pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

// Copyright (c) 2017, 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.

/// This file declares a "shadow hierarchy" of concrete classes which extend
/// the kernel class hierarchy, adding methods and fields needed by the
/// BodyBuilder.
///
/// Instances of these classes may be created using the factory methods in
/// `ast_factory.dart`.
///
/// Note that these classes represent the Dart language prior to desugaring.
/// When a single Dart construct desugars to a tree containing multiple kernel
/// AST nodes, the shadow class extends the kernel object at the top of the
/// desugared tree.
///
/// This means that in some cases multiple shadow classes may extend the same
/// kernel class, because multiple constructs in Dart may desugar to a tree
/// with the same kind of root node.

import 'dart:core' hide MapEntry;

import 'package:kernel/ast.dart' as kernel show Expression, Initializer;

import 'package:kernel/ast.dart';

import 'package:kernel/type_algebra.dart' show Substitution;

import '../../base/instrumentation.dart'
    show
        Instrumentation,
        InstrumentationValueForMember,
        InstrumentationValueForType,
        InstrumentationValueForTypeArgs;

import '../fasta_codes.dart'
    show
        messageCantDisambiguateAmbiguousInformation,
        messageCantDisambiguateNotEnoughInformation,
        messageNonNullAwareSpreadIsNull,
        messageSwitchExpressionNotAssignableCause,
        messageVoidExpression,
        noLength,
        templateCantInferTypeDueToCircularity,
        templateForInLoopElementTypeNotAssignable,
        templateForInLoopTypeNotIterable,
        templateIntegerLiteralIsOutOfRange,
        templateSpreadElementTypeMismatch,
        templateSpreadMapEntryElementKeyTypeMismatch,
        templateSpreadMapEntryElementValueTypeMismatch,
        templateSpreadMapEntryTypeMismatch,
        templateSpreadTypeMismatch,
        templateSwitchExpressionNotAssignable,
        templateWebLiteralCannotBeRepresentedExactly;

import '../problems.dart' show getFileUri, unhandled, unsupported;

import '../source/source_class_builder.dart' show SourceClassBuilder;

import '../type_inference/inference_helper.dart' show InferenceHelper;

import '../type_inference/interface_resolver.dart' show InterfaceResolver;

import '../type_inference/type_inference_engine.dart'
    show IncludesTypeParametersCovariantly, InferenceNode, TypeInferenceEngine;

import '../type_inference/type_inferrer.dart'
    show ExpressionInferenceResult, TypeInferrer, TypeInferrerImpl;

import '../type_inference/type_promotion.dart'
    show TypePromoter, TypePromoterImpl, TypePromotionFact, TypePromotionScope;

import '../type_inference/type_schema.dart' show UnknownType;

import '../type_inference/type_schema_elimination.dart' show greatestClosure;

import '../type_inference/type_schema_environment.dart'
    show TypeSchemaEnvironment, getPositionalParameterType;

import 'body_builder.dart' show combineStatements;

import 'collections.dart'
    show
        ControlFlowMapEntry,
        ForElement,
        ForInElement,
        ForInMapEntry,
        ForMapEntry,
        IfElement,
        IfMapEntry,
        SpreadElement,
        SpreadMapEntry,
        convertToElement;

import 'implicit_type_argument.dart' show ImplicitTypeArgument;

import 'kernel_builder.dart' show KernelLibraryBuilder;

import 'kernel_expression_generator.dart' show makeLet;

part "inference_visitor.dart";
part "inferred_type_visitor.dart";

/// Computes the return type of a (possibly factory) constructor.
InterfaceType computeConstructorReturnType(Member constructor) {
  if (constructor is Constructor) {
    return constructor.enclosingClass.thisType;
  } else {
    return constructor.function.returnType;
  }
}

List<DartType> getExplicitTypeArguments(Arguments arguments) {
  if (arguments is ArgumentsJudgment) {
    return arguments._hasExplicitTypeArguments ? arguments.types : null;
  } else {
    // This code path should only be taken in situations where there are no
    // type arguments at all, e.g. calling a user-definable operator.
    assert(arguments.types.isEmpty);
    return null;
  }
}

/// Information associated with a class during type inference.
class ClassInferenceInfo {
  /// The builder associated with this class.
  final SourceClassBuilder builder;

  /// The visitor for determining if a given type makes covariant use of one of
  /// the class's generic parameters, and therefore requires covariant checks.
  IncludesTypeParametersCovariantly needsCheckVisitor;

  /// Getters and methods in the class's API.  May include forwarding nodes.
  final gettersAndMethods = <Member>[];

  /// Setters in the class's API.  May include forwarding nodes.
  final setters = <Member>[];

  ClassInferenceInfo(this.builder);
}

/// Concrete shadow object representing a set of invocation arguments.
class ArgumentsJudgment extends Arguments {
  bool _hasExplicitTypeArguments;

  List<Expression> get positionalJudgments => positional.cast();

  ArgumentsJudgment(List<Expression> positional,
      {List<DartType> types, List<NamedExpression> named})
      : _hasExplicitTypeArguments = types != null && types.isNotEmpty,
        super(positional, types: types, named: named);

  static void setNonInferrableArgumentTypes(
      ArgumentsJudgment arguments, List<DartType> types) {
    arguments.types.clear();
    arguments.types.addAll(types);
    arguments._hasExplicitTypeArguments = true;
  }

  static void removeNonInferrableArgumentTypes(ArgumentsJudgment arguments) {
    arguments.types.clear();
    arguments._hasExplicitTypeArguments = false;
  }
}

/// Concrete shadow object representing an assert initializer in kernel form.
class AssertInitializerJudgment extends AssertInitializer
    implements InitializerJudgment {
  AssertInitializerJudgment(AssertStatement statement) : super(statement);

  AssertStatementJudgment get judgment => statement;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitAssertInitializerJudgment(this);
  }
}

/// Concrete shadow object representing an assertion statement in kernel form.
class AssertStatementJudgment extends AssertStatement
    implements StatementJudgment {
  AssertStatementJudgment(Expression condition,
      {Expression message, int conditionStartOffset, int conditionEndOffset})
      : super(condition,
            message: message,
            conditionStartOffset: conditionStartOffset,
            conditionEndOffset: conditionEndOffset);

  Expression get conditionJudgment => condition;

  Expression get messageJudgment => message;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitAssertStatementJudgment(this);
  }
}

/// Concrete shadow object representing a statement block in kernel form.
class BlockJudgment extends Block implements StatementJudgment {
  BlockJudgment(List<Statement> statements) : super(statements);

  List<Statement> get judgments => statements;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitBlockJudgment(this);
  }
}

/// Concrete shadow object representing a cascade expression.
///
/// A cascade expression of the form `a..b()..c()` is represented as the kernel
/// expression:
///
///     let v = a in
///         let _ = v.b() in
///             let _ = v.c() in
///                 v
///
/// In the documentation that follows, `v` is referred to as the "cascade
/// variable"--this is the variable that remembers the value of the expression
/// preceding the first `..` while the cascades are being evaluated.
///
/// After constructing a [CascadeJudgment], the caller should
/// call [finalize] with an expression representing the expression after the
/// `..`.  If a further `..` follows that expression, the caller should call
/// [extend] followed by [finalize] for each subsequent cascade.
class CascadeJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  /// Pointer to the last "let" expression in the cascade.
  Let nextCascade;

  /// Creates a [CascadeJudgment] using [variable] as the cascade
  /// variable.  Caller is responsible for ensuring that [variable]'s
  /// initializer is the expression preceding the first `..` of the cascade
  /// expression.
  CascadeJudgment(VariableDeclarationJudgment variable)
      : super(
            variable,
            makeLet(new VariableDeclaration.forValue(new _UnfinishedCascade()),
                new VariableGet(variable))) {
    nextCascade = body;
  }

  Expression get targetJudgment => variable.initializer;

  Iterable<Expression> get cascadeJudgments sync* {
    Let section = body;
    while (true) {
      yield section.variable.initializer;
      if (section.body is! Let) break;
      section = section.body;
    }
  }

  /// Adds a new unfinalized section to the end of the cascade.  Should be
  /// called after the previous cascade section has been finalized.
  void extend() {
    assert(nextCascade.variable.initializer is! _UnfinishedCascade);
    Let newCascade = makeLet(
        new VariableDeclaration.forValue(new _UnfinishedCascade()),
        nextCascade.body);
    nextCascade.body = newCascade;
    newCascade.parent = nextCascade;
    nextCascade = newCascade;
  }

  /// Finalizes the last cascade section with the given [expression].
  void finalize(Expression expression) {
    assert(nextCascade.variable.initializer is _UnfinishedCascade);
    nextCascade.variable.initializer = expression;
    expression.parent = nextCascade.variable;
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitCascadeJudgment(this, typeContext);
  }
}

/// Shadow object representing a class in kernel form.
class ShadowClass extends Class {
  ClassInferenceInfo _inferenceInfo;

  ShadowClass(
      {String name,
      Supertype supertype,
      Supertype mixedInType,
      List<TypeParameter> typeParameters,
      List<Supertype> implementedTypes,
      List<Procedure> procedures,
      List<Field> fields})
      : super(
            name: name,
            supertype: supertype,
            mixedInType: mixedInType,
            typeParameters: typeParameters,
            implementedTypes: implementedTypes,
            procedures: procedures,
            fields: fields);

  /// Resolves all forwarding nodes for this class, propagates covariance
  /// annotations, and creates forwarding stubs as needed.
  void finalizeCovariance(InterfaceResolver interfaceResolver) {
    interfaceResolver.finalizeCovariance(
        this, _inferenceInfo.gettersAndMethods, _inferenceInfo.builder.library);
    interfaceResolver.finalizeCovariance(
        this, _inferenceInfo.setters, _inferenceInfo.builder.library);
  }

  /// Creates API members for this class.
  void setupApiMembers(InterfaceResolver interfaceResolver) {
    interfaceResolver.createApiMembers(this, _inferenceInfo.gettersAndMethods,
        _inferenceInfo.setters, _inferenceInfo.builder.library);
  }

  static void clearClassInferenceInfo(ShadowClass class_) {
    class_._inferenceInfo = null;
  }

  static ClassInferenceInfo getClassInferenceInfo(Class class_) {
    if (class_ is ShadowClass) return class_._inferenceInfo;
    return null;
  }

  /// Initializes the class inference information associated with the given
  /// [class_], starting with the fact that it is associated with the given
  /// [builder].
  static void setBuilder(ShadowClass class_, SourceClassBuilder builder) {
    class_._inferenceInfo = new ClassInferenceInfo(builder);
  }
}

/// Abstract shadow object representing a complex assignment in kernel form.
///
/// Since there are many forms a complex assignment might have been desugared
/// to, this class wraps the desugared assignment rather than extending it.
///
/// TODO(paulberry): once we know exactly what constitutes a "complex
/// assignment", document it here.
abstract class ComplexAssignmentJudgment extends SyntheticExpressionJudgment {
  /// In a compound assignment, the expression that reads the old value, or
  /// `null` if this is not a compound assignment.
  Expression read;

  /// The expression appearing on the RHS of the assignment.
  final Expression rhs;

  /// The expression that performs the write (e.g. `a.[]=(b, a.[](b) + 1)` in
  /// `++a[b]`).
  Expression write;

  /// In a compound assignment without shortcut semantics, the expression that
  /// combines the old and new values, or `null` if this is not a compound
  /// assignment.
  ///
  /// Note that in a compound assignment with shortcut semantics, this is not
  /// used; [nullAwareCombiner] is used instead.
  MethodInvocation combiner;

  /// In a compound assignment with shortcut semantics, the conditional
  /// expression that determines whether the assignment occurs.
  ///
  /// Note that in a compound assignment without shortcut semantics, this is not
  /// used; [combiner] is used instead.
  ConditionalExpression nullAwareCombiner;

  /// Indicates whether the expression arose from a post-increment or
  /// post-decrement.
  bool isPostIncDec = false;

  /// Indicates whether the expression arose from a pre-increment or
  /// pre-decrement.
  bool isPreIncDec = false;

  ComplexAssignmentJudgment._(this.rhs) : super._(null);

  String toString() {
    var parts = _getToStringParts();
    return '${runtimeType}(${parts.join(', ')})';
  }

  List<String> _getToStringParts() {
    List<String> parts = [];
    if (desugared != null) parts.add('desugared=$desugared');
    if (read != null) parts.add('read=$read');
    if (rhs != null) parts.add('rhs=$rhs');
    if (write != null) parts.add('write=$write');
    if (combiner != null) parts.add('combiner=$combiner');
    if (nullAwareCombiner != null) {
      parts.add('nullAwareCombiner=$nullAwareCombiner');
    }
    if (isPostIncDec) parts.add('isPostIncDec=true');
    if (isPreIncDec) parts.add('isPreIncDec=true');
    return parts;
  }

  _ComplexAssignmentInferenceResult _inferRhs(
      ShadowTypeInferrer inferrer, DartType readType, DartType writeContext) {
    assert(writeContext != null);
    if (readType is VoidType &&
        (combiner != null || nullAwareCombiner != null)) {
      inferrer.helper
          ?.addProblem(messageVoidExpression, read.fileOffset, noLength);
    }
    var writeOffset = write == null ? -1 : write.fileOffset;
    Procedure combinerMember;
    DartType combinedType;
    if (combiner != null) {
      bool isOverloadedArithmeticOperator = false;
      combinerMember = inferrer.findMethodInvocationMember(readType, combiner,
          instrumented: false);
      if (combinerMember is Procedure) {
        isOverloadedArithmeticOperator = inferrer.typeSchemaEnvironment
            .isOverloadedArithmeticOperatorAndType(combinerMember, readType);
      }
      DartType rhsType;
      var combinerType = inferrer.getCalleeFunctionType(
          inferrer.getCalleeType(combinerMember, readType), false);
      if (isPreIncDec || isPostIncDec) {
        rhsType = inferrer.coreTypes.intClass.rawType;
      } else {
        // It's not necessary to call _storeLetType for [rhs] because the RHS
        // is always passed directly to the combiner; it's never stored in a
        // temporary variable first.
        assert(identical(combiner.arguments.positional.first, rhs));
        // Analyzer uses a null context for the RHS here.
        // TODO(paulberry): improve on this.
        inferrer.inferExpression(rhs, const UnknownType(), true);
        rhsType = getInferredType(rhs, inferrer);
        // Do not use rhs after this point because it may be a Shadow node
        // that has been replaced in the tree with its desugaring.
        var expectedType = getPositionalParameterType(combinerType, 0);
        inferrer.ensureAssignable(expectedType, rhsType,
            combiner.arguments.positional.first, combiner.fileOffset);
      }
      if (isOverloadedArithmeticOperator) {
        combinedType = inferrer.typeSchemaEnvironment
            .getTypeOfOverloadedArithmetic(readType, rhsType);
      } else {
        combinedType = combinerType.returnType;
      }
      var checkKind = inferrer.preCheckInvocationContravariance(read, readType,
          combinerMember, combiner, combiner.arguments, combiner);
      var replacedCombiner = inferrer.handleInvocationContravariance(
          checkKind,
          combiner,
          combiner.arguments,
          combiner,
          combinedType,
          combinerType,
          combiner.fileOffset);
      var replacedCombiner2 = inferrer.ensureAssignable(
          writeContext, combinedType, replacedCombiner, writeOffset);
      if (replacedCombiner2 != null) {
        replacedCombiner = replacedCombiner2;
      }
      _storeLetType(inferrer, replacedCombiner, combinedType);
    } else {
      inferrer.inferExpression(rhs, writeContext ?? const UnknownType(), true,
          isVoidAllowed: true);
      var rhsType = getInferredType(rhs, inferrer);
      var replacedRhs = inferrer.ensureAssignable(
          writeContext, rhsType, rhs, writeOffset,
          isVoidAllowed: writeContext is VoidType);
      _storeLetType(inferrer, replacedRhs ?? rhs, rhsType);
      if (nullAwareCombiner != null) {
        MethodInvocation equalsInvocation = nullAwareCombiner.condition;
        inferrer.findMethodInvocationMember(
            greatestClosure(inferrer.coreTypes, writeContext), equalsInvocation,
            instrumented: false);
        // Note: the case of readType=null only happens for erroneous code.
        combinedType = readType == null
            ? rhsType
            : inferrer.typeSchemaEnvironment
                .getStandardUpperBound(readType, rhsType);
        nullAwareCombiner.staticType = combinedType;
      } else {
        combinedType = rhsType;
      }
    }
    if (this is IndexAssignmentJudgment) {
      _storeLetType(inferrer, write, const VoidType());
    } else {
      _storeLetType(inferrer, write, combinedType);
    }
    inferredType =
        isPostIncDec ? (readType ?? const DynamicType()) : combinedType;
    return new _ComplexAssignmentInferenceResult(combinerMember);
  }
}

/// Abstract shadow object representing a complex assignment involving a
/// receiver.
abstract class ComplexAssignmentJudgmentWithReceiver
    extends ComplexAssignmentJudgment {
  /// The receiver of the assignment target (e.g. `a` in `a[b] = c`).
  final Expression receiver;

  /// Indicates whether this assignment uses `super`.
  final bool isSuper;

  ComplexAssignmentJudgmentWithReceiver._(
      this.receiver, Expression rhs, this.isSuper)
      : super._(rhs);

  @override
  List<String> _getToStringParts() {
    var parts = super._getToStringParts();
    if (receiver != null) parts.add('receiver=$receiver');
    if (isSuper) parts.add('isSuper=true');
    return parts;
  }

  DartType _inferReceiver(ShadowTypeInferrer inferrer) {
    if (receiver != null) {
      inferrer.inferExpression(receiver, const UnknownType(), true);
      var receiverType = getInferredType(receiver, inferrer);
      _storeLetType(inferrer, receiver, receiverType);
      return receiverType;
    } else if (isSuper) {
      return inferrer.classHierarchy.getTypeAsInstanceOf(
          inferrer.thisType, inferrer.thisType.classNode.supertype.classNode);
    } else {
      return inferrer.thisType;
    }
  }
}

/// Concrete shadow object representing a continue statement from a switch
/// statement, in kernel form.
class ContinueSwitchJudgment extends ContinueSwitchStatement
    implements StatementJudgment {
  ContinueSwitchJudgment(SwitchCase target) : super(target);

  SwitchCaseJudgment get targetJudgment => target;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitContinueSwitchJudgment(this);
  }
}

/// Shadow object representing a deferred check in kernel form.
class DeferredCheckJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  DeferredCheckJudgment(VariableDeclaration variable, Expression body)
      : super(variable, body);

  Expression get judgment => body;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitDeferredCheckJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a do loop in kernel form.
class DoJudgment extends DoStatement implements StatementJudgment {
  DoJudgment(Statement body, Expression condition) : super(body, condition);

  Expression get conditionJudgment => condition;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitDoJudgment(this);
  }
}

/// Concrete shadow object representing a double literal in kernel form.
class DoubleJudgment extends DoubleLiteral implements ExpressionJudgment {
  DartType inferredType;

  DoubleJudgment(double value) : super(value);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitDoubleJudgment(this, typeContext);
  }
}

/// Common base class for shadow objects representing expressions in kernel
/// form.
abstract class ExpressionJudgment extends Expression {
  DartType inferredType;

  /// Calls back to [inferrer] to perform type inference for whatever concrete
  /// type of [Expression] this is.
  void acceptInference(InferenceVisitor visitor, DartType typeContext);
}

/// Concrete shadow object representing an empty statement in kernel form.
class EmptyStatementJudgment extends EmptyStatement
    implements StatementJudgment {
  EmptyStatementJudgment();

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitEmptyStatementJudgment(this);
  }
}

/// Concrete shadow object representing an expression statement in kernel form.
class ExpressionStatementJudgment extends ExpressionStatement
    implements StatementJudgment {
  ExpressionStatementJudgment(Expression expression) : super(expression);

  Expression get judgment => expression;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitExpressionStatementJudgment(this);
  }
}

/// Shadow object for [StaticInvocation] when the procedure being invoked is a
/// factory constructor.
class FactoryConstructorInvocationJudgment extends StaticInvocation
    implements ExpressionJudgment {
  DartType inferredType;

  FactoryConstructorInvocationJudgment(
      Procedure target, ArgumentsJudgment arguments,
      {bool isConst: false})
      : super(target, arguments, isConst: isConst);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitFactoryConstructorInvocationJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a field in kernel form.
class ShadowField extends Field implements ShadowMember {
  @override
  InferenceNode inferenceNode;

  ShadowTypeInferrer _typeInferrer;

  final bool _isImplicitlyTyped;

  ShadowField(Name name, this._isImplicitlyTyped, {Uri fileUri})
      : super(name, fileUri: fileUri) {}

  @override
  void setInferredType(
      TypeInferenceEngine engine, Uri uri, DartType inferredType) {
    type = inferredType;
  }

  static bool isImplicitlyTyped(ShadowField field) => field._isImplicitlyTyped;

  static void setInferenceNode(ShadowField field, InferenceNode node) {
    assert(field.inferenceNode == null);
    field.inferenceNode = node;
  }
}

/// Concrete shadow object representing a field initializer in kernel form.
class ShadowFieldInitializer extends FieldInitializer
    implements InitializerJudgment {
  ShadowFieldInitializer(Field field, Expression value) : super(field, value);

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitShadowFieldInitializer(this);
  }
}

/// Concrete shadow object representing a classic for loop in kernel form.
class ForJudgment extends ForStatement implements StatementJudgment {
  ForJudgment(List<VariableDeclaration> variables, Expression condition,
      List<Expression> updates, Statement body)
      : super(variables ?? [], condition, updates, body);

  Expression get conditionJudgment => condition;

  List<Expression> get updateJudgments => updates.cast();

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitForJudgment(this);
  }
}

/// Concrete shadow object representing a function expression in kernel form.
class FunctionNodeJudgment extends FunctionNode {
  FunctionNodeJudgment(Statement body,
      {List<TypeParameter> typeParameters,
      List<VariableDeclaration> positionalParameters,
      List<VariableDeclaration> namedParameters,
      int requiredParameterCount,
      DartType returnType: const DynamicType(),
      AsyncMarker asyncMarker: AsyncMarker.Sync,
      AsyncMarker dartAsyncMarker})
      : super(body,
            typeParameters: typeParameters,
            positionalParameters: positionalParameters,
            namedParameters: namedParameters,
            requiredParameterCount: requiredParameterCount,
            returnType: returnType,
            asyncMarker: asyncMarker,
            dartAsyncMarker: dartAsyncMarker);
}

/// Concrete shadow object representing a local function declaration in kernel
/// form.
class FunctionDeclarationJudgment extends FunctionDeclaration
    implements StatementJudgment {
  bool _hasImplicitReturnType = false;

  FunctionDeclarationJudgment(
      VariableDeclarationJudgment variable, FunctionNodeJudgment function)
      : super(variable, function);

  VariableDeclarationJudgment get variableJudgment => variable;

  FunctionNodeJudgment get functionJudgment => function;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitFunctionDeclarationJudgment(this);
  }

  static void setHasImplicitReturnType(
      FunctionDeclarationJudgment declaration, bool hasImplicitReturnType) {
    declaration._hasImplicitReturnType = hasImplicitReturnType;
  }
}

/// Concrete shadow object representing a super initializer in kernel form.
class InvalidSuperInitializerJudgment extends LocalInitializer
    implements InitializerJudgment {
  final Constructor target;
  final ArgumentsJudgment argumentsJudgment;

  InvalidSuperInitializerJudgment(
      this.target, this.argumentsJudgment, VariableDeclaration variable)
      : super(variable);

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitInvalidSuperInitializerJudgment(this);
  }
}

/// Concrete shadow object representing an if-null expression.
///
/// An if-null expression of the form `a ?? b` is represented as the kernel
/// expression:
///
///     let v = a in v == null ? b : v
class IfNullJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  IfNullJudgment(VariableDeclaration variable, Expression body)
      : super(variable, body);

  @override
  ConditionalExpression get body => super.body;

  /// Returns the expression to the left of `??`.
  Expression get leftJudgment => variable.initializer;

  /// Returns the expression to the right of `??`.
  Expression get rightJudgment => body.then;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitIfNullJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing an if statement in kernel form.
class IfJudgment extends IfStatement implements StatementJudgment {
  IfJudgment(Expression condition, Statement then, Statement otherwise)
      : super(condition, then, otherwise);

  Expression get conditionJudgment => condition;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitIfJudgment(this);
  }
}

/// Concrete shadow object representing an assignment to a target for which
/// assignment is not allowed.
class IllegalAssignmentJudgment extends ComplexAssignmentJudgment {
  /// The offset at which the invalid assignment should be stored.
  /// If `-1`, then there is no separate location for invalid assignment.
  final int assignmentOffset;

  IllegalAssignmentJudgment._(Expression rhs, {this.assignmentOffset: -1})
      : super._(rhs) {
    rhs.parent = this;
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitIllegalAssignmentJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing an assignment to a target of the form
/// `a[b]`.
class IndexAssignmentJudgment extends ComplexAssignmentJudgmentWithReceiver {
  /// In an assignment to an index expression, the index expression.
  final Expression index;

  IndexAssignmentJudgment._(Expression receiver, this.index, Expression rhs,
      {bool isSuper: false})
      : super._(receiver, rhs, isSuper);

  Arguments _getInvocationArguments(
      ShadowTypeInferrer inferrer, Expression invocation) {
    if (invocation is MethodInvocation) {
      return invocation.arguments;
    } else if (invocation is SuperMethodInvocation) {
      return invocation.arguments;
    } else {
      throw unhandled("${invocation.runtimeType}", "_getInvocationArguments",
          fileOffset, inferrer.uri);
    }
  }

  @override
  List<String> _getToStringParts() {
    var parts = super._getToStringParts();
    if (index != null) parts.add('index=$index');
    return parts;
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitIndexAssignmentJudgment(this, typeContext);
  }
}

/// Common base class for shadow objects representing initializers in kernel
/// form.
abstract class InitializerJudgment implements Initializer {
  /// Performs type inference for whatever concrete type of [InitializerJudgment]
  /// this is.
  void acceptInference(InferenceVisitor visitor);
}

Expression checkWebIntLiteralsErrorIfUnexact(
    ShadowTypeInferrer inferrer, int value, String literal, int charOffset) {
  if (value >= 0 && value <= (1 << 53)) return null;
  if (inferrer.library == null) return null;
  if (!inferrer.library.loader.target.backendTarget
      .errorOnUnexactWebIntLiterals) return null;
  BigInt asInt = BigInt.from(value).toUnsigned(64);
  BigInt asDouble = BigInt.from(asInt.toDouble());
  if (asInt == asDouble) return null;
  String text = literal ?? value.toString();
  String nearest = text.startsWith('0x') || text.startsWith('0X')
      ? '0x${asDouble.toRadixString(16)}'
      : asDouble.toString();
  int length = literal?.length ?? noLength;
  return inferrer.helper.desugarSyntheticExpression(inferrer.helper
      .buildProblem(
          templateWebLiteralCannotBeRepresentedExactly.withArguments(
              text, nearest),
          charOffset,
          length));
}

/// Concrete shadow object representing an integer literal in kernel form.
class IntJudgment extends IntLiteral implements ExpressionJudgment {
  DartType inferredType;
  final String literal;

  IntJudgment(int value, this.literal) : super(value);

  double asDouble({bool negated: false}) {
    if (value == 0 && negated) return -0.0;
    BigInt intValue = BigInt.from(negated ? -value : value);
    double doubleValue = intValue.toDouble();
    return intValue == BigInt.from(doubleValue) ? doubleValue : null;
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitIntJudgment(this, typeContext);
  }
}

class ShadowLargeIntLiteral extends IntLiteral implements ExpressionJudgment {
  final String literal;
  final int fileOffset;
  bool isParenthesized = false;

  DartType inferredType;

  ShadowLargeIntLiteral(this.literal, this.fileOffset) : super(0);

  double asDouble({bool negated: false}) {
    BigInt intValue = BigInt.tryParse(negated ? '-${literal}' : literal);
    if (intValue == null) return null;
    double doubleValue = intValue.toDouble();
    return !doubleValue.isNaN &&
            !doubleValue.isInfinite &&
            intValue == BigInt.from(doubleValue)
        ? doubleValue
        : null;
  }

  int asInt64({bool negated: false}) {
    return int.tryParse(negated ? '-${literal}' : literal);
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitShadowLargeIntLiteral(this, typeContext);
  }
}

/// Concrete shadow object representing an invalid initializer in kernel form.
class ShadowInvalidInitializer extends LocalInitializer
    implements InitializerJudgment {
  ShadowInvalidInitializer(VariableDeclaration variable) : super(variable);

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitShadowInvalidInitializer(this);
  }
}

/// Concrete shadow object representing an invalid initializer in kernel form.
class ShadowInvalidFieldInitializer extends LocalInitializer
    implements InitializerJudgment {
  final Field field;
  final Expression value;

  ShadowInvalidFieldInitializer(
      this.field, this.value, VariableDeclaration variable)
      : super(variable) {
    value?.parent = this;
  }

  Expression get judgment => value;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitShadowInvalidFieldInitializer(this);
  }
}

/// Type inference derivation for [ListLiteral].
class ListLiteralJudgment extends ListLiteral implements ExpressionJudgment {
  DartType inferredType;

  ListLiteralJudgment(List<Expression> expressions,
      {DartType typeArgument, bool isConst: false})
      : assert(typeArgument != null),
        super(expressions, typeArgument: typeArgument, isConst: isConst);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitListLiteralJudgment(this, typeContext);
  }
}

/// Type inference derivation for [SetLiteral].
class SetLiteralJudgment extends SetLiteral implements ExpressionJudgment {
  DartType inferredType;

  SetLiteralJudgment(List<Expression> expressions,
      {DartType typeArgument, bool isConst: false})
      : assert(typeArgument != null),
        super(expressions, typeArgument: typeArgument, isConst: isConst);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitSetLiteralJudgment(this, typeContext);
  }
}

/// Type inference derivation for [MapLiteral].
class MapLiteralJudgment extends MapLiteral implements ExpressionJudgment {
  DartType inferredType;

  MapLiteralJudgment(List<MapEntry> judgments,
      {DartType keyType, DartType valueType, bool isConst: false})
      : assert(keyType != null),
        assert(valueType != null),
        super(judgments,
            keyType: keyType, valueType: valueType, isConst: isConst);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitMapLiteralJudgment(this, typeContext);
  }
}

/// Abstract shadow object representing a field or procedure in kernel form.
abstract class ShadowMember implements Member {
  Uri get fileUri;

  InferenceNode get inferenceNode;

  void set inferenceNode(InferenceNode value);

  void setInferredType(
      TypeInferenceEngine engine, Uri uri, DartType inferredType);
}

/// Shadow object for [MethodInvocation].
class MethodInvocationJudgment extends MethodInvocation
    implements ExpressionJudgment {
  DartType inferredType;

  /// Indicates whether this method invocation is a call to a `call` method
  /// resulting from the invocation of a function expression.
  final bool _isImplicitCall;

  MethodInvocationJudgment(
      Expression receiver, Name name, ArgumentsJudgment arguments,
      {bool isImplicitCall: false, Member interfaceTarget})
      : _isImplicitCall = isImplicitCall,
        super(receiver, name, arguments, interfaceTarget);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitMethodInvocationJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a named function expression.
///
/// Named function expressions are not legal in Dart, but they are accepted by
/// the parser and BodyBuilder for error recovery purposes.
///
/// A named function expression of the form `f() { ... }` is represented as the
/// kernel expression:
///
///     let f = () { ... } in f
class NamedFunctionExpressionJudgment extends Let
    implements ExpressionJudgment {
  DartType inferredType;

  NamedFunctionExpressionJudgment(VariableDeclarationJudgment variable)
      : super(variable, new VariableGet(variable));

  VariableDeclarationJudgment get variableJudgment => variable;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitNamedFunctionExpressionJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a null-aware method invocation.
///
/// A null-aware method invocation of the form `a?.b(...)` is represented as the
/// expression:
///
///     let v = a in v == null ? null : v.b(...)
class NullAwareMethodInvocationJudgment extends Let
    implements ExpressionJudgment {
  DartType inferredType;

  NullAwareMethodInvocationJudgment(
      VariableDeclaration variable, Expression body)
      : super(variable, body);

  @override
  ConditionalExpression get body => super.body;

  MethodInvocation get _desugaredInvocation => body.otherwise;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitNullAwareMethodInvocationJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a null-aware read from a property.
///
/// A null-aware property get of the form `a?.b` is represented as the kernel
/// expression:
///
///     let v = a in v == null ? null : v.b
class NullAwarePropertyGetJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  NullAwarePropertyGetJudgment(
      VariableDeclaration variable, ConditionalExpression body)
      : super(variable, body);

  @override
  ConditionalExpression get body => super.body;

  PropertyGet get _desugaredGet => body.otherwise;

  Expression get receiverJudgment => variable.initializer;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitNullAwarePropertyGetJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a procedure in kernel form.
class ShadowProcedure extends Procedure implements ShadowMember {
  @override
  InferenceNode inferenceNode;

  final bool _hasImplicitReturnType;

  ShadowProcedure(Name name, ProcedureKind kind, FunctionNode function,
      this._hasImplicitReturnType,
      {Uri fileUri, bool isAbstract: false})
      : super(name, kind, function, fileUri: fileUri, isAbstract: isAbstract);

  @override
  void setInferredType(
      TypeInferenceEngine engine, Uri uri, DartType inferredType) {
    if (isSetter) {
      if (function.positionalParameters.length > 0) {
        function.positionalParameters[0].type = inferredType;
      }
    } else if (isGetter) {
      function.returnType = inferredType;
    } else {
      unhandled("setInferredType", "not accessor", fileOffset, uri);
    }
  }

  static bool hasImplicitReturnType(ShadowProcedure procedure) {
    return procedure._hasImplicitReturnType;
  }
}

/// Concrete shadow object representing an assignment to a property.
class PropertyAssignmentJudgment extends ComplexAssignmentJudgmentWithReceiver {
  /// If this assignment uses null-aware access (`?.`), the conditional
  /// expression that guards the access; otherwise `null`.
  ConditionalExpression nullAwareGuard;

  PropertyAssignmentJudgment._(Expression receiver, Expression rhs,
      {bool isSuper: false})
      : super._(receiver, rhs, isSuper);

  @override
  List<String> _getToStringParts() {
    var parts = super._getToStringParts();
    if (nullAwareGuard != null) parts.add('nullAwareGuard=$nullAwareGuard');
    return parts;
  }

  Object _handleWriteContravariance(
      ShadowTypeInferrer inferrer, DartType receiverType) {
    return inferrer.findPropertySetMember(receiverType, write);
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitPropertyAssignmentJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a redirecting initializer in kernel
/// form.
class RedirectingInitializerJudgment extends RedirectingInitializer
    implements InitializerJudgment {
  RedirectingInitializerJudgment(
      Constructor target, ArgumentsJudgment arguments)
      : super(target, arguments);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitRedirectingInitializerJudgment(this);
  }
}

/// Concrete shadow object representing a return statement in kernel form.
class ReturnJudgment extends ReturnStatement implements StatementJudgment {
  final String returnKeywordLexeme;

  ReturnJudgment(this.returnKeywordLexeme, [Expression expression])
      : super(expression);

  Expression get judgment => expression;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitReturnJudgment(this);
  }
}

/// Common base class for shadow objects representing statements in kernel
/// form.
abstract class StatementJudgment extends Statement {
  /// Calls back to [inferrer] to perform type inference for whatever concrete
  /// type of [StatementJudgment] this is.
  void acceptInference(InferenceVisitor visitor);
}

/// Concrete shadow object representing an assignment to a static variable.
class StaticAssignmentJudgment extends ComplexAssignmentJudgment {
  StaticAssignmentJudgment._(Expression rhs) : super._(rhs);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitStaticAssignmentJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a super initializer in kernel form.
class SuperInitializerJudgment extends SuperInitializer
    implements InitializerJudgment {
  SuperInitializerJudgment(Constructor target, ArgumentsJudgment arguments)
      : super(target, arguments);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitSuperInitializerJudgment(this);
  }
}

/// Shadow object for [SuperMethodInvocation].
class SuperMethodInvocationJudgment extends SuperMethodInvocation
    implements ExpressionJudgment {
  DartType inferredType;

  SuperMethodInvocationJudgment(Name name, ArgumentsJudgment arguments,
      {Procedure interfaceTarget})
      : super(name, arguments, interfaceTarget);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitSuperMethodInvocationJudgment(this, typeContext);
  }
}

/// Shadow object for [SuperPropertyGet].
class SuperPropertyGetJudgment extends SuperPropertyGet
    implements ExpressionJudgment {
  DartType inferredType;

  SuperPropertyGetJudgment(Name name, {Member interfaceTarget})
      : super(name, interfaceTarget);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitSuperPropertyGetJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a switch case.
class SwitchCaseJudgment extends SwitchCase {
  SwitchCaseJudgment(
      List<Expression> expressions, List<int> expressionOffsets, Statement body,
      {bool isDefault: false})
      : super(expressions, expressionOffsets, body, isDefault: isDefault);

  SwitchCaseJudgment.defaultCase(Statement body) : super.defaultCase(body);

  SwitchCaseJudgment.empty() : super.empty();

  List<Expression> get expressionJudgments => expressions.cast();
}

/// Concrete shadow object representing a switch statement in kernel form.
class SwitchStatementJudgment extends SwitchStatement
    implements StatementJudgment {
  SwitchStatementJudgment(Expression expression, List<SwitchCase> cases)
      : super(expression, cases);

  Expression get expressionJudgment => expression;

  List<SwitchCaseJudgment> get caseJudgments => cases.cast();

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitSwitchStatementJudgment(this);
  }
}

/// Shadow object for [SymbolLiteral].
class SymbolLiteralJudgment extends SymbolLiteral
    implements ExpressionJudgment {
  DartType inferredType;

  SymbolLiteralJudgment(String value) : super(value);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitSymbolLiteralJudgment(this, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to invoke an unresolved
/// constructor, or a constructor that cannot be invoked, or a resolved
/// constructor with wrong number of arguments.
// TODO(ahe): Remove this?
class InvalidConstructorInvocationJudgment extends SyntheticExpressionJudgment {
  final Member constructor;
  final Arguments arguments;

  InvalidConstructorInvocationJudgment._(
      kernel.Expression desugared, this.constructor, this.arguments)
      : super._(desugared);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitInvalidConstructorInvocationJudgment(this, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to assign to the
/// [expression] which is not assignable.
class InvalidWriteJudgment extends SyntheticExpressionJudgment {
  final Expression expression;

  InvalidWriteJudgment._(kernel.Expression desugared, this.expression)
      : super._(desugared);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitInvalidWriteJudgment(this, typeContext);
  }
}

/// Shadow object for expressions that are introduced by the front end as part
/// of desugaring or the handling of error conditions.
///
/// These expressions are removed by type inference and replaced with their
/// desugared equivalents.
class SyntheticExpressionJudgment extends Let implements ExpressionJudgment {
  DartType inferredType;

  SyntheticExpressionJudgment._(Expression desugared)
      : super(new VariableDeclaration('_', initializer: new NullLiteral()),
            desugared);

  /// The desugared kernel representation of this synthetic expression.
  Expression get desugared => body;

  void set desugared(Expression value) {
    this.body = value;
    value.parent = this;
  }

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitSyntheticExpressionJudgment(this, typeContext);
  }

  /// Removes this expression from the expression tree, replacing it with
  /// [desugared].
  void _replaceWithDesugared() {
    parent.replaceChild(this, desugared);
    parent = null;
  }

  /// Updates any [Let] nodes in the desugared expression to account for the
  /// fact that [expression] has the given [type].
  void _storeLetType(
      TypeInferrerImpl inferrer, Expression expression, DartType type) {
    Expression desugared = this.desugared;
    while (true) {
      if (desugared is Let) {
        Let desugaredLet = desugared;
        var variable = desugaredLet.variable;
        if (identical(variable.initializer, expression)) {
          variable.type = type;
          return;
        }
        desugared = desugaredLet.body;
      } else if (desugared is ConditionalExpression) {
        // When a null-aware assignment is desugared, often the "then" or "else"
        // branch of the conditional expression often contains "let" nodes that
        // need to be updated.
        ConditionalExpression desugaredConditionalExpression = desugared;
        if (desugaredConditionalExpression.then is Let) {
          desugared = desugaredConditionalExpression.then;
        } else {
          desugared = desugaredConditionalExpression.otherwise;
        }
      } else {
        break;
      }
    }
  }

  @override
  accept(ExpressionVisitor v) {
    // This is designed to throw an exception during serialization. It can also
    // lead to exceptions during transformations, but we have to accept a
    // [Transformer] as this is used to implement `replaceChild`.
    if (v is Transformer) return super.accept(v);
    unsupported("accept", fileOffset, getFileUri(this));
  }

  @override
  accept1(ExpressionVisitor1 v, arg) {
    unsupported("accept1", fileOffset, getFileUri(this));
  }

  @override
  visitChildren(Visitor v) {
    unsupported("visitChildren", fileOffset, getFileUri(this));
  }
}

/// Concrete shadow object representing a catch clause.
class CatchJudgment extends Catch {
  CatchJudgment(VariableDeclaration exception, Statement body,
      {DartType guard: const DynamicType(), VariableDeclaration stackTrace})
      : super(exception, body, guard: guard, stackTrace: stackTrace);

  VariableDeclarationJudgment get exceptionJudgment => exception;

  VariableDeclarationJudgment get stackTraceJudgment => stackTrace;
}

/// Concrete shadow object representing a try-catch block in kernel form.
class TryCatchJudgment extends TryCatch implements StatementJudgment {
  TryCatchJudgment(Statement body, List<Catch> catches) : super(body, catches);

  List<CatchJudgment> get catchJudgments => catches.cast();

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitTryCatchJudgment(this);
  }
}

/// Concrete shadow object representing a try-finally block in kernel form.
class TryFinallyJudgment extends TryFinally implements StatementJudgment {
  TryFinallyJudgment(Statement body, Statement finalizer)
      : super(body, finalizer);

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitTryFinallyJudgment(this);
  }
}

/// Concrete implementation of [TypeInferenceEngine] specialized to work with
/// kernel objects.
class ShadowTypeInferenceEngine extends TypeInferenceEngine {
  ShadowTypeInferenceEngine(Instrumentation instrumentation)
      : super(instrumentation);

  @override
  ShadowTypeInferrer createLocalTypeInferrer(
      Uri uri, InterfaceType thisType, KernelLibraryBuilder library) {
    return new TypeInferrer(this, uri, false, thisType, library);
  }

  @override
  ShadowTypeInferrer createTopLevelTypeInferrer(
      InterfaceType thisType, ShadowField field, KernelLibraryBuilder library) {
    return field._typeInferrer =
        new TypeInferrer(this, field.fileUri, true, thisType, library);
  }

  @override
  ShadowTypeInferrer getFieldTypeInferrer(ShadowField field) {
    return field._typeInferrer;
  }
}

/// Concrete implementation of [TypeInferrer] specialized to work with kernel
/// objects.
class ShadowTypeInferrer extends TypeInferrerImpl {
  @override
  final typePromoter;

  ShadowTypeInferrer.private(ShadowTypeInferenceEngine engine, Uri uri,
      bool topLevel, InterfaceType thisType, KernelLibraryBuilder library)
      : typePromoter = new TypePromoter(engine.typeSchemaEnvironment),
        super.private(engine, uri, topLevel, thisType, library);

  @override
  Expression getFieldInitializer(ShadowField field) {
    return field.initializer;
  }

  @override
  DartType inferExpression(
      kernel.Expression expression, DartType typeContext, bool typeNeeded,
      {bool isVoidAllowed: false}) {
    // `null` should never be used as the type context.  An instance of
    // `UnknownType` should be used instead.
    assert(typeContext != null);

    // It isn't safe to do type inference on an expression without a parent,
    // because type inference might cause us to have to replace one expression
    // with another, and we can only replace a node if it has a parent pointer.
    assert(expression.parent != null);

    // For full (non-top level) inference, we need access to the
    // ExpressionGeneratorHelper so that we can perform error recovery.
    assert(isTopLevel || helper != null);

    // When doing top level inference, we skip subexpressions whose type isn't
    // needed so that we don't induce bogus dependencies on fields mentioned in
    // those subexpressions.
    if (!typeNeeded) return null;

    InferenceVisitor visitor = new InferenceVisitor(this);
    if (expression is ExpressionJudgment) {
      expression.acceptInference(visitor, typeContext);
    } else {
      expression.accept1(visitor, typeContext);
    }
    DartType inferredType = getInferredType(expression, this);
    if (inferredType is VoidType && !isVoidAllowed) {
      if (expression.parent is! ArgumentsJudgment) {
        helper?.addProblem(
            messageVoidExpression, expression.fileOffset, noLength);
      }
    }
    return inferredType;
  }

  @override
  DartType inferFieldTopLevel(ShadowField field) {
    if (field.initializer == null) return const DynamicType();
    return inferExpression(field.initializer, const UnknownType(), true,
        isVoidAllowed: true);
  }

  @override
  void inferInitializer(
      InferenceHelper helper, kernel.Initializer initializer) {
    assert(initializer is InitializerJudgment);
    this.helper = helper;
    // Use polymorphic dispatch on [KernelInitializer] to perform whatever
    // kind of type inference is correct for this kind of initializer.
    // TODO(paulberry): experiment to see if dynamic dispatch would be better,
    // so that the type hierarchy will be simpler (which may speed up "is"
    // checks).
    InitializerJudgment kernelInitializer = initializer;
    kernelInitializer.acceptInference(new InferenceVisitor(this));
    this.helper = null;
  }

  @override
  void inferStatement(Statement statement) {
    // For full (non-top level) inference, we need access to the
    // ExpressionGeneratorHelper so that we can perform error recovery.
    if (!isTopLevel) assert(helper != null);

    if (statement is StatementJudgment) {
      // Use polymorphic dispatch on [KernelStatement] to perform whatever kind
      // of type inference is correct for this kind of statement.
      // TODO(paulberry): experiment to see if dynamic dispatch would be better,
      // so that the type hierarchy will be simpler (which may speed up "is"
      // checks).
      return statement.acceptInference(new InferenceVisitor(this));
    } else if (statement is ForInStatement) {
      return statement.accept1(new InferenceVisitor(this), null);
    } else if (statement is LabeledStatement) {
      return statement.accept1(new InferenceVisitor(this), null);
    } else if (statement is BreakStatement) {
      return statement.accept1(new InferenceVisitor(this), null);
    } else {
      // Encountered a statement type for which type inference is not yet
      // implemented, so just skip it for now.
      // TODO(paulberry): once the BodyBuilder uses shadow classes for
      // everything, this case should no longer be needed.
    }
  }
}

class TypeLiteralJudgment extends TypeLiteral implements ExpressionJudgment {
  DartType inferredType;

  TypeLiteralJudgment(DartType type) : super(type);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitTypeLiteralJudgment(this, typeContext);
  }
}

/// Concrete implementation of [TypePromoter] specialized to work with kernel
/// objects.
class ShadowTypePromoter extends TypePromoterImpl {
  ShadowTypePromoter.private(TypeSchemaEnvironment typeSchemaEnvironment)
      : super.private(typeSchemaEnvironment);

  @override
  int getVariableFunctionNestingLevel(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      return variable._functionNestingLevel;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
      return 0;
    }
  }

  @override
  bool isPromotionCandidate(VariableDeclaration variable) {
    assert(variable is VariableDeclarationJudgment);
    VariableDeclarationJudgment kernelVariableDeclaration = variable;
    return !kernelVariableDeclaration._isLocalFunction;
  }

  @override
  bool sameExpressions(Expression a, Expression b) {
    return identical(a, b);
  }

  @override
  void setVariableMutatedAnywhere(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      variable._mutatedAnywhere = true;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
    }
  }

  @override
  void setVariableMutatedInClosure(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      variable._mutatedInClosure = true;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
    }
  }

  @override
  bool wasVariableMutatedAnywhere(VariableDeclaration variable) {
    if (variable is VariableDeclarationJudgment) {
      return variable._mutatedAnywhere;
    } else {
      // Hack to deal with the fact that BodyBuilder still creates raw
      // VariableDeclaration objects sometimes.
      // TODO(paulberry): get rid of this once the type parameter is
      // KernelVariableDeclaration.
      return true;
    }
  }
}

class VariableAssignmentJudgment extends ComplexAssignmentJudgment {
  VariableAssignmentJudgment._(Expression rhs) : super._(rhs);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitVariableAssignmentJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a variable declaration in kernel form.
class VariableDeclarationJudgment extends VariableDeclaration
    implements StatementJudgment {
  final bool forSyntheticToken;

  final bool _implicitlyTyped;

  // TODO(ahe): Remove this field. We can get rid of it by recording closure
  // mutation in [BodyBuilder].
  final int _functionNestingLevel;

  // TODO(ahe): Remove this field. It's only used locally when compiling a
  // method, and this can thus be tracked in a [Set] (actually, tracking this
  // information in a [List] is probably even faster as the average size will
  // be close to zero).
  bool _mutatedInClosure = false;

  // TODO(ahe): Investigate if this can be removed.
  bool _mutatedAnywhere = false;

  // TODO(ahe): Investigate if this can be removed.
  final bool _isLocalFunction;

  VariableDeclarationJudgment(String name, this._functionNestingLevel,
      {this.forSyntheticToken: false,
      Expression initializer,
      DartType type,
      bool isFinal: false,
      bool isConst: false,
      bool isFieldFormal: false,
      bool isCovariant: false,
      bool isLocalFunction: false})
      : _implicitlyTyped = type == null,
        _isLocalFunction = isLocalFunction,
        super(name,
            initializer: initializer,
            type: type ?? const DynamicType(),
            isFinal: isFinal,
            isConst: isConst,
            isFieldFormal: isFieldFormal,
            isCovariant: isCovariant);

  VariableDeclarationJudgment.forEffect(
      Expression initializer, this._functionNestingLevel)
      : forSyntheticToken = false,
        _implicitlyTyped = false,
        _isLocalFunction = false,
        super.forValue(initializer);

  VariableDeclarationJudgment.forValue(
      Expression initializer, this._functionNestingLevel)
      : forSyntheticToken = false,
        _implicitlyTyped = true,
        _isLocalFunction = false,
        super.forValue(initializer);

  Expression get initializerJudgment => initializer;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitVariableDeclarationJudgment(this);
  }

  /// Determine whether the given [VariableDeclarationJudgment] had an implicit
  /// type.
  ///
  /// This is static to avoid introducing a method that would be visible to
  /// the kernel.
  static bool isImplicitlyTyped(VariableDeclarationJudgment variable) =>
      variable._implicitlyTyped;

  /// Determines whether the given [VariableDeclarationJudgment] represents a
  /// local function.
  ///
  /// This is static to avoid introducing a method that would be visible to the
  /// kernel.
  static bool isLocalFunction(VariableDeclarationJudgment variable) =>
      variable._isLocalFunction;
}

/// Synthetic judgment class representing an attempt to invoke an unresolved
/// target.
class UnresolvedTargetInvocationJudgment extends SyntheticExpressionJudgment {
  final ArgumentsJudgment argumentsJudgment;

  UnresolvedTargetInvocationJudgment._(
      kernel.Expression desugared, this.argumentsJudgment)
      : super._(desugared);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitUnresolvedTargetInvocationJudgment(this, typeContext);
  }
}

/// Synthetic judgment class representing an attempt to assign to an unresolved
/// variable.
class UnresolvedVariableAssignmentJudgment extends SyntheticExpressionJudgment {
  final bool isCompound;
  final Expression rhs;

  UnresolvedVariableAssignmentJudgment._(
      kernel.Expression desugared, this.isCompound, this.rhs)
      : super._(desugared);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitUnresolvedVariableAssignmentJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a read from a variable in kernel form.
class VariableGetJudgment extends VariableGet implements ExpressionJudgment {
  DartType inferredType;

  final TypePromotionFact _fact;

  final TypePromotionScope _scope;

  VariableGetJudgment(VariableDeclaration variable, this._fact, this._scope)
      : super(variable);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitVariableGetJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a while loop in kernel form.
class WhileJudgment extends WhileStatement implements StatementJudgment {
  WhileJudgment(Expression condition, Statement body) : super(condition, body);

  Expression get conditionJudgment => condition;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitWhileJudgment(this);
  }
}

/// Concrete shadow object representing a yield statement in kernel form.
class YieldJudgment extends YieldStatement implements StatementJudgment {
  YieldJudgment(bool isYieldStar, Expression expression)
      : super(expression, isYieldStar: isYieldStar);

  Expression get judgment => expression;

  @override
  void acceptInference(InferenceVisitor visitor) {
    return visitor.visitYieldJudgment(this);
  }
}

/// Concrete shadow object representing a deferred load library call.
class LoadLibraryJudgment extends LoadLibrary implements ExpressionJudgment {
  final Arguments arguments;

  DartType inferredType;

  LoadLibraryJudgment(LibraryDependency import, this.arguments) : super(import);

  ArgumentsJudgment get argumentJudgments => arguments;

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitLoadLibraryJudgment(this, typeContext);
  }
}

/// Concrete shadow object representing a tear-off of a `loadLibrary` function.
class LoadLibraryTearOffJudgment extends StaticGet
    implements ExpressionJudgment {
  final LibraryDependency import;

  DartType inferredType;

  LoadLibraryTearOffJudgment(this.import, Procedure target) : super(target);

  @override
  void acceptInference(InferenceVisitor visitor, DartType typeContext) {
    return visitor.visitLoadLibraryTearOffJudgment(this, typeContext);
  }
}

/// The result of inference for a RHS of an assignment.
class _ComplexAssignmentInferenceResult {
  /// The resolved combiner [Procedure], e.g. `operator+` for `a += 2`, or
  /// `null` if the assignment is not compound.
  final Procedure combiner;

  _ComplexAssignmentInferenceResult(this.combiner);
}

class _UnfinishedCascade extends Expression {
  accept(v) => unsupported("accept", -1, null);

  accept1(v, arg) => unsupported("accept1", -1, null);

  getStaticType(types) => unsupported("getStaticType", -1, null);

  transformChildren(v) => unsupported("transformChildren", -1, null);

  visitChildren(v) => unsupported("visitChildren", -1, null);
}

class SyntheticWrapper {
  static Expression wrapIllegalAssignment(Expression rhs,
      {int assignmentOffset: -1}) {
    return new IllegalAssignmentJudgment._(rhs,
        assignmentOffset: assignmentOffset)
      ..fileOffset = rhs.fileOffset;
  }

  static Expression wrapIndexAssignment(
      Expression receiver, Expression index, Expression rhs,
      {bool isSuper: false}) {
    return new IndexAssignmentJudgment._(receiver, index, rhs, isSuper: isSuper)
      ..fileOffset = index.fileOffset;
  }

  static Expression wrapInvalidConstructorInvocation(
      kernel.Expression desugared, Member constructor, Arguments arguments) {
    return new InvalidConstructorInvocationJudgment._(
        desugared, constructor, arguments)
      ..fileOffset = desugared.fileOffset;
  }

  static Expression wrapInvalidWrite(
      Expression desugared, Expression expression) {
    return new InvalidWriteJudgment._(desugared, expression)
      ..fileOffset = desugared.fileOffset;
  }

  static Expression wrapPropertyAssignment(Expression receiver, Expression rhs,
      {bool isSuper: false}) {
    return new PropertyAssignmentJudgment._(receiver, rhs, isSuper: isSuper)
      ..fileOffset = rhs.fileOffset;
  }

  static Expression wrapStaticAssignment(Expression rhs) {
    return new StaticAssignmentJudgment._(rhs)..fileOffset = rhs.fileOffset;
  }

  static Expression wrapSyntheticExpression(Expression desugared) {
    return new SyntheticExpressionJudgment._(desugared)
      ..fileOffset = desugared.fileOffset;
  }

  static Expression wrapUnresolvedTargetInvocation(
      Expression desugared, Arguments arguments) {
    return new UnresolvedTargetInvocationJudgment._(desugared, arguments)
      ..fileOffset = desugared.fileOffset;
  }

  static Expression wrapUnresolvedVariableAssignment(
      Expression desugared, bool isCompound, Expression rhs) {
    return new UnresolvedVariableAssignmentJudgment._(
        desugared, isCompound, rhs)
      ..fileOffset = desugared.fileOffset;
  }

  static Expression wrapVariableAssignment(Expression rhs) {
    return new VariableAssignmentJudgment._(rhs)..fileOffset = rhs.fileOffset;
  }
}