .invalid) {
        state.evaluation_diagnostic = resolved.data.diagnostic
        return no_node
    }
    if (is_type(state, annotation)) {
        value :: try evaluate(state, expression)
        if (value.kind != .type) {
            if (state.evaluation_diagnostic == diagnostics.no_diagnostic)
                state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                    .type_mismatch, ast.node_span(&state.tree, expression))
            return no_node
        }
        concrete :: try materialize(state, value)
        constraint :: try ast.add_type_operand(state.builder, ast.node_span(&state.tree, expression), concrete.node, true)
        try refresh(state)
        try arraylist.append(&state.rewrites.constraints, constraint)
        return concrete.node
    }
    value :: try evaluate_ir(state, expression, annotation)
    if (value.kind == .invalid) return no_node
    concrete :: try materialize(state, value)
    result :: try ast.add_typed_constant(state.builder, ast.node_span(&state.tree, expression), annotation, concrete.node)
    try refresh(state)
    return result
}

# check capture guards with ordinary IR without elaborating either source body.
@hide:file
type_capture_guard proc(state @mut State, expression, capture ast.NodeId) Value ! Error {
    resolved :: binding(state, capture)
    if (resolved.kind == .invalid) {
        state.evaluation_diagnostic = resolved.data.diagnostic
        return Value{}
    }
    span :: ast.node_span(&state.tree, expression)
    yes :: try ast.add_boolean(state.builder, span, true)
    no :: try ast.add_boolean(state.builder, span, false)
    taken :: try ast.add_return_stmt(state.builder, span, yes)
    skipped :: try ast.add_return_stmt(state.builder, span, no)
    taken_statements [1]ast.NodeId :: [taken]
    skipped_statements [1]ast.NodeId :: [skipped]
    then_block :: try ast.add_block(state.builder, span, taken_statements[..])
    else_block :: try ast.add_block(state.builder, span, skipped_statements[..])
    selector :: try ast.add_if_stmt(state.builder, span, expression, then_block, else_block)
    ast.set_capture(state.builder, selector, capture)
    statements [1]ast.NodeId :: [selector]
    body :: try ast.add_block(state.builder, span, statements[..])
    try refresh(state)
    return try evaluate_ir_body(state, expression, body, try builtin(state, expression, .bool))
}

# update one owned aggregate through ordinary assignment checking and execution.
# only the target's root becomes a temporary local; indices and RHS retain their
# pre-assignment snapshots, and no interpreter storage survives the evaluation.
@hide:file
type_assignment_value proc(
    state @mut State, execution @mut TypeExecution, statement ast.NodeId, index usize,
) ast.NodeId ! Error {
    assignment :: ast.assignment_stmt(&state.tree, statement)
    local :: execution.locals.items[index]
    span :: ast.node_span(&state.tree, statement)
    value :: try type_snapshot(state, execution, assignment.value)
    mutability :: if (ast.node_class(&state.tree, local.declaration) == .statement)
        ast.statement_decl(&state.tree, local.declaration).mutability else ast.Mutability.constant
    temporary :: try ast.add_statement_decl(state.builder, span, span, strpool.no_str,
        local.annotation, mutability, local.value)
    try refresh(state)
    target := no_node
    {
        execution.locals.items[index].value = try reference(state, assignment.target, temporary)
        defer execution.locals.items[index].value = local.value
        target = try type_snapshot(state, execution, assignment.target)
    }
    if (!try expand_type_matches(state, target) or !try expand_type_matches(state, value)) return no_node
    update :: try ast.add_assignment_stmt(state.builder, span, target, value)
    returned :: try ast.add_return_stmt(state.builder, span, try reference(state, statement, temporary))
    statements [3]ast.NodeId :: [temporary, update, returned]
    body :: try ast.add_block(state.builder, span, statements[..])
    try refresh(state)
    result :: try evaluate_ir_body(state, statement, body, local.annotation)
    if (result.kind == .invalid) return no_node
    concrete :: try materialize(state, result)
    owned :: try ast.add_typed_constant(state.builder, span, local.annotation, concrete.node)
    try refresh(state)
    return owned
}

@hide:file
execute_type_block proc(state @mut State, execution @mut TypeExecution, block ast.NodeId) TypeFlow ! Error {
    saved :: execution.locals.items.len
    defer execution.locals.items = execution.locals.items[..saved]
    for (0..ast.block_statement_count(&state.tree, block)) |i| {
        statement :: ast.block_statement(&state.tree, block, i)
        if (!try type_step(state, statement)) return .invalid
        flow :: try execute_type_statement(state, execution, statement)
        if (flow != .next) return flow
    }
    return .next
}

@hide:file
execute_type_statement proc(state @mut State, execution @mut TypeExecution, statement ast.NodeId) TypeFlow ! Error {
    match ast.stmt_kind(&state.tree, statement) {
        .return_stmt: {
            if (ast.return_stmt(&state.tree, statement).value) |value| {
                expression :: try type_snapshot(state, execution, value)
                execution.returned = try type_execution_value(state, expression, try builtin(state, expression, .type))
                if (execution.returned != no_node) return .returned
            }
            return .invalid
        }
        .declaration: {
            declaration :: ast.statement_decl(&state.tree, statement)
            snapshot :: try type_snapshot(state, execution, declaration.initializer)
            expression :: try type_value_expression(state, snapshot)
            if (expression == no_node) return .invalid
            annotation := no_node
            if (declaration.type) |type| annotation = try type_snapshot(state, execution, type)
            else if (nominal_type_value(state, expression)) annotation = try builtin(state, expression, .type)
            else {
                if (!try expand_type_matches(state, expression)) return .invalid
                try expand(state, expression)
                annotation = try expression_type(state, expression, 0)
                if (annotation == no_node) {
                    value :: try evaluate(state, expression)
                    if (value.kind == .type) annotation = try builtin(state, expression, .type)
                }
            }
            if (annotation == no_node) return .invalid
            value :: try type_execution_value(state, expression, annotation)
            if (value == no_node) return .invalid
            if (ast.expr_kind(&state.tree, value) == .typed_constant)
                annotation = ast.typed_constant(&state.tree, value).type
            try arraylist.append(&execution.locals, TypeLocal{
                declaration = statement, value = value, annotation = annotation,
            })
        }
        .assignment_stmt: {
            assignment :: ast.assignment_stmt(&state.tree, statement)
            root := assignment.target
            while (true) {
                kind :: ast.expr_kind(&state.tree, root)
                if (kind == .selection and binding(state, root).kind == .none)
                    root = ast.selection_expr(&state.tree, root).base
                else if (kind == .index) root = ast.index_expr(&state.tree, root).base
                else break
            }
            resolved :: binding(state, root)
            index := no_index
            if (resolved.kind == .declaration) {
                for (execution.locals.items) |local, i| {
                    if (local.declaration == resolved.data.declaration) index = i
                }
            }
            if (
                index == no_index
                or (root == assignment.target
                    and (ast.node_class(&state.tree, execution.locals.items[index].declaration) != .statement
                        or ast.statement_decl(&state.tree, execution.locals.items[index].declaration).mutability != .variable))
            ) {
                state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                    .invalid_assignment_target, ast.node_span(&state.tree, assignment.target))
                return .invalid
            }
            if (root != assignment.target) {
                if (try type_array_storage(state, execution.locals.items[index].annotation)) {
                    local :: execution.locals.items[index]
                    if (
                        ast.node_class(&state.tree, local.declaration) != .statement
                        or ast.statement_decl(&state.tree, local.declaration).mutability != .variable
                    ) {
                        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics,
                            .invalid_index_assignment, ast.node_span(&state.tree, assignment.target))
                        return .invalid
                    }
                    input :: try type_snapshot(state, execution, assignment.value)
                    if (!try expand_type_matches(state, input)) return .invalid
                    value :: try type_array_update(state, execution, assignment.target, root, local.value, input)
                    if (value == no_node) return .invalid
                    execution.locals.items[index].value = value
                    return .next
                }
                value :: try type_assignment_value(state, execution, statement, index)
                if (value == no_node) return .invalid
                execution.locals.items[index].value = value
                return .next
            }
            expression :: try type_snapshot(state, execution, assignment.value)
            value :: try type_execution_value(state, expression, execution.locals.items[index].annotation)
            if (value == no_node) return .invalid
            execution.locals.items[index].value = value
        }
        .if_stmt, .while_stmt: {
            looping :: ast.stmt_kind(&state.tree, statement) == .while_stmt
            condition := no_node
            body := no_node
            other := no_node
            capture ?ast.NodeId := null
            if (looping) {
                view :: ast.while_stmt(&state.tree, statement)
                capture = view.capture
                condition = view.condition
                body = view.body
            } else {
                view :: ast.if_stmt(&state.tree, statement)
                capture = view.capture
                condition = view.condition
                body = view.then_block
                if (view.else_block) |branch| other = branch
            }
            if (looping) execution.loop_depth += 1
            defer if (looping) execution.loop_depth -= 1
            saved :: execution.locals.items.len
            defer execution.locals.items = execution.locals.items[..saved]
            invariant :: branch_constant(state, condition, 0)
            value := Value{}
            while (true) {
                if (!try type_step(state, condition)) return .invalid
                if (!invariant or value.kind == .invalid) {
                    expression :: try type_snapshot(state, execution, condition)
                    if (capture) |name| value = try type_capture_guard(state, expression, name)
                    else {
                        boolean :: try builtin(state, expression, .bool)
                        value = try evaluate_ir(state, expression, boolean)
                    }
                }
                if (value.kind != .boolean) return .invalid
                if (!value.boolean) {
                    if (!looping and other != no_node) return try execute_type_block(state, execution, other)
                    return .next
                }
                execution.locals.items = execution.locals.items[..saved]
                if (capture) |name| {
                    expression :: try type_snapshot(state, execution, condition)
                    unwrapped :: try ast.add_unary(state.builder,
                        ast.node_span(&state.tree, condition), .unwrap, expression)
                    try refresh(state)
                    annotation :: try expression_type(state, unwrapped, 0)
                    payload :: try type_execution_value(state, unwrapped, annotation)
                    if (payload == no_node) return .invalid
                    try arraylist.append(&execution.locals, TypeLocal{
                        declaration = name, value = payload, annotation = annotation,
                    })
                }
                flow :: try execute_type_block(state, execution, body)
                if (!looping or flow == .returned or flow == .invalid) return flow
                if (flow == .break) return .next
            }
        }
        .expression: {
            expression :: ast.statement_expr(&state.tree, statement)
            if (ast.expr_kind(&state.tree, expression) != .match_expr) return .invalid
            snapshot :: try type_snapshot(state, execution, expression)
            index :: try type_match_arm(state, snapshot)
            if (index == no_index) return .invalid
            arm :: ast.match_arm_view(&state.tree, ast.match_arm(&state.tree, expression, index))
            saved :: execution.locals.items.len
            defer execution.locals.items = execution.locals.items[..saved]
            if (arm.capture) |capture| {
                value :: try type_match_capture(state, snapshot)
                if (value == no_node) return .invalid
                try arraylist.append(&execution.locals, TypeLocal{
                    declaration = capture, value = value, annotation = ast.typed_constant(&state.tree, value).type,
                })
            }
            if (!arm.expression) return try execute_type_block(state, execution, arm.body)
            body :: try type_snapshot(state, execution, arm.body)
            value :: try type_value_expression(state, body)
            if (value == no_node) return .invalid
            annotation :: try expression_type(state, value, 0)
            if (try type_execution_value(state, value, annotation) == no_node) return .invalid
        }
        .for_stmt: return try execute_type_for(state, execution, statement)
        .block_stmt: return try execute_type_block(state, execution, ast.statement_block(&state.tree, statement))
        .break_stmt, .continue_stmt: {
            if (execution.loop_depth == 0) {
                code :: if (ast.stmt_kind(&state.tree, statement) == .break_stmt)
                    diagnostics.Code.break_outside_loop else diagnostics.Code.continue_outside_loop
                state.evaluation_diagnostic = try diagnostics.add(state.diagnostics, code, ast.node_span(&state.tree, statement))
                return .invalid
            }
            return if (ast.stmt_kind(&state.tree, statement) == .break_stmt) TypeFlow.break else TypeFlow.continue
        }
        else: return .invalid
    }
    return .next
}

# retain an array place, but freeze its indices at loop entry. snapshots then read
# the current owned root after body assignments without re-evaluating the place.
@hide:file
type_collection_place proc(state @mut State, execution @mut TypeExecution, id ast.NodeId) ast.NodeId ! Error {
    resolved :: binding(state, id)
    if (resolved.kind == .declaration) {
        for (execution.locals.items) |local| {
            if (local.declaration == resolved.data.declaration) return id
        }
        return no_node
    }
    kind :: ast.expr_kind(&state.tree, id)
    if (kind != .selection and kind != .index) return no_node
    base :: if (kind == .selection) ast.selection_expr(&state.tree, id).base else ast.index_expr(&state.tree, id).base
    place :: try type_collection_place(state, execution, base)
    if (place == no_node) return no_node
    result := no_node
    if (kind == .selection) {
        result = try ast.add_selection(state.builder, ast.node_span(&state.tree, id),
            place, ast.selection_expr(&state.tree, id).member)
    } else {
        index :: try type_snapshot(state, execution, ast.index_expr(&state.tree, id).index)
        value :: try type_execution_value(state, index, try builtin(state, index, .usize))
        if (value == no_node) return no_node
        result = try ast.add_index(state.builder, ast.node_span(&state.tree, id), place, value)
    }
    try refresh(state)
    return result
}

@hide:file
type_collection_read proc(state @mut State, execution @mut TypeExecution, id ast.NodeId) ast.NodeId ! Error {
    if (binding(state, id).kind == .declaration) return try type_snapshot(state, execution, id)
    kind :: ast.expr_kind(&state.tree, id)
    base :: if (kind == .selection) ast.selection_expr(&state.tree, id).base else ast.index_expr(&state.tree, id).base
    value :: try type_collection_read(state, execution, base)
    result := no_node
    if (kind == .selection)
        result = try ast.add_selection(state.builder, ast.node_span(&state.tree, id),
            value, ast.selection_expr(&state.tree, id).member)
    else result = try ast.add_index(state.builder, ast.node_span(&state.tree, id),
        value, ast.index_expr(&state.tree, id).index)
    try refresh(state)
    return result
}

@hide:file
execute_type_collection proc(state @mut State, execution @mut TypeExecution, statement ast.NodeId) TypeFlow ! Error {
    loop :: ast.for_stmt(&state.tree, statement)
    span :: ast.node_span(&state.tree, statement)
    if (loop.capture) |capture| {
        if (binding(state, capture).kind == .invalid) {
            state.evaluation_diagnostic = binding(state, capture).data.diagnostic
            return .invalid
        }
    }
    if (loop.index) |index| {
        if (binding(state, index).kind == .invalid) {
            state.evaluation_diagnostic = binding(state, index).data.diagnostic
            return .invalid
        }
    }
    snapshot :: try type_snapshot(state, execution, loop.iterable)
    if (!try expand_type_matches(state, snapshot)) return .invalid
    try expand(state, snapshot)
    annotation :: try expression_type(state, snapshot, 0)
    if (annotation == no_node) return .invalid
    collection :: try type_execution_value(state, snapshot, annotation)
    if (collection == no_node) return .invalid
    count := Value{}
    if (try type_array_storage(state, annotation)) {
        count = Value{ kind = .integer,
            magnitude = u64(ast.array_element_count(&state.tree, ast.typed_constant(&state.tree, collection).value)) }
    } else {
        # validate the loop contract even for empty collections, without executing
        # its type-valued body. the synthetic body exits before reading an element.
        stop :: try ast.add_break_stmt(state.builder, span)
        stops [1]ast.NodeId :: [stop]
        body :: try ast.add_block(state.builder, span, stops[..])
        selector :: try ast.add_for_stmt(state.builder, span, collection, loop.capture, loop.index,
            loop.pointer_capture, loop.index_capture, body)
        length := no_node
        if (ast.expr_kind(&state.tree, annotation) == .array_type) length = ast.array_type(&state.tree, annotation).count?
        else {
            value :: try evaluate(state, collection)
            if (value.kind == .string) {
                text :: ast.literal_text(&state.tree, value.node)
                cursor := usize(1)
                size := u64(0)
                while (cursor < text.len - 1) {
                    _ = ast.string_next(text, &cursor) catch { return .invalid }
                    size += 1
                }
                length = try ast.add_integer(state.builder, span, size)
            } else length = try ast.add_integer(state.builder, span, 0)
        }
        returned :: try ast.add_return_stmt(state.builder, span, length)
        statements [2]ast.NodeId :: [selector, returned]
        wrapper :: try ast.add_block(state.builder, span, statements[..])
        try refresh(state)
        count = try evaluate_ir_body(state, statement, wrapper, try builtin(state, statement, .usize))
        if (count.kind != .integer) return .invalid
    }
    if (loop.pointer_capture and loop.capture != null) {
        state.evaluation_diagnostic = try diagnostics.add(state.diagnostics, .comptime_value_escape, span)
        return .invalid
    }
    place := no_node
    if (ast.expr_kind(&state.tree, annotation) == .array_type)
        place = try type_collection_place(state, execution, loop.iterable)
    if (state.evaluation_diagnostic != diagnostics.no_diagnostic) return .invalid
    saved :: execution.locals.items.len
    defer execution.locals.items = execution.locals.items[..saved]
    execution.loop_depth += 1
    defer execution.loop_depth -= 1
    for (0..usize(count.magnitude)) |i| {
        if (!try type_step(state, statement)) return .invalid
        execution.locals.items = execution.locals.items[..saved]
        index :: try ast.add_integer(state.builder, span, u64(i))
        try refresh(state)
        if (loop.capture) |capture| {
            current := collection
            if (place != no_node) current = try type_collection_read(state, execution, place)
            element :: try ast.add_index(state.builder, span, current, index)
            try refresh(state)
            element_type :: try expression_type(state, element, 0)
            value :: try type_execution_value(state, element, element_type)
            if (value == no_node) return .invalid
            try arraylist.append(&execution.locals, TypeLocal{
                declaration = capture, value = value, annotation = element_type,
            })
        }
        if (loop.index) |capture| {
            type :: try builtin(state, capture, .usize)
            value :: try type_execution_value(state, index, type)
            if (value == no_node) return .invalid
            try arraylist.append(&execution.locals, TypeLocal{
                declaration = capture, value = value, annotation = type,
            })
        }
        flow :: try execute_type_block(state, execution, loop.body)
        if (flow == .returned or flow == .invalid) return flow
        if (flow == .break) return .next
    }
    return .next
}

@hide:file
execute_type_for proc(state @mut State, execution @mut TypeExecution, statement ast.NodeId) TypeFlow ! Error {
    loop :: ast.for_stmt(&state.tree, statement)
    if (ast.expr_kind(&state.tree, loop.iterable) != .range)
        return try execute_type_collection(state, execution, statement)
    if (loop.pointer_capture or loop.index_capture) return .invalid
    bounds :: ast.range_expr(&state.tree, loop.iterable)
    start :: try type_snapshot(state, execution, bounds.start)
    end :: try type_snapshot(state, execution, bounds.end)
    typed_bound :: if (flexible(state, start)) end else start
    annotation :: try expression_type(state, typed_bound, 0)
    if (annotation == no_node or !try integer_type(state, annotation)) return .invalid
    current := try type_execution_value(state, start, annotation)
    limit :: try type_execution_value(state, end, annotation)
    if (current == no_node or limit == no_node) return .invalid
    saved :: execution.locals.items.len
    defer execution.locals.items = execution.locals.items[..saved]
    if (loop.capture) |capture| try arraylist.append(&execution.locals, TypeLocal{
        declaration = capture, value = current, annotation = annotation,
    })
    execution.loop_depth += 1
    defer execution.loop_depth -= 1
    while (true) {
        if (!try type_step(state, statement)) return .invalid
        span :: ast.node_span(&state.tree, statement)
        operator ast.BinaryOperator :: if (bounds.inclusive) .less_equal else .less
        comparison :: try ast.add_binary(state.builder, span, operator, current, limit)
        try refresh(state)
        keep :: try evaluate(state, comparison)
        if (keep.kind != .boolean) return .invalid
        if (!keep.boolean) return .next
        if (loop.capture != null) execution.locals.items[saved].value = current
        flow :: try execute_type_block(state, execution, loop.body)
        if (flow == .returned or flow == .invalid) return flow
        if (flow == .break) return .next
        if (try value_equal(state, try evaluate(state, current), try evaluate(state, limit))) return .next
        one :: try ast.add_integer(state.builder, span, 1)
        increment :: try ast.add_binary(state.builder, span, .add, current, one)
        try refresh(state)
        current = try type_execution_value(state, increment, annotation)
        if (current == no_node) return .invalid
    }
}

@hide:file
mix proc(left, right usize) usize {
    return (left << 7) xor (left >> (sizeof!(usize) * 8 - 7)) xor right
}

@hide:file
evaluate proc(state @mut State, id ast.NodeId) Value ! Error {
    if (state.depth >= 128) return Value{}
    state.depth += 1
    defer state.depth -= 1
    try expand(state, id)
    kind :: ast.expr_kind(&state.tree, id)
    resolved :: binding(state, id)
    if (resolved.kind == .builtin and resolved.data.builtin != .type) return Value{ kind = .type, node = id }
    if (resolved.kind == .declaration) {
        target :: resolved.data.declaration
        class :: ast.node_class(&state.tree, target)
        if (class == .function) {
            if (template(state, target)) return Value{}
            return Value{ kind = .function, node = id }
        }
        if (class == .statement) {
            declaration :: ast.statement_decl(&state.tree, target)
            if (declaration.mutability != .constant) return Value{}
            value := try evaluate(state, declaration.initializer)
            if (declaration.type) |annotation| value.annotation = annotation
            return value
        }
        return Value{}
    }
    match kind {
        .typed_constant: {
            view :: ast.typed_constant(&state.tree, id)
            value := try evaluate_expected(state, view.value, view.type)
            value.annotation = view.type
            return value
        }
        .literal_int: {
            value := u64(0)
            for ast.literal_text(&state.tree, id) |byte| {
                if (byte < '0' or byte > '9') return Value{}
                digit :: u64(byte - '0')
                if (value > divtrunc!(maxval!(u64) - digit, u64(10))) return Value{}
                value = value * 10 + digit
            }
            return Value{ kind = .integer, node = id, magnitude = value }
        }
        .literal_float: {
            if (ast.float_value(&state.tree, id)) |value| return Value{
                kind = .real, node = id, magnitude = ptrcast!(@u64, &value)^,
            }
            return try evaluate_ir(state, id, try builtin(state, id, .float))
        }
        .literal_bool: return Value{ kind = .boolean, node = id,
            boolean = mem.eql(ast.literal_text(&state.tree, id), "true") }
        .literal_string: {
            text :: ast.literal_text(&state.tree, id)
            cursor := usize(1)
            while (cursor < text.len - 1) {
                _ = ast.string_next(text, &cursor) catch {
                    try reject(state, id, .invalid_escape)
                    return Value{}
                }
            }
            return Value{ kind = .string, node = id }
        }
        .literal_null: return Value{ kind = .null, node = id }
        .literal_void: return Value{ kind = .aggregate, node = id, annotation = try builtin(state, id, .void) }
        .record_type, .array_type, .slice_type, .pointer_type, .function_type, .optional_type,
        .enum_type, .union_type, .distinct_type, .fallible_type, .sum_type:
            return Value{ kind = .type, node = id }
        .alias_expr: return try evaluate(state, ast.type_operand(&state.tree, id))
        .unary: {
            view :: ast.unary_expr(&state.tree, id)
            value := try evaluate(state, view.operand)
            if (view.operator == .negate and value.kind == .integer) value.negative = value.magnitude != 0 and !value.negative
            else if (view.operator == .negate and value.kind == .real) value.magnitude = value.magnitude xor (u64(1) << 63)
            else if (view.operator == .logical_not and value.kind == .boolean) value.boolean = !value.boolean
            else return Value{}
            value.node = id
            return value
        }
        .binary: return try evaluate_binary(state, id)
        .call: return try evaluate_call(state, id)
        .record_literal, .array_literal, .selection, .index, .match_expr, .builtin_call, .catch_expr:
            return try evaluate_ir(state, id, try expression_type(state, id, 0))
        else: return Value{}
    }
}

@hide:file
evaluate_binary proc(state @mut State, id ast.NodeId) Value ! Error {
    view :: ast.binary_expr(&state.tree, id)
    if (view.operator == .divide) return try evaluate_ir(state, id, try builtin(state, id, .float))
    left :: try evaluate(state, view.left)
    if (left.kind == .invalid) return Value{}
    if (left.kind == .boolean) {
        if (view.operator == .logical_and and !left.boolean) return Value{ kind = .boolean, node = id, boolean = false }
        if (view.operator == .logical_or and left.boolean) return Value{ kind = .boolean, node = id, boolean = true }
    }
    right := try evaluate(state, view.right)
    if (left.kind == .invalid or right.kind == .invalid) return Value{}
    if (left.kind == .real or right.kind == .real) {
        boolean :: (
            view.operator == .equal or view.operator == .not_equal or view.operator == .less
            or view.operator == .less_equal or view.operator == .greater or view.operator == .greater_equal
        )
        kind sema.BuiltinType :: if (boolean) .bool else .float
        result_type :: try builtin(state, id, kind)
        return try evaluate_ir(state, id, result_type)
    }
    if (
        left.annotation != no_node and right.annotation != no_node
        and left.kind == .integer and right.kind == .integer
        and !try same_type(state, left.annotation, right.annotation)
    ) return Value{}
    if (view.operator == .equal or view.operator == .not_equal) {
        if (left.kind != right.kind) return Value{}
        equal :: try value_equal(state, left, right)
        if (left.kind == .type) {
            operands [2]ast.NodeId :: [view.left, view.right]
            for (operands) |operand| {
                constraint :: try ast.add_type_operand(state.builder,
                    ast.node_span(&state.tree, operand), operand, true)
                try refresh(state)
                try arraylist.append(&state.rewrites.constraints, constraint)
            }
            folded :: try ast.add_boolean(state.builder, ast.node_span(&state.tree, id),
                equal == (view.operator == .equal))
            try replace(state, id, folded)
            try refresh(state)
        }
        return Value{ kind = .boolean, node = id, boolean = equal == (view.operator == .equal) }
    }
    if (left.kind == .boolean and right.kind == .boolean) {
        if (view.operator == .logical_and) return Value{ kind = .boolean, node = id, boolean = left.boolean and right.boolean }
        if (view.operator == .logical_or) return Value{ kind = .boolean, node = id, boolean = left.boolean or right.boolean }
        return Value{}
    }
    if (left.kind != .integer or right.kind != .integer) return Value{}
    annotation :: if (left.annotation != no_node) left.annotation else right.annotation
    less :: if (left.negative != right.negative) left.negative
        else if (left.negative) left.magnitude > right.magnitude else left.magnitude < right.magnitude
    equal :: left.magnitude == right.magnitude and left.negative == right.negative
    match view.operator {
        .less: return Value{ kind = .boolean, node = id, boolean = less }
        .less_equal: return Value{ kind = .boolean, node = id, boolean = less or equal }
        .greater: return Value{ kind = .boolean, node = id, boolean = !less and !equal }
        .greater_equal: return Value{ kind = .boolean, node = id, boolean = !less }
        .multiply: {
            if (right.magnitude != 0 and left.magnitude > divtrunc!(maxval!(u64), right.magnitude)) return Value{}
            magnitude :: left.magnitude * right.magnitude
            return try integer_value(state, id, magnitude, magnitude != 0 and left.negative != right.negative, annotation)
        }
        .add, .subtract: {
            if (view.operator == .subtract and right.magnitude != 0) right.negative = !right.negative
            if (left.negative == right.negative) {
                if (left.magnitude > maxval!(u64) - right.magnitude) return Value{}
                return try integer_value(state, id, left.magnitude + right.magnitude, left.negative, annotation)
            }
            if (left.magnitude < right.magnitude) return try integer_value(state, id, right.magnitude - left.magnitude, right.negative, annotation)
            return try integer_value(state, id, left.magnitude - right.magnitude,
                left.magnitude != right.magnitude and left.negative, annotation)
        }
        else: return Value{}
    }
}

@hide:file
canonical_type proc(state @mut State, id ast.NodeId) ast.NodeId ! Error {
    value :: try evaluate(state, id)
    return if (value.kind == .type) value.node else no_node
}

@hide:file
same_type proc(state @mut State, left, right ast.NodeId) bool ! Error {
    a :: try canonical_type(state, left)
    b :: try canonical_type(state, right)
    if (a == no_node or b == no_node) return false
    if (a == b) return true
    x :: binding(state, a)
    y :: binding(state, b)
    if (x.kind == .builtin or y.kind == .builtin) return x.kind == .builtin and y.kind == .builtin and builtin_identity(x.data.builtin) == builtin_identity(y.data.builtin)
    kind :: ast.expr_kind(&state.tree, a)
    if (kind == .sum_type or ast.expr_kind(&state.tree, b) == .sum_type)
        return try type_subset(state, a, b) and try type_subset(state, b, a)
    if (kind != ast.expr_kind(&state.tree, b)) return false
    match kind {
        .pointer_type: {
            p :: ast.pointer_type(&state.tree, a)
            q :: ast.pointer_type(&state.tree, b)
            return p.mutable == q.mutable and p.many == q.many and try same_type(state, p.element, q.element)
        }
        .array_type: {
            p :: ast.array_type(&state.tree, a)
            q :: ast.array_type(&state.tree, b)
            if (p.mutable != q.mutable or p.count == null or q.count == null) return false
            pv :: try evaluate(state, p.count?)
            qv :: try evaluate(state, q.count?)
            return try value_equal(state, pv, qv) and try same_type(state, p.element, q.element)
        }
        .slice_type: {
            p :: ast.slice_type(&state.tree, a)
            q :: ast.slice_type(&state.tree, b)
            if (p.mutable != q.mutable or (p.sentinel == null) != (q.sentinel == null)) return false
            if (p.sentinel) |sentinel| {
                if (!try value_equal(state, try evaluate(state, sentinel), try evaluate(state, q.sentinel?))) return false
            }
            return try same_type(state, p.element, q.element)
        }
        .optional_type: return try same_type(state, ast.type_operand(&state.tree, a), ast.type_operand(&state.tree, b))
        .fallible_type: {
            p :: ast.binary_expr(&state.tree, a)
            q :: ast.binary_expr(&state.tree, b)
            return try same_type(state, p.left, q.left) and try same_type(state, p.right, q.right)
        }
        .function_type: {
            count :: ast.function_param_count(&state.tree, a)
            if (count != ast.function_param_count(&state.tree, b)) return false
            if (!try same_type(state, ast.callable_result(&state.tree, a), ast.callable_result(&state.tree, b))) return false
            for (0..count) |i| {
                p :: ast.param(&state.tree, ast.function_param(&state.tree, a, i)).type
                q :: ast.param(&state.tree, ast.function_param(&state.tree, b, i)).type
                if (!try same_type(state, p, q)) return false
            }
            return true
        }
        else: return false
    }
}

@hide:file
type_subset proc(state @mut State, left, right ast.NodeId) bool ! Error {
    a :: try canonical_type(state, left)
    b :: try canonical_type(state, right)
    if (a == no_node or b == no_node) return false
    if (ast.expr_kind(&state.tree, a) == .sum_type) {
        view :: ast.binary_expr(&state.tree, a)
        return try type_subset(state, view.left, b) and try type_subset(state, view.right, b)
    }
    if (ast.expr_kind(&state.tree, b) == .sum_type) {
        view :: ast.binary_expr(&state.tree, b)
        return try type_subset(state, a, view.left) or try type_subset(state, a, view.right)
    }
    return try same_type(state, a, b)
}

@hide:file
value_equal proc(state @mut State, left, right Value) bool ! Error {
    if (left.kind != right.kind) return false
    return match left.kind {
        .invalid, .null: true
        .type: try same_type(state, left.node, right.node)
        .integer: left.magnitude == right.magnitude and left.negative == right.negative
        .boolean: left.boolean == right.boolean
        .function: binding(state, left.node).data.declaration == binding(state, right.node).data.declaration
        .real: left.magnitude == right.magnitude
        .aggregate: (
            try same_type(state, left.annotation, right.annotation)
            and try constant_equal(state, left.node, right.node)
        )
        .string: try same_string(state, left.node, right.node)
    }
}

@hide:file
value_hash proc(state @mut State, value Value) usize ! Error {
    hash :: enum_index(value.kind)
    negative :: if (value.negative) usize(1) else usize(0)
    boolean :: if (value.boolean) usize(1) else usize(0)
    return match value.kind {
        .type: mix(hash, try type_hash(state, value.node))
        .integer: mix(mix(hash, usize(value.magnitude)), negative)
        .boolean: mix(hash, boolean)
        .function: mix(hash, usize(binding(state, value.node).data.declaration))
        .string: string_hash(hash, ast.literal_text(&state.tree, value.node))
        .real: mix(hash, usize(value.magnitude))
        .aggregate: mix(mix(hash, try type_hash(state, value.annotation)), try constant_hash(state, value.node))
        .invalid, .null: hash
    }
}

@hide:file
string_hash proc(seed usize, text []u8) usize {
    result := seed
    cursor := usize(1)
    while (cursor < text.len - 1) {
        byte :: ast.string_next(text, &cursor) catch { return 0 }
        result = mix(result, usize(byte))
    }
    return result
}

@hide:file
type_hash proc(state @mut State, id ast.NodeId) usize ! Error {
    node :: try canonical_type(state, id)
    if (node == no_node) return 0
    resolved :: binding(state, node)
    if (resolved.kind == .builtin) return mix(1, enum_index(builtin_identity(resolved.data.builtin)))
    kind :: ast.expr_kind(&state.tree, node)
    hash := enum_index(kind)
    match kind {
        .pointer_type: {
            view :: ast.pointer_type(&state.tree, node)
            mutable :: if (view.mutable) usize(1) else usize(0)
            many :: if (view.many) usize(1) else usize(0)
            hash = mix(hash, mutable)
            hash = mix(hash, many)
            return mix(hash, try type_hash(state, view.element))
        }
        .array_type: {
            view :: ast.array_type(&state.tree, node)
            mutable :: if (view.mutable) usize(1) else usize(0)
            hash = mix(hash, mutable)
            if (view.count) |count| hash = mix(hash, try value_hash(state, try evaluate(state, count)))
            return mix(hash, try type_hash(state, view.element))
        }
        .slice_type: {
            view :: ast.slice_type(&state.tree, node)
            mutable :: if (view.mutable) usize(1) else usize(0)
            hash = mix(hash, mutable)
            if (view.sentinel) |sentinel| hash = mix(hash, try value_hash(state, try evaluate(state, sentinel)))
            return mix(hash, try type_hash(state, view.element))
        }
        .optional_type: return mix(hash, try type_hash(state, ast.type_operand(&state.tree, node)))
        .sum_type: {
            view :: ast.binary_expr(&state.tree, node)
            # set fingerprints must ignore ordering, grouping and repeated members.
            # colliding fingerprints still use full type equality in the cache chain.
            return try type_hash(state, view.left) | try type_hash(state, view.right)
        }
        .fallible_type: {
            view :: ast.binary_expr(&state.tree, node)
            return mix(mix(hash, try type_hash(state, view.left)), try type_hash(state, view.right))
        }
        .function_type: {
            hash = mix(hash, try type_hash(state, ast.callable_result(&state.tree, node)))
            for (0..ast.function_param_count(&state.tree, node)) |i| {
                parameter :: ast.function_param(&state.tree, node, i)
                hash = mix(hash, try type_hash(state, ast.param(&state.tree, parameter).type))
            }
            return hash
        }
        else: return mix(hash, usize(node))
    }
}

@hide:file
expression_type proc(state @mut State, id ast.NodeId, depth usize) ast.NodeId ! Error {
    if (depth >= 128) return no_node
    try expand(state, id)
    resolved :: binding(state, id)
    if (resolved.kind == .builtin and resolved.data.builtin != .type) return try builtin(state, id, .type)
    if (resolved.kind == .declaration) {
        target :: resolved.data.declaration
        class :: ast.node_class(&state.tree, target)
        if (class == .parameter) {
            annotation :: ast.param(&state.tree, target).type
            return try canonical_type(state, annotation)
        }
        if (class == .statement) {
            declaration :: ast.statement_decl(&state.tree, target)
            if (declaration.type) |annotation| return try canonical_type(state, annotation)
            return try expression_type(state, declaration.initializer, depth + 1)
        }
        if (class == .function) {
            params := arraylist.init(ast.NodeId, state.allocator)
            defer arraylist.deinit(&params)
            for (0..ast.function_param_count(&state.tree, target)) |i|
                try arraylist.append(&params, ast.function_param(&state.tree, target, i))
            result :: try ast.add_function_type(state.builder, ast.node_span(&state.tree, id),
                ast.function_decl(&state.tree, target).return_type, params.items)
            try refresh(state)
            return result
        }
        return no_node
    }
    kind :: ast.expr_kind(&state.tree, id)
    match kind {
        .record_type, .array_type, .slice_type, .pointer_type, .function_type, .optional_type,
        .enum_type, .union_type, .distinct_type, .fallible_type, .sum_type, .alias_expr:
            return try builtin(state, id, .type)
        .typed_constant: return try canonical_type(state, ast.typed_constant(&state.tree, id).type)
        .literal_int: return try builtin(state, id, .i64)
        .literal_float: return try builtin(state, id, .f64)
        .literal_bool: return try builtin(state, id, .bool)
        .literal_string: return try builtin(state, id, .string)
        .literal_void: return try builtin(state, id, .void)
        .catch_expr: {
            type :: try expression_type(state, ast.catch_expr(&state.tree, id).operand, depth + 1)
            if (type == no_node or ast.expr_kind(&state.tree, type) != .fallible_type) return no_node
            return try canonical_type(state, ast.binary_expr(&state.tree, type).left)
        }
        .record_literal: {
            view :: ast.record_literal(&state.tree, id)
            if (view.type) |annotation| return try canonical_type(state, annotation)
            fields := arraylist.init(ast.NodeId, state.allocator)
            defer arraylist.deinit(&fields)
            for (0..ast.record_field_count(&state.tree, id)) |i| {
                field :: ast.field(&state.tree, ast.record_field(&state.tree, id, i))
                if (field.value == null) return no_node
                type :: try expression_type(state, field.value?, depth + 1)
                if (type == no_node) return no_node
                generated :: try ast.add_field(state.builder, field.span, field.name, type, null)
                try refresh(state)
                try arraylist.append(&fields, generated)
            }
            result :: try ast.add_record_type(state.builder, ast.node_span(&state.tree, id), fields.items)
            try refresh(state)
            return result
        }
        .array_literal: {
            count :: ast.array_element_count(&state.tree, id)
            if (count == 0) return no_node
            element :: try expression_type(state, ast.array_element(&state.tree, id, 0), depth + 1)
            if (element == no_node) return no_node
            length :: try ast.add_integer(state.builder, ast.node_span(&state.tree, id), u64(count))
            try refresh(state)
            result :: try ast.add_array_type(state.builder, ast.node_span(&state.tree, id), length, element, false)
            try refresh(state)
            return result
        }
        .call: {
            callee :: ast.call_expr(&state.tree, id).callee
            destination :: binding(state, callee)
            if (destination.kind == .builtin) return callee
            if (destination.kind == .declaration and ast.node_class(&state.tree, destination.data.declaration) == .function) return try canonical_type(state, ast.function_decl(&state.tree, destination.data.declaration).return_type)
            signature :: try expression_type(state, callee, depth + 1)
            if (signature != no_node and ast.expr_kind(&state.tree, signature) == .function_type)
                return try canonical_type(state, ast.callable_result(&state.tree, signature))
            return no_node
        }
        .unary: {
            view :: ast.unary_expr(&state.tree, id)
            operand :: try expression_type(state, view.operand, depth + 1)
            if (operand == no_node) return no_node
            if (view.operator == .address_of) {
                mutable := false
                target :: binding(state, view.operand)
                if (target.kind == .declaration and ast.node_class(&state.tree, target.data.declaration) == .statement)
                    mutable = ast.statement_decl(&state.tree, target.data.declaration).mutability == .variable
                result :: try ast.add_pointer_type(state.builder, view.span, operand, mutable, false)
                try refresh(state)
                return result
            }
            if (view.operator == .dereference and ast.expr_kind(&state.tree, operand) == .pointer_type)
                return try canonical_type(state, ast.pointer_type(&state.tree, operand).element)
            if (view.operator == .unwrap and ast.expr_kind(&state.tree, operand) == .optional_type)
                return try canonical_type(state, ast.type_operand(&state.tree, operand))
            return operand
        }
        .binary: {
            view :: ast.binary_expr(&state.tree, id)
            if (
                view.operator == .equal or view.operator == .not_equal or view.operator == .less
                or view.operator == .less_equal or view.operator == .greater or view.operator == .greater_equal
                or view.operator == .logical_and or view.operator == .logical_or
            ) return try builtin(state, id, .bool)
            return try expression_type(state, view.left, depth + 1)
        }
        .selection: {
            view :: ast.selection_expr(&state.tree, id)
            base := try expression_type(state, view.base, depth + 1)
            if (base == no_node) return no_node
            if (ast.expr_kind(&state.tree, base) == .pointer_type) base = try canonical_type(state, ast.pointer_type(&state.tree, base).element)
            if (base == no_node) return no_node
            member :: ast.source_text(&state.tree, ast.node_span(&state.tree, view.member))
            base_kind :: ast.expr_kind(&state.tree, base)
            if (
                mem.eql(member, "len") and (
                    base_kind == .array_type or base_kind == .slice_type
                    or (binding(state, base).kind == .builtin and binding(state, base).data.builtin == .string)
                )
            )
                return try builtin(state, id, .usize)
            if (base_kind != .record_type and base_kind != .union_type) return no_node
            for (0..ast.record_field_count(&state.tree, base)) |i| {
                field :: ast.field(&state.tree, ast.record_field(&state.tree, base, i))
                if (field.name) |name| {
                    if (ast.name_id(&state.tree, name) == ast.name_id(&state.tree, view.member))
                        return try canonical_type(state, field.type?)
                }
            }
            return no_node
        }
        .index: {
            view :: ast.index_expr(&state.tree, id)
            base :: try expression_type(state, view.base, depth + 1)
            if (base == no_node) return no_node
            base_kind :: ast.expr_kind(&state.tree, base)
            if (base_kind == .array_type) {
                element :: ast.array_type(&state.tree, base).element
                return if (is_type(state, element)) element else try canonical_type(state, element)
            }
            if (base_kind == .slice_type) return try canonical_type(state, ast.slice_type(&state.tree, base).element)
            if (base_kind == .pointer_type) return try canonical_type(state, ast.pointer_type(&state.tree, base).element)
            if (binding(state, base).kind == .builtin and binding(state, base).data.builtin == .string) return try builtin(state, id, .u8)
            return no_node
        }
        .slice: {
            view :: ast.slice_expr(&state.tree, id)
            base :: try expression_type(state, view.base, depth + 1)
            if (base == no_node) return no_node
            base_kind :: ast.expr_kind(&state.tree, base)
            element := no_node
            mutable := false
            if (base_kind == .array_type) {
                element = ast.array_type(&state.tree, base).element
                mutable = ast.array_type(&state.tree, base).mutable
            }
            if (base_kind == .pointer_type) {
                element = ast.pointer_type(&state.tree, base).element
                mutable = ast.pointer_type(&state.tree, base).mutable
            }
            if (base_kind == .slice_type) {
                element = ast.slice_type(&state.tree, base).element
                mutable = ast.slice_type(&state.tree, base).mutable
            }
            if (element == no_node) return no_node
            result :: try ast.add_slice_type(state.builder, ast.node_span(&state.tree, id), element, mutable, null)
            try refresh(state)
            return result
        }
        else: return no_node
    }
}

@hide:file
parameter_index proc(state @State, function, id ast.NodeId) usize {
    for (0..ast.function_param_count(&state.tree, function)) |i| {
        if (ast.function_param(&state.tree, function, i) == id and ast.param(&state.tree, id).comptime) return i
    }
    return no_index
}

@hide:file
infer_value proc(state @mut State, function, pattern ast.NodeId, actual Value, arguments []mut Value, contextual bool) bool ! Error {
    resolved :: binding(state, pattern)
    if (resolved.kind == .declaration) {
        index :: parameter_index(state, function, resolved.data.declaration)
        if (index != no_index) {
            if (arguments[index].kind == .invalid) {
                arguments[index] = actual
                return true
            }
            if (contextual and arguments[index].kind == .type and actual.kind == .type) return true
            return try value_equal(state, arguments[index], actual)
        }
    }
    return try value_equal(state, try evaluate(state, pattern), actual)
}

@hide:file
infer proc(state @mut State, function, pattern, actual ast.NodeId, arguments []mut Value, contextual bool) bool ! Error {
    resolved :: binding(state, pattern)
    if (resolved.kind == .declaration and parameter_index(state, function, resolved.data.declaration) != no_index)
        return try infer_value(state, function, pattern, Value{ kind = .type, node = actual }, arguments, contextual)
    kind :: ast.expr_kind(&state.tree, pattern)
    actual_kind :: ast.expr_kind(&state.tree, actual)
    match kind {
        .pointer_type: {
            if (actual_kind != .pointer_type) return false
            return try infer(state, function, ast.pointer_type(&state.tree, pattern).element,
                ast.pointer_type(&state.tree, actual).element, arguments, contextual)
        }
        .slice_type: {
            element := no_node
            if (actual_kind == .slice_type) element = ast.slice_type(&state.tree, actual).element
            if (actual_kind == .array_type) element = ast.array_type(&state.tree, actual).element
            if (element == no_node) return false
            return try infer(state, function, ast.slice_type(&state.tree, pattern).element, element, arguments, contextual)
        }
        .array_type: {
            if (actual_kind != .array_type) return false
            p :: ast.array_type(&state.tree, pattern)
            a :: ast.array_type(&state.tree, actual)
            if (p.count) |count| {
                if (a.count == null or !try infer_value(state, function, count, try evaluate(state, a.count?), arguments, false)) return false
            }
            return try infer(state, function, p.element, a.element, arguments, contextual)
        }
        .optional_type: {
            element :: if (actual_kind == .optional_type) ast.type_operand(&state.tree, actual) else actual
            return try infer(state, function, ast.type_operand(&state.tree, pattern), element, arguments, contextual)
        }
        .call: {
            callee :: binding(state, ast.call_expr(&state.tree, pattern).callee)
            if (callee.kind != .declaration) return false
            for (0..(state.instances.items.len)) |i| {
                instance :: state.instances.items[i]
                if (instance.type == no_node or instance.function != callee.data.declaration) continue
                if (try canonical_type(state, instance.type) != actual) continue
                count :: ast.call_arg_count(&state.tree, pattern)
                if (count != instance.arguments.len) return false
                for (0..count) |j| {
                    if (!try infer_value(state, function, ast.call_arg(&state.tree, pattern, j), instance.arguments[j], arguments, contextual)) return false
                }
                return true
            }
            return false
        }
        else: return true
    }
}

@hide:file
materialize proc(state @mut State, value Value) Value ! Error {
    result := value
    span :: ast.node_span(&state.tree, value.node)
    if (value.kind == .type and nominal_type_value(state, value.node)) {
        # refer to declared nominal types, rather than reusing their definition as
        # an annotation and turning its value-role checker cell into an annotation.
        for (state.declarations.items) |declaration| {
            if (
                ast.node_class(&state.tree, declaration) == .statement
                and ast.statement_decl(&state.tree, declaration).initializer == value.node
            ) {
                result.node = try reference(state, value.node, declaration)
                return result
            }
        }
    }
    if (value.kind == .integer) {
        result.node = try ast.add_integer(state.builder, span, value.magnitude)
        if (value.negative) result.node = try ast.add_unary(state.builder, span, .negate, result.node)
    } else if (value.kind == .boolean) {
        result.node = try ast.add_boolean(state.builder, span, value.boolean)
    } else if (value.kind == .real) {
        result.node = try ast.add_float(state.builder, span, ptrcast!(@f64, &value.magnitude)^)
    }
    try refresh(state)
    return result
}

@hide:file
valid_value proc(state @mut State, annotation ast.NodeId, value Value) bool ! Error {
    type :: try canonical_type(state, annotation)
    if (type == no_node) return false
    resolved :: binding(state, type)
    if (resolved.kind == .builtin) {
        kind :: resolved.data.builtin
        if (kind == .bool) return value.kind == .boolean
        if (kind == .string) return value.kind == .string
        if (kind == .float or kind == .f64) return value.kind == .real
        if (kind == .void) return (
            value.kind == .aggregate and value.annotation != no_node
            and try same_type(state, type, value.annotation)
        )
        if (value.kind != .integer) return false
        bits := u32(64)
        signed := false
        match kind {
            .i8, .c_char: {
                bits = 8
                signed = true
            }
            .i16, .c_short: {
                bits = 16
                signed = true
            }
            .i32, .c_int: {
                bits = 32
                signed = true
            }
            .int, .i64, .isize, .c_long, .c_longlong: signed = true
            .u8, .c_uchar: bits = 8
            .u16, .c_ushort: bits = 16
            .u32, .c_uint: bits = 32
            .u64, .uint, .usize, .c_ulong, .c_ulonglong: {}
            else: return false
        }
        limit := maxval!(u64)
        if (signed) {
            limit = u64(1) << (bits - 1)
            if (!value.negative) limit -= 1
        } else {
            if (value.negative) return false
            if (bits != 64) limit = (u64(1) << bits) - 1
        }
        return value.magnitude <= limit
    }
    kind :: ast.expr_kind(&state.tree, type)
    if (value.kind == .aggregate) return value.annotation != no_node and try same_type(state, type, value.annotation)
    if (kind == .optional_type) return value.kind == .null or try valid_value(state, ast.type_operand(&state.tree, type), value)
    if (kind == .slice_type and value.kind == .string) {
        view :: ast.slice_type(&state.tree, type)
        child :: try canonical_type(state, view.element)
        return (
            !view.mutable and child != no_node and binding(state, child).kind == .builtin
            and binding(state, child).data.builtin == .u8
        )
    }
    if (kind == .function_type and value.kind == .function) {
        actual :: try expression_type(state, value.node, 0)
        return actual != no_node and try same_type(state, type, actual)
    }
    if (kind == .distinct_type) return try valid_value(state, ast.type_operand(&state.tree, type), value)
    return false
}

@hide:file
enum_index proc($T type, value T) usize {
    match typeinfo!(T) {
        .enum |info|: {
            inline for (info.fields) |field| {
                if (value == field!(T, field.name)) return field.index
            }
            unreachable
        }
        else: unreachable
    }
}

@hide:file
remember proc(state @mut State, target ast.NodeId) void ! mem.AllocError {
    if (usize(target) >= state.original_count) return
    saved :: try ast.copy_node(state.builder, target)
    try arraylist.append(&state.rewrites.changes, Rewrite{ target = target, saved = saved })
}

@hide:file
replace proc(state @mut State, target, source ast.NodeId) void ! mem.AllocError {
    try remember(state, target)
    ast.replace_node(state.builder, target, source)
}

@hide:file
builtin_identity proc(kind sema.BuiltinType) sema.BuiltinType {
    return match kind {
        .i64, .c_long, .c_longlong: .int
        .f64: .float
        .uint, .c_ulong, .c_ulonglong: .u64
        .c_char: .i8
        .c_uchar: .u8
        .c_short: .i16
        .c_ushort: .u16
        .c_int: .i32
        .c_uint: .u32
        else: kind
    }
}

@hide:file
integer_type proc(state @mut State, annotation ast.NodeId) bool ! Error {
    type :: try canonical_type(state, annotation)
    if (type == no_node) return false
    resolved :: binding(state, type)
    if (resolved.kind == .builtin) {
        return match builtin_identity(resolved.data.builtin) {
            .int, .i8, .i16, .i32, .isize, .u8, .u16, .u32, .u64, .usize: true
            else: false
        }
    }
    if (ast.expr_kind(&state.tree, type) == .distinct_type) return try integer_type(state, ast.type_operand(&state.tree, type))
    return false
}

@hide:file
integer_value proc(state @mut State, id ast.NodeId, magnitude u64, negative bool, annotation ast.NodeId) Value ! Error {
    result :: Value{ kind = .integer, node = id, magnitude = magnitude, negative = negative, annotation = annotation }
    if (annotation != no_node and !try valid_value(state, annotation, result)) return Value{}
    return result
}

@hide:file
same_string proc(state @State, left, right ast.NodeId) bool ! Error {
    a :: ast.literal_text(&state.tree, left)
    b :: ast.literal_text(&state.tree, right)
    ac := usize(1)
    bc := usize(1)
    while (ac < a.len - 1 and bc < b.len - 1) {
        av :: ast.string_next(a, &ac) catch { return false }
        bv :: ast.string_next(b, &bc) catch { return false }
        if (av != bv) return false
    }
    return ac == a.len - 1 and bc == b.len - 1
}

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flexible proc(state @State, id ast.NodeId) bool {
    kind :: ast.expr_kind(&state.tree, id)
    if (kind == .literal_int or kind == .literal_float) return true
    if (kind == .unary) {
        view :: ast.unary_expr(&state.tree, id)
        return view.operator == .negate and flexible(state, view.operand)
    }
    if (kind == .binary) {
        view :: ast.binary_expr(&state.tree, id)
        return (
            (view.operator == .add or view.operator == .subtract or view.operator == .multiply or view.operator == .divide)
            and flexible(state, view.left) and flexible(state, view.right)
        )
    }
    return false
}

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template proc(state @State, id ast.NodeId) bool {
    return ast.function_generic(&state.tree, id) or is_type(state, ast.function_decl(&state.tree, id).return_type)
}