submilli_engine/codegen/

cast_check.rs

//! The structural conformance test and its consumers: `x as T`, the `!`
//! non-null assertion, and the read guard on a narrowed field redeclaration.

use wasm_encoder::{BlockType, Function, HeapType, Instruction, RefType, ValType};

use crate::codegen::function_emitter::expr::emit_expr;
use crate::codegen::function_emitter::json::emit_inline_const_raw_string;
use crate::codegen::function_emitter::{FunctionEmitter, cast};
use crate::codegen::{CodegenCtx, cast_diagnostics as diagnostic};
use crate::{ExprId, Ident, Span, Type};

pub const TYPE_TAG_STRINGS: &[&str] = &[
    "string",
    "number",
    "boolean",
    "function",
    "object",
    "null",
    "undefined",
];
pub(super) const RECURSIVE_VALIDATOR_CAPACITY: i32 = 1024;

#[derive(Clone, Copy)]
struct ValidatorState {
    visited: u32,
    validator_ids: u32,
    depth: u32,
}

#[derive(Clone, Copy)]
struct FieldConformanceLocals {
    object: u32,
    field: u32,
}

#[derive(Clone, Copy)]
struct CollectionStorage {
    backing: u32,
    backing_type: u32,
    order_len_field: u32,
    order: u32,
}

pub fn error_prefix(target_ty: &Type) -> String {
    format!("type mismatch: expected {target_ty}, got ")
}

/// `value as target_ty`. `check` is `Some(shape)` when a runtime structural check is needed
/// (the source isn't a static subtype of the target — `unknown`/downcasts); `None` for a
/// statically-proven upcast (repr-only narrow). `shape` has interfaces reduced to their
/// object shapes so this walks plain structural types.
/// `cast_span` is the cast expression's own span, mapped while its runtime
/// check is emitted so a limit the check reaches, and a failed check, are
/// reported at the cast rather than at the last part of the value. The code
/// after the cast maps to the enclosing span again.
pub fn emit_cast(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value: ExprId,
    target_ty: &Type,
    check: Option<&Type>,
    cast_span: crate::Span,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let source_ty = ctx
        .ta
        .try_expr(value)
        .map_err(crate::codegen::arena_failure)?
        .ty
        .clone();

    // unknown accepts every value — box to $Object, no test.
    if matches!(target_ty.peel(), Type::Unknown) {
        emit_expr(emitter, ctx, value)?;
        cast::emit_box(emitter, ctx, &source_ty)?;
        return Ok(());
    }

    let Some(check_shape) = check else {
        let static_ty = ctx
            .ta
            .source_type(value)
            .map_err(crate::codegen::arena_failure)?;
        if crate::codegen::closure_coercions::is_union_call_cast(static_ty, target_ty) {
            emit_expr(emitter, ctx, value)?;
            cast::emit_box(emitter, ctx, &source_ty)?;
            return crate::codegen::closure_coercions::emit_union_call_cast(
                emitter, ctx, target_ty,
            );
        }
        if &source_ty != static_ty {
            emit_expr(emitter, ctx, value)?;
            let enclosing = map_cast_span(emitter, cast_span);
            emit_checked_cast_on_stack(emitter, ctx, &source_ty, target_ty)?;
            restore_span(emitter, enclosing);
            return Ok(());
        }
        // Statically-proven upcast: box then narrow representation, no runtime test.
        emit_expr(emitter, ctx, value)?;
        cast::emit_box(emitter, ctx, &source_ty)?;
        cast::emit_cast_to(emitter, ctx, target_ty)?;
        return Ok(());
    };

    let mark = emitter.single_evaluation_mark();
    emit_expr(emitter, ctx, value)?;
    emitter.end_single_evaluations(mark)?;
    let enclosing = map_cast_span(emitter, cast_span);
    cast::emit_box(emitter, ctx, &source_ty)?;
    emit_shape_cast_on_stack(emitter, ctx, target_ty, check_shape)?;
    restore_span(emitter, enclosing);
    Ok(())
}

/// The runtime half of `value as target_ty` for the boxed value on the stack:
/// test it against `check_shape` and narrow it, or throw the cast's
/// `TypeError`.
pub(crate) fn emit_shape_cast_on_stack(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    target_ty: &Type,
    check_shape: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let object_idx = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?
        .object;
    let scratch_ty = ValType::Ref(RefType {
        nullable: true,
        heap_type: HeapType::Concrete(object_idx),
    });
    let scratch = emitter.add_anonymous_local(scratch_ty)?;
    emitter.instruction(Instruction::LocalSet(scratch));
    let previous_diagnostic = diagnostic::begin(emitter, ctx);

    emit_structural_test(emitter, ctx, scratch, check_shape, target_ty)?;

    let target_val = ctx.symbols.value_type(target_ty)?;
    let block_ty = BlockType::Result(target_val);
    emitter.emit_if(block_ty);
    emitter.instruction(Instruction::LocalGet(scratch));
    cast::emit_cast_to(emitter, ctx, target_ty)?;
    emitter.emit_else();
    emit_cast_throw(emitter, ctx, scratch, target_ty);
    emitter.emit_end();
    emitter.cast_diagnostic = previous_diagnostic;
    Ok(())
}

/// Maps `cast_span` to the code that follows, returning the span it replaces.
fn map_cast_span(emitter: &mut FunctionEmitter, cast_span: crate::Span) -> Option<crate::Span> {
    let enclosing = emitter.last_mapped_span();
    emitter.record_span(cast_span);
    enclosing
}

fn restore_span(emitter: &mut FunctionEmitter, enclosing: Option<crate::Span>) {
    if let Some(span) = enclosing {
        emitter.record_span(span);
    }
}

/// `value!`. Converts a runtime `null` into a catchable `TypeError` instead
/// of relying on `ref.as_non_null`, which would trap.
pub fn emit_non_null_assert(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value: ExprId,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let source_ty = ctx
        .ta
        .try_expr(value)
        .map_err(crate::codegen::arena_failure)?
        .ty
        .clone();
    emit_expr(emitter, ctx, value)?;
    emit_non_null_assert_on_stack(emitter, ctx, &source_ty, target_ty)?;
    Ok(())
}

/// `!` applied to a value already on the stack at `source_ty`'s slot — the
/// optional-chain form, where the operand is the step before rather than an
/// expression this can emit itself.
pub fn emit_non_null_assert_on_stack(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    source_ty: &Type,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    cast::emit_box(emitter, ctx, source_ty)?;

    let object_idx = object_idx_of(ctx)?;
    let scratch = emitter.add_anonymous_local(scratch_object_ty(object_idx))?;
    emitter.instruction(Instruction::LocalSet(scratch));

    emitter.instruction(Instruction::LocalGet(scratch));
    crate::codegen::function_emitter::expr::emit_is_nullish(emitter, ctx)?;
    emitter.emit_if(BlockType::Result(ctx.symbols.value_type(target_ty)?));
    crate::codegen::throw::emit_type_error_throw(
        emitter,
        ctx,
        crate::codegen::throw::NON_NULL_ASSERT_MESSAGE,
    );
    emitter.emit_else();
    emitter.instruction(Instruction::LocalGet(scratch));
    cast::emit_cast_to(emitter, ctx, target_ty)?;
    emitter.emit_end();

    Ok(())
}

/// Pushes i32 1 if the `(ref null $Object)` in `value_local` structurally conforms to
/// `ty`, 0 otherwise — no throw, so it composes inside unions/fields/elements. Recurses
/// into object fields, array elements, tuple slots, and union members; verifies number and
/// string literal values.
///
/// `typed_ast::runtime_type_is_testable` is the shared allowlist deciding which
/// types may reach this function. `codegen::analysis::note_shape_member_names`
/// separately registers the per-name globals an arm's field scan reads. A new
/// arm must update that registration when it reads names, and may enter the
/// allowlist only if every value the type admits is one this test accepts.
///
/// `named` is the type a limit error names: the type the program wrote, where
/// `ty` is the shape it was reduced to.
pub(super) fn emit_structural_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    named: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    ctx.checking(named, || {
        emit_structural_test_inner(
            emitter,
            ctx,
            value_local,
            ty,
            &mut std::collections::BTreeSet::new(),
            None,
        )?;
        // A step's own code follows its charge, so the finished check is
        // measured again before the function can reach the engine.
        ctx.check_function_limits(emitter)
    })
}

fn emit_structural_test_inner(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    interface_stack: &mut std::collections::BTreeSet<Type>,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    ctx.charge_validator_step(emitter)?;
    let checkpoint = diagnostic::checkpoint(emitter, ctx);
    let i32_block = BlockType::Result(ValType::I32);
    match ty.lowered() {
        Type::Union(members) => {
            let union = diagnostic::union_start(emitter, ctx, members);
            let mut first = true;
            for m in members {
                diagnostic::union_next(emitter, ctx);
                emit_structural_test_inner(
                    emitter,
                    ctx,
                    value_local,
                    m,
                    interface_stack,
                    validator_state,
                )?;
                diagnostic::union_keep(emitter, ctx, union);
                if first {
                    first = false;
                } else {
                    emitter.instruction(Instruction::I32Or);
                }
            }
            if first {
                emitter.instruction(Instruction::I32Const(0));
            }
            diagnostic::union_end(emitter, ctx, union)?;
        }
        Type::Null => {
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefIsNull);
        }
        Type::Undefined | Type::Void => {
            emitter.instruction(Instruction::LocalGet(value_local));
            crate::codegen::function_emitter::expr::emit_is_undefined(emitter, ctx)?;
        }
        // `{}` holds every value but `null` and `undefined`.
        Type::Unknown if ty.is_empty_object() => {
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefIsNull);
            emitter.instruction(Instruction::LocalGet(value_local));
            crate::codegen::function_emitter::expr::emit_is_undefined(emitter, ctx)?;
            emitter.instruction(Instruction::I32Or);
            emitter.instruction(Instruction::I32Eqz);
        }
        // `object` holds every value but the primitives, `null` and `undefined`.
        Type::Unknown if matches!(ty.peel(), Type::NonPrimitive) => {
            crate::codegen::function_emitter::expr::emit_is_non_primitive(
                emitter,
                ctx,
                value_local,
                crate::codegen::function_emitter::expr::Closures::AreObjects,
            )?;
        }
        Type::Unknown => emitter.instruction(Instruction::I32Const(1)),
        Type::Number => emit_ref_test(emitter, value_local, boxed_number_idx(ctx)?),
        Type::Boolean => emit_ref_test(emitter, value_local, boxed_boolean_idx(ctx)?),
        Type::String => emit_ref_test(emitter, value_local, string_idx(ctx)?),
        Type::NumberEnum { .. } | Type::StringEnum { .. } => {
            if let Some(crate::FieldNarrowingTest::Shape(members)) =
                ctx.ta.runtime_type_tests.get(ty.lowered())
                && members.lowered() != ty.lowered()
            {
                emit_structural_test_inner(
                    emitter,
                    ctx,
                    value_local,
                    members,
                    interface_stack,
                    validator_state,
                )?;
            } else {
                emit_representation_test(emitter, ctx, value_local, ty)?;
            }
        }
        Type::BigInt => emit_ref_test(
            emitter,
            value_local,
            ctx.symbols
                .bigint_type_idx()
                .ok_or_else(|| crate::codegen::internal_failure("$BigInt intrinsic registered"))?,
        ),
        Type::Uint8Array => emit_ref_test(
            emitter,
            value_local,
            ctx.symbols.uint8_array_type_idx().ok_or_else(|| {
                crate::codegen::internal_failure("$Uint8Array intrinsic registered")
            })?,
        ),
        Type::Function { has_rest, .. } => {
            emit_function_structural_test(emitter, ctx, value_local, ty, *has_rest)?;
        }
        // Generic leaves use the caller's concrete predicate when available.
        // Legacy entry points without descriptors retain their erased checks.
        Type::TypeVar(name) | Type::GenericParam { name, .. } => {
            super::runtime_descriptors::test_parameter(emitter, ctx, name, value_local)?;
        }
        Type::NumberLiteral(n) => {
            let boxed = boxed_number_idx(ctx)?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(boxed)));
            emitter.emit_if(i32_block);
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(boxed)));
            emitter.instruction(Instruction::StructGet {
                struct_type_index: boxed,
                field_index: 1,
            });
            emitter.instruction(Instruction::F64Const(n.0.into()));
            emitter.instruction(Instruction::F64Eq);
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
        }
        Type::BooleanLiteral(value) => {
            let boxed = boxed_boolean_idx(ctx)?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(boxed)));
            emitter.emit_if(i32_block);
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(boxed)));
            emitter.instruction(Instruction::StructGet {
                struct_type_index: boxed,
                field_index: 1,
            });
            emitter.instruction(Instruction::I32Const(i32::from(*value)));
            emitter.instruction(Instruction::I32Eq);
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
        }
        Type::TemplateLiteral { .. } => {
            let string = string_idx(ctx)?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(string)));
            emitter.emit_if(i32_block);
            emitter.instruction(Instruction::LocalGet(value_local));
            let domain = serde_json::to_string(&crate::property_keys::runtime_domain(ty).map_err(
                |error| error.into_failure(crate::compiler_error::CompilerStage::Codegen, None),
            )?)
            .map_err(|error| {
                crate::codegen::internal_failure(format!("could not encode template type: {error}"))
            })?;
            emit_inline_string_checked(emitter, ctx, &domain)?;
            let helper = ctx
                .symbols
                .prelude_func_idx("ObjectConstructor##matchesKeyDomain")
                .ok_or_else(|| crate::codegen::internal_failure("template matcher imported"))?;
            emitter.instruction(Instruction::Call(helper));
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
        }
        Type::StringLiteral(s) => {
            let str_idx = string_idx(ctx)?;
            let intr = ctx
                .symbols
                .intrinsic_type_indices()
                .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(str_idx)));
            emitter.emit_if(i32_block);
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(str_idx)));
            emitter.instruction(Instruction::StructGet {
                struct_type_index: str_idx,
                field_index: 1,
            });
            let raw_local = emitter.add_anonymous_local(ValType::Ref(RefType {
                nullable: false,
                heap_type: HeapType::Concrete(intr.raw_string),
            }))?;
            emitter.instruction(Instruction::LocalSet(raw_local));
            crate::codegen::function_emitter::json::emit_raw_string_matches_literal(
                emitter, intr, raw_local, s,
            );
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
        }
        Type::Object { fields, index } => {
            let shape_idx = ctx.symbols.object_shape_type_idx().ok_or_else(|| {
                crate::codegen::internal_failure("$ObjectShape intrinsic registered")
            })?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(shape_idx)));
            emitter.emit_if(i32_block);
            let obj_local = emitter.add_anonymous_local(ValType::Ref(RefType {
                nullable: false,
                heap_type: HeapType::Concrete(shape_idx),
            }))?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(shape_idx)));
            emitter.instruction(Instruction::LocalSet(obj_local));
            let field_local =
                emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
            emitter.instruction(Instruction::I32Const(1)); // accumulator
            for ordered in diagnostic::check_order(emitter, fields) {
                emit_field_conformance(
                    emitter,
                    ctx,
                    FieldConformanceLocals {
                        object: obj_local,
                        field: field_local,
                    },
                    ordered,
                    interface_stack,
                    validator_state,
                )?;
                emitter.instruction(Instruction::I32And);
            }
            if let Some(index) = index {
                emit_index_conformance(
                    emitter,
                    ctx,
                    obj_local,
                    index,
                    interface_stack,
                    validator_state,
                )?;
                emitter.instruction(Instruction::I32And);
            }
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
        }
        Type::Array(elem) => {
            emit_elements_test(
                emitter,
                ctx,
                value_local,
                elem,
                interface_stack,
                validator_state,
                None,
            )?;
        }
        Type::Tuple(elems) => {
            let intr = ctx
                .symbols
                .intrinsic_type_indices()
                .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
            let array_idx = ctx
                .symbols
                .array_type_idx()
                .ok_or_else(|| crate::codegen::internal_failure("$Array intrinsic registered"))?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(array_idx)));
            emitter.emit_if(i32_block);
            let raw_local = emitter.add_anonymous_local(ValType::Ref(RefType {
                nullable: false,
                heap_type: HeapType::Concrete(intr.raw_array),
            }))?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(array_idx)));
            emitter.instruction(Instruction::StructGet {
                struct_type_index: array_idx,
                field_index: 1,
            });
            emitter.instruction(Instruction::LocalSet(raw_local));
            // Validate the logical length before reading any tuple position.
            let length = emitter.add_anonymous_local(ValType::I32)?;
            emitter.instruction(Instruction::LocalGet(value_local));
            emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(array_idx)));
            emitter.instruction(Instruction::StructGet {
                struct_type_index: array_idx,
                field_index: 2,
            });
            emitter.instruction(Instruction::LocalTee(length));
            emitter.instruction(Instruction::I32Const(
                crate::codegen::wasm_u32(elems.len())?.cast_signed(),
            ));
            emitter.instruction(Instruction::I32LeU);
            emitter.instruction(Instruction::LocalGet(length));
            emitter.instruction(Instruction::I32Const(
                crate::codegen::wasm_u32(elems.required_len())?.cast_signed(),
            ));
            emitter.instruction(Instruction::I32GeU);
            emitter.instruction(Instruction::I32And);
            emitter.emit_if(i32_block);
            let elem_local = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
            emitter.instruction(Instruction::I32Const(1));
            for (idx, et) in elems.iter().enumerate() {
                if idx >= elems.required_len() {
                    emitter.instruction(Instruction::LocalGet(length));
                    emitter.instruction(Instruction::I32Const(
                        crate::codegen::wasm_u32(idx)?.cast_signed(),
                    ));
                    emitter.instruction(Instruction::I32LeU);
                    emitter.emit_if(i32_block);
                    emitter.instruction(Instruction::I32Const(1));
                    emitter.emit_else();
                }
                let index = emitter.add_anonymous_local(ValType::I32)?;
                emitter.instruction(Instruction::I32Const(
                    crate::codegen::wasm_u32(idx)?.cast_signed(),
                ));
                emitter.instruction(Instruction::LocalSet(index));
                let parent_path = diagnostic::index(emitter, ctx, index);
                emitter.instruction(Instruction::LocalGet(raw_local));
                emitter.instruction(Instruction::I32Const(
                    crate::codegen::wasm_u32(idx)?.cast_signed(),
                ));
                emitter.instruction(Instruction::ArrayGet(intr.raw_array));
                emitter.instruction(Instruction::LocalSet(elem_local));
                emit_structural_test_inner(
                    emitter,
                    ctx,
                    elem_local,
                    et,
                    interface_stack,
                    validator_state,
                )?;
                diagnostic::pop(emitter, parent_path);
                if idx >= elems.required_len() {
                    emitter.emit_end();
                }
                emitter.instruction(Instruction::I32And);
            }
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
            emitter.emit_else();
            emitter.instruction(Instruction::I32Const(0));
            emitter.emit_end();
        }
        // `keyof T`, `T[K]` and a utility over `T` are erased like `T`, with no
        // descriptor of their own to test against (casts to them are rejected
        // at typecheck, SUB-2152), so a read guard over one admits the value.
        // So does one codegen instantiated (`Partial<{ a: number }>` for a
        // generic class's field): it has no type table to evaluate it with.
        operator
            if operator.is_utility_application()
                || matches!(operator, Type::KeyOf(_) | Type::IndexedAccess { .. }) =>
        {
            emitter.instruction(Instruction::I32Const(1));
        }
        // Recursion back-edge: delegate to the alias's generated validator, which
        // re-enters this test on the alias body. Terminates on the finite value.
        Type::AliasRef { .. } => {
            let func_idx = ctx
                .symbols
                .runtime_validator_idx(ty.lowered())
                .ok_or_else(|| {
                    crate::codegen::internal_failure(
                        "recursive runtime validator pre-allocated during discovery",
                    )
                })?;
            emitter.instruction(Instruction::LocalGet(value_local));
            let visited = emit_validator_state(emitter, ctx, validator_state)?;
            super::runtime_descriptors::validator_environment(emitter, ctx, ty)?;
            emitter.instruction(Instruction::Call(func_idx));
            diagnostic::load_recursive(emitter, ctx, visited);
        }
        Type::InterfaceRef { .. } => emit_interface_ref_test(
            emitter,
            ctx,
            value_local,
            ty,
            interface_stack,
            validator_state,
        )?,
        // Nominal, not structural: same-shape sibling classes canonicalize to one
        // WasmGC type, so membership is the vtable-parent walk `instanceof` uses.
        // Unreachable from `as` (class targets are rejected at typecheck); reached
        // from the narrowed-field read guard.
        Type::ClassRef { mangled, .. } => {
            let vtable_global = ctx
                .symbols
                .class_vtable_global_idx(mangled)
                .ok_or_else(|| crate::codegen::internal_failure("class vtable global recorded"))?;
            emitter.instruction(Instruction::LocalGet(value_local));
            cast::emit_nominal_instance_test(emitter, ctx, vtable_global)?;
            if let Some(validator) = ctx.symbols.runtime_validator_idx(ty.lowered()) {
                emitter.emit_if(BlockType::Result(ValType::I32));
                emitter.instruction(Instruction::LocalGet(value_local));
                let visited = emit_validator_state(emitter, ctx, validator_state)?;
                super::runtime_descriptors::validator_environment(emitter, ctx, ty)?;
                emitter.instruction(Instruction::Call(validator));
                diagnostic::load_recursive(emitter, ctx, visited);
                emitter.emit_else();
                emitter.instruction(Instruction::I32Const(0));
                emitter.emit_end();
            }
        }
        // Rejected at typecheck (`unsupported_cast_target_reason`); defensive 0.
        _ => emitter.instruction(Instruction::I32Const(0)),
    }
    diagnostic::finish(
        emitter,
        ctx,
        checkpoint,
        diagnostic::Tested::Value {
            ty,
            local: value_local,
        },
    )?;
    Ok(())
}

/// Pushes whether the value in `value_local` is a closure that can stand for
/// the function type `ty`. A rest closure shares the struct of its Wasm arity
/// but takes its arguments packed, and only a closure whose environment carries
/// argument metadata can have one. So a closure of the target's struct without
/// metadata passes outright unless the target ends in a rest parameter, and
/// `__value_defaults_fit` decides every other case.
fn emit_function_structural_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    has_rest: bool,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let signature = crate::codegen::closures::classify(ty)?;
    let closure_struct = ctx
        .symbols
        .closure_struct_type_idx(signature)
        .ok_or_else(|| {
            crate::codegen::internal_failure("closure signature registered during analysis")
        })?;
    let target = super::closure_coercions::DefaultsFitTarget::exact(signature, has_rest);
    // A `void` function fits a function type returning `unknown`, as in
    // TypeScript; the cast adapts its convention.
    let returns_unknown = matches!(ty.lowered(), Type::Function { ret, .. } if matches!(ret.lowered(), Type::Unknown));
    let target = if returns_unknown {
        target.either_convention()
    } else {
        target
    };
    let fit = |emitter: &mut FunctionEmitter| {
        super::closure_coercions::emit_defaults_fit(emitter, ctx, value_local, target)
    };
    emit_ref_test(emitter, value_local, closure_struct);
    emitter.emit_if(BlockType::Result(ValType::I32));
    super::closure_coercions::emit_may_have_argument_metadata(
        emitter,
        ctx,
        value_local,
        closure_struct,
    )?;
    emitter.emit_if(BlockType::Result(ValType::I32));
    fit(emitter)?;
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(i32::from(!has_rest)));
    emitter.emit_end();
    emitter.emit_else();
    if has_rest {
        emitter.instruction(Instruction::I32Const(0));
    } else {
        fit(emitter)?;
    }
    emitter.emit_end();
    Ok(())
}

fn emit_interface_ref_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    interface_stack: &mut std::collections::BTreeSet<Type>,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let key = ty.lowered().clone();
    if ctx.symbols.generic_runtime_validator(&key).is_some() {
        emit_validator_or_representation(emitter, ctx, value_local, ty, &key, validator_state)?;
        return Ok(());
    }
    let _: () = match ctx.ta.runtime_type_tests.get(&key) {
        Some(crate::FieldNarrowingTest::Shape(shape)) => emit_structural_test_inner(
            emitter,
            ctx,
            value_local,
            shape,
            interface_stack,
            validator_state,
        )?,
        Some(crate::FieldNarrowingTest::Interface(test)) if interface_stack.insert(key.clone()) => {
            emit_interface_test_inner(
                emitter,
                ctx,
                value_local,
                ty,
                test,
                interface_stack,
                validator_state,
            )?;
            interface_stack.remove(&key);
        }
        _ => {
            emit_validator_or_representation(emitter, ctx, value_local, ty, &key, validator_state)?;
        }
    };
    Ok(())
}

fn emit_validator_or_representation(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    key: &Type,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    if let Some(func_idx) = ctx.symbols.runtime_validator_idx(key) {
        emitter.instruction(Instruction::LocalGet(value_local));
        let visited = emit_validator_state(emitter, ctx, validator_state)?;
        super::runtime_descriptors::validator_environment(emitter, ctx, key)?;
        emitter.instruction(Instruction::Call(func_idx));
        diagnostic::load_recursive(emitter, ctx, visited);
    } else {
        emit_representation_test(emitter, ctx, value_local, ty)?;
    };
    Ok(())
}

/// Validate an erased value against its declared target before casting it.
/// Mismatches become a catchable `TypeError` instead of an unchecked Wasm trap.
pub(crate) fn emit_checked_cast_on_stack(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    source_ty: &Type,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    cast::emit_box(emitter, ctx, source_ty)?;
    let scratch = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::LocalSet(scratch));
    let previous_diagnostic = emitter.cast_diagnostic.take();
    if diagnostic::has_nested_paths(target_ty) {
        diagnostic::begin(emitter, ctx);
    }
    match ctx.ta.runtime_type_tests.get(target_ty.lowered()) {
        Some(crate::FieldNarrowingTest::Interface(test)) => {
            emit_interface_test(emitter, ctx, scratch, target_ty, test)?;
        }
        Some(crate::FieldNarrowingTest::Shape(shape)) => {
            emit_structural_test(emitter, ctx, scratch, shape, target_ty)?;
        }
        // A function that declares fewer parameters than the target, or
        // trailing defaults, still fits when its adapter can stand for the
        // target, which the cast then builds.
        _ if matches!(target_ty.lowered(), Type::Function { .. }) => {
            emit_structural_test(emitter, ctx, scratch, target_ty, target_ty)?;
        }
        _ => emit_target_test(emitter, ctx, scratch, target_ty)?,
    }
    emitter.emit_if(BlockType::Result(ctx.symbols.value_type(target_ty)?));
    emitter.instruction(Instruction::LocalGet(scratch));
    cast::emit_cast_to(emitter, ctx, target_ty)?;
    emitter.emit_else();
    emit_cast_throw(emitter, ctx, scratch, target_ty);
    emitter.emit_end();
    emitter.cast_diagnostic = previous_diagnostic;
    Ok(())
}

/// Validate an erased parameter before rebinding it to the declaration's
/// physical slot. Interface values may have host-owned carriers that cannot be
/// reconstructed structurally here (for example Temporal plain-date structs),
/// so parameters require representation safety; concrete value types retain
/// the stronger structural check.
pub(crate) fn emit_checked_parameter_cast_on_stack(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    source_ty: &Type,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    if !is_interface_parameter(target_ty) {
        emit_checked_cast_on_stack(emitter, ctx, source_ty, target_ty)?;
        return Ok(());
    }

    cast::emit_box(emitter, ctx, source_ty)?;
    let scratch = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::LocalSet(scratch));
    emit_representation_test(emitter, ctx, scratch, target_ty)?;
    emitter.emit_if(BlockType::Result(ctx.symbols.value_type(target_ty)?));
    emitter.instruction(Instruction::LocalGet(scratch));
    cast::emit_cast_to(emitter, ctx, target_ty)?;
    emitter.emit_else();
    emit_cast_throw(emitter, ctx, scratch, target_ty);
    emitter.emit_end();
    Ok(())
}

fn is_interface_parameter(ty: &Type) -> bool {
    match ty.lowered() {
        Type::InterfaceRef { .. } => true,
        Type::Union(members) => {
            let mut non_null = members
                .iter()
                .filter(|member| !matches!(member.lowered(), Type::Null | Type::Undefined));
            non_null
                .next()
                .is_some_and(|member| matches!(member.lowered(), Type::InterfaceRef { .. }))
                && non_null.next().is_none()
        }
        _ => false,
    }
}

/// Require the carrier an operation consumes, without validating its contents
/// or invoking getters. Reading a value itself does not perform this check.
pub(crate) fn emit_operation_cast_on_stack(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    source_ty: &Type,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    cast::emit_box(emitter, ctx, source_ty)?;
    let scratch = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::LocalSet(scratch));
    // Operation checks validate only the carrier, so they have no nested path.
    let previous_diagnostic = emitter.cast_diagnostic.take();
    if matches!(target_ty.lowered(), Type::Function { .. }) {
        let signature = crate::codegen::closures::classify(target_ty.lowered())?;
        emit_representation_test(emitter, ctx, scratch, target_ty)?;
        let opposite = crate::codegen::closures::ClosureSig {
            is_void: !signature.is_void,
            ..signature
        };
        if let Some(structure) = ctx.symbols.closure_struct_type_idx(opposite) {
            emit_ref_test(emitter, scratch, structure);
            emitter.instruction(Instruction::I32Or);
        }
        if matches!(
            target_ty.lowered(),
            Type::Function {
                has_rest: false,
                ..
            }
        ) {
            emitter.emit_if(BlockType::Result(ValType::I32));
            emitter.instruction(Instruction::I32Const(1));
            emitter.emit_else();
            super::closure_coercions::emit_defaults_fit(
                emitter,
                ctx,
                scratch,
                super::closure_coercions::DefaultsFitTarget::any_convention(signature.arity),
            )?;
            emitter.emit_end();
        }
    } else {
        emit_target_test(emitter, ctx, scratch, target_ty)?;
    }
    emitter.emit_if(BlockType::Result(ctx.symbols.value_type(target_ty)?));
    emitter.instruction(Instruction::LocalGet(scratch));
    cast::emit_cast_to(emitter, ctx, target_ty)?;
    emitter.emit_else();
    emit_cast_throw(emitter, ctx, scratch, target_ty);
    emitter.emit_end();
    emitter.cast_diagnostic = previous_diagnostic;
    Ok(())
}

/// Pushes whether `value_local` holds a `target_ty`, for a target without a
/// recorded runtime test. A primitive slot (`1 | 2` is an `f64`) has no
/// reference shape to test, so its values are told apart structurally.
fn emit_target_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    if matches!(
        ctx.symbols.value_type(target_ty)?,
        ValType::F64 | ValType::I32
    ) {
        emit_structural_test(emitter, ctx, value_local, target_ty, target_ty)
    } else {
        emit_representation_test(emitter, ctx, value_local, target_ty)
    }
}

/// The conservative fallback for types without a full structural validator.
/// It cannot distinguish two interface values sharing `$Object`, but it proves
/// the following representation cast safe and turns representation mismatches
/// into a catchable `TypeError` rather than a Wasm trap.
fn emit_representation_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let ValType::Ref(target) = ctx.symbols.value_type(target_ty)? else {
        return Err(crate::codegen::internal_failure(
            "representation test requires a reference type",
        ));
    };
    let allows_null = cast::target_allows_null(ctx, target_ty)?;
    if allows_null {
        emitter.instruction(Instruction::LocalGet(value_local));
        emitter.instruction(Instruction::RefIsNull);
        emitter.emit_if(BlockType::Result(ValType::I32));
        emitter.instruction(Instruction::I32Const(1));
        emitter.emit_else();
    }
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefTestNonNull(target.heap_type));
    if allows_null {
        emitter.emit_end();
    };
    Ok(())
}

/// The read of a class field whose declaration narrows an inherited one. The
/// slot is shared with the parent's declaration, so it can hold a value this
/// declaration does not admit (spec.md §Classes) — test before casting and throw
/// a named, catchable `TypeError` instead of letting `ref.cast` raise a bare
/// `cast failure`. The raw payload slot value is on the stack.
///
/// Pushes i32 1 if the object in `obj_local` carries property `fname` at a value
/// conforming to `field`, 0 otherwise.
///
/// A data slot is read and tested. No data slot is not the same as "absent": an
/// accessor-backed property has no slot, and the value it would answer with is
/// behind a getter this must not call — a conformance predicate that runs user
/// code could throw or have side effects. The accessor slot's existence is
/// therefore accepted here; the accessor call validates its erased return at
/// the actual read. Neither slot means the property really is missing, and only
/// an optional field tolerates that.
///
/// The read counterpart is `expr::emit_object_property_read`, which makes the
/// same three-way distinction and calls the getter because it needs the value.
fn emit_field_conformance(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    locals: FieldConformanceLocals,
    ordered: diagnostic::OrderedField<'_>,
    interface_stack: &mut std::collections::BTreeSet<Type>,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let (fname, field) = (ordered.name, ordered.field);
    let checkpoint = diagnostic::checkpoint(emitter, ctx);
    let parent_path = diagnostic::field(emitter, ctx, ordered);
    let i32_block = BlockType::Result(ValType::I32);
    let intrinsics = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    let name_global = ctx
        .symbols
        .field_name_string_global_idx(fname)
        .ok_or_else(|| {
            crate::codegen::internal_failure(
                "per-name string global recorded for cast-target field",
            )
        })?;
    let index_local = emitter.add_anonymous_local(ValType::I32)?;
    crate::codegen::function_emitter::expr::emit_object_field_index_by_name(
        emitter,
        ctx,
        locals.object,
        name_global,
    );
    emitter.instruction(Instruction::LocalTee(index_local));
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::I32GeS);
    emitter.emit_if(i32_block);
    crate::codegen::function_emitter::expr::emit_field_slot_get(
        emitter,
        intrinsics,
        locals.object,
        index_local,
    );
    emitter.instruction(Instruction::LocalSet(locals.field));
    emit_structural_test_inner(
        emitter,
        ctx,
        locals.field,
        &field.ty,
        interface_stack,
        validator_state,
    )?;
    // A missing key reads as `undefined`, so a field that admits it conforms
    // without one: `{ parent: number | undefined }` round-trips through JSON.
    let may_be_absent = field.optional || admits_undefined(&field.ty);
    if may_be_absent {
        emitter.instruction(Instruction::LocalGet(locals.field));
        crate::codegen::function_emitter::expr::emit_is_undefined(emitter, ctx)?;
        emitter.instruction(Instruction::I32Or);
    } else {
        // A required undefined-valued property must still exist on the object.
        emitter.instruction(Instruction::LocalGet(locals.object));
        emitter.instruction(Instruction::StructGet {
            struct_type_index: intrinsics.object_shape,
            field_index: 1,
        });
        emitter.instruction(Instruction::LocalGet(index_local));
        emitter.instruction(Instruction::ArrayGet(intrinsics.field_names));
        crate::codegen::field_names::emit_name_presence(emitter, ctx)?;
        emitter.instruction(Instruction::LocalGet(locals.field));
        crate::codegen::function_emitter::expr::emit_is_nullish(emitter, ctx)?;
        emitter.instruction(Instruction::I32Eqz);
        emitter.instruction(Instruction::I32Or);
        emitter.instruction(Instruction::I32And);
    }
    emitter.emit_else();
    let getter = crate::codegen::classes::accessor_getter_name(fname);
    if may_be_absent {
        // Absent and accessor-backed both conform, so the branch is constant.
        emitter.instruction(Instruction::I32Const(1));
    } else if crate::codegen::function_emitter::expr::accessor_branch_emittable(ctx, &getter) {
        let _ = crate::codegen::function_emitter::expr::emit_is_accessor_backed(
            emitter,
            ctx,
            locals.object,
            &getter,
            crate::AccessorKind::Get,
        )?;
    } else {
        emitter.instruction(Instruction::I32Const(0));
    }
    emitter.emit_end();
    // Reached unset only when the field itself is missing: a present value's
    // own test records what it expected.
    diagnostic::finish(
        emitter,
        ctx,
        checkpoint,
        diagnostic::Tested::Absent { ty: &field.ty },
    )?;
    diagnostic::pop(emitter, parent_path);
    Ok(())
}

pub fn emit_narrowed_field_read(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    check: &crate::FieldNarrowingCheck,
    result_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    emit_narrowed_field_read_as(emitter, ctx, check, result_ty, result_ty)?;
    Ok(())
}

pub(crate) fn emit_narrowed_field_read_as(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    check: &crate::FieldNarrowingCheck,
    result_ty: &Type,
    check_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let result_val = ctx.symbols.value_type(result_ty)?;
    let scratch = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::LocalSet(scratch));
    // Prefer the concrete read-time descriptor. It includes generic
    // substitutions and nested interface metadata that the declaration could
    // not know; imported legacy declarations fall back to their recorded test.
    let runtime_test = ctx.ta.runtime_type_tests.get(check_ty.lowered());
    let test = if check
        .minimal_test_target
        .as_ref()
        .is_some_and(|target| target.lowered() == check_ty.lowered())
    {
        if matches!(check.test, crate::FieldNarrowingTest::NonNull) {
            &check.test
        } else {
            &crate::FieldNarrowingTest::Representation
        }
    } else {
        match runtime_test {
            Some(crate::FieldNarrowingTest::Representation)
                if !matches!(&check.test, crate::FieldNarrowingTest::Representation) =>
            {
                &check.test
            }
            Some(test) => test,
            None => &check.test,
        }
    };
    // Representation and null checks cannot descend to a nested location.
    // Keep ordinary property reads free of unused diagnostic state.
    let previous_diagnostic = emitter.cast_diagnostic.take();
    let nested_check = match test {
        crate::FieldNarrowingTest::Shape(ty) => diagnostic::has_nested_paths(ty),
        crate::FieldNarrowingTest::Interface(_) => true,
        _ => false,
    };
    if nested_check {
        diagnostic::begin(emitter, ctx);
    }
    match test {
        // Presence is the check only where the read's type rejects `null`. On a
        // generic class the declaration cannot say: `Sub<T>`'s `v: T` is
        // `string | null` at `Sub<string | null>`, where a `null` is legal and
        // the cast lets it through anyway. `result_ty` is substituted, so it
        // answers what the declaration could not.
        crate::FieldNarrowingTest::NonNull => {
            if cast::target_allows_null(ctx, check_ty)? {
                emitter.instruction(Instruction::I32Const(1));
            } else {
                emit_is_non_null(emitter, scratch);
            }
        }
        crate::FieldNarrowingTest::Shape(shape) => {
            emit_structural_test(emitter, ctx, scratch, shape, check_ty)?;
            // `emit_cast_to` lifts to non-null for every target it does not admit
            // a null for, so those are exactly the targets a `null` in the slot
            // would trap on.
            if !cast::target_allows_null(ctx, check_ty)? {
                emit_is_non_null(emitter, scratch);
                emitter.instruction(Instruction::I32And);
            }
        }
        crate::FieldNarrowingTest::Interface(test) => {
            emit_interface_test(emitter, ctx, scratch, check_ty, test)?;
        }
        crate::FieldNarrowingTest::Substituted => {
            emit_representation_test(emitter, ctx, scratch, check_ty)?;
        }
        crate::FieldNarrowingTest::Representation => {
            emit_representation_test(emitter, ctx, scratch, check_ty)?;
        }
    }
    emitter.emit_if(BlockType::Result(result_val));
    emitter.instruction(Instruction::LocalGet(scratch));
    cast::emit_cast_to(emitter, ctx, result_ty)?;
    emitter.emit_else();
    emit_inline_string(emitter, ctx, &check.message);
    diagnostic::append_failure(emitter, ctx);
    emit_type_error_from_message(emitter, ctx);
    emitter.emit_end();
    emitter.cast_diagnostic = previous_diagnostic;
    Ok(())
}

/// `ty` is the type `test` checks, named if the check exhausts its step limit.
fn emit_interface_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    test: &crate::InterfaceNarrowingTest,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    ctx.checking(ty, || {
        emit_interface_test_inner(
            emitter,
            ctx,
            value_local,
            ty,
            test,
            &mut std::collections::BTreeSet::new(),
            None,
        )?;
        ctx.check_function_limits(emitter)
    })
}

fn emit_interface_test_inner(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    ty: &Type,
    test: &crate::InterfaceNarrowingTest,
    interface_stack: &mut std::collections::BTreeSet<Type>,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    ctx.charge_validator_step(emitter)?;
    let checkpoint = diagnostic::checkpoint(emitter, ctx);
    if test.nullable || test.undefined {
        emitter.instruction(Instruction::LocalGet(value_local));
        if test.nullable && test.undefined {
            crate::codegen::function_emitter::expr::emit_is_nullish(emitter, ctx)?;
        } else if test.undefined {
            crate::codegen::function_emitter::expr::emit_is_undefined(emitter, ctx)?;
        } else {
            emitter.instruction(Instruction::RefIsNull);
        }
        emitter.emit_if(BlockType::Result(ValType::I32));
        emitter.instruction(Instruction::I32Const(1));
        emitter.emit_else();
    }

    if !test.shape_allowed {
        emit_non_shape_interface_test(emitter, ctx, value_local, test, validator_state)?;
        if test.nullable || test.undefined {
            emitter.emit_end();
        }
        return Ok(());
    }

    let shape_idx = ctx
        .symbols
        .object_shape_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$ObjectShape intrinsic registered"))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(shape_idx)));
    emitter.emit_if(BlockType::Result(ValType::I32));
    let object_local = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(shape_idx),
    }))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(shape_idx)));
    emitter.instruction(Instruction::LocalSet(object_local));
    let field_local = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::I32Const(1));
    for ordered in diagnostic::check_order(emitter, &test.members) {
        let (name, field) = (ordered.name, ordered.field);
        if test.methods.contains(name) {
            let parent_path = diagnostic::field(emitter, ctx, ordered);
            let name_global = ctx
                .symbols
                .field_name_string_global_idx(name)
                .ok_or_else(|| {
                    crate::codegen::internal_failure("interface narrowing method name registered")
                })?;
            crate::codegen::function_emitter::expr::emit_object_field_read_by_name(
                emitter,
                ctx,
                object_local,
                name_global,
            );
            emitter.instruction(Instruction::LocalSet(field_local));
            emit_structural_test_inner(
                emitter,
                ctx,
                field_local,
                &field.ty,
                interface_stack,
                validator_state,
            )?;
            diagnostic::pop(emitter, parent_path);
        } else {
            emit_field_conformance(
                emitter,
                ctx,
                FieldConformanceLocals {
                    object: object_local,
                    field: field_local,
                },
                ordered,
                interface_stack,
                validator_state,
            )?;
        }
        emitter.instruction(Instruction::I32And);
    }
    if let Some(index) = &test.index {
        emit_index_conformance(
            emitter,
            ctx,
            object_local,
            index,
            interface_stack,
            validator_state,
        )?;
        emitter.instruction(Instruction::I32And);
    }
    // Some direct-dispatch interfaces intentionally use `$ObjectShape` only as
    // an inert receiver carrier (for example TextEncoder/TextDecoder); their
    // methods are host imports, not payload slots. Admit those exact carriers
    // alongside ordinary structural conformance.
    emit_non_shape_interface_test(emitter, ctx, value_local, test, validator_state)?;
    emitter.instruction(Instruction::I32Or);
    emitter.emit_else();
    emit_non_shape_interface_test(emitter, ctx, value_local, test, validator_state)?;
    emitter.emit_end();

    if test.nullable || test.undefined {
        emitter.emit_end();
    }
    diagnostic::finish(
        emitter,
        ctx,
        checkpoint,
        diagnostic::Tested::Value {
            ty,
            local: value_local,
        },
    )?;
    Ok(())
}

fn emit_non_shape_interface_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    test: &crate::InterfaceNarrowingTest,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    use crate::InterfaceCarrier;

    let matches_any = emitter.add_anonymous_local(ValType::I32)?;
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::LocalSet(matches_any));
    for carrier in &test.non_shape_carriers {
        let intr = ctx
            .symbols
            .intrinsic_type_indices()
            .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
        match carrier {
            InterfaceCarrier::ArrayAny => emit_structural_test_inner(
                emitter,
                ctx,
                value_local,
                &Type::Array(Box::new(Type::Unknown)),
                &mut std::collections::BTreeSet::new(),
                validator_state,
            )?,
            InterfaceCarrier::Array(ty) => {
                emit_structural_test_inner(
                    emitter,
                    ctx,
                    value_local,
                    ty,
                    &mut std::collections::BTreeSet::new(),
                    validator_state,
                )?;
            }
            InterfaceCarrier::Number => emit_ref_test(emitter, value_local, boxed_number_idx(ctx)?),
            InterfaceCarrier::Boolean => {
                emit_ref_test(emitter, value_local, boxed_boolean_idx(ctx)?);
            }
            InterfaceCarrier::String => emit_ref_test(emitter, value_local, string_idx(ctx)?),
            InterfaceCarrier::BigInt => emit_ref_test(emitter, value_local, intr.bigint),
            InterfaceCarrier::Uint8Array => emit_ref_test(emitter, value_local, intr.uint8_array),
            InterfaceCarrier::MapAny => emit_ref_test(emitter, value_local, intr.map),
            InterfaceCarrier::Map(key, value) => {
                emit_map_carrier_test(emitter, ctx, value_local, key, value, validator_state)?;
            }
            InterfaceCarrier::SetAny => emit_ref_test(emitter, value_local, intr.set),
            InterfaceCarrier::Set(element) => {
                emit_set_carrier_test(emitter, ctx, value_local, element, validator_state)?;
            }
            InterfaceCarrier::RegExp => emit_ref_test(emitter, value_local, intr.regex),
            InterfaceCarrier::RegExpMatch => {
                emit_regexp_array_test(emitter, ctx, value_local, test, validator_state)?;
                emit_ref_test(emitter, value_local, intr.regex_match_box);
                emitter.instruction(Instruction::I32And);
            }
            InterfaceCarrier::RegExpMatchArray => {
                emit_regexp_array_test(emitter, ctx, value_local, test, validator_state)?;
            }
            InterfaceCarrier::TemporalInstant => {
                emit_ref_test(emitter, value_local, intr.temporal_instant);
            }
            InterfaceCarrier::TemporalDuration => {
                emit_ref_test(emitter, value_local, intr.temporal_duration);
            }
            InterfaceCarrier::TemporalZonedDateTime => {
                emit_ref_test(emitter, value_local, intr.temporal_zdt);
            }
            InterfaceCarrier::TemporalPlainDate => {
                emit_ref_test(emitter, value_local, intr.temporal_plain_date);
            }
            InterfaceCarrier::TemporalPlainTime => {
                emit_ref_test(emitter, value_local, intr.temporal_plain_time);
            }
            InterfaceCarrier::TemporalPlainDateTime => {
                emit_ref_test(emitter, value_local, intr.temporal_plain_date_time);
            }
            InterfaceCarrier::TemporalPlainYearMonth => {
                emit_ref_test(emitter, value_local, intr.temporal_plain_year_month);
            }
            InterfaceCarrier::TemporalPlainMonthDay => {
                emit_ref_test(emitter, value_local, intr.temporal_plain_month_day);
            }
            InterfaceCarrier::ObjectShape => {
                emit_ref_test(emitter, value_local, intr.object_shape);
            }
            InterfaceCarrier::Url => emit_ref_test(emitter, value_local, intr.url),
            InterfaceCarrier::FsStat => emit_ref_test(emitter, value_local, intr.fs_stat),
            InterfaceCarrier::FsPeek => emit_ref_test(emitter, value_local, intr.fs_peek),
            InterfaceCarrier::FsDirEntry => emit_ref_test(emitter, value_local, intr.fs_dir_entry),
            InterfaceCarrier::FsInfo => emit_ref_test(emitter, value_local, intr.fs_info),
            InterfaceCarrier::FsMountInfo => {
                emit_ref_test(emitter, value_local, intr.fs_mount_info);
            }
            InterfaceCarrier::FsFileWriter => {
                emit_ref_test(emitter, value_local, intr.fs_file_writer);
            }
            InterfaceCarrier::HttpResponse => {
                emit_ref_test(emitter, value_local, intr.http_response);
            }
            InterfaceCarrier::HttpDownloadResult => {
                emit_ref_test(emitter, value_local, intr.http_download_result);
            }
            InterfaceCarrier::SessionEntry => {
                emit_ref_test(emitter, value_local, intr.session_entry);
            }
            InterfaceCarrier::SessionPage => emit_ref_test(emitter, value_local, intr.session_page),
        }
        emitter.instruction(Instruction::LocalGet(matches_any));
        emitter.instruction(Instruction::I32Or);
        emitter.instruction(Instruction::LocalSet(matches_any));
    }
    emitter.instruction(Instruction::LocalGet(matches_any));
    Ok(())
}

/// The whole match, captures and named groups remain mutable through widened
/// aliases, so a checked cast validates their contents even on a branded result.
fn emit_regexp_array_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    test: &crate::InterfaceNarrowingTest,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    emit_structural_test_inner(
        emitter,
        ctx,
        value_local,
        &Type::Array(Box::new(Type::regexp_element())),
        &mut std::collections::BTreeSet::new(),
        validator_state,
    )?;
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    // The successful element test establishes the array carrier. Nothing calls
    // guest code between that test and these length/storage reads.
    emitter.emit_if(BlockType::Result(ValType::I32));
    emit_regexp_whole_match_test(emitter, ctx, value_local)?;
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(0));
    emitter.emit_end();
    emit_ref_test(emitter, value_local, intr.regex_match_box);
    emitter.emit_if(BlockType::Result(ValType::I32));
    let groups_local = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(
        intr.regex_match_box,
    )));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: intr.regex_match_box,
        field_index: 5,
    });
    emitter.instruction(Instruction::LocalSet(groups_local));
    let groups = test
        .members
        .get("groups")
        .ok_or_else(|| crate::codegen::internal_failure("regex array declares groups"))?;
    emit_structural_test_inner(
        emitter,
        ctx,
        groups_local,
        &groups.ty,
        &mut std::collections::BTreeSet::new(),
        validator_state,
    )?;
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(1));
    emitter.emit_end();
    emitter.instruction(Instruction::I32And);
    Ok(())
}

/// Called only after the array carrier and its elements have been validated.
fn emit_regexp_whole_match_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    let array_idx = ctx
        .symbols
        .array_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$Array intrinsic registered"))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(array_idx)));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: array_idx,
        field_index: 2,
    });
    // A nonempty logical array has a backing slot zero. Check before loading it
    // so empty/truncated arrays fail the cast instead of trapping on the read.
    emitter.emit_if(BlockType::Result(ValType::I32));
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(array_idx)));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: array_idx,
        field_index: 1,
    });
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::ArrayGet(intr.raw_array));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(string_idx(
        ctx,
    )?)));
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(0));
    emitter.emit_end();
    Ok(())
}

fn emit_set_carrier_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    element: &Type,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    emit_ref_test(emitter, value_local, intr.set);
    emitter.emit_if(BlockType::Result(ValType::I32));
    let backing = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.set),
    }))?;
    let elements = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_array),
    }))?;
    let order = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_index_array),
    }))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(intr.set)));
    emitter.instruction(Instruction::LocalTee(backing));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: intr.set,
        field_index: 1,
    });
    emitter.instruction(Instruction::LocalSet(elements));
    emitter.instruction(Instruction::LocalGet(backing));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: intr.set,
        field_index: 3,
    });
    emitter.instruction(Instruction::LocalSet(order));
    emit_ordered_collection_members_test(
        emitter,
        ctx,
        CollectionStorage {
            backing,
            backing_type: intr.set,
            order_len_field: 4,
            order,
        },
        &[(elements, element)],
        validator_state,
    )?;
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(0));
    emitter.emit_end();
    Ok(())
}

fn emit_map_carrier_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    key: &Type,
    value: &Type,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    emit_ref_test(emitter, value_local, intr.map);
    emitter.emit_if(BlockType::Result(ValType::I32));
    let backing = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.map),
    }))?;
    let keys = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_array),
    }))?;
    let values = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_array),
    }))?;
    let order = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_index_array),
    }))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(intr.map)));
    emitter.instruction(Instruction::LocalTee(backing));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: intr.map,
        field_index: 1,
    });
    emitter.instruction(Instruction::LocalSet(keys));
    emitter.instruction(Instruction::LocalGet(backing));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: intr.map,
        field_index: 2,
    });
    emitter.instruction(Instruction::LocalSet(values));
    emitter.instruction(Instruction::LocalGet(backing));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: intr.map,
        field_index: 4,
    });
    emitter.instruction(Instruction::LocalSet(order));
    emit_ordered_collection_members_test(
        emitter,
        ctx,
        CollectionStorage {
            backing,
            backing_type: intr.map,
            order_len_field: 5,
            order,
        },
        &[(keys, key), (values, value)],
        validator_state,
    )?;
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(0));
    emitter.emit_end();
    Ok(())
}

fn emit_ordered_collection_members_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    storage: CollectionStorage,
    payloads: &[(u32, &Type)],
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    let acc = emitter.add_anonymous_local(ValType::I32)?;
    let cursor = emitter.add_anonymous_local(ValType::I32)?;
    let slot = emitter.add_anonymous_local(ValType::I32)?;
    let member = emitter.add_anonymous_local(scratch_object_ty(intr.object))?;
    emitter.instruction(Instruction::I32Const(1));
    emitter.instruction(Instruction::LocalSet(acc));
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::LocalSet(cursor));
    emitter.emit_block(BlockType::Empty);
    emitter.emit_loop(BlockType::Empty);
    emitter.instruction(Instruction::LocalGet(cursor));
    emitter.instruction(Instruction::LocalGet(storage.backing));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: storage.backing_type,
        field_index: storage.order_len_field,
    });
    emitter.instruction(Instruction::I32GeU);
    emitter.instruction(Instruction::BrIf(1));
    emitter.instruction(Instruction::LocalGet(storage.order));
    emitter.instruction(Instruction::LocalGet(cursor));
    emitter.instruction(Instruction::ArrayGet(intr.raw_index_array));
    emitter.instruction(Instruction::LocalTee(slot));
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::I32GeS);
    emitter.emit_if(BlockType::Empty);
    for (position, (payload, ty)) in payloads.iter().enumerate() {
        let parent_path = diagnostic::index(emitter, ctx, cursor);
        let member_path = diagnostic::entry_part(
            emitter,
            ctx,
            if payloads.len() == 1 || position == 1 {
                "value"
            } else {
                "key"
            },
        );
        emitter.instruction(Instruction::LocalGet(*payload));
        emitter.instruction(Instruction::LocalGet(slot));
        emitter.instruction(Instruction::ArrayGet(intr.raw_array));
        emitter.instruction(Instruction::LocalSet(member));
        emitter.instruction(Instruction::LocalGet(acc));
        emit_structural_test_inner(
            emitter,
            ctx,
            member,
            ty,
            &mut std::collections::BTreeSet::new(),
            validator_state,
        )?;
        diagnostic::pop(emitter, member_path);
        diagnostic::pop(emitter, parent_path);
        emitter.instruction(Instruction::I32And);
        emitter.instruction(Instruction::LocalSet(acc));
    }
    emitter.emit_end();
    emitter.instruction(Instruction::LocalGet(cursor));
    emitter.instruction(Instruction::I32Const(1));
    emitter.instruction(Instruction::I32Add);
    emitter.instruction(Instruction::LocalSet(cursor));
    emitter.instruction(Instruction::Br(0));
    emitter.emit_end();
    emitter.emit_end();
    emitter.instruction(Instruction::LocalGet(acc));
    Ok(())
}

fn emit_validator_state(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    validator_state: Option<ValidatorState>,
) -> Result<u32, crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| super::internal_failure("intrinsic types are not declared"))?;
    let visited = if let Some(state) = validator_state {
        state.visited
    } else {
        let local = emitter.add_anonymous_local(ValType::Ref(RefType {
            nullable: false,
            heap_type: HeapType::Concrete(intr.raw_array),
        }))?;
        emitter.instruction(Instruction::I32Const(
            RECURSIVE_VALIDATOR_CAPACITY * 2 + diagnostic::SLOTS,
        ));
        emitter.instruction(Instruction::ArrayNewDefault(intr.raw_array));
        emitter.instruction(Instruction::LocalSet(local));
        local
    };
    diagnostic::save_recursive(emitter, ctx, visited)?;
    emitter.instruction(Instruction::LocalGet(visited));
    if let Some(state) = validator_state {
        emitter.instruction(Instruction::LocalGet(state.validator_ids));
        emitter.instruction(Instruction::LocalGet(state.depth));
        emitter.instruction(Instruction::I32Const(1));
        emitter.instruction(Instruction::I32Add);
    } else {
        emitter.instruction(Instruction::I32Const(RECURSIVE_VALIDATOR_CAPACITY));
        emitter.instruction(Instruction::ArrayNewDefault(intr.raw_index_array));
        emitter.instruction(Instruction::I32Const(0));
    }
    Ok(visited)
}

/// Body of a recursive runtime validator:
/// `(ref null $Object, ref $rawArray visited, ref $rawIndexArray ids, i32 depth, ref $ObjectFields types) -> i32`.
/// A `(value, validator)` pair already present on the active recursion path
/// closes a valid cycle without conflating mutually recursive shapes.
/// The fixed-capacity path rejects an implausibly deep acyclic graph rather than
/// overflowing the Wasm stack or accepting an unchecked tail.
pub(crate) fn emit_runtime_validator_body(
    ctx: &CodegenCtx,
    key: &Type,
    body: &Type,
    rejects_polymorphic_edge: bool,
    validator_id: i32,
) -> Result<Function, crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    let object_idx = intr.object;
    let object_ref_null = ValType::Ref(RefType {
        nullable: true,
        heap_type: HeapType::Concrete(object_idx),
    });
    let raw_array_ref = ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_array),
    });
    let raw_index_array_ref = ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_index_array),
    });
    let ident = |name: &str| Ident {
        name: name.to_string(),
        span: Span::at(ctx.file),
    };
    let mut emitter = FunctionEmitter::new(
        ctx,
        &[
            (ident("v"), object_ref_null),
            (ident("visited"), raw_array_ref),
            (ident("validator_ids"), raw_index_array_ref),
            (ident("depth"), ValType::I32),
            (
                ident("types"),
                super::runtime_descriptors::environment_type(ctx.symbols)?,
            ),
        ],
    )?;
    super::runtime_descriptors::bind(
        &mut emitter,
        &super::runtime_descriptors::parameters(key),
        4,
    )?;
    if rejects_polymorphic_edge {
        emitter.instruction(Instruction::I32Const(0));
        return emitter.build();
    }
    diagnostic::enter_recursive(&mut emitter, ctx);
    let checkpoint = diagnostic::checkpoint(&mut emitter, ctx);
    let parameters = super::runtime_descriptors::parameters(key);
    let seen = emitter.add_anonymous_local(ValType::I32)?;
    let index = emitter.add_anonymous_local(ValType::I32)?;
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::LocalSet(seen));
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::LocalSet(index));
    emitter.emit_block(BlockType::Empty);
    emitter.emit_loop(BlockType::Empty);
    emitter.instruction(Instruction::LocalGet(index));
    emitter.instruction(Instruction::LocalGet(3));
    emitter.instruction(Instruction::I32GeU);
    emitter.instruction(Instruction::BrIf(1));
    emitter.instruction(Instruction::LocalGet(1));
    emitter.instruction(Instruction::LocalGet(index));
    emitter.instruction(Instruction::I32Const(2));
    emitter.instruction(Instruction::I32Mul);
    emitter.instruction(Instruction::ArrayGet(intr.raw_array));
    emitter.instruction(Instruction::LocalGet(0));
    emitter.instruction(Instruction::RefEq);
    emitter.instruction(Instruction::LocalGet(2));
    emitter.instruction(Instruction::LocalGet(index));
    emitter.instruction(Instruction::ArrayGet(intr.raw_index_array));
    emitter.instruction(Instruction::I32Const(validator_id));
    emitter.instruction(Instruction::I32Eq);
    emitter.instruction(Instruction::I32And);
    emitter.emit_if(BlockType::Empty);
    emit_same_validator_environment(&mut emitter, ctx, index, parameters.len())?;
    emitter.instruction(Instruction::LocalSet(seen));
    emitter.instruction(Instruction::LocalGet(seen));
    emitter.emit_if(BlockType::Empty);
    emitter.instruction(Instruction::Br(3));
    emitter.emit_end();
    emitter.emit_end();
    emitter.instruction(Instruction::LocalGet(index));
    emitter.instruction(Instruction::I32Const(1));
    emitter.instruction(Instruction::I32Add);
    emitter.instruction(Instruction::LocalSet(index));
    emitter.instruction(Instruction::Br(0));
    emitter.emit_end();
    emitter.emit_end();
    emitter.instruction(Instruction::LocalGet(seen));
    emitter.emit_if(BlockType::Result(ValType::I32));
    emitter.instruction(Instruction::I32Const(1));
    emitter.emit_else();
    emitter.instruction(Instruction::LocalGet(3));
    emitter.instruction(Instruction::I32Const(RECURSIVE_VALIDATOR_CAPACITY));
    emitter.instruction(Instruction::I32GeU);
    emitter.emit_if(BlockType::Result(ValType::I32));
    emitter.instruction(Instruction::I32Const(0));
    emitter.emit_else();
    emitter.instruction(Instruction::LocalGet(1));
    emitter.instruction(Instruction::LocalGet(3));
    emitter.instruction(Instruction::I32Const(2));
    emitter.instruction(Instruction::I32Mul);
    emitter.instruction(Instruction::LocalGet(0));
    emitter.instruction(Instruction::ArraySet(intr.raw_array));
    if !parameters.is_empty() {
        emit_store_validator_environment(&mut emitter, ctx)?;
    }
    emitter.instruction(Instruction::LocalGet(2));
    emitter.instruction(Instruction::LocalGet(3));
    emitter.instruction(Instruction::I32Const(validator_id));
    emitter.instruction(Instruction::ArraySet(intr.raw_index_array));
    let mut interface_stack = std::collections::BTreeSet::new();
    if key.lowered() == body.lowered()
        && let Some(crate::FieldNarrowingTest::Interface(test)) =
            ctx.ta.runtime_type_tests.get(key.lowered())
    {
        emit_interface_test_inner(
            &mut emitter,
            ctx,
            0,
            key,
            test,
            &mut interface_stack,
            Some(ValidatorState {
                visited: 1,
                validator_ids: 2,
                depth: 3,
            }),
        )?;
    } else {
        // A recursive alias's body is checked under the alias's name, so a
        // failure at its value names `Tree`, not the expanded shape.
        let named_body = match key {
            Type::AliasRef {
                mangled,
                package,
                name,
                args,
            } => Some(Type::alias_ty(
                package.clone(),
                name.clone(),
                mangled.clone(),
                args.clone(),
                Box::new(body.clone()),
            )),
            _ => None,
        };
        emit_structural_test_inner(
            &mut emitter,
            ctx,
            0,
            named_body.as_ref().unwrap_or(body),
            &mut interface_stack,
            Some(ValidatorState {
                visited: 1,
                validator_ids: 2,
                depth: 3,
            }),
        )?;
    }
    emitter.emit_end();
    emitter.emit_end();
    diagnostic::finish(
        &mut emitter,
        ctx,
        checkpoint,
        diagnostic::Tested::Value { ty: key, local: 0 },
    )?;
    diagnostic::save_recursive(&mut emitter, ctx, 1)?;
    ctx.check_function_limits(&mut emitter)?;
    emitter.build()
}

fn emit_is_non_null(emitter: &mut FunctionEmitter, value_local: u32) {
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefIsNull);
    emitter.instruction(Instruction::I32Eqz);
}

fn emit_ref_test(emitter: &mut FunctionEmitter, value_local: u32, type_idx: u32) {
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(type_idx)));
}

fn boxed_number_idx(ctx: &CodegenCtx) -> Result<u32, crate::compiler_error::CompilerFailure> {
    ctx.symbols
        .boxed_number_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$BoxedNumber intrinsic registered"))
}

fn boxed_boolean_idx(ctx: &CodegenCtx) -> Result<u32, crate::compiler_error::CompilerFailure> {
    ctx.symbols
        .boxed_boolean_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$BoxedBoolean intrinsic registered"))
}

fn string_idx(ctx: &CodegenCtx) -> Result<u32, crate::compiler_error::CompilerFailure> {
    ctx.symbols
        .string_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$string intrinsic registered"))
}

fn object_idx_of(ctx: &CodegenCtx) -> Result<u32, crate::compiler_error::CompilerFailure> {
    Ok(ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?
        .object)
}

fn scratch_object_ty(object_idx: u32) -> ValType {
    ValType::Ref(RefType {
        nullable: true,
        heap_type: HeapType::Concrete(object_idx),
    })
}

fn emit_cast_throw(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    scratch: u32,
    target_ty: &Type,
) {
    ctx.latch(emit_cast_throw_checked(emitter, ctx, scratch, target_ty));
}

fn emit_cast_throw_checked(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    scratch: u32,
    target_ty: &Type,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let prefix = error_prefix(target_ty);
    emit_inline_string(emitter, ctx, &prefix);

    emit_runtime_type_string(emitter, ctx, scratch);

    let concat_idx = ctx
        .symbols
        .prelude_func_idx("string_concat")
        .ok_or_else(|| crate::codegen::internal_failure("string_concat imported from prelude"))?;
    emitter.instruction(Instruction::Call(concat_idx));

    diagnostic::append_failure(emitter, ctx);

    emit_type_error_from_message(emitter, ctx);

    Ok(())
}

fn emit_type_error_from_message(emitter: &mut FunctionEmitter, ctx: &CodegenCtx) {
    ctx.latch(emit_type_error_from_message_checked(emitter, ctx));
}

fn emit_type_error_from_message_checked(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let new_idx = ctx
        .symbols
        .prelude_func_idx("TypeError#constructor")
        .ok_or_else(|| {
            crate::codegen::internal_failure("TypeError#constructor imported from prelude")
        })?;
    crate::codegen::throw::emit_no_error_options(emitter);
    emitter.instruction(Instruction::Call(new_idx));

    // Constructor returns (ref null $Object); the throw helper narrows to
    // $Error so the throw carries the payload type the catch expects.
    crate::codegen::throw::emit_error_throw(emitter, ctx);

    Ok(())
}

pub(super) fn emit_inline_string(emitter: &mut FunctionEmitter, ctx: &CodegenCtx, text: &str) {
    ctx.latch(emit_inline_string_checked(emitter, ctx, text));
}

fn emit_inline_string_checked(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    text: &str,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    let string_vtable_idx = ctx
        .symbols
        .prelude_global_idx("string_vtable")
        .ok_or_else(|| crate::codegen::internal_failure("string_vtable imported"))?;
    emitter.instruction(Instruction::GlobalGet(string_vtable_idx));
    ctx.latch(emit_inline_const_raw_string(emitter, ctx, text));
    emitter.instruction(Instruction::I64Const(0));
    emitter.instruction(Instruction::StructNew(intr.string));

    Ok(())
}

/// Cascades ref.test on scratch to produce a JS-typeof-style tag string:
/// undefined, null, string, number, boolean, function, then array, which typeof
/// would call "object", then the "object" fallthrough.
pub(super) fn emit_runtime_type_string(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    scratch: u32,
) {
    ctx.latch(emit_runtime_type_string_checked(emitter, ctx, scratch));
}

fn emit_runtime_type_string_checked(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    scratch: u32,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let string_type_idx = ctx
        .symbols
        .string_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$string intrinsic registered"))?;
    let string_ref = ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(string_type_idx),
    });
    let block = BlockType::Result(string_ref);

    emitter.instruction(Instruction::LocalGet(scratch));
    crate::codegen::function_emitter::expr::emit_is_undefined(emitter, ctx)?;
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "undefined")?;
    emitter.emit_else();

    // null gets its own arm rather than JS's "object" tag — clearer cast error messages.
    emitter.instruction(Instruction::LocalGet(scratch));
    emitter.instruction(Instruction::RefIsNull);
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "null")?;
    emitter.emit_else();

    emitter.instruction(Instruction::LocalGet(scratch));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(
        string_type_idx,
    )));
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "string")?;
    emitter.emit_else();

    let num_idx = ctx
        .symbols
        .boxed_number_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$BoxedNumber intrinsic registered"))?;
    emitter.instruction(Instruction::LocalGet(scratch));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(num_idx)));
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "number")?;
    emitter.emit_else();

    let bool_idx = ctx
        .symbols
        .boxed_boolean_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$BoxedBoolean intrinsic registered"))?;
    emitter.instruction(Instruction::LocalGet(scratch));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(bool_idx)));
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "boolean")?;
    emitter.emit_else();

    let closure_idx = ctx
        .symbols
        .closure_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$Closure intrinsic registered"))?;
    emitter.instruction(Instruction::LocalGet(scratch));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(closure_idx)));
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "function")?;
    emitter.emit_else();

    // Arrays too, so `got object` never answers an expected array or tuple.
    let array_idx = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsic types registered"))?
        .array;
    emitter.instruction(Instruction::LocalGet(scratch));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(array_idx)));
    emitter.emit_if(block);
    emit_type_tag(emitter, ctx, "array")?;
    emitter.emit_else();

    emit_type_tag(emitter, ctx, "object")?;

    for _ in 0..6 {
        emitter.emit_end();
    }

    emitter.emit_end();
    Ok(())
}

/// Whether `ty` spells `undefined`. A required `unknown` field still needs its key.
fn admits_undefined(ty: &Type) -> bool {
    match ty.lowered() {
        Type::Undefined | Type::Void => true,
        Type::Union(members) => members.iter().any(admits_undefined),
        _ => false,
    }
}

/// A tag the string pool interned is read from its data segment, which is
/// smaller than spelling it out at each of the many failure paths that name it.
fn emit_type_tag(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    tag: &str,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    match ctx.strings.lookup_text(tag) {
        Some(pool_idx) => {
            crate::codegen::function_emitter::emit_pooled_string(emitter, ctx, pool_idx)
        }
        None => emit_inline_string_checked(emitter, ctx, tag),
    }
}

/// Visited entries pair the value with its effective type arguments. Forwarded
/// descriptors preserve identity; growing arguments construct fresh ones.
fn emit_same_validator_environment(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    index: u32,
    count: usize,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    emitter.instruction(Instruction::I32Const(1));
    if count == 0 {
        return Ok(());
    }
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics"))?;
    let closure = ctx
        .symbols
        .closure_struct_type_idx(super::field_guards::signature())
        .ok_or_else(|| crate::codegen::internal_failure("predicate closure type"))?;
    for slot in 0..count {
        emitter.instruction(Instruction::LocalGet(1));
        emitter.instruction(Instruction::LocalGet(index));
        emitter.instruction(Instruction::I32Const(2));
        emitter.instruction(Instruction::I32Mul);
        emitter.instruction(Instruction::I32Const(1));
        emitter.instruction(Instruction::I32Add);
        emitter.instruction(Instruction::ArrayGet(intr.raw_array));
        emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(closure)));
        emitter.instruction(Instruction::StructGet {
            struct_type_index: closure,
            field_index: 2,
        });
        emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(
            intr.object_fields,
        )));
        emitter.instruction(Instruction::I32Const(slot as i32));
        emitter.instruction(Instruction::ArrayGet(intr.object_fields));
        emitter.instruction(Instruction::LocalGet(4));
        emitter.instruction(Instruction::I32Const(slot as i32));
        emitter.instruction(Instruction::ArrayGet(intr.object_fields));
        emitter.instruction(Instruction::RefEq);
        emitter.instruction(Instruction::I32And);
    }
    Ok(())
}

fn emit_store_validator_environment(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics"))?;
    let closure = ctx
        .symbols
        .closure_struct_type_idx(super::field_guards::signature())
        .ok_or_else(|| crate::codegen::internal_failure("predicate closure type"))?;
    emitter.instruction(Instruction::LocalGet(1));
    emitter.instruction(Instruction::LocalGet(3));
    emitter.instruction(Instruction::I32Const(2));
    emitter.instruction(Instruction::I32Mul);
    emitter.instruction(Instruction::I32Const(1));
    emitter.instruction(Instruction::I32Add);
    emitter.instruction(Instruction::GlobalGet(
        ctx.symbols
            .closure_vtable_global_idx()
            .ok_or_else(|| crate::codegen::internal_failure("closure vtable"))?,
    ));
    emitter.instruction(Instruction::RefFunc(
        *ctx.symbols
            .type_descriptor_functions
            .get(&Type::Unknown)
            .ok_or_else(|| {
                crate::codegen::internal_failure(
                    "validator environment descriptor is not registered",
                )
            })?,
    ));
    emitter.instruction(Instruction::LocalGet(4));
    emitter.instruction(Instruction::I64Const(0));
    emitter.instruction(Instruction::StructNew(closure));
    emitter.instruction(Instruction::ArraySet(intr.raw_array));

    Ok(())
}

/// Tests each element of the array in `value_local`. When the array holds a
/// record's values, `record_keys` names a failing element by its key instead of
/// its index.
fn emit_elements_test(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    value_local: u32,
    elem: &Type,
    interface_stack: &mut std::collections::BTreeSet<Type>,
    validator_state: Option<ValidatorState>,
    record_keys: Option<diagnostic::RecordKeys>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let i32_block = BlockType::Result(ValType::I32);
    let intr = ctx
        .symbols
        .intrinsic_type_indices()
        .ok_or_else(|| crate::codegen::internal_failure("intrinsics declared"))?;
    let array_idx = ctx
        .symbols
        .array_type_idx()
        .ok_or_else(|| crate::codegen::internal_failure("$Array intrinsic registered"))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefTestNonNull(HeapType::Concrete(array_idx)));
    emitter.emit_if(i32_block);
    let raw_local = emitter.add_anonymous_local(ValType::Ref(RefType {
        nullable: false,
        heap_type: HeapType::Concrete(intr.raw_array),
    }))?;
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(array_idx)));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: array_idx,
        field_index: 1,
    });
    emitter.instruction(Instruction::LocalSet(raw_local));
    let acc = emitter.add_anonymous_local(ValType::I32)?;
    let i = emitter.add_anonymous_local(ValType::I32)?;
    let len = emitter.add_anonymous_local(ValType::I32)?;
    let elem_local = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::I32Const(1));
    emitter.instruction(Instruction::LocalSet(acc));
    emitter.instruction(Instruction::I32Const(0));
    emitter.instruction(Instruction::LocalSet(i));
    emitter.instruction(Instruction::LocalGet(value_local));
    emitter.instruction(Instruction::RefCastNonNull(HeapType::Concrete(array_idx)));
    emitter.instruction(Instruction::StructGet {
        struct_type_index: array_idx,
        field_index: 2,
    });
    emitter.instruction(Instruction::LocalSet(len));
    emitter.emit_block(BlockType::Empty);
    emitter.emit_loop(BlockType::Empty);
    emitter.instruction(Instruction::LocalGet(i));
    emitter.instruction(Instruction::LocalGet(len));
    emitter.instruction(Instruction::I32GeU);
    emitter.instruction(Instruction::BrIf(1));
    emitter.instruction(Instruction::LocalGet(raw_local));
    emitter.instruction(Instruction::LocalGet(i));
    emitter.instruction(Instruction::ArrayGet(intr.raw_array));
    emitter.instruction(Instruction::LocalSet(elem_local));
    emitter.instruction(Instruction::LocalGet(acc));
    let parent_path = match record_keys {
        Some(keys) => diagnostic::record_key(emitter, keys, i),
        None => diagnostic::index(emitter, ctx, i),
    };
    emit_structural_test_inner(
        emitter,
        ctx,
        elem_local,
        elem,
        interface_stack,
        validator_state,
    )?;
    diagnostic::pop(emitter, parent_path);
    emitter.instruction(Instruction::I32And);
    emitter.instruction(Instruction::LocalSet(acc));
    emitter.instruction(Instruction::LocalGet(i));
    emitter.instruction(Instruction::I32Const(1));
    emitter.instruction(Instruction::I32Add);
    emitter.instruction(Instruction::LocalSet(i));
    emitter.instruction(Instruction::Br(0));
    emitter.emit_end();
    emitter.emit_end();
    emitter.instruction(Instruction::LocalGet(acc));
    emitter.emit_else();
    emitter.instruction(Instruction::I32Const(0));
    emitter.emit_end();
    Ok(())
}

fn emit_index_conformance(
    emitter: &mut FunctionEmitter,
    ctx: &CodegenCtx,
    object: u32,
    index: &crate::IndexSignature,
    interface_stack: &mut std::collections::BTreeSet<Type>,
    validator_state: Option<ValidatorState>,
) -> Result<(), crate::compiler_error::CompilerFailure> {
    let domain = serde_json::to_string(&crate::property_keys::runtime_domain(&index.key).map_err(
        |error| error.into_failure(crate::compiler_error::CompilerStage::Codegen, None),
    )?)
    .map_err(|error| {
        crate::codegen::internal_failure(format!("could not encode index key domain: {error}"))
    })?;
    emit_inline_string_checked(emitter, ctx, &domain)?;
    let domain_local = emitter.add_anonymous_local(ValType::Ref(wasm_encoder::RefType {
        nullable: false,
        heap_type: HeapType::Concrete(string_idx(ctx)?),
    }))?;
    emitter.instruction(Instruction::LocalSet(domain_local));
    emitter.instruction(Instruction::LocalGet(object));
    let Some(symbol) = ctx.require(
        ctx.symbols
            .prelude_func_idx("ObjectConstructor##recordValues"),
        "record validator imported",
    ) else {
        return Ok(());
    };
    emitter.instruction(Instruction::LocalGet(domain_local));
    emitter.instruction(Instruction::Call(symbol));
    let values = emitter.add_anonymous_local(scratch_object_ty(object_idx_of(ctx)?))?;
    emitter.instruction(Instruction::LocalSet(values));
    let record_keys = diagnostic::record_keys(emitter, ctx, object, values, domain_local);
    emit_elements_test(
        emitter,
        ctx,
        values,
        &index.value,
        interface_stack,
        validator_state,
        record_keys,
    )?;
    Ok(())
}

#[cfg(test)]
mod representation_invariant_tests {
    use super::*;
    use crate::codegen::invariant_tests::{assert_internal, with_context};

    #[test]
    fn primitive_representation_test_returns_internal_failure() {
        with_context(
            &crate::TypedAst::new(),
            &crate::codegen::SymbolTable::default(),
            |ctx| {
                let mut emitter = FunctionEmitter::new(ctx, &[]).unwrap();
                assert_internal(
                    emit_representation_test(&mut emitter, ctx, 0, &Type::Number).unwrap_err(),
                );
            },
        );
    }
}