pkg/dart2wasm/lib/types.dart

// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:math' show max;

import 'package:dart2wasm/class_info.dart';
import 'package:dart2wasm/code_generator.dart';
import 'package:dart2wasm/translator.dart';

import 'package:kernel/ast.dart';
import 'package:kernel/core_types.dart';

import 'package:wasm_builder/wasm_builder.dart' as w;

/// Values for the `_kind` field in `_TopType`. Must match the definitions in
/// `_TopType`.
class TopTypeKind {
  static const int objectKind = 0;
  static const int dynamicKind = 1;
  static const int voidKind = 2;
}

class InterfaceTypeEnvironment {
  final Map<TypeParameter, int> _typeOffsets = {};

  void _add(InterfaceType type) {
    Class cls = type.classNode;
    int i = 0;
    for (TypeParameter typeParameter in cls.typeParameters) {
      _typeOffsets[typeParameter] = i++;
    }
  }

  int lookup(TypeParameter typeParameter) => _typeOffsets[typeParameter]!;
}

/// Helper class for building runtime types.
class Types {
  final Translator translator;

  /// Class info for `_Type`
  late final ClassInfo typeClassInfo =
      translator.classInfo[translator.typeClass]!;

  /// Wasm value type of `List<_Type>`
  late final w.ValueType typeListExpectedType =
      translator.classInfo[translator.listBaseClass]!.nonNullableType;

  /// Wasm array type of `WasmArray<_Type>`
  late final w.ArrayType typeArrayArrayType =
      translator.arrayTypeForDartType(typeType);

  /// Wasm value type of `WasmArray<_Type>`
  late final w.ValueType typeArrayExpectedType =
      w.RefType.def(typeArrayArrayType, nullable: false);

  /// Wasm value type of `WasmArray<_NamedParameter>`
  late final w.ValueType namedParametersExpectedType = classAndFieldToType(
      translator.functionTypeClass, FieldIndex.functionTypeNamedParameters);

  /// Wasm value type of `_RecordType.names` field.
  late final w.ValueType recordTypeNamesFieldExpectedType = classAndFieldToType(
      translator.recordTypeClass, FieldIndex.recordTypeNames);

  /// A mapping from concrete subclass `classID` to [Map]s of superclass
  /// `classID` and the necessary substitutions which must be performed to test
  /// for a valid subtyping relationship.
  late final Map<int, Map<int, List<DartType>>> typeRules = _buildTypeRules();

  /// We will build the [interfaceTypeEnvironment] when building the
  /// [typeRules].
  final InterfaceTypeEnvironment interfaceTypeEnvironment =
      InterfaceTypeEnvironment();

  /// Because we can't currently support [Map]s in our `TypeUniverse`, we have
  /// to decompose [typeRules] into two [Map]s based on [List]s.
  ///
  /// [typeRulesSupers] is a [List] where the index in the list is a subclasses'
  /// `classID` and the value at that index is a [List] of superclass
  /// `classID`s.
  late final List<List<int>> typeRulesSupers = _buildTypeRulesSupers();

  /// [typeRulesSubstitutions] is a [List] where the index in the list is a
  /// subclasses' `classID` and the value at that index is a [List] indexed by
  /// the index of the superclasses' `classID` in [typeRulesSuper] and the value
  /// at that index is a [List] of [DartType]s which must be substituted for the
  /// subtyping relationship to be valid.
  late final List<List<List<DartType>>> typeRulesSubstitutions =
      _buildTypeRulesSubstitutions();

  /// A list which maps class ID to the classes [String] name.
  late final List<String> typeNames = _buildTypeNames();

  /// Type parameter offset for function types, specifying the lower end of
  /// their index range for type parameter types.
  Map<FunctionType, int> functionTypeParameterOffset = Map.identity();

  /// Index value for function type parameter types, indexing into the type
  /// parameter index range of their corresponding function type.
  Map<StructuralParameter, int> functionTypeParameterIndex = Map.identity();

  Types(this.translator);

  w.ValueType classAndFieldToType(Class cls, int fieldIndex) =>
      translator.classInfo[cls]!.struct.fields[fieldIndex].type.unpacked;

  /// Wasm value type for non-nullable `_Type` values
  w.ValueType get nonNullableTypeType => typeClassInfo.nonNullableType;

  InterfaceType get namedParameterType =>
      InterfaceType(translator.namedParameterClass, Nullability.nonNullable);

  InterfaceType get typeType =>
      InterfaceType(translator.typeClass, Nullability.nonNullable);

  CoreTypes get coreTypes => translator.coreTypes;

  /// Builds a [Map<int, Map<int, List<DartType>>>] to store subtype
  /// information.  The first key is the class id of a subtype. This returns a
  /// map where each key is the class id of a transitively implemented super
  /// type and each value is a list of the necessary type substitutions required
  /// for the subtyping relationship to be valid.
  Map<int, Map<int, List<DartType>>> _buildTypeRules() {
    List<ClassInfo> classes = translator.classes;
    Map<int, Map<int, List<DartType>>> subtypeMap = {};
    for (ClassInfo classInfo in classes) {
      ClassInfo superclassInfo = classInfo;

      // We don't need type rules for any class without a superclass, or for
      // classes whose supertype is [Object]. The latter case will be handled
      // directly in the subtype checking algorithm.
      if (superclassInfo.cls == null ||
          superclassInfo.cls == coreTypes.objectClass) continue;
      Class superclass = superclassInfo.cls!;

      // TODO(joshualitt): This includes abstract types that can't be
      // instantiated, but might be needed for subtype checks. The majority of
      // abstract classes are probably unnecessary though. We should filter
      // these cases to reduce the size of the type rules.
      Iterable<Class> subclasses = translator.subtypes
          .getSubtypesOf(superclass)
          .where((cls) => cls != superclass);
      Iterable<InterfaceType> subtypes = subclasses.map(
          (Class cls) => cls.getThisType(coreTypes, Nullability.nonNullable));
      for (InterfaceType subtype in subtypes) {
        interfaceTypeEnvironment._add(subtype);
        List<DartType>? typeArguments = translator.hierarchy
            .getInterfaceTypeArgumentsAsInstanceOfClass(subtype, superclass)
            ?.map(normalize)
            .toList();
        ClassInfo subclassInfo = translator.classInfo[subtype.classNode]!;
        Map<int, List<DartType>> substitutionMap =
            subtypeMap[subclassInfo.classId] ??= {};
        substitutionMap[superclassInfo.classId] = typeArguments ?? const [];
      }
    }
    return subtypeMap;
  }

  List<List<int>> _buildTypeRulesSupers() {
    List<List<int>> typeRulesSupers = [];
    for (int classId = 0; classId < translator.classes.length; classId++) {
      List<int>? superclassIds = typeRules[classId]?.keys.toList();
      if (superclassIds == null) {
        typeRulesSupers.add(const []);
      } else {
        superclassIds.sort();
        typeRulesSupers.add(superclassIds);
      }
    }
    return typeRulesSupers;
  }

  List<List<List<DartType>>> _buildTypeRulesSubstitutions() {
    List<List<List<DartType>>> typeRulesSubstitutions = [];
    for (int classId = 0; classId < translator.classes.length; classId++) {
      List<int> supers = typeRulesSupers[classId];
      typeRulesSubstitutions.add(supers.isEmpty ? const [] : []);
      for (int j = 0; j < supers.length; j++) {
        int superId = supers[j];
        typeRulesSubstitutions.last.add(typeRules[classId]![superId]!);
      }
    }
    return typeRulesSubstitutions;
  }

  List<String> _buildTypeNames() {
    // This logic assumes `translator.classes` returns the classes indexed by
    // class ID. If we ever change that logic, we will need to change this code.
    List<String> typeNames = [];
    for (ClassInfo classInfo in translator.classes) {
      Class? cls = classInfo.cls;
      if (cls == null || cls.isAnonymousMixin) {
        typeNames.add("");
      } else {
        typeNames.add(cls.name);
      }
    }
    return typeNames;
  }

  /// Builds a map of subclasses to the transitive set of superclasses they
  /// implement.
  /// TODO(joshualitt): This implementation is just temporary. Eventually we
  /// should move to a data structure more closely resembling [typeRules].
  w.ValueType makeTypeRulesSupers(w.InstructionsBuilder b) {
    final wasmI32Type =
        InterfaceType(translator.wasmI32Class, Nullability.nonNullable);

    final supersOfClasses = <Constant>[];
    for (List<int> supers in typeRulesSupers) {
      supersOfClasses.add(translator.constants.makeArrayOf(
          wasmI32Type, [for (final cid in supers) IntConstant(cid)]));
    }

    final arrayOfWasmI32Type = InterfaceType(
        translator.wasmArrayClass, Nullability.nonNullable, [wasmI32Type]);
    final typeRuleSupers =
        translator.constants.makeArrayOf(arrayOfWasmI32Type, supersOfClasses);

    final arrayOfArrayOfWasmI32Type = InterfaceType(translator.wasmArrayClass,
        Nullability.nonNullable, [arrayOfWasmI32Type]);

    final typeRulesSupersType =
        translator.translateStorageType(arrayOfArrayOfWasmI32Type).unpacked;
    translator.constants
        .instantiateConstant(null, b, typeRuleSupers, typeRulesSupersType);
    return typeRulesSupersType;
  }

  /// Similar to the above, but provides the substitutions required for each
  /// supertype.
  /// TODO(joshualitt): Like [makeTypeRulesSupers], this is just temporary.
  w.ValueType makeTypeRulesSubstitutions(w.InstructionsBuilder b) {
    final typeType =
        InterfaceType(translator.typeClass, Nullability.nonNullable);
    final arrayOfType = InterfaceType(
        translator.wasmArrayClass, Nullability.nonNullable, [typeType]);
    final arrayOfArrayOfType = InterfaceType(
        translator.wasmArrayClass, Nullability.nonNullable, [arrayOfType]);
    final arrayOfArrayOfArrayOfType = InterfaceType(translator.wasmArrayClass,
        Nullability.nonNullable, [arrayOfArrayOfType]);

    final substitutionsConstantL0 = <Constant>[];
    for (List<List<DartType>> substitutionsL1 in typeRulesSubstitutions) {
      final substitutionsConstantL1 = <Constant>[];
      for (List<DartType> substitutionsL2 in substitutionsL1) {
        substitutionsConstantL1.add(translator.constants.makeArrayOf(typeType,
            [for (final t in substitutionsL2) TypeLiteralConstant(t)]));
      }
      substitutionsConstantL0.add(translator.constants
          .makeArrayOf(arrayOfType, substitutionsConstantL1));
    }

    final typeRulesSubstitutionsType =
        translator.translateStorageType(arrayOfArrayOfArrayOfType).unpacked;
    translator.constants.instantiateConstant(
        null,
        b,
        translator.constants
            .makeArrayOf(arrayOfArrayOfType, substitutionsConstantL0),
        typeRulesSubstitutionsType);
    return typeRulesSubstitutionsType;
  }

  /// Returns a list of string type names for pretty printing types.
  w.ValueType makeTypeNames(w.InstructionsBuilder b) {
    final stringType =
        translator.coreTypes.stringRawType(Nullability.nonNullable);
    final arrayOfStringType = InterfaceType(
        translator.wasmArrayClass, Nullability.nonNullable, [stringType]);

    final typeNamesType =
        translator.translateStorageType(arrayOfStringType).unpacked;
    if (translator.options.minify) {
      b.ref_null((typeNamesType as w.RefType).heapType);
    } else {
      final arrayOfStrings = translator.constants.makeArrayOf(
          stringType, [for (final name in typeNames) StringConstant(name)]);
      translator.constants
          .instantiateConstant(null, b, arrayOfStrings, typeNamesType);
    }
    return typeNamesType;
  }

  bool _isTypeConstant(DartType type) {
    return type is DynamicType ||
        type is VoidType ||
        type is NeverType ||
        type is NullType ||
        type is FutureOrType && _isTypeConstant(type.typeArgument) ||
        (type is FunctionType &&
            type.typeParameters.every((p) => _isTypeConstant(p.bound)) &&
            _isTypeConstant(type.returnType) &&
            type.positionalParameters.every(_isTypeConstant) &&
            type.namedParameters.every((n) => _isTypeConstant(n.type))) ||
        type is InterfaceType && type.typeArguments.every(_isTypeConstant) ||
        (type is RecordType &&
            type.positional.every(_isTypeConstant) &&
            type.named.every((n) => _isTypeConstant(n.type))) ||
        type is StructuralParameterType ||
        type is ExtensionType && _isTypeConstant(type.extensionTypeErasure);
  }

  Class classForType(DartType type) {
    if (type is DynamicType) {
      return translator.topTypeClass;
    } else if (type is VoidType) {
      return translator.topTypeClass;
    } else if (type is NeverType) {
      return translator.bottomTypeClass;
    } else if (type is NullType) {
      return translator.bottomTypeClass;
    } else if (type is FutureOrType) {
      return translator.futureOrTypeClass;
    } else if (type is InterfaceType) {
      if (type.classNode == coreTypes.objectClass) {
        return translator.topTypeClass;
      }
      if (type.classNode == coreTypes.functionClass) {
        return translator.abstractFunctionTypeClass;
      }
      if (type.classNode == coreTypes.recordClass) {
        return translator.abstractRecordTypeClass;
      }
      return translator.interfaceTypeClass;
    } else if (type is FunctionType) {
      return translator.functionTypeClass;
    } else if (type is TypeParameterType) {
      return translator.interfaceTypeParameterTypeClass;
    } else if (type is StructuralParameterType) {
      return translator.functionTypeParameterTypeClass;
    } else if (type is ExtensionType) {
      return classForType(type.extensionTypeErasure);
    } else if (type is RecordType) {
      return translator.recordTypeClass;
    }
    throw "Unexpected DartType: $type";
  }

  bool isSpecializedClass(Class cls) {
    return cls == coreTypes.objectClass ||
        cls == coreTypes.functionClass ||
        cls == coreTypes.recordClass;
  }

  int topTypeKind(DartType type) {
    return type is VoidType
        ? TopTypeKind.voidKind
        : type is DynamicType
            ? TopTypeKind.dynamicKind
            : TopTypeKind.objectKind;
  }

  /// Allocates a `WasmArray<_Type>` from [types] and pushes it to the
  /// stack.
  void _makeTypeArray(CodeGenerator codeGen, Iterable<DartType> types) {
    if (types.every(_isTypeConstant)) {
      translator.constants.instantiateConstant(codeGen.function, codeGen.b,
          translator.constants.makeTypeArray(types), typeArrayExpectedType);
    } else {
      for (DartType type in types) {
        makeType(codeGen, type);
      }
      codeGen.b.array_new_fixed(typeArrayArrayType, types.length);
    }
  }

  void _makeInterfaceType(CodeGenerator codeGen, InterfaceType type) {
    final b = codeGen.b;
    ClassInfo typeInfo = translator.classInfo[type.classNode]!;
    b.i32_const(encodedNullability(type));
    b.i64_const(typeInfo.classId);
    _makeTypeArray(codeGen, type.typeArguments);
  }

  void _makeRecordType(CodeGenerator codeGen, RecordType type) {
    codeGen.b.i32_const(encodedNullability(type));

    final names = translator.constants.makeArrayOf(
        translator.coreTypes.stringNonNullableRawType,
        type.named.map((t) => StringConstant(t.name)).toList());

    translator.constants.instantiateConstant(
        codeGen.function, codeGen.b, names, recordTypeNamesFieldExpectedType);
    _makeTypeArray(
        codeGen, type.positional.followedBy(type.named.map((t) => t.type)));
  }

  /// Normalizes a Dart type. Many rules are already applied for us, but we
  /// still have to manually turn `Never?` into `Null` and normalize `FutureOr`.
  DartType normalize(DartType type) {
    if (type is NeverType && type.declaredNullability == Nullability.nullable) {
      return const NullType();
    }

    if (type is! FutureOrType) return type;

    final s = normalize(type.typeArgument);

    // `coreTypes.isTop` and `coreTypes.isObject` take into account the
    // normalization rules of `FutureOr`.
    if (coreTypes.isTop(type) || coreTypes.isObject(type)) {
      return type.declaredNullability == Nullability.nullable
          ? s.withDeclaredNullability(Nullability.nullable)
          : s;
    } else if (s is NeverType) {
      return InterfaceType(coreTypes.futureClass, Nullability.nonNullable,
          const [const NeverType.nonNullable()]);
    } else if (s is NullType) {
      return InterfaceType(coreTypes.futureClass, Nullability.nullable,
          const [const NullType()]);
    }

    // The type is normalized, and remains a `FutureOr` so now we normalize its
    // nullability.
    // Note: We diverge from the spec here and normalize the type to nullable if
    // its type argument is nullable, since this simplifies subtype checking.
    // We compensate for this difference when converting the type to a string,
    // making the discrepancy invisible to the user.
    final declaredNullability = s.nullability == Nullability.nullable
        ? Nullability.nullable
        : type.declaredNullability;
    return FutureOrType(s, declaredNullability);
  }

  void _makeFutureOrType(CodeGenerator codeGen, FutureOrType type) {
    final b = codeGen.b;
    b.i32_const(encodedNullability(type));
    makeType(codeGen, type.typeArgument);
    codeGen.call(translator.createNormalizedFutureOrType.reference);
  }

  void _makeFunctionType(CodeGenerator codeGen, FunctionType type) {
    int typeParameterOffset = computeFunctionTypeParameterOffset(type);
    final b = codeGen.b;
    b.i32_const(encodedNullability(type));
    b.i64_const(typeParameterOffset);

    // WasmArray<_Type> typeParameterBounds
    _makeTypeArray(codeGen, type.typeParameters.map((p) => p.bound));

    // WasmArray<_Type> typeParameterDefaults
    _makeTypeArray(codeGen, type.typeParameters.map((p) => p.defaultType));

    // _Type returnType
    makeType(codeGen, type.returnType);

    // WasmArray<_Type> positionalParameters
    _makeTypeArray(codeGen, type.positionalParameters);

    // int requiredParameterCount
    b.i64_const(type.requiredParameterCount);

    // WasmArray<_NamedParameter> namedParameters
    if (type.namedParameters.every((n) => _isTypeConstant(n.type))) {
      translator.constants.instantiateConstant(
          codeGen.function,
          b,
          translator.constants.makeNamedParametersArray(type),
          namedParametersExpectedType);
    } else {
      Class namedParameterClass = translator.namedParameterClass;
      Constructor namedParameterConstructor =
          namedParameterClass.constructors.single;
      List<Expression> expressions = [];
      for (NamedType n in type.namedParameters) {
        expressions.add(_isTypeConstant(n.type)
            ? ConstantExpression(
                translator.constants.makeNamedParameterConstant(n),
                namedParameterType)
            : ConstructorInvocation(
                namedParameterConstructor,
                Arguments([
                  StringLiteral(n.name),
                  TypeLiteral(n.type),
                  BoolLiteral(n.isRequired)
                ])));
      }
      w.ValueType namedParametersListType =
          codeGen.makeArrayFromExpressions(expressions, namedParameterType);
      translator.convertType(codeGen.function, namedParametersListType,
          namedParametersExpectedType);
    }
  }

  /// Makes a `_Type` object on the stack.
  /// TODO(joshualitt): Refactor this logic to remove the dependency on
  /// CodeGenerator.
  w.ValueType makeType(CodeGenerator codeGen, DartType type) {
    // Always ensure type is normalized before making a type.
    type = normalize(type);
    final b = codeGen.b;
    if (_isTypeConstant(type)) {
      translator.constants.instantiateConstant(
          codeGen.function, b, TypeLiteralConstant(type), nonNullableTypeType);
      return nonNullableTypeType;
    }
    // All of the singleton types represented by canonical objects should be
    // created const.
    assert(type is TypeParameterType ||
        type is ExtensionType ||
        type is InterfaceType ||
        type is FutureOrType ||
        type is FunctionType ||
        type is RecordType);
    if (type is TypeParameterType) {
      codeGen.instantiateTypeParameter(type.parameter);
      if (type.declaredNullability == Nullability.nullable) {
        codeGen.call(translator.typeAsNullable.reference);
      }
      return nonNullableTypeType;
    }

    if (type is ExtensionType) {
      return makeType(codeGen, type.extensionTypeErasure);
    }

    ClassInfo info = translator.classInfo[classForType(type)]!;
    if (type is FutureOrType) {
      _makeFutureOrType(codeGen, type);
      return info.nonNullableType;
    }

    translator.functions.recordClassAllocation(info.classId);
    b.i32_const(info.classId);
    b.i32_const(initialIdentityHash);
    if (type is InterfaceType) {
      _makeInterfaceType(codeGen, type);
    } else if (type is FunctionType) {
      _makeFunctionType(codeGen, type);
    } else if (type is RecordType) {
      _makeRecordType(codeGen, type);
    } else {
      throw '`$type` should have already been handled.';
    }
    b.struct_new(info.struct);
    return info.nonNullableType;
  }

  /// Compute the lower end of the type parameter index range for this function
  /// type. This is computed such that it avoids overlap between the index range
  /// of this function type and the index ranges of all generic function types
  /// nested within it that contain references to the type parameters of this
  /// function type.
  ///
  /// This will also compute the index values for all of the function's type
  /// parameters, which can subsequently be queried using
  /// [getFunctionTypeParameterIndex].
  int computeFunctionTypeParameterOffset(FunctionType type) {
    if (type.typeParameters.isEmpty) return 0;
    int? offset = functionTypeParameterOffset[type];
    if (offset != null) return offset;
    _FunctionTypeParameterOffsetCollector(this).visitFunctionType(type);
    return functionTypeParameterOffset[type]!;
  }

  /// Get the index value for a function type parameter, indexing into the
  /// type parameter index range of its corresponding function type.
  int getFunctionTypeParameterIndex(StructuralParameter type) {
    assert(functionTypeParameterIndex.containsKey(type),
        "Type parameter offset has not been computed for function type");
    return functionTypeParameterIndex[type]!;
  }

  /// Emit code for testing a value against a Dart type. Expects the value on
  /// the stack as a (ref null #Top) and leaves the result on the stack as an
  /// i32.
  void emitTypeCheck(CodeGenerator codeGen, DartType type, DartType operandType,
      [TreeNode? node]) {
    final b = codeGen.b;
    b.comment("Type check against $type");
    w.Local? operandTemp;
    if (translator.options.verifyTypeChecks) {
      operandTemp = codeGen.addLocal(translator.topInfo.nullableType);
      b.local_tee(operandTemp);
    }
    if (!_emitOptimizedTypeCheck(codeGen, type, operandType)) {
      // General fallback path
      makeType(codeGen, type);
      codeGen.call(translator.isSubtype.reference);
    }
    if (translator.options.verifyTypeChecks) {
      b.local_get(operandTemp!);
      makeType(codeGen, type);
      if (node != null && node.location != null) {
        w.FunctionType verifyFunctionType = translator.functions
            .getFunctionType(translator.verifyOptimizedTypeCheck.reference);
        String location = node.location.toString();
        translator.constants.instantiateConstant(codeGen.function, b,
            StringConstant(location), verifyFunctionType.inputs.last);
      } else {
        b.ref_null(w.HeapType.none);
      }
      codeGen.call(translator.verifyOptimizedTypeCheck.reference);
    }
  }

  /// Emit optimized code for testing a value against a Dart type. If the type
  /// to be tested against is of a shape where we can generate more efficient
  /// code than the general fallback path, generate such code and return `true`.
  /// Otherwise, return `false` to indicate that the general path should be
  /// taken.
  bool _emitOptimizedTypeCheck(
      CodeGenerator codeGen, DartType type, DartType operandType) {
    if (type is! InterfaceType) return false;

    if (type.typeArguments.any((t) => t is! DynamicType)) {
      // Type has at least one type argument that is not `dynamic`.
      //
      // In cases like `x is List<T>` where `x : Iterable<T>` (tested-against
      // type is a subtype of the operand's static type and the types have same
      // number of type arguments), it is not necessary to test the type
      // arguments.
      Class cls = translator.classForType(operandType);
      InterfaceType? base = translator.hierarchy
          .getInterfaceTypeAsInstanceOfClass(type, cls,
              isNonNullableByDefault:
                  codeGen.member.enclosingLibrary.isNonNullableByDefault)
          ?.withDeclaredNullability(operandType.declaredNullability);

      final sameNumTypeParams = operandType is InterfaceType &&
          operandType.typeArguments.length == type.typeArguments.length;

      if (!(sameNumTypeParams && base == operandType)) {
        return false;
      }
    }

    final b = codeGen.b;
    bool isPotentiallyNullable = operandType.isPotentiallyNullable;
    w.Label? resultLabel;
    if (isPotentiallyNullable) {
      // Store operand in a temporary variable, since Binaryen does not support
      // block inputs.
      w.Local operand = codeGen.addLocal(translator.topInfo.nullableType);
      b.local_set(operand);
      resultLabel = b.block(const [], const [w.NumType.i32]);
      w.Label nullLabel = b.block(const [], const []);
      b.local_get(operand);
      b.br_on_null(nullLabel);
    }

    final interfaceClass = type.classNode;

    if (interfaceClass == coreTypes.objectClass) {
      b.drop();
      b.i32_const(1);
    } else if (interfaceClass == coreTypes.functionClass) {
      b.ref_test(translator.closureInfo.nonNullableType);
    } else {
      final ranges =
          translator.classIdNumbering.getConcreteClassIdRanges(interfaceClass);
      b.struct_get(translator.topInfo.struct, FieldIndex.classId);
      b.emitClassIdRangeCheck(codeGen, ranges);
    }

    if (isPotentiallyNullable) {
      b.br(resultLabel!);
      b.end(); // nullLabel
      b.i32_const(encodedNullability(type));
      b.end(); // resultLabel
    }

    return true;
  }

  int encodedNullability(DartType type) =>
      type.declaredNullability == Nullability.nullable ? 1 : 0;
}

/// For a function type F = `... Function<X0, ..., Xn-1>(...)` compute offset(F)
/// such that for any function type G = `... Function<Y0, ..., Ym-1>(...)`
/// nested inside F, if G contains a reference to any type parameters of F, then
/// offset(F) >= offset(G) + m.
///
/// Conceptually, the type parameters of F are indexed from offset(F) inclusive
/// to offset(F) + n exclusive.
///
/// Also assign to each type parameter Xi the index offset(F) + i such that it
/// indexes the correct type parameter in the conceptual type parameter index
/// range of F.
///
/// This ensures that for every reference to a type parameter, its corresponding
/// function type is the innermost function type enclosing it for which the
/// index falls within the type parameter index range of the function type.
class _FunctionTypeParameterOffsetCollector extends RecursiveVisitor {
  final Types types;

  final List<FunctionType> _functionStack = [];
  final List<Set<FunctionType>> _functionsContainingParameters = [];
  final Map<StructuralParameter, int> _functionForParameter = {};

  _FunctionTypeParameterOffsetCollector(this.types);

  @override
  void visitFunctionType(FunctionType node) {
    int slot = _functionStack.length;
    _functionStack.add(node);
    _functionsContainingParameters.add({});

    for (int i = 0; i < node.typeParameters.length; i++) {
      StructuralParameter parameter = node.typeParameters[i];
      _functionForParameter[parameter] = slot;
    }

    super.visitFunctionType(node);

    int offset = 0;
    for (FunctionType inner in _functionsContainingParameters.last) {
      offset = max(
          offset,
          types.functionTypeParameterOffset[inner]! +
              inner.typeParameters.length);
    }
    types.functionTypeParameterOffset[node] = offset;

    for (int i = 0; i < node.typeParameters.length; i++) {
      StructuralParameter parameter = node.typeParameters[i];
      types.functionTypeParameterIndex[parameter] = offset + i;
    }

    _functionsContainingParameters.removeLast();
    _functionStack.removeLast();
  }

  @override
  void visitStructuralParameterType(StructuralParameterType node) {
    int slot = _functionForParameter[node.parameter]!;
    for (int inner = slot + 1; inner < _functionStack.length; inner++) {
      _functionsContainingParameters[slot].add(_functionStack[inner]);
    }
  }
}