pkg/compiler/lib/src/js_backend/runtime_types.dart

// Copyright (c) 2013, 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.

library js_backend.runtime_types;

import '../common.dart';
import '../common/names.dart' show Identifiers;
import '../common_elements.dart'
    show
        CommonElements,
        ElementEnvironment,
        JCommonElements,
        JElementEnvironment;
import '../elements/entities.dart';
import '../elements/types.dart';
import '../js/js.dart' as jsAst;
import '../js/js.dart' show js;
import '../js_emitter/js_emitter.dart' show ModularEmitter;
import '../options.dart';
import '../universe/codegen_world_builder.dart';
import '../universe/feature.dart';
import '../world.dart';
import 'namer.dart';
import 'native_data.dart';
import 'runtime_types_codegen.dart';
import 'runtime_types_resolution.dart';

typedef jsAst.Expression OnVariableCallback(TypeVariableType variable);
typedef bool ShouldEncodeTypedefCallback(TypedefType variable);

/// Interface for the needed runtime type checks.
abstract class RuntimeTypesChecks {
  /// Returns the required runtime type checks.
  TypeChecks get requiredChecks;

  /// Return all classes needed for runtime type information.
  Iterable<ClassEntity> get requiredClasses;
}

class TrivialTypesChecks implements RuntimeTypesChecks {
  final TypeChecks _typeChecks;
  final Set<ClassEntity> _allClasses;

  TrivialTypesChecks(this._typeChecks)
      : _allClasses = _typeChecks.classes.toSet();

  @override
  TypeChecks get requiredChecks => _typeChecks;

  @override
  Iterable<ClassEntity> get requiredClasses => _allClasses;
}

/// Interface for computing the needed runtime type checks.
abstract class RuntimeTypesChecksBuilder {
  void registerTypeVariableBoundsSubtypeCheck(
      DartType typeArgument, DartType bound);

  /// Registers that a generic [instantiation] is used.
  void registerGenericInstantiation(GenericInstantiation instantiation);

  /// Computes the [RuntimeTypesChecks] for the data in this builder.
  RuntimeTypesChecks computeRequiredChecks(
      CodegenWorld codegenWorld, CompilerOptions options);

  bool get rtiChecksBuilderClosed;
}

class TrivialRuntimeTypesChecksBuilder implements RuntimeTypesChecksBuilder {
  final JClosedWorld _closedWorld;
  final TrivialRuntimeTypesSubstitutions _substitutions;
  @override
  bool rtiChecksBuilderClosed = false;

  TrivialRuntimeTypesChecksBuilder(this._closedWorld, this._substitutions);

  ElementEnvironment get _elementEnvironment => _closedWorld.elementEnvironment;

  @override
  void registerTypeVariableBoundsSubtypeCheck(
      DartType typeArgument, DartType bound) {}

  @override
  void registerGenericInstantiation(GenericInstantiation instantiation) {}

  @override
  RuntimeTypesChecks computeRequiredChecks(
      CodegenWorld codegenWorld, CompilerOptions options) {
    rtiChecksBuilderClosed = true;

    Map<ClassEntity, ClassUse> classUseMap = <ClassEntity, ClassUse>{};
    for (ClassEntity cls in _closedWorld.classHierarchy
        .getClassSet(_closedWorld.commonElements.objectClass)
        .subtypes()) {
      ClassUse classUse = new ClassUse()
        ..directInstance = true
        ..checkedInstance = true
        ..typeArgument = true
        ..checkedTypeArgument = true
        ..typeLiteral = true
        ..functionType = _computeFunctionType(_elementEnvironment, cls);
      classUseMap[cls] = classUse;
    }
    TypeChecks typeChecks = _substitutions._requiredChecks =
        _substitutions._computeChecks(classUseMap);
    return new TrivialTypesChecks(typeChecks);
  }

  Set<ClassEntity> computeCheckedClasses(
      CodegenWorldBuilder codegenWorldBuilder, Set<DartType> implicitIsChecks) {
    return _closedWorld.classHierarchy
        .getClassSet(_closedWorld.commonElements.objectClass)
        .subtypes()
        .toSet();
  }

  Set<FunctionType> computeCheckedFunctions(
      CodegenWorldBuilder codegenWorldBuilder, Set<DartType> implicitIsChecks) {
    return new Set<FunctionType>();
  }
}

abstract class RuntimeTypesSubstitutionsMixin
    implements RuntimeTypesSubstitutions {
  JClosedWorld get _closedWorld;
  TypeChecks get _requiredChecks;

  JElementEnvironment get _elementEnvironment =>
      _closedWorld.elementEnvironment;
  DartTypes get _types => _closedWorld.dartTypes;
  RuntimeTypesNeed get _rtiNeed => _closedWorld.rtiNeed;

  /// Compute the required type checks and substitutions for the given
  /// instantiated and checked classes.
  TypeChecks _computeChecks(Map<ClassEntity, ClassUse> classUseMap) {
    // Run through the combination of instantiated and checked
    // arguments and record all combination where the element of a checked
    // argument is a superclass of the element of an instantiated type.
    TypeCheckMapping result = new TypeCheckMapping();
    Set<ClassEntity> handled = new Set<ClassEntity>();

    // Empty usage object for classes with no direct rti usage.
    final ClassUse emptyUse = new ClassUse();

    /// Compute the $isX and $asX functions need for [cls].
    ClassChecks computeChecks(ClassEntity cls) {
      if (!handled.add(cls)) return result[cls];

      ClassUse classUse = classUseMap[cls] ?? emptyUse;
      ClassChecks checks = new ClassChecks(classUse.functionType);
      result[cls] = checks;

      // Find the superclass from which [cls] inherits checks.
      ClassEntity superClass = _elementEnvironment.getSuperClass(cls,
          skipUnnamedMixinApplications: true);
      ClassChecks superChecks;
      bool extendsSuperClassTrivially = false;
      if (superClass != null) {
        // Compute the checks inherited from [superClass].
        superChecks = computeChecks(superClass);

        // Does [cls] extend [superClass] trivially?
        //
        // For instance:
        //
        //     class A<T> {}
        //     class B<S> extends A<S> {}
        //     class C<U, V> extends A<U> {}
        //     class D extends A<int> {}
        //
        // here `B` extends `A` trivially, but `C` and `D` don't.
        extendsSuperClassTrivially = isTrivialSubstitution(cls, superClass);
      }

      bool isNativeClass = _closedWorld.nativeData.isNativeClass(cls);
      if (classUse.typeArgument ||
          classUse.typeLiteral ||
          (isNativeClass && classUse.checkedInstance)) {
        Substitution substitution = computeSubstitution(cls, cls);
        // We need [cls] at runtime - even if [cls] is not instantiated. Either
        // as a type argument, for a type literal or for an is-test if [cls] is
        // native.
        checks.add(new TypeCheck(cls, substitution, needsIs: isNativeClass));
      }

      // Compute the set of classes that [cls] inherited properties from.
      //
      // This set reflects the emitted class hierarchy and therefore uses
      // `getEffectiveMixinClass` to find the inherited mixins.
      Set<ClassEntity> inheritedClasses = new Set<ClassEntity>();
      ClassEntity other = cls;
      while (other != null) {
        inheritedClasses.add(other);
        if (classUse.instance &&
            _elementEnvironment.isMixinApplication(other)) {
          // We don't mixin [other] if [cls] isn't instantiated, directly or
          // indirectly.
          inheritedClasses
              .add(_elementEnvironment.getEffectiveMixinClass(other));
        }
        other = _elementEnvironment.getSuperClass(other);
      }

      /// Compute the needed check for [cls] against the class of the super
      /// [type].
      void processSupertype(InterfaceType type) {
        ClassEntity checkedClass = type.element;
        ClassUse checkedClassUse = classUseMap[checkedClass] ?? emptyUse;

        // Where [cls] inherits properties for [checkedClass].
        bool inheritsFromCheckedClass = inheritedClasses.contains(checkedClass);

        // If [cls] inherits properties from [checkedClass] and [checkedClass]
        // needs type arguments, [cls] must provide a substitution for
        // [checkedClass].
        //
        // For instance:
        //
        //     class M<T> {
        //        m() => T;
        //     }
        //     class S {}
        //     class C extends S with M<int> {}
        //
        // Here `C` needs an `$asM` substitution function to provide the value
        // of `T` in `M.m`.
        bool needsTypeArgumentsForCheckedClass = inheritsFromCheckedClass &&
            _rtiNeed.classNeedsTypeArguments(checkedClass);

        // Whether [checkedClass] is used in an instance test or type argument
        // test.
        //
        // For instance:
        //
        //    class A {}
        //    class B {}
        //    test(o) => o is A || o is List<B>;
        //
        // Here `A` is used in an instance test and `B` is used in a type
        // argument test.
        bool isChecked = checkedClassUse.checkedTypeArgument ||
            checkedClassUse.checkedInstance;

        if (isChecked || needsTypeArgumentsForCheckedClass) {
          // We need an $isX and/or $asX property on [cls] for [checkedClass].

          // Whether `cls` implements `checkedClass` trivially.
          //
          // For instance:
          //
          //     class A<T> {}
          //     class B<S> implements A<S> {}
          //     class C<U, V> implements A<U> {}
          //     class D implements A<int> {}
          //
          // here `B` implements `A` trivially, but `C` and `D` don't.
          bool implementsCheckedTrivially =
              isTrivialSubstitution(cls, checkedClass);

          // Whether [checkedClass] is generic.
          //
          // Currently [isTrivialSubstitution] reports that [cls] implements
          // [checkedClass] trivially if [checkedClass] is not generic. In this
          // case the substitution is not only trivial it is also not needed.
          bool isCheckedGeneric =
              _elementEnvironment.isGenericClass(checkedClass);

          // The checks for [checkedClass] inherited for [superClass].
          TypeCheck checkFromSuperClass =
              superChecks != null ? superChecks[checkedClass] : null;

          // Whether [cls] need an explicit $isX property for [checkedClass].
          //
          // If [cls] inherits from [checkedClass] it also inherits the $isX
          // property automatically generated on [checkedClass].
          bool needsIs = !inheritsFromCheckedClass && isChecked;

          if (checkFromSuperClass != null) {
            // The superclass has a substitution function for [checkedClass].
            // Check if we can reuse this it of need to override it.
            //
            // The inherited $isX property does _not_ need to be overriding.
            if (extendsSuperClassTrivially) {
              // [cls] implements [checkedClass] the same way as [superClass]
              // so the inherited substitution function already works.
              checks.add(new TypeCheck(checkedClass, null, needsIs: false));
            } else {
              // [cls] implements [checkedClass] differently from [superClass]
              // so the inherited substitution function needs to be replaced.
              if (implementsCheckedTrivially) {
                // We need an explicit trivial substitution function for
                // [checkedClass] that overrides the inherited function.
                checks.add(new TypeCheck(checkedClass,
                    isCheckedGeneric ? const Substitution.trivial() : null,
                    needsIs: false));
              } else {
                // We need a non-trivial substitution function for
                // [checkedClass].
                checks.add(new TypeCheck(
                    checkedClass, computeSubstitution(cls, checkedClass),
                    needsIs: false));
              }
            }
          } else {
            // The superclass has no substitution function for [checkedClass].
            if (implementsCheckedTrivially) {
              // We don't add an explicit substitution function for
              // [checkedClass] because the substitution is trivial and doesn't
              // need to override an inherited function.
              checks.add(new TypeCheck(checkedClass, null, needsIs: needsIs));
            } else {
              // We need a non-trivial substitution function for
              // [checkedClass].
              checks.add(new TypeCheck(
                  checkedClass, computeSubstitution(cls, checkedClass),
                  needsIs: needsIs));
            }
          }
        }
      }

      for (InterfaceType type in _types.getSupertypes(cls)) {
        processSupertype(type);
      }
      FunctionType callType = _types.getCallType(_types.getThisType(cls));
      if (callType != null) {
        processSupertype(_closedWorld.commonElements.functionType);
      }
      return checks;
    }

    for (ClassEntity cls in classUseMap.keys) {
      ClassUse classUse = classUseMap[cls] ?? emptyUse;
      if (classUse.directInstance ||
          classUse.typeArgument ||
          classUse.typeLiteral) {
        // Add checks only for classes that are live either as instantiated
        // classes or type arguments passed at runtime.
        computeChecks(cls);
      }
    }

    return result;
  }

  @override
  Set<ClassEntity> getClassesUsedInSubstitutions(TypeChecks checks) {
    Set<ClassEntity> instantiated = new Set<ClassEntity>();
    ArgumentCollector collector = new ArgumentCollector();
    for (ClassEntity target in checks.classes) {
      ClassChecks classChecks = checks[target];
      for (TypeCheck check in classChecks.checks) {
        Substitution substitution = check.substitution;
        if (substitution != null) {
          collector.collectAll(substitution.arguments);
        }
      }
    }
    return instantiated..addAll(collector.classes);

    // TODO(sra): This computation misses substitutions for reading type
    // parameters.
  }

  // TODO(karlklose): maybe precompute this value and store it in typeChecks?
  @override
  bool isTrivialSubstitution(ClassEntity cls, ClassEntity check) {
    if (cls.isClosure) {
      // TODO(karlklose): handle closures.
      return true;
    }

    // If there are no type variables, we do not need a substitution.
    if (!_elementEnvironment.isGenericClass(check)) {
      return true;
    }

    // JS-interop classes need an explicit substitution to mark the type
    // arguments as `any` type.
    if (_closedWorld.nativeData.isJsInteropClass(cls)) {
      return false;
    }

    // If the type is the same, we do not need a substitution.
    if (cls == check) {
      return true;
    }

    InterfaceType originalType = _elementEnvironment.getThisType(cls);
    InterfaceType type = _types.asInstanceOf(originalType, check);
    // [type] is not a subtype of [check]. we do not generate a check and do not
    // need a substitution.
    if (type == null) return true;

    // Run through both lists of type variables and check if the type variables
    // are identical at each position. If they are not, we need to calculate a
    // substitution function.
    List<DartType> variables = originalType.typeArguments;
    List<DartType> arguments = type.typeArguments;
    if (variables.length != arguments.length) {
      return false;
    }
    for (int index = 0; index < variables.length; index++) {
      if (variables[index] != arguments[index]) {
        return false;
      }
    }
    return true;
  }

  @override
  Substitution getSubstitution(ClassEntity cls, ClassEntity other) {
    // Look for a precomputed check.
    for (TypeCheck check in _requiredChecks[cls].checks) {
      if (check.cls == other) {
        return check.substitution;
      }
    }
    // There is no precomputed check for this pair (because the check is not
    // done on type arguments only.  Compute a new substitution.
    return computeSubstitution(cls, other);
  }

  Substitution computeSubstitution(ClassEntity cls, ClassEntity check,
      {bool alwaysGenerateFunction: false}) {
    if (isTrivialSubstitution(cls, check)) return null;

    // Unnamed mixin application classes do not need substitutions, because they
    // are never instantiated and their checks are overwritten by the class that
    // they are mixed into.
    InterfaceType type = _elementEnvironment.getThisType(cls);
    InterfaceType target = _types.asInstanceOf(type, check);
    List<DartType> typeVariables = type.typeArguments;
    if (_closedWorld.nativeData.isJsInteropClass(cls)) {
      int typeArguments = target.typeArguments.length;
      // Generic JS-interop class need an explicit substitution to mark
      // the type arguments as `any` type.
      return new Substitution.jsInterop(typeArguments);
    } else if (typeVariables.isEmpty && !alwaysGenerateFunction) {
      return new Substitution.list(target.typeArguments);
    } else {
      return new Substitution.function(target.typeArguments, typeVariables);
    }
  }
}

class TrivialRuntimeTypesSubstitutions extends RuntimeTypesSubstitutionsMixin {
  @override
  final JClosedWorld _closedWorld;
  @override
  TypeChecks _requiredChecks;

  TrivialRuntimeTypesSubstitutions(this._closedWorld);
}

abstract class RuntimeTypesEncoder {
  jsAst.Expression getSignatureEncoding(ModularNamer namer,
      ModularEmitter emitter, DartType type, jsAst.Expression this_);

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a function type.
  jsAst.Template get templateForIsFunctionType;

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a FutureOr type.
  jsAst.Template get templateForIsFutureOrType;

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is the void type.
  jsAst.Template get templateForIsVoidType;

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is the dynamic type.
  jsAst.Template get templateForIsDynamicType;

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a type argument of js-interop class.
  jsAst.Template get templateForIsJsInteropTypeArgument;

  /// Returns a [jsAst.Expression] representing the given [type]. Type variables
  /// are replaced by the [jsAst.Expression] returned by [onVariable].
  jsAst.Expression getTypeRepresentation(
      ModularEmitter emitter, DartType type, OnVariableCallback onVariable,
      [ShouldEncodeTypedefCallback shouldEncodeTypedef]);

  jsAst.Expression getJsInteropTypeArguments(int count);

  /// Fixed strings used for runtime types encoding.
  RuntimeTypeTags get rtiTags;
}

class _RuntimeTypesChecks implements RuntimeTypesChecks {
  final RuntimeTypesSubstitutions _substitutions;
  @override
  final TypeChecks requiredChecks;
  final Iterable<ClassEntity> _typeLiterals;
  final Iterable<ClassEntity> _typeArguments;

  _RuntimeTypesChecks(this._substitutions, this.requiredChecks,
      this._typeLiterals, this._typeArguments);

  @override
  Iterable<ClassEntity> get requiredClasses {
    Set<ClassEntity> required = new Set<ClassEntity>();
    required.addAll(_typeArguments);
    required.addAll(_typeLiterals);
    required
        .addAll(_substitutions.getClassesUsedInSubstitutions(requiredChecks));
    return required;
  }
}

class RuntimeTypesImpl
    with RuntimeTypesSubstitutionsMixin
    implements RuntimeTypesChecksBuilder {
  @override
  final JClosedWorld _closedWorld;

  // The set of type arguments tested against type variable bounds.
  final Set<DartType> checkedTypeArguments = new Set<DartType>();
  // The set of tested type variable bounds.
  final Set<DartType> checkedBounds = new Set<DartType>();

  TypeChecks cachedRequiredChecks;

  @override
  bool rtiChecksBuilderClosed = false;

  RuntimeTypesImpl(this._closedWorld);

  JCommonElements get _commonElements => _closedWorld.commonElements;
  @override
  JElementEnvironment get _elementEnvironment =>
      _closedWorld.elementEnvironment;
  @override
  RuntimeTypesNeed get _rtiNeed => _closedWorld.rtiNeed;

  @override
  TypeChecks get _requiredChecks => cachedRequiredChecks;

  Map<ClassEntity, ClassUse> classUseMapForTesting;

  final Set<GenericInstantiation> _genericInstantiations =
      new Set<GenericInstantiation>();

  @override
  void registerTypeVariableBoundsSubtypeCheck(
      DartType typeArgument, DartType bound) {
    checkedTypeArguments.add(typeArgument);
    checkedBounds.add(bound);
  }

  @override
  void registerGenericInstantiation(GenericInstantiation instantiation) {
    _genericInstantiations.add(instantiation);
  }

  @override
  RuntimeTypesChecks computeRequiredChecks(
      CodegenWorld codegenWorld, CompilerOptions options) {
    TypeVariableTests typeVariableTests = new TypeVariableTests(
        _elementEnvironment,
        _commonElements,
        _types,
        codegenWorld,
        _genericInstantiations,
        forRtiNeeds: false);
    Set<DartType> explicitIsChecks = typeVariableTests.explicitIsChecks;
    Set<DartType> implicitIsChecks = typeVariableTests.implicitIsChecks;

    Map<ClassEntity, ClassUse> classUseMap = <ClassEntity, ClassUse>{};
    if (retainDataForTesting) {
      classUseMapForTesting = classUseMap;
    }

    Set<FunctionType> checkedFunctionTypes = new Set<FunctionType>();
    Set<ClassEntity> typeLiterals = new Set<ClassEntity>();
    Set<ClassEntity> typeArguments = new Set<ClassEntity>();

    // The [liveTypeVisitor] is used to register class use in the type of
    // instantiated objects like `new T` and the function types of
    // tear offs and closures.
    //
    // A type found in a covariant position of such types is considered live
    // whereas a type found in a contravariant position of such types is
    // considered tested.
    //
    // For instance
    //
    //    new A<B Function(C)>();
    //
    // makes A and B live but C tested.
    TypeVisitor liveTypeVisitor =
        new TypeVisitor(onClass: (ClassEntity cls, {TypeVisitorState state}) {
      ClassUse classUse = classUseMap.putIfAbsent(cls, () => new ClassUse());
      switch (state) {
        case TypeVisitorState.covariantTypeArgument:
          classUse.typeArgument = true;
          typeArguments.add(cls);
          break;
        case TypeVisitorState.contravariantTypeArgument:
          classUse.typeArgument = true;
          classUse.checkedTypeArgument = true;
          typeArguments.add(cls);
          break;
        case TypeVisitorState.typeLiteral:
          classUse.typeLiteral = true;
          typeLiterals.add(cls);
          break;
        case TypeVisitorState.direct:
          break;
      }
    });

    // The [testedTypeVisitor] is used to register class use in type tests like
    // `o is T` and `o as T` (both implicit and explicit).
    //
    // A type found in a covariant position of such types is considered tested
    // whereas a type found in a contravariant position of such types is
    // considered live.
    //
    // For instance
    //
    //    o is A<B Function(C)>;
    //
    // makes A and B tested but C live.
    TypeVisitor testedTypeVisitor =
        new TypeVisitor(onClass: (ClassEntity cls, {TypeVisitorState state}) {
      ClassUse classUse = classUseMap.putIfAbsent(cls, () => new ClassUse());
      switch (state) {
        case TypeVisitorState.covariantTypeArgument:
          classUse.typeArgument = true;
          classUse.checkedTypeArgument = true;
          typeArguments.add(cls);
          break;
        case TypeVisitorState.contravariantTypeArgument:
          classUse.typeArgument = true;
          typeArguments.add(cls);
          break;
        case TypeVisitorState.typeLiteral:
          break;
        case TypeVisitorState.direct:
          classUse.checkedInstance = true;
          break;
      }
    });

    codegenWorld.instantiatedClasses.forEach((ClassEntity cls) {
      ClassUse classUse = classUseMap.putIfAbsent(cls, () => new ClassUse());
      classUse.instance = true;
    });

    Set<ClassEntity> visitedSuperClasses = {};
    codegenWorld.instantiatedTypes.forEach((InterfaceType type) {
      liveTypeVisitor.visitType(type, TypeVisitorState.direct);
      ClassUse classUse =
          classUseMap.putIfAbsent(type.element, () => new ClassUse());
      classUse.directInstance = true;
      FunctionType callType = _types.getCallType(type);
      if (callType != null) {
        liveTypeVisitor.visitType(callType, TypeVisitorState.direct);
      }

      // Superclass might make classes live as type arguments. For instance
      //
      //    class A {}
      //    class B<T> {}
      //    class C implements B<A> {}
      //    main() => new C();
      //
      // Here `A` is live as a type argument through the liveness of `C`.
      for (InterfaceType supertype
          in _closedWorld.dartTypes.getSupertypes(type.element)) {
        if (supertype.typeArguments.isEmpty &&
            visitedSuperClasses.contains(supertype.element)) {
          // If [superclass] is not generic then a second visit cannot add more
          // information that the first. In the example above, visiting `C`
          // twice can only result in a second registration of `A` as live
          // type argument.
          break;
        }
        visitedSuperClasses.add(supertype.element);
        liveTypeVisitor.visitType(supertype, TypeVisitorState.direct);
      }
    });

    for (FunctionEntity element in codegenWorld.closurizedStatics) {
      FunctionType functionType = _elementEnvironment.getFunctionType(element);
      liveTypeVisitor.visitType(functionType, TypeVisitorState.direct);
    }

    for (FunctionEntity element in codegenWorld.closurizedMembers) {
      FunctionType functionType = _elementEnvironment.getFunctionType(element);
      liveTypeVisitor.visitType(functionType, TypeVisitorState.direct);
    }

    void processMethodTypeArguments(_, Set<DartType> typeArguments) {
      for (DartType typeArgument in typeArguments) {
        liveTypeVisitor.visit(
            typeArgument, TypeVisitorState.covariantTypeArgument);
      }
    }

    codegenWorld.forEachStaticTypeArgument(processMethodTypeArguments);
    codegenWorld.forEachDynamicTypeArgument(processMethodTypeArguments);
    codegenWorld.liveTypeArguments.forEach((DartType type) {
      liveTypeVisitor.visitType(type, TypeVisitorState.covariantTypeArgument);
    });
    codegenWorld.constTypeLiterals.forEach((DartType type) {
      liveTypeVisitor.visitType(type, TypeVisitorState.typeLiteral);
    });

    bool isFunctionChecked = false;

    void processCheckedType(DartType t) {
      if (t is FunctionType) {
        checkedFunctionTypes.add(t);
      } else if (t is InterfaceType) {
        isFunctionChecked =
            isFunctionChecked || t.element == _commonElements.functionClass;
      }
      testedTypeVisitor.visitType(t, TypeVisitorState.direct);
    }

    explicitIsChecks.forEach(processCheckedType);
    implicitIsChecks.forEach(processCheckedType);

    // A closure class implements the function type of its `call`
    // method and needs a signature function for testing its function type
    // against typedefs and function types that are used in is-checks. Since
    // closures have a signature method iff they need it and should have a
    // function type iff they have a signature, we process all classes.
    void processClass(ClassEntity cls) {
      ClassFunctionType functionType =
          _computeFunctionType(_elementEnvironment, cls);
      if (functionType != null) {
        ClassUse classUse = classUseMap.putIfAbsent(cls, () => new ClassUse());
        classUse.functionType = functionType;
      }
    }

    // Collect classes that are 'live' either through instantiation or use in
    // type arguments.
    List<ClassEntity> liveClasses = <ClassEntity>[];
    classUseMap.forEach((ClassEntity cls, ClassUse classUse) {
      if (classUse.isLive) {
        liveClasses.add(cls);
      }
    });
    liveClasses.forEach(processClass);

    codegenWorld.forEachGenericMethod((FunctionEntity method) {
      if (_closedWorld.annotationsData
          .getParameterCheckPolicy(method)
          .isEmitted) {
        if (_rtiNeed.methodNeedsTypeArguments(method)) {
          for (TypeVariableType typeVariable
              in _elementEnvironment.getFunctionTypeVariables(method)) {
            DartType bound =
                _elementEnvironment.getTypeVariableBound(typeVariable.element);
            processCheckedType(bound);
            liveTypeVisitor.visit(
                bound, TypeVisitorState.covariantTypeArgument);
          }
        }
      }
    });

    cachedRequiredChecks = _computeChecks(classUseMap);
    rtiChecksBuilderClosed = true;
    return new _RuntimeTypesChecks(
        this, cachedRequiredChecks, typeArguments, typeLiterals);
  }
}

/// Computes the function type of [cls], if any.
///
/// In Dart 1, any class with a `call` method has a function type, in Dart 2
/// only closure classes have a function type.
ClassFunctionType _computeFunctionType(
    ElementEnvironment elementEnvironment, ClassEntity cls) {
  FunctionEntity signatureFunction;
  if (cls.isClosure) {
    // Use signature function if available.
    signatureFunction =
        elementEnvironment.lookupLocalClassMember(cls, Identifiers.signature);
    if (signatureFunction == null) {
      // In Dart 2, a closure only needs its function type if it has a
      // signature function.
      return null;
    }
  } else {
    // Only closures have function type in Dart 2.
    return null;
  }
  MemberEntity call =
      elementEnvironment.lookupLocalClassMember(cls, Identifiers.call);
  if (call != null && call.isFunction) {
    FunctionEntity callFunction = call;
    FunctionType callType = elementEnvironment.getFunctionType(callFunction);
    return new ClassFunctionType(callFunction, callType, signatureFunction);
  }
  return null;
}

class RuntimeTypesEncoderImpl implements RuntimeTypesEncoder {
  final ElementEnvironment _elementEnvironment;
  final CommonElements commonElements;
  final TypeRepresentationGenerator _representationGenerator;
  final RuntimeTypesNeed _rtiNeed;
  @override
  final RuntimeTypeTags rtiTags;

  RuntimeTypesEncoderImpl(this.rtiTags, NativeBasicData nativeData,
      this._elementEnvironment, this.commonElements, this._rtiNeed)
      : _representationGenerator =
            new TypeRepresentationGenerator(rtiTags, nativeData);

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a function type.
  @override
  jsAst.Template get templateForIsFunctionType {
    return _representationGenerator.templateForIsFunctionType;
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a FutureOr type.
  @override
  jsAst.Template get templateForIsFutureOrType {
    return _representationGenerator.templateForIsFutureOrType;
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is the void type.
  @override
  jsAst.Template get templateForIsVoidType {
    return _representationGenerator.templateForIsVoidType;
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is the dynamic type.
  @override
  jsAst.Template get templateForIsDynamicType {
    return _representationGenerator.templateForIsDynamicType;
  }

  @override
  jsAst.Template get templateForIsJsInteropTypeArgument {
    return _representationGenerator.templateForIsJsInteropTypeArgument;
  }

  @override
  jsAst.Expression getTypeRepresentation(
      ModularEmitter emitter, DartType type, OnVariableCallback onVariable,
      [ShouldEncodeTypedefCallback shouldEncodeTypedef]) {
    return _representationGenerator.getTypeRepresentation(
        emitter, type, onVariable, shouldEncodeTypedef);
  }

  String getTypeVariableName(TypeVariableType type) {
    String name = type.element.name;
    return name.replaceAll('#', '_');
  }

  jsAst.Expression getTypeEncoding(ModularEmitter emitter, DartType type,
      {bool alwaysGenerateFunction: false}) {
    ClassEntity contextClass = DartTypes.getClassContext(type);
    jsAst.Expression onVariable(TypeVariableType v) {
      return new jsAst.VariableUse(getTypeVariableName(v));
    }

    jsAst.Expression encoding =
        getTypeRepresentation(emitter, type, onVariable);
    if (contextClass == null && !alwaysGenerateFunction) {
      return encoding;
    } else {
      List<String> parameters = const <String>[];
      if (contextClass != null) {
        parameters = _elementEnvironment
            .getThisType(contextClass)
            .typeArguments
            .map((DartType _type) {
          TypeVariableType type = _type;
          return getTypeVariableName(type);
        }).toList();
      }
      return js('function(#) { return # }', [parameters, encoding]);
    }
  }

  @override
  jsAst.Expression getSignatureEncoding(ModularNamer namer,
      ModularEmitter emitter, DartType type, jsAst.Expression this_) {
    ClassEntity contextClass = DartTypes.getClassContext(type);
    jsAst.Expression encoding =
        getTypeEncoding(emitter, type, alwaysGenerateFunction: true);
    if (contextClass != null &&
        // We only generate folding using 'computeSignature' if [contextClass]
        // has reified type arguments. The 'computeSignature' function might not
        // be emitted (if it's not needed elsewhere) and the generated signature
        // will have `undefined` as its type variables in any case.
        //
        // This is needed specifically for --lax-runtime-type-to-string which
        // may require a signature on a method that uses class type variables
        // while at the same time not needing type arguments on the class.
        _rtiNeed.classNeedsTypeArguments(contextClass)) {
      jsAst.Name contextName = namer.className(contextClass);
      return js('function () { return #(#, #, #); }', [
        emitter.staticFunctionAccess(commonElements.computeSignature),
        encoding,
        this_,
        js.quoteName(contextName)
      ]);
    } else {
      return encoding;
    }
  }

  @override
  jsAst.Expression getJsInteropTypeArguments(int count) {
    return _representationGenerator.getJsInteropTypeArguments(count);
  }
}

/// Fixed strings used for runtime types encoding.
// TODO(johnniwinther): Use different names for minified code?
class RuntimeTypeTags {
  const RuntimeTypeTags();

  String get typedefTag => r'typedef';

  String get functionTypeTag => r'func';

  String get functionTypeVoidReturnTag => r'v';

  String get functionTypeReturnTypeTag => r'ret';

  String get functionTypeRequiredParametersTag => r'args';

  String get functionTypeOptionalParametersTag => r'opt';

  String get functionTypeNamedParametersTag => r'named';

  String get functionTypeGenericBoundsTag => r'bounds';

  String get futureOrTag => r'futureOr';

  String get futureOrTypeTag => r'type';
}

class TypeRepresentationGenerator
    implements DartTypeVisitor<jsAst.Expression, ModularEmitter> {
  final RuntimeTypeTags _rtiTags;
  final NativeBasicData _nativeData;

  OnVariableCallback onVariable;
  ShouldEncodeTypedefCallback shouldEncodeTypedef;
  Map<TypeVariableType, jsAst.Expression> typedefBindings;
  List<FunctionTypeVariable> functionTypeVariables = <FunctionTypeVariable>[];

  TypeRepresentationGenerator(this._rtiTags, this._nativeData);

  /// Creates a type representation for [type]. [onVariable] is called to
  /// provide the type representation for type variables.
  jsAst.Expression getTypeRepresentation(
      ModularEmitter emitter,
      DartType type,
      OnVariableCallback onVariable,
      ShouldEncodeTypedefCallback encodeTypedef) {
    assert(typedefBindings == null);
    this.onVariable = onVariable;
    this.shouldEncodeTypedef =
        (encodeTypedef != null) ? encodeTypedef : (TypedefType type) => false;
    jsAst.Expression representation = visit(type, emitter);
    this.onVariable = null;
    this.shouldEncodeTypedef = null;
    assert(functionTypeVariables.isEmpty);
    return representation;
  }

  jsAst.Expression getJavaScriptClassName(
      Entity element, ModularEmitter emitter) {
    return emitter.typeAccess(element);
  }

  jsAst.Expression getDynamicValue() => js('null');

  jsAst.Expression getVoidValue() => js('-1');

  jsAst.Expression getJsInteropTypeArgumentValue() => js('-2');

  @override
  jsAst.Expression visit(DartType type, ModularEmitter emitter) =>
      type.accept(this, emitter);

  @override
  jsAst.Expression visitTypeVariableType(
      TypeVariableType type, ModularEmitter emitter) {
    if (typedefBindings != null) {
      assert(typedefBindings[type] != null);
      return typedefBindings[type];
    }
    return onVariable(type);
  }

  @override
  jsAst.Expression visitFunctionTypeVariable(
      FunctionTypeVariable type, ModularEmitter emitter) {
    int position = functionTypeVariables.indexOf(type);
    assert(position >= 0);
    return js.number(functionTypeVariables.length - position - 1);
  }

  @override
  jsAst.Expression visitDynamicType(DynamicType type, ModularEmitter emitter) {
    return getDynamicValue();
  }

  @override
  jsAst.Expression visitAnyType(AnyType type, ModularEmitter emitter) =>
      getJsInteropTypeArgumentValue();

  jsAst.Expression getJsInteropTypeArguments(int count,
      {jsAst.Expression name}) {
    List<jsAst.Expression> elements = <jsAst.Expression>[];
    if (name != null) {
      elements.add(name);
    }
    for (int i = 0; i < count; i++) {
      elements.add(getJsInteropTypeArgumentValue());
    }
    return new jsAst.ArrayInitializer(elements);
  }

  @override
  jsAst.Expression visitInterfaceType(
      InterfaceType type, ModularEmitter emitter) {
    jsAst.Expression name = getJavaScriptClassName(type.element, emitter);
    jsAst.Expression result;
    if (type.typeArguments.isEmpty) {
      result = name;
    } else {
      // Visit all type arguments. This is done even for jsinterop classes to
      // enforce the invariant that [onVariable] is called for each type
      // variable in the type.
      result = visitList(type.typeArguments, emitter, head: name);
      if (_nativeData.isJsInteropClass(type.element)) {
        // Replace type arguments of generic jsinterop classes with 'any' type.
        result =
            getJsInteropTypeArguments(type.typeArguments.length, name: name);
      }
    }
    return result;
  }

  jsAst.Expression visitList(List<DartType> types, ModularEmitter emitter,
      {jsAst.Expression head}) {
    List<jsAst.Expression> elements = <jsAst.Expression>[];
    if (head != null) {
      elements.add(head);
    }
    for (DartType type in types) {
      jsAst.Expression element = visit(type, emitter);
      if (element is jsAst.LiteralNull) {
        elements.add(new jsAst.ArrayHole());
      } else {
        elements.add(element);
      }
    }
    return new jsAst.ArrayInitializer(elements);
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a function type.
  jsAst.Template get templateForIsFunctionType {
    return jsAst.js.expressionTemplateFor("'${_rtiTags.functionTypeTag}' in #");
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is a FutureOr type.
  jsAst.Template get templateForIsFutureOrType {
    return jsAst.js.expressionTemplateFor("'${_rtiTags.futureOrTag}' in #");
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is the void type.
  jsAst.Template get templateForIsVoidType {
    return jsAst.js.expressionTemplateFor("# === -1");
  }

  /// Returns the JavaScript template to determine at runtime if a type object
  /// is the dynamic type.
  jsAst.Template get templateForIsDynamicType {
    return jsAst.js.expressionTemplateFor("# == null");
  }

  jsAst.Template get templateForIsJsInteropTypeArgument {
    return jsAst.js.expressionTemplateFor("# === -2");
  }

  @override
  jsAst.Expression visitFunctionType(
      FunctionType type, ModularEmitter emitter) {
    List<jsAst.Property> properties = <jsAst.Property>[];

    void addProperty(String name, jsAst.Expression value) {
      properties.add(new jsAst.Property(js.string(name), value));
    }

    // Type representations for functions have a property which is a tag marking
    // them as function types. The value is not used, so '1' is just a dummy.
    addProperty(_rtiTags.functionTypeTag, js.number(1));

    if (type.typeVariables.isNotEmpty) {
      // Generic function types have type parameters which are reduced to de
      // Bruijn indexes.
      for (FunctionTypeVariable variable in type.typeVariables.reversed) {
        functionTypeVariables.add(variable);
      }
      // TODO(sra): This emits `P.Object` for the common unbounded case. We
      // could replace the Object bounds with an array hole for a compact `[,,]`
      // representation.
      addProperty(_rtiTags.functionTypeGenericBoundsTag,
          visitList(type.typeVariables.map((v) => v.bound).toList(), emitter));
    }

    if (!type.returnType.treatAsDynamic) {
      addProperty(
          _rtiTags.functionTypeReturnTypeTag, visit(type.returnType, emitter));
    }
    if (!type.parameterTypes.isEmpty) {
      addProperty(_rtiTags.functionTypeRequiredParametersTag,
          visitList(type.parameterTypes, emitter));
    }
    if (!type.optionalParameterTypes.isEmpty) {
      addProperty(_rtiTags.functionTypeOptionalParametersTag,
          visitList(type.optionalParameterTypes, emitter));
    }
    if (!type.namedParameterTypes.isEmpty) {
      List<jsAst.Property> namedArguments = <jsAst.Property>[];
      List<String> names = type.namedParameters;
      List<DartType> types = type.namedParameterTypes;
      assert(types.length == names.length);
      for (int index = 0; index < types.length; index++) {
        jsAst.Expression name = js.string(names[index]);
        namedArguments
            .add(new jsAst.Property(name, visit(types[index], emitter)));
      }
      addProperty(_rtiTags.functionTypeNamedParametersTag,
          new jsAst.ObjectInitializer(namedArguments));
    }

    // Exit generic function scope.
    if (type.typeVariables.isNotEmpty) {
      functionTypeVariables.length -= type.typeVariables.length;
    }

    return new jsAst.ObjectInitializer(properties);
  }

  @override
  jsAst.Expression visitVoidType(VoidType type, ModularEmitter emitter) {
    return getVoidValue();
  }

  @override
  jsAst.Expression visitTypedefType(TypedefType type, ModularEmitter emitter) {
    bool shouldEncode = shouldEncodeTypedef(type);
    DartType unaliasedType = type.unaliased;

    var oldBindings = typedefBindings;
    if (typedefBindings == null) {
      // First level typedef - capture arguments for re-use within typedef body.
      //
      // The type `Map<T, Foo<Set<T>>>` contains one type variable referenced
      // twice, so there are two inputs into the HTypeInfoExpression
      // instruction.
      //
      // If Foo is a typedef, T can be reused, e.g.
      //
      //     typedef E Foo<E>(E a, E b);
      //
      // As the typedef is expanded (to (Set<T>, Set<T>) => Set<T>) it should
      // not consume additional types from the to-level input.  We prevent this
      // by capturing the types and using the captured type expressions inside
      // the typedef expansion.
      //
      // TODO(sra): We should make the type subexpression Foo<...> be a second
      // HTypeInfoExpression, with Set<T> as its input (a third
      // HTypeInfoExpression). This would share all the Set<T> subexpressions
      // instead of duplicating them. This would require HTypeInfoExpression
      // inputs to correspond to type variables AND typedefs.
      typedefBindings = <TypeVariableType, jsAst.Expression>{};
      type.forEachTypeVariable((TypeVariableType variable) {
        typedefBindings[variable] = onVariable(variable);
      });
    }

    jsAst.Expression finish(jsAst.Expression result) {
      typedefBindings = oldBindings;
      return result;
    }

    if (shouldEncode) {
      jsAst.ObjectInitializer initializer = visit(unaliasedType, emitter);
      // We have to encode the aliased type.
      jsAst.Expression name = getJavaScriptClassName(type.element, emitter);
      jsAst.Expression encodedTypedef = type.treatAsRaw
          ? name
          : visitList(type.typeArguments, emitter, head: name);

      // Add it to the function-type object.
      jsAst.LiteralString tag = js.string(_rtiTags.typedefTag);
      initializer.properties.add(new jsAst.Property(tag, encodedTypedef));
      return finish(initializer);
    } else {
      return finish(visit(unaliasedType, emitter));
    }
  }

  @override
  jsAst.Expression visitFutureOrType(
      FutureOrType type, ModularEmitter emitter) {
    List<jsAst.Property> properties = <jsAst.Property>[];

    void addProperty(String name, jsAst.Expression value) {
      properties.add(new jsAst.Property(js.string(name), value));
    }

    // Type representations for FutureOr have a property which is a tag marking
    // them as FutureOr types. The value is not used, so '1' is just a dummy.
    addProperty(_rtiTags.futureOrTag, js.number(1));
    if (!type.typeArgument.treatAsDynamic) {
      addProperty(_rtiTags.futureOrTypeTag, visit(type.typeArgument, emitter));
    }

    return new jsAst.ObjectInitializer(properties);
  }
}

class TypeCheckMapping implements TypeChecks {
  final Map<ClassEntity, ClassChecks> map = new Map<ClassEntity, ClassChecks>();

  @override
  ClassChecks operator [](ClassEntity element) {
    ClassChecks result = map[element];
    return result != null ? result : const ClassChecks.empty();
  }

  void operator []=(ClassEntity element, ClassChecks checks) {
    map[element] = checks;
  }

  @override
  Iterable<ClassEntity> get classes => map.keys;

  @override
  String toString() {
    StringBuffer sb = new StringBuffer();
    for (ClassEntity holder in classes) {
      for (TypeCheck check in this[holder].checks) {
        sb.write('${holder.name} <: ${check.cls.name}, ');
      }
    }
    return '[$sb]';
  }
}

class ArgumentCollector extends DartTypeVisitor<dynamic, bool> {
  final Set<ClassEntity> classes = new Set<ClassEntity>();

  void addClass(ClassEntity cls) {
    classes.add(cls);
  }

  collect(DartType type, {bool isTypeArgument: false}) {
    visit(type, isTypeArgument);
  }

  /// Collect all types in the list as if they were arguments of an
  /// InterfaceType.
  collectAll(List<DartType> types, {bool isTypeArgument: false}) {
    for (DartType type in types) {
      visit(type, true);
    }
  }

  @override
  visitTypedefType(TypedefType type, bool isTypeArgument) {
    collect(type.unaliased, isTypeArgument: isTypeArgument);
  }

  @override
  visitInterfaceType(InterfaceType type, bool isTypeArgument) {
    if (isTypeArgument) addClass(type.element);
    collectAll(type.typeArguments, isTypeArgument: true);
  }

  @override
  visitFunctionType(FunctionType type, _) {
    collect(type.returnType, isTypeArgument: true);
    collectAll(type.parameterTypes, isTypeArgument: true);
    collectAll(type.optionalParameterTypes, isTypeArgument: true);
    collectAll(type.namedParameterTypes, isTypeArgument: true);
  }
}

class FunctionArgumentCollector extends DartTypeVisitor<dynamic, bool> {
  final Set<ClassEntity> classes = new Set<ClassEntity>();

  FunctionArgumentCollector();

  collect(DartType type, {bool inFunctionType: false}) {
    visit(type, inFunctionType);
  }

  collectAll(Iterable<DartType> types, {bool inFunctionType: false}) {
    for (DartType type in types) {
      visit(type, inFunctionType);
    }
  }

  @override
  visitTypedefType(TypedefType type, bool inFunctionType) {
    collect(type.unaliased, inFunctionType: inFunctionType);
  }

  @override
  visitInterfaceType(InterfaceType type, bool inFunctionType) {
    if (inFunctionType) {
      classes.add(type.element);
    }
    collectAll(type.typeArguments, inFunctionType: inFunctionType);
  }

  @override
  visitFunctionType(FunctionType type, _) {
    collect(type.returnType, inFunctionType: true);
    collectAll(type.parameterTypes, inFunctionType: true);
    collectAll(type.optionalParameterTypes, inFunctionType: true);
    collectAll(type.namedParameterTypes, inFunctionType: true);
    collectAll(type.typeVariables.map((type) => type.bound),
        inFunctionType: true);
  }
}

enum TypeVisitorState {
  direct,
  covariantTypeArgument,
  contravariantTypeArgument,
  typeLiteral,
}

class TypeVisitor extends DartTypeVisitor<void, TypeVisitorState> {
  Set<FunctionTypeVariable> _visitedFunctionTypeVariables =
      new Set<FunctionTypeVariable>();

  final void Function(ClassEntity entity, {TypeVisitorState state}) onClass;
  final void Function(TypeVariableEntity entity, {TypeVisitorState state})
      onTypeVariable;
  final void Function(FunctionType type, {TypeVisitorState state})
      onFunctionType;

  TypeVisitor({this.onClass, this.onTypeVariable, this.onFunctionType});

  visitType(DartType type, TypeVisitorState state) => type.accept(this, state);

  TypeVisitorState covariantArgument(TypeVisitorState state) {
    switch (state) {
      case TypeVisitorState.direct:
        return TypeVisitorState.covariantTypeArgument;
      case TypeVisitorState.covariantTypeArgument:
        return TypeVisitorState.covariantTypeArgument;
      case TypeVisitorState.contravariantTypeArgument:
        return TypeVisitorState.contravariantTypeArgument;
      case TypeVisitorState.typeLiteral:
        return TypeVisitorState.typeLiteral;
    }
    throw new UnsupportedError("Unexpected TypeVisitorState $state");
  }

  TypeVisitorState contravariantArgument(TypeVisitorState state) {
    switch (state) {
      case TypeVisitorState.direct:
        return TypeVisitorState.contravariantTypeArgument;
      case TypeVisitorState.covariantTypeArgument:
        return TypeVisitorState.contravariantTypeArgument;
      case TypeVisitorState.contravariantTypeArgument:
        return TypeVisitorState.covariantTypeArgument;
      case TypeVisitorState.typeLiteral:
        return TypeVisitorState.typeLiteral;
    }
    throw new UnsupportedError("Unexpected TypeVisitorState $state");
  }

  visitTypes(List<DartType> types, TypeVisitorState state) {
    for (DartType type in types) {
      visitType(type, state);
    }
  }

  @override
  void visitTypeVariableType(TypeVariableType type, TypeVisitorState state) {
    if (onTypeVariable != null) {
      onTypeVariable(type.element, state: state);
    }
  }

  @override
  visitInterfaceType(InterfaceType type, TypeVisitorState state) {
    if (onClass != null) {
      onClass(type.element, state: state);
    }
    visitTypes(type.typeArguments, covariantArgument(state));
  }

  @override
  visitFunctionType(FunctionType type, TypeVisitorState state) {
    if (onFunctionType != null) {
      onFunctionType(type, state: state);
    }
    // Visit all nested types as type arguments; these types are not runtime
    // instances but runtime type representations.
    visitType(type.returnType, covariantArgument(state));
    visitTypes(type.parameterTypes, contravariantArgument(state));
    visitTypes(type.optionalParameterTypes, contravariantArgument(state));
    visitTypes(type.namedParameterTypes, contravariantArgument(state));
    _visitedFunctionTypeVariables.removeAll(type.typeVariables);
  }

  @override
  visitTypedefType(TypedefType type, TypeVisitorState state) {
    visitType(type.unaliased, state);
  }

  @override
  visitFunctionTypeVariable(FunctionTypeVariable type, TypeVisitorState state) {
    if (_visitedFunctionTypeVariables.add(type)) {
      visitType(type.bound, state);
    }
  }
}

/// Runtime type usage for a class.
class ClassUse {
  /// Whether the class is directly or indirectly instantiated.
  ///
  /// For instance `A` and `B` in:
  ///
  ///     class A {}
  ///     class B extends A {}
  ///     main() => new B();
  ///
  bool instance = false;

  /// Whether the class is directly instantiated.
  ///
  /// For instance `B` in:
  ///
  ///     class A {}
  ///     class B extends A {}
  ///     main() => new B();
  ///
  bool directInstance = false;

  /// Whether objects are checked to be instances of the class.
  ///
  /// For instance `A` in:
  ///
  ///     class A {}
  ///     main() => null is A;
  ///
  bool checkedInstance = false;

  /// Whether the class is passed as a type argument at runtime.
  ///
  /// For instance `A` in:
  ///
  ///     class A {}
  ///     main() => new List<A>() is List<String>;
  ///
  bool typeArgument = false;

  /// Whether the class is checked as a type argument at runtime.
  ///
  /// For instance `A` in:
  ///
  ///     class A {}
  ///     main() => new List<String>() is List<A>;
  ///
  bool checkedTypeArgument = false;

  /// Whether the class is used in a constant type literal.
  ///
  /// For instance `A`:
  ///
  ///     class A {}
  ///     main() => A;
  ///
  bool typeLiteral = false;

  /// The function type of the class, if any.
  ///
  /// This is only set if the function type is needed at runtime. For instance,
  /// if no function types are checked at runtime then the function type isn't
  /// needed.
  ///
  /// Furthermore optimization might also omit function type that are known not
  /// to be valid in any subtype test.
  ClassFunctionType functionType;

  /// `true` if the class is 'live' either through instantiation or use in
  /// type arguments.
  bool get isLive => directInstance || typeArgument;

  @override
  String toString() {
    List<String> properties = <String>[];
    if (instance) {
      properties.add('instance');
    }
    if (directInstance) {
      properties.add('directInstance');
    }
    if (checkedInstance) {
      properties.add('checkedInstance');
    }
    if (typeArgument) {
      properties.add('typeArgument');
    }
    if (checkedTypeArgument) {
      properties.add('checkedTypeArgument');
    }
    if (typeLiteral) {
      properties.add('rtiValue');
    }
    if (functionType != null) {
      properties.add('functionType');
    }
    return 'ClassUse(${properties.join(',')})';
  }
}