text: //===--- ConstraintSystem.h - Constraint-based Type Checking ----*- C++ -*-===//
  //
  // This source file is part of the Swift.org open source project
  //
  // Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
  // Licensed under Apache License v2.0 with Runtime Library Exception
  //
  // See https://swift.org/LICENSE.txt for license information
  // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
  //
  //===----------------------------------------------------------------------===//
  //
  // This file provides the constraint-based type checker, anchored by the
  // \c ConstraintSystem class, which provides type checking and type
  // inference for expressions.
  //
  //===----------------------------------------------------------------------===//
  #ifndef SWIFT_SEMA_CONSTRAINT_SYSTEM_H
  #define SWIFT_SEMA_CONSTRAINT_SYSTEM_H
  
  #include "CSFix.h"
  #include "Constraint.h"
  #include "ConstraintGraph.h"
  #include "ConstraintGraphScope.h"
  #include "ConstraintLocator.h"
  #include "OverloadChoice.h"
  #include "TypeChecker.h"
  #include "swift/AST/ASTVisitor.h"
  #include "swift/AST/ASTWalker.h"
  #include "swift/AST/NameLookup.h"
  #include "swift/AST/PropertyWrappers.h"
  #include "swift/AST/TypeCheckerDebugConsumer.h"
  #include "swift/AST/Types.h"
  #include "swift/Basic/LLVM.h"
  #include "swift/Basic/OptionSet.h"
  #include "llvm/ADT/PointerUnion.h"
  #include "llvm/ADT/STLExtras.h"
  #include "llvm/ADT/SetOperations.h"
  #include "llvm/ADT/SetVector.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/ilist.h"
  #include "llvm/Support/ErrorHandling.h"
  #include "llvm/Support/Timer.h"
  #include "llvm/Support/raw_ostream.h"
  #include <cstddef>
  #include <functional>
  
  namespace swift {
  
  class Expr;
  
  namespace constraints {
  
  class ConstraintGraph;
  class ConstraintGraphNode;
  class ConstraintSystem;
  
  } // end namespace constraints
  
  } // end namespace swift
  
  /// Allocate memory within the given constraint system.
  void *operator new(size_t bytes, swift::constraints::ConstraintSystem& cs,
                     size_t alignment = 8);
  
  namespace swift {
  
  namespace constraints {
  
  /// A handle that holds the saved state of a type variable, which
  /// can be restored.
  class SavedTypeVariableBinding {
    /// The type variable and type variable options.
    llvm::PointerIntPair<TypeVariableType *, 3> TypeVarAndOptions;
    
    /// The parent or fixed type.
    llvm::PointerUnion<TypeVariableType *, TypeBase *> ParentOrFixed;
  
  public:
    explicit SavedTypeVariableBinding(TypeVariableType *typeVar);
  
    /// Restore the state of the type variable to the saved state.
    void restore();
  
    TypeVariableType *getTypeVariable() { return TypeVarAndOptions.getPointer(); }
    unsigned getOptions() { return TypeVarAndOptions.getInt(); }
  };
  
  /// A set of saved type variable bindings.
  using SavedTypeVariableBindings = SmallVector<SavedTypeVariableBinding, 16>;
  
  class ConstraintLocator;
  
  /// Describes a conversion restriction or a fix.
  struct RestrictionOrFix {
    union {
      ConversionRestrictionKind Restriction;
      ConstraintFix *TheFix;
    };
    bool IsRestriction;
  
  public:
    RestrictionOrFix(ConversionRestrictionKind restriction)
    : Restriction(restriction), IsRestriction(true) { }
  
    RestrictionOrFix(ConstraintFix *fix) : TheFix(fix), IsRestriction(false) {}
  
    Optional<ConversionRestrictionKind> getRestriction() const {
      if (IsRestriction)
        return Restriction;
  
      return None;
    }
  
    Optional<ConstraintFix *> getFix() const {
      if (!IsRestriction)
        return TheFix;
  
      return None;
    }
  };
  
  
  class ExpressionTimer {
    Expr* E;
    unsigned WarnLimit;
    ASTContext &Context;
    llvm::TimeRecord StartTime;
  
    bool PrintDebugTiming;
    bool PrintWarning;
  
  public:
    ExpressionTimer(Expr *E, ConstraintSystem &CS);
  
    ~ExpressionTimer();
  
    llvm::TimeRecord startedAt() const { return StartTime; }
  
    /// Return the elapsed process time (including fractional seconds)
    /// as a double.
    double getElapsedProcessTimeInFractionalSeconds() const {
      llvm::TimeRecord endTime = llvm::TimeRecord::getCurrentTime(false);
  
      return endTime.getProcessTime() - StartTime.getProcessTime();
    }
  
    // Disable emission of warnings about expressions that take longer
    // than the warning threshold.
    void disableWarning() { PrintWarning = false; }
  
    bool isExpired(unsigned thresholdInMillis) const {
      auto elapsed = getElapsedProcessTimeInFractionalSeconds();
      return unsigned(elapsed) > thresholdInMillis;
    }
  };
  
  } // end namespace constraints
  
  /// Options that describe how a type variable can be used.
  enum TypeVariableOptions {
    /// Whether the type variable can be bound to an lvalue type or not.
    TVO_CanBindToLValue = 0x01,
  
    /// Whether the type variable can be bound to an inout type or not.
    TVO_CanBindToInOut = 0x02,
  
    /// Whether a more specific deduction for this type variable implies a
    /// better solution to the constraint system.
    TVO_PrefersSubtypeBinding = 0x04
  };
  
  /// The implementation object for a type variable used within the
  /// constraint-solving type checker.
  ///
  /// The implementation object for a type variable contains information about
  /// the type variable, where it was generated, what protocols it must conform
  /// to, what specific types it might be and, eventually, the fixed type to
  /// which it is assigned.
  class TypeVariableType::Implementation {
    /// The locator that describes where this type variable was generated.
    constraints::ConstraintLocator *locator;
  
    /// Either the parent of this type variable within an equivalence
    /// class of type variables, or the fixed type to which this type variable
    /// type is bound.
    llvm::PointerUnion<TypeVariableType *, TypeBase *> ParentOrFixed;
  
    /// The corresponding node in the constraint graph.
    constraints::ConstraintGraphNode *GraphNode = nullptr;
  
    friend class constraints::SavedTypeVariableBinding;
  
  public:
    /// Retrieve the type variable associated with this implementation.
    TypeVariableType *getTypeVariable() {
      return reinterpret_cast<TypeVariableType *>(this) - 1;
    }
  
    /// Retrieve the type variable associated with this implementation.
    const TypeVariableType *getTypeVariable() const {
      return reinterpret_cast<const TypeVariableType *>(this) - 1;
    }
  
    explicit Implementation(constraints::ConstraintLocator *locator,
                            unsigned options)
      : locator(locator), ParentOrFixed(getTypeVariable()) {
      getTypeVariable()->Bits.TypeVariableType.Options = options;
    }
  
    /// Retrieve the unique ID corresponding to this type variable.
    unsigned getID() const { return getTypeVariable()->getID(); }
  
    unsigned getRawOptions() const {
      return getTypeVariable()->Bits.TypeVariableType.Options;
    }
  
    void setRawOptions(unsigned bits) {
      getTypeVariable()->Bits.TypeVariableType.Options = bits;
      assert(getTypeVariable()->Bits.TypeVariableType.Options == bits
             && "Trucation");
    }
  
    /// Whether this type variable can bind to an lvalue type.
    bool canBindToLValue() const { return getRawOptions() & TVO_CanBindToLValue; }
  
    /// Whether this type variable can bind to an inout type.
    bool canBindToInOut() const { return getRawOptions() & TVO_CanBindToInOut; }
  
    /// Whether this type variable prefers a subtype binding over a supertype
    /// binding.
    bool prefersSubtypeBinding() const {
      return getRawOptions() & TVO_PrefersSubtypeBinding;
    }
  
    /// Retrieve the corresponding node in the constraint graph.
    constraints::ConstraintGraphNode *getGraphNode() const { return GraphNode; }
  
    /// Set the corresponding node in the constraint graph.
    void setGraphNode(constraints::ConstraintGraphNode *newNode) { 
      GraphNode = newNode; 
    }
  
    /// Retrieve the index into the constraint graph's list of type variables.
    unsigned getGraphIndex() const { 
      assert(GraphNode && "Graph node isn't set");
      return getTypeVariable()->Bits.TypeVariableType.GraphIndex; 
    }
  
    /// Set the index into the constraint graph's list of type variables.
    void setGraphIndex(unsigned newIndex) {
      getTypeVariable()->Bits.TypeVariableType.GraphIndex = newIndex;
    }
    
    /// Check whether this type variable either has a representative that
    /// is not itself or has a fixed type binding.
    bool hasRepresentativeOrFixed() const {
      // If we have a fixed type, we're done.
      if (!ParentOrFixed.is<TypeVariableType *>())
        return true;
  
      // Check whether the representative is different from our own type
      // variable.
      return ParentOrFixed.get<TypeVariableType *>() != getTypeVariable();
    }
  
    /// Record the current type-variable binding.
    void recordBinding(constraints::SavedTypeVariableBindings &record) {
      record.push_back(constraints::SavedTypeVariableBinding(getTypeVariable()));
    }
  
    /// Retrieve the locator describing where this type variable was
    /// created.
    constraints::ConstraintLocator *getLocator() const {
      return locator;
    }
  
    /// Retrieve the generic parameter opened by this type variable.
    GenericTypeParamType *getGenericParameter() const;
  
    /// Retrieve the representative of the equivalence class to which this
    /// type variable belongs.
    ///
    /// \param record The record of changes made by retrieving the representative,
    /// which can happen due to path compression. If null, path compression is
    /// not performed.
    TypeVariableType *
    getRepresentative(constraints::SavedTypeVariableBindings *record) {
      // Find the representative type variable.
      auto result = getTypeVariable();
      Implementation *impl = this;
      while (impl->ParentOrFixed.is<TypeVariableType *>()) {
        // Extract the representative.
        auto nextTV = impl->ParentOrFixed.get<TypeVariableType *>();
        if (nextTV == result)
          break;
  
        result = nextTV;
        impl = &nextTV->getImpl();
      }
  
      if (impl == this || !record)
        return result;
  
      // Perform path compression.
      impl = this;
      while (impl->ParentOrFixed.is<TypeVariableType *>()) {
        // Extract the representative.
        auto nextTV = impl->ParentOrFixed.get<TypeVariableType *>();
        if (nextTV == result)
          break;
  
        // Record the state change.
        impl->recordBinding(*record);
  
        impl->ParentOrFixed = result;
        impl = &nextTV->getImpl();
      }
  
      return result;
    }
  
    /// Merge the equivalence class of this type variable with the
    /// equivalence class of another type variable.
    ///
    /// \param other The type variable to merge with.
    ///
    /// \param record The record of state changes.
    void mergeEquivalenceClasses(TypeVariableType *other,
                                 constraints::SavedTypeVariableBindings *record) {
      // Merge the equivalence classes corresponding to these two type
      // variables. Always merge 'up' the constraint stack, because it is simpler.
      if (getID() > other->getImpl().getID()) {
        other->getImpl().mergeEquivalenceClasses(getTypeVariable(), record);
        return;
      }
  
      auto otherRep = other->getImpl().getRepresentative(record);
      if (record)
        otherRep->getImpl().recordBinding(*record);
      otherRep->getImpl().ParentOrFixed = getTypeVariable();
  
      if (canBindToLValue() && !otherRep->getImpl().canBindToLValue()) {
        if (record)
          recordBinding(*record);
        getTypeVariable()->Bits.TypeVariableType.Options &= ~TVO_CanBindToLValue;
      }
  
      if (canBindToInOut() && !otherRep->getImpl().canBindToInOut()) {
        if (record)
          recordBinding(*record);
        getTypeVariable()->Bits.TypeVariableType.Options &= ~TVO_CanBindToInOut;
      }
    }
  
    /// Retrieve the fixed type that corresponds to this type variable,
    /// if there is one.
    ///
    /// \returns the fixed type associated with this type variable, or a null
    /// type if there is no fixed type.
    ///
    /// \param record The record of changes made by retrieving the representative,
    /// which can happen due to path compression. If null, path compression is
    /// not performed.
    Type getFixedType(constraints::SavedTypeVariableBindings *record) {
      // Find the representative type variable.
      auto rep = getRepresentative(record);
      Implementation &repImpl = rep->getImpl();
  
      // Return the bound type if there is one, otherwise, null.
      return repImpl.ParentOrFixed.dyn_cast<TypeBase *>();
    }
  
    /// Assign a fixed type to this equivalence class.
    void assignFixedType(Type type,
                         constraints::SavedTypeVariableBindings *record) {
      assert((!getFixedType(0) || getFixedType(0)->isEqual(type)) &&
             "Already has a fixed type!");
      auto rep = getRepresentative(record);
      if (record)
        rep->getImpl().recordBinding(*record);
      rep->getImpl().ParentOrFixed = type.getPointer();
    }
  
    void setCanBindToLValue(constraints::SavedTypeVariableBindings *record,
                            bool enabled) {
      auto &impl = getRepresentative(record)->getImpl();
      if (record)
        impl.recordBinding(*record);
  
      if (enabled)
        impl.getTypeVariable()->Bits.TypeVariableType.Options |=
            TVO_CanBindToLValue;
      else
        impl.getTypeVariable()->Bits.TypeVariableType.Options &=
            ~TVO_CanBindToLValue;
    }
  
    /// Print the type variable to the given output stream.
    void print(llvm::raw_ostream &OS);
  };
  
  namespace constraints {
  
  struct ResolvedOverloadSetListItem;
  
  /// The result of comparing two constraint systems that are a solutions
  /// to the given set of constraints.
  enum class SolutionCompareResult {
    /// The two solutions are incomparable, because, e.g., because one
    /// solution has some better decisions and some worse decisions than the
    /// other.
    Incomparable,
    /// The two solutions are identical.
    Identical,
    /// The first solution is better than the second.
    Better,
    /// The second solution is better than the first.
    Worse
  };
  
  /// An overload that has been selected in a particular solution.
  ///
  /// A selected overload captures the specific overload choice (e.g., a
  /// particular declaration) as well as the type to which the reference to the
  /// declaration was opened, which may involve type variables.
  struct SelectedOverload {
    /// The overload choice.
    OverloadChoice choice;
  
    /// The opened type of the base of the reference to this overload, if
    /// we're referencing a member.
    Type openedFullType;
  
    /// The opened type produced by referring to this overload.
    Type openedType;
  };
  
  /// Describes an aspect of a solution that affects its overall score, i.e., a
  /// user-defined conversions.
  enum ScoreKind {
    // These values are used as indices into a Score value.
  
    /// A fix needs to be applied to the source.
    SK_Fix,
    /// A reference to an @unavailable declaration.
    SK_Unavailable,
    /// A use of a disfavored overload.
    SK_DisfavoredOverload,
    /// An implicit force of an implicitly unwrapped optional value.
    SK_ForceUnchecked,
    /// A user-defined conversion.
    SK_UserConversion,
    /// A non-trivial function conversion.
    SK_FunctionConversion,
    /// A literal expression bound to a non-default literal type.
    SK_NonDefaultLiteral,
    /// An implicit upcast conversion between collection types.
    SK_CollectionUpcastConversion,
    /// A value-to-optional conversion.
    SK_ValueToOptional,
    /// A conversion to an empty existential type ('Any' or '{}').
    SK_EmptyExistentialConversion,
    /// A key path application subscript.
    SK_KeyPathSubscript,
    /// A conversion from a string, array, or inout to a pointer.
    SK_ValueToPointerConversion,
  
    SK_LastScoreKind = SK_ValueToPointerConversion,
  };
  
  /// The number of score kinds.
  const unsigned NumScoreKinds = SK_LastScoreKind + 1;
  
  /// Describes the fixed score of a solution to the constraint system.
  struct Score {
    unsigned Data[NumScoreKinds] = {};
  
    friend Score &operator+=(Score &x, const Score &y) {
      for (unsigned i = 0; i != NumScoreKinds; ++i) {
        x.Data[i] += y.Data[i];
      }
      return x;
    }
  
    friend Score operator+(const Score &x, const Score &y) {
      Score result;
      for (unsigned i = 0; i != NumScoreKinds; ++i) {
        result.Data[i] = x.Data[i] + y.Data[i];
      }
      return result;
    }
  
    friend Score operator-(const Score &x, const Score &y) {
      Score result;
      for (unsigned i = 0; i != NumScoreKinds; ++i) {
        result.Data[i] = x.Data[i] - y.Data[i];
      }
      return result;
    }
  
    friend Score &operator-=(Score &x, const Score &y) {
      for (unsigned i = 0; i != NumScoreKinds; ++i) {
        x.Data[i] -= y.Data[i];
      }
      return x;
    }
  
    friend bool operator==(const Score &x, const Score &y) {
      for (unsigned i = 0; i != NumScoreKinds; ++i) {
        if (x.Data[i] != y.Data[i])
          return false;
      }
  
      return true;
    }
  
    friend bool operator!=(const Score &x, const Score &y) {
      return !(x == y);
    }
  
    friend bool operator<(const Score &x, const Score &y) {
      for (unsigned i = 0; i != NumScoreKinds; ++i) {
        if (x.Data[i] < y.Data[i])
          return true;
  
        if (x.Data[i] > y.Data[i])
          return false;
      }
  
      return false;
    }
  
    friend bool operator<=(const Score &x, const Score &y) {
      return !(y < x);
    }
  
    friend bool operator>(const Score &x, const Score &y) {
      return y < x;
    }
  
    friend bool operator>=(const Score &x, const Score &y) {
      return !(x < y);
    }
  
  };
  
  /// An AST node that can gain type information while solving.
  using TypedNode =
      llvm::PointerUnion3<const Expr *, const TypeLoc *,
                          const ParamDecl *>;
  
  /// Display a score.
  llvm::raw_ostream &operator<<(llvm::raw_ostream &out, const Score &score);
  
  /// Describes a dependent type that has been opened to a particular type
  /// variable.
  using OpenedType = std::pair<GenericTypeParamType *, TypeVariableType *>;
  
  using OpenedTypeMap =
      llvm::DenseMap<GenericTypeParamType *, TypeVariableType *>;
  
  /// A complete solution to a constraint system.
  ///
  /// A solution to a constraint system consists of type variable bindings to
  /// concrete types for every type variable that is used in the constraint
  /// system along with a set of mappings from each constraint locator
  /// involving an overload set to the selected overload.
  class Solution {
    /// The constraint system this solution solves.
    ConstraintSystem *constraintSystem;
  
    /// The fixed score for this solution.
    Score FixedScore;
  
  public:
    /// Create a solution for the given constraint system.
    Solution(ConstraintSystem &cs, const Score &score)
      : constraintSystem(&cs), FixedScore(score) {}
  
    // Solution is a non-copyable type for performance reasons.
    Solution(const Solution &other) = delete;
    Solution &operator=(const Solution &other) = delete;
  
    Solution(Solution &&other) = default;
    Solution &operator=(Solution &&other) = default;
  
    size_t getTotalMemory() const;
  
    /// Retrieve the constraint system that this solution solves.
    ConstraintSystem &getConstraintSystem() const { return *constraintSystem; }
  
    /// The set of type bindings.
    llvm::DenseMap<TypeVariableType *, Type> typeBindings;
    
    /// The set of overload choices along with their types.
    llvm::DenseMap<ConstraintLocator *, SelectedOverload> overloadChoices;
  
    /// The set of constraint restrictions used to arrive at this restriction,
    /// which informs constraint application.
    llvm::DenseMap<std::pair<CanType, CanType>, ConversionRestrictionKind>
      ConstraintRestrictions;
  
    /// The list of fixes that need to be applied to the initial expression
    /// to make the solution work.
    llvm::SmallVector<ConstraintFix *, 4> Fixes;
    /// The list of member references which couldn't be resolved,
    /// and had to be assumed based on their use.
    llvm::SmallVector<ConstraintLocator *, 4> MissingMembers;
  
    /// The set of disjunction choices used to arrive at this solution,
    /// which informs constraint application.
    llvm::DenseMap<ConstraintLocator *, unsigned> DisjunctionChoices;
  
    /// The set of opened types for a given locator.
    llvm::DenseMap<ConstraintLocator *, ArrayRef<OpenedType>> OpenedTypes;
  
    /// The opened existential type for a given locator.
    llvm::DenseMap<ConstraintLocator *, OpenedArchetypeType *>
      OpenedExistentialTypes;
  
    /// The locators of \c Defaultable constraints whose defaults were used.
    llvm::SmallPtrSet<ConstraintLocator *, 2> DefaultedConstraints;
  
    /// The node -> type mappings introduced by this solution.
    llvm::SmallVector<std::pair<TypedNode, Type>, 8> addedNodeTypes;
  
    std::vector<std::pair<ConstraintLocator *, ProtocolConformanceRef>>
        Conformances;
  
    /// The set of closures that have been transformed by a function builder.
    llvm::MapVector<ClosureExpr *, std::pair<Type, Expr *>>
        builderTransformedClosures;
  
    /// Simplify the given type by substituting all occurrences of
    /// type variables for their fixed types.
    Type simplifyType(Type type) const;
  
    /// Coerce the given expression to the given type.
    ///
    /// This operation cannot fail.
    ///
    /// \param expr The expression to coerce.
    /// \param toType The type to coerce the expression to.
    /// \param locator Locator used to describe the location of this expression.
    ///
    /// \param ignoreTopLevelInjection Whether to suppress diagnostics
    /// on a suspicious top-level optional injection (because the caller already
    /// diagnosed it).
    ///
    /// \param typeFromPattern Optionally, the caller can specify the pattern
    /// from where the toType is derived, so that we can deliver better fixit.
    ///
    /// \returns the coerced expression, which will have type \c ToType.
    Expr *coerceToType(Expr *expr, Type toType,
                       ConstraintLocator *locator,
                       bool ignoreTopLevelInjection = false,
                       Optional<Pattern*> typeFromPattern = None) const;
  
    /// Convert the given optional-producing expression to a Bool
    /// indicating whether the optional has a value.
    ///
    /// This operation cannot fail.
    ///
    /// \param expr The expression to coerce. The type of this expression
    /// must be T?.
    ///
    /// \param locator Locator used to describe the location of this expression.
    ///
    /// \returns a Bool expression indicating whether the optional
    /// contains a value.
    Expr *convertOptionalToBool(Expr *expr, ConstraintLocator *locator) const;
  
    /// Compute the set of substitutions for a generic signature opened at the
    /// given locator.
    ///
    /// \param sig The generic signature.
    ///
    /// \param locator The locator that describes where the substitutions came
    /// from.
    SubstitutionMap computeSubstitutions(GenericSignature *sig,
                                         ConstraintLocatorBuilder locator) const;
  
    /// Return the disjunction choice for the given constraint location.
    unsigned getDisjunctionChoice(ConstraintLocator *locator) const {
      assert(DisjunctionChoices.count(locator));
      return DisjunctionChoices.find(locator)->second;
    }
  
    /// Retrieve the fixed score of this solution
    const Score &getFixedScore() const { return FixedScore; }
  
    /// Retrieve the fixed score of this solution
    Score &getFixedScore() { return FixedScore; }
  
    /// Retrieve the fixed type for the given type variable.
    Type getFixedType(TypeVariableType *typeVar) const;
  
    /// Try to resolve the given locator to a declaration within this
    /// solution.
    ConcreteDeclRef resolveLocatorToDecl(ConstraintLocator *locator) const;
  
    /// Retrieve the overload choice associated with the given
    /// locator.
    SelectedOverload getOverloadChoice(ConstraintLocator *locator) const {
      return *getOverloadChoiceIfAvailable(locator);
    }
  
    /// Retrieve the overload choice associated with the given
    /// locator.
    Optional<SelectedOverload>
    getOverloadChoiceIfAvailable(ConstraintLocator *locator) const {
      auto known = overloadChoices.find(locator);
      if (known != overloadChoices.end())
        return known->second;
      return None;
    }
  
    LLVM_ATTRIBUTE_DEPRECATED(
        void dump() const LLVM_ATTRIBUTE_USED,
        "only for use within the debugger");
  
    /// Dump this solution.
    void dump(raw_ostream &OS) const LLVM_ATTRIBUTE_USED;
  };
  
  /// Describes the differences between several solutions to the same
  /// constraint system.
  class SolutionDiff {
  public:
    /// A difference between two overloads.
    struct OverloadDiff {
      /// The locator that describes where the overload comes from.
      ConstraintLocator *locator;
  
      /// The choices that each solution made.
      SmallVector<OverloadChoice, 2> choices;
    };
  
    /// A difference between two type variable bindings.
    struct TypeBindingDiff {
      /// The type variable.
      TypeVariableType *typeVar;
  
      /// The bindings that each solution made.
      SmallVector<Type, 2> bindings;
    };
  
    /// The differences between the overload choices between the
    /// solutions.
    SmallVector<OverloadDiff, 4> overloads;
  
    /// The differences between the type variable bindings of the
    /// solutions.
    SmallVector<TypeBindingDiff, 4> typeBindings;
  
    /// Compute the differences between the given set of solutions.
    ///
    /// \param solutions The set of solutions.
    explicit SolutionDiff(ArrayRef<Solution> solutions);
  };
  
  /// Describes one resolved overload set within the list of overload sets
  /// resolved by the solver.
  struct ResolvedOverloadSetListItem {
    /// The previously resolved overload set in the list.
    ResolvedOverloadSetListItem *Previous;
  
    /// The type that this overload binds.
    Type BoundType;
  
    /// The overload choice.
    OverloadChoice Choice;
  
    /// The locator for this choice.
    ConstraintLocator *Locator;
  
    /// The type of the fully-opened base, if any.
    Type OpenedFullType;
  
    /// The type of the referenced choice.
    Type ImpliedType;
  
    // Make vanilla new/delete illegal for overload set items.
    void *operator new(size_t Bytes) = delete;
    void operator delete(void *Data) = delete;
  
    // Only allow allocation of list items using the allocator in the
    // constraint system.
    void *operator new(size_t bytes, ConstraintSystem &cs,
                       unsigned alignment
                         = alignof(ResolvedOverloadSetListItem));
  };
    
  
  
  /// Identifies a specific conversion from
  struct SpecificConstraint {
    CanType First;
    CanType Second;
    ConstraintKind Kind;
  };
  
  /// An intrusive, doubly-linked list of constraints.
  using ConstraintList = llvm::ilist<Constraint>;
  
  enum class ConstraintSystemFlags {
    /// Whether we allow the solver to attempt fixes to the system.
    AllowFixes = 0x01,
    
    /// If set, this is going to prevent constraint system from erasing all
    /// discovered solutions except the best one.
    ReturnAllDiscoveredSolutions = 0x04,
  
    /// Set if the client wants diagnostics suppressed.
    SuppressDiagnostics = 0x08,
  
    /// If set, the client wants a best-effort solution to the constraint system,
    /// but can tolerate a solution where all of the constraints are solved, but
    /// not all type variables have been determined.  In this case, the constraint
    /// system is not applied to the expression AST, but the ConstraintSystem is
    /// left in-tact.
    AllowUnresolvedTypeVariables = 0x10,
  
    /// If set, constraint system always reuses type of pre-typechecked
    /// expression, and doesn't dig into its subexpressions.
    ReusePrecheckedType = 0x20,
    
    /// If set, the top-level expression may be able to provide an underlying
    /// type for the contextual opaque archetype.
    UnderlyingTypeForOpaqueReturnType = 0x40,
  
    /// FIXME(diagnostics): Once diagnostics are completely switched to new
    /// framework, this flag could be removed as obsolete.
    ///
    /// If set, this identifies constraint system as being used to re-typecheck
    /// one of the sub-expressions as part of the expression diagnostics, which
    /// is attempting to narrow down failure location.
    SubExpressionDiagnostics = 0x80,
  };
  
  /// Options that affect the constraint system as a whole.
  using ConstraintSystemOptions = OptionSet<ConstraintSystemFlags>;
  
  /// This struct represents the results of a member lookup of
  struct MemberLookupResult {
    enum {
      /// This result indicates that we cannot begin to solve this, because the
      /// base expression is a type variable.
      Unsolved,
      
      /// This result indicates that the member reference is erroneous, but was
      /// already diagnosed.  Don't emit another error.
      ErrorAlreadyDiagnosed,
      
      /// This result indicates that the lookup produced candidate lists,
      /// potentially of viable results, potentially of error candidates, and
      /// potentially empty lists, indicating that there were no matches.
      HasResults
    } OverallResult;
    
    /// This is a list of viable candidates that were matched.
    ///
    SmallVector<OverloadChoice, 4> ViableCandidates;
    
    /// If there is a favored candidate in the viable list, this indicates its
    /// index.
    unsigned FavoredChoice = ~0U;
    
    
    /// This enum tracks reasons why a candidate is not viable.
    enum UnviableReason {
      /// Argument labels don't match.
      UR_LabelMismatch,
  
      /// This uses a type like Self in its signature that cannot be used on an
      /// existential box.
      UR_UnavailableInExistential,
  
      /// This is an instance member being accessed through something of metatype
      /// type.
      UR_InstanceMemberOnType,
  
      /// This is a static/class member being accessed through an instance.
      UR_TypeMemberOnInstance,
  
      /// This is a mutating member, being used on an rvalue.
      UR_MutatingMemberOnRValue,
  
      /// The getter for this subscript or computed property is mutating and we
      /// only have an rvalue base.  This is more specific than the former one.
      UR_MutatingGetterOnRValue,
  
      /// The member is inaccessible (e.g. a private member in another file).
      UR_Inaccessible,
  
      /// This is a `WritableKeyPath` being used to look up read-only member,
      /// used in situations involving dynamic member lookup via keypath,
      /// because it's not known upfront what access capability would the
      /// member have.
      UR_WritableKeyPathOnReadOnlyMember,
  
      /// This is a `ReferenceWritableKeyPath` being used to look up mutating
      /// member, used in situations involving dynamic member lookup via keypath,
      /// because it's not known upfront what access capability would the
      /// member have.
      UR_ReferenceWritableKeyPathOnMutatingMember,
  
      /// This is a KeyPath whose root type is AnyObject
      UR_KeyPathWithAnyObjectRootType
    };
  
    /// This is a list of considered (but rejected) candidates, along with a
    /// reason for their rejection. Split into separate collections to make
    /// it easier to use in conjunction with viable candidates.
    SmallVector<OverloadChoice, 4> UnviableCandidates;
    SmallVector<UnviableReason, 4> UnviableReasons;
  
    /// Mark this as being an already-diagnosed error and return itself.
    MemberLookupResult &markErrorAlreadyDiagnosed() {
      OverallResult = ErrorAlreadyDiagnosed;
      return *this;
    }
    
    void addViable(OverloadChoice candidate) {
      ViableCandidates.push_back(candidate);
    }
    
    void addUnviable(OverloadChoice candidate, UnviableReason reason) {
      UnviableCandidates.push_back(candidate);
      UnviableReasons.push_back(reason);
    }
  
    Optional<unsigned> getFavoredIndex() const {
      return (FavoredChoice == ~0U) ? Optional<unsigned>() : FavoredChoice;
    }
  };
  
  /// Stores the required methods for @dynamicCallable types.
  struct DynamicCallableMethods {
    llvm::DenseSet<FuncDecl *> argumentsMethods;
    llvm::DenseSet<FuncDecl *> keywordArgumentsMethods;
  
    void addArgumentsMethod(FuncDecl *method) {
      argumentsMethods.insert(method);
    }
  
    void addKeywordArgumentsMethod(FuncDecl *method) {
      keywordArgumentsMethods.insert(method);
    }
  
    /// Returns true if type defines either of the @dynamicCallable
    /// required methods. Returns false iff type does not satisfy @dynamicCallable
    /// requirements.
    bool isValid() const {
      return !argumentsMethods.empty() || !keywordArgumentsMethods.empty();
    }
  };
  
  /// Describes a system of constraints on type variables, the
  /// solution of which assigns concrete types to each of the type variables.
  /// Constraint systems are typically generated given an (untyped) expression.
  class ConstraintSystem {
  public:
    TypeChecker &TC;
    DeclContext *DC;
    ConstraintSystemOptions Options;
    Optional<ExpressionTimer> Timer;
    
    friend class ConstraintFix;
    friend class OverloadChoice;
    friend class ConstraintGraph;
    friend class DisjunctionChoice;
    friend class Component;
    friend class FailureDiagnostic;
    friend class TypeVarBindingProducer;
    friend class TypeVariableBinding;
    friend class StepScope;
    friend class SolverStep;
    friend class SplitterStep;
    friend class ComponentStep;
    friend class TypeVariableStep;
    friend class RequirementFailure;
    friend class MissingMemberFailure;
  
    class SolverScope;
  
    Constraint *failedConstraint = nullptr;
  
    /// Expressions that are known to be unevaluated.
    /// Note: this is only used to support ObjCSelectorExpr at the moment.
    llvm::SmallPtrSet<Expr *, 2> UnevaluatedRootExprs;
  
    /// The original CS if this CS was created as a simplification of another CS
    ConstraintSystem *baseCS = nullptr;
  
    /// The total number of disjunctions created.
    unsigned CountDisjunctions = 0;
  
  private:
  
    llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> ExprWeights;
  
    /// Allocator used for all of the related constraint systems.
    llvm::BumpPtrAllocator Allocator;
  
    /// Arena used for memory management of constraint-checker-related
    /// allocations.
    ConstraintCheckerArenaRAII Arena;
  
    /// Counter for type variables introduced.
    unsigned TypeCounter = 0;
  
    /// The number of scopes created so far during the solution
    /// of this constraint system.
    ///
    /// This is a measure of complexity of the solution space. A new
    /// scope is created every time we attempt a type variable binding
    /// or explore an option in a disjunction.
    unsigned CountScopes = 0;
  
    /// High-water mark of measured memory usage in any sub-scope we
    /// explored.
    size_t MaxMemory = 0;
  
    /// Cached member lookups.
    llvm::DenseMap<std::pair<Type, DeclName>, Optional<LookupResult>>
      MemberLookups;
  
    /// Cached sets of "alternative" literal types.
    static const unsigned NumAlternativeLiteralTypes = 13;
    Optional<ArrayRef<Type>> AlternativeLiteralTypes[NumAlternativeLiteralTypes];
  
    /// Folding set containing all of the locators used in this
    /// constraint system.
    llvm::FoldingSetVector<ConstraintLocator> ConstraintLocators;
  
    /// The overload sets that have been resolved along the current path.
    ResolvedOverloadSetListItem *resolvedOverloadSets = nullptr;
  
    /// The current fixed score for this constraint system and the (partial)
    /// solution it represents.
    Score CurrentScore;
  
    llvm::SetVector<TypeVariableType *> TypeVariables;
  
    /// Maps expressions to types for choosing a favored overload
    /// type in a disjunction constraint.
    llvm::DenseMap<Expr *, TypeBase *> FavoredTypes;
  
    /// Maps expression types used within all portions of the constraint
    /// system, instead of directly using the types on the expression
    /// nodes themselves. This allows us to typecheck an expression and
    /// run through various diagnostics passes without actually mutating
    /// the types on the expression nodes.
    llvm::DenseMap<const Expr *, TypeBase *> ExprTypes;
    llvm::DenseMap<const TypeLoc *, TypeBase *> TypeLocTypes;
    llvm::DenseMap<const ParamDecl *, TypeBase *> ParamTypes;
  
    /// Maps closure parameters to type variables.
    llvm::DenseMap<const ParamDecl *, TypeVariableType *>
      OpenedParameterTypes;
  
    /// There can only be a single contextual type on the root of the expression
    /// being checked.  If specified, this holds its type along with the base
    /// expression, and the purpose of it.
    TypeLoc contextualType;
    Expr *contextualTypeNode = nullptr;
    ContextualTypePurpose contextualTypePurpose = CTP_Unused;
    
    /// The set of constraint restrictions used to reach the
    /// current constraint system.
    ///
    /// Constraint restrictions help describe which path the solver took when
    /// there are multiple ways in which one type could convert to another, e.g.,
    /// given class types A and B, the solver might choose either a superclass
    /// conversion or a user-defined conversion.
    std::vector<std::tuple<Type, Type, ConversionRestrictionKind>>
        ConstraintRestrictions;
  
    /// The set of fixes applied to make the solution work.
    llvm::SmallVector<ConstraintFix *, 4> Fixes;
    /// The set of type variables representing types of missing members.
    llvm::SmallSetVector<ConstraintLocator *, 4> MissingMembers;
  
    /// The set of remembered disjunction choices used to reach
    /// the current constraint system.
    std::vector<std::pair<ConstraintLocator*, unsigned>>
        DisjunctionChoices;
  
    /// The worklist of "active" constraints that should be revisited
    /// due to a change.
    ConstraintList ActiveConstraints;
  
    /// The list of "inactive" constraints that still need to be solved,
    /// but will not be revisited until one of their inputs changes.
    ConstraintList InactiveConstraints;
  
    /// The constraint graph.
    ConstraintGraph &CG;
  
    /// A mapping from constraint locators to the set of opened types associated
    /// with that locator.
    SmallVector<std::pair<ConstraintLocator *, ArrayRef<OpenedType>>, 4>
      OpenedTypes;
  
    /// A mapping from constraint locators to the opened existential archetype
    /// used for the 'self' of an existential type.
    SmallVector<std::pair<ConstraintLocator *, OpenedArchetypeType *>, 4>
      OpenedExistentialTypes;
  
    /// The node -> type mappings introduced by generating constraints.
    llvm::SmallVector<std::pair<TypedNode, Type>, 8> addedNodeTypes;
  
    std::vector<std::pair<ConstraintLocator *, ProtocolConformanceRef>>
        CheckedConformances;
  
    /// The set of closures that have been transformed by a function builder.
    SmallVector<std::tuple<ClosureExpr *, Type, Expr *>, 4>
        builderTransformedClosures;
  
  public:
    /// The locators of \c Defaultable constraints whose defaults were used.
    std::vector<ConstraintLocator *> DefaultedConstraints;
  
    /// A cache that stores the @dynamicCallable required methods implemented by
    /// types.
    llvm::DenseMap<CanType, DynamicCallableMethods> DynamicCallableCache;
  
    /// A cache that stores whether types are valid @dynamicMemberLookup types.
    llvm::DenseMap<CanType, bool> DynamicMemberLookupCache;
  
  private:
    /// Describe the candidate expression for partial solving.
    /// This class used by shrink & solve methods which apply
    /// variation of directional path consistency algorithm in attempt
    /// to reduce scopes of the overload sets (disjunctions) in the system.
    class Candidate {
      Expr *E;
      TypeChecker &TC;
      DeclContext *DC;
      llvm::BumpPtrAllocator &Allocator;
  
      ConstraintSystem &BaseCS;
  
      // Contextual Information.
      Type CT;
      ContextualTypePurpose CTP;
  
    public:
      Candidate(ConstraintSystem &cs, Expr *expr, Type ct = Type(),
                ContextualTypePurpose ctp = ContextualTypePurpose::CTP_Unused)
          : E(expr), TC(cs.TC), DC(cs.DC), Allocator(cs.Allocator), BaseCS(cs),
            CT(ct), CTP(ctp) {}
  
      /// Return underlying expression.
      Expr *getExpr() const { return E; }
  
      /// Try to solve this candidate sub-expression
      /// and re-write it's OSR domains afterwards.
      ///
      /// \param shrunkExprs The set of expressions which
      /// domains have been successfully shrunk so far.
      ///
      /// \returns true on solver failure, false otherwise.
      bool solve(llvm::SmallDenseSet<OverloadSetRefExpr *> &shrunkExprs);
  
      /// Apply solutions found by solver as reduced OSR sets for
      /// for current and all of it's sub-expressions.
      ///
      /// \param solutions The solutions found by running solver on the
      /// this candidate expression.
      ///
      /// \param shrunkExprs The set of expressions which
      /// domains have been successfully shrunk so far.
      void applySolutions(
          llvm::SmallVectorImpl<Solution> &solutions,
          llvm::SmallDenseSet<OverloadSetRefExpr *> &shrunkExprs) const;
  
      /// Check if attempt at solving of the candidate makes sense given
      /// the current conditions - number of shrunk domains which is related
      /// to the given candidate over the total number of disjunctions present.
      static bool
      isTooComplexGiven(ConstraintSystem *const cs,
                        llvm::SmallDenseSet<OverloadSetRefExpr *> &shrunkExprs) {
        SmallVector<Constraint *, 8> disjunctions;
        cs->collectDisjunctions(disjunctions);
  
        unsigned unsolvedDisjunctions = disjunctions.size();
        for (auto *disjunction : disjunctions) {
          auto *locator = disjunction->getLocator();
          if (!locator)
            continue;
  
          if (auto *anchor = locator->getAnchor()) {
            auto *OSR = dyn_cast<OverloadSetRefExpr>(anchor);
            if (!OSR)
              continue;
  
            if (shrunkExprs.count(OSR) > 0)
              --unsolvedDisjunctions;
          }
        }
  
        unsigned threshold = cs->TC.getLangOpts().SolverShrinkUnsolvedThreshold;
        return unsolvedDisjunctions >= threshold;
      }
    };
  
    /// Describes the current solver state.
    struct SolverState {
      SolverState(ConstraintSystem &cs,
                  FreeTypeVariableBinding allowFreeTypeVariables);
      ~SolverState();
  
      /// The constraint system.
      ConstraintSystem &CS;
  
      FreeTypeVariableBinding AllowFreeTypeVariables;
  
      /// Old value of DebugConstraintSolver.
      /// FIXME: Move the "debug constraint solver" bit into the constraint 
      /// system itself.
      bool OldDebugConstraintSolver;
  
      /// Depth of the solution stack.
      unsigned depth = 0;
  
      /// Maximum depth reached so far in exploring solutions.
      unsigned maxDepth = 0;
  
      /// Whether to record failures or not.
      bool recordFixes = false;
  
      /// The set of type variable bindings that have changed while
      /// processing this constraint system.
      SavedTypeVariableBindings savedBindings;
  
       /// The best solution computed so far.
      Optional<Score> BestScore;
  
      /// The number of the solution attempt we're looking at.
      unsigned SolutionAttempt;
  
      /// Refers to the innermost partial solution scope.
      SolverScope *PartialSolutionScope = nullptr;
  
      // Statistics
      #define CS_STATISTIC(Name, Description) unsigned Name = 0;
      #include "ConstraintSolverStats.def"
  
      /// Check whether there are any retired constraints present.
      bool hasRetiredConstraints() const {
        return !retiredConstraints.empty();
      }
  
      /// Mark given constraint as retired along current solver path.
      ///
      /// \param constraint The constraint to retire temporarily.
      void retireConstraint(Constraint *constraint) {
        retiredConstraints.push_front(constraint);
      }
  
      /// Iterate over all of the retired constraints registered with
      /// current solver state.
      ///
      /// \param processor The processor function to be applied to each of
      /// the constraints retrieved.
      void forEachRetired(llvm::function_ref<void(Constraint &)> processor) {
        for (auto &constraint : retiredConstraints)
          processor(constraint);
      }
  
      /// Add new "generated" constraint along the current solver path.
      ///
      /// \param constraint The newly generated constraint.
      void addGeneratedConstraint(Constraint *constraint) {
        assert(constraint && "Null generated constraint?");
        generatedConstraints.push_back(constraint);
      }
  
      /// Erase given constraint from the list of generated constraints
      /// along the current solver path. Note that this operation doesn't
      /// guarantee any ordering of the after it's application.
      ///
      /// \param constraint The constraint to erase.
      void removeGeneratedConstraint(Constraint *constraint) {
        for (auto *&generated : generatedConstraints) {
          // When we find the constraint we're erasing, overwrite its
          // value with the last element in the generated constraints
          // vector and then pop that element from the vector.
          if (generated == constraint) {
            generated = generatedConstraints.back();
            generatedConstraints.pop_back();
            return;
          }
        }
      }
  
      /// Register given scope to be tracked by the current solver state,
      /// this helps to make sure that all of the retired/generated constraints
      /// are dealt with correctly when the life time of the scope ends.
      ///
      /// \param scope The scope to associate with current solver state.
      void registerScope(SolverScope *scope) {
        ++depth;
        maxDepth = std::max(maxDepth, depth);
        scope->scopeNumber = NumStatesExplored++;
  
        CS.incrementScopeCounter();
        auto scopeInfo =
          std::make_tuple(scope, retiredConstraints.begin(),
                          generatedConstraints.size());
        scopes.push_back(scopeInfo);
      }
  
      /// Restore all of the retired/generated constraints to the state
      /// before given scope. This is required because retired constraints have
      /// to be re-introduced to the system in order of arrival (LIFO) and list
      /// of the generated constraints has to be truncated back to the
      /// original size.
      ///
      /// \param scope The solver scope to rollback.
      void rollback(SolverScope *scope) {
        --depth;
  
        unsigned countScopesExplored = NumStatesExplored - scope->scopeNumber;
        if (countScopesExplored == 1)
          CS.incrementLeafScopes();
  
        SolverScope *savedScope;
        // The position of last retired constraint before given scope.
        ConstraintList::iterator lastRetiredPos;
        // The original number of generated constraints before given scope.
        unsigned numGenerated;
  
        std::tie(savedScope, lastRetiredPos, numGenerated) =
          scopes.pop_back_val();
  
        assert(savedScope == scope && "Scope rollback not in LIFO order!");
  
        // Restore all of the retired constraints.
        CS.InactiveConstraints.splice(CS.InactiveConstraints.end(),
                                      retiredConstraints,
                                      retiredConstraints.begin(), lastRetiredPos);
  
        // And remove all of the generated constraints.
        auto genStart = generatedConstraints.begin() + numGenerated,
             genEnd = generatedConstraints.end();
        for (auto genI = genStart; genI != genEnd; ++genI) {
          CS.InactiveConstraints.erase(ConstraintList::iterator(*genI));
        }
  
        generatedConstraints.erase(genStart, genEnd);
  
        for (unsigned constraintIdx :
               range(scope->numDisabledConstraints, disabledConstraints.size())) {
          if (disabledConstraints[constraintIdx]->isDisabled())
            disabledConstraints[constraintIdx]->setEnabled();
        }
        disabledConstraints.erase(
            disabledConstraints.begin() + scope->numDisabledConstraints,
            disabledConstraints.end());
  
        for (unsigned constraintIdx :
               range(scope->numFavoredConstraints, favoredConstraints.size())) {
          if (favoredConstraints[constraintIdx]->isFavored())
            favoredConstraints[constraintIdx]->setFavored(false);
        }
        favoredConstraints.erase(
            favoredConstraints.begin() + scope->numFavoredConstraints,
            favoredConstraints.end());
      }
  
      /// Check whether constraint system is allowed to form solutions
      /// even with unbound type variables present.
      bool allowsFreeTypeVariables() const {
        return AllowFreeTypeVariables != FreeTypeVariableBinding::Disallow;
      }
  
      unsigned getNumDisabledConstraints() const {
        return disabledConstraints.size();
      }
  
      /// Disable the given constraint; this change will be rolled back
      /// when we exit the current solver scope.
      void disableContraint(Constraint *constraint) {
        constraint->setDisabled();
        disabledConstraints.push_back(constraint);
      }
  
      unsigned getNumFavoredConstraints() const {
        return favoredConstraints.size();
      }
  
      /// Favor the given constraint; this change will be rolled back
      /// when we exit the current solver scope.
      void favorConstraint(Constraint *constraint) {
        assert(!constraint->isFavored());
  
        constraint->setFavored();
        favoredConstraints.push_back(constraint);
      }
  
    private:
      /// The list of constraints that have been retired along the
      /// current path, this list is used in LIFO fashion when constraints
      /// are added back to the circulation.
      ConstraintList retiredConstraints;
  
      /// The set of constraints which were active at the time of this state
      /// creating, it's used to re-activate them on destruction.
      SmallVector<Constraint *, 4> activeConstraints;
  
      /// The current set of generated constraints.
      SmallVector<Constraint *, 4> generatedConstraints;
  
      /// The collection which holds association between solver scope
      /// and position of the last retired constraint and number of
      /// constraints generated before registration of given scope,
      /// this helps to rollback all of the constraints retired/generated
      /// each of the registered scopes correct (LIFO) order.
      llvm::SmallVector<
        std::tuple<SolverScope *, ConstraintList::iterator, unsigned>, 4> scopes;
  
      SmallVector<Constraint *, 4> disabledConstraints;
      SmallVector<Constraint *, 4> favoredConstraints;
    };
  
    class CacheExprTypes : public ASTWalker {
      Expr *RootExpr;
      ConstraintSystem &CS;
      bool ExcludeRoot;
  
    public:
      CacheExprTypes(Expr *expr, ConstraintSystem &cs, bool excludeRoot)
          : RootExpr(expr), CS(cs), ExcludeRoot(excludeRoot) {}
  
      Expr *walkToExprPost(Expr *expr) override {
        if (ExcludeRoot && expr == RootExpr) {
          assert(!expr->getType() && "Unexpected type in root of expression!");
          return expr;
        }
  
        if (expr->getType())
          CS.cacheType(expr);
  
        return expr;
      }
  
      /// Ignore statements.
      std::pair<bool, Stmt *> walkToStmtPre(Stmt *stmt) override {
        return { false, stmt };
      }
  
      /// Ignore declarations.
      bool walkToDeclPre(Decl *decl) override { return false; }
    };
  
    class SetExprTypes : public ASTWalker {
      Expr *RootExpr;
      ConstraintSystem &CS;
      bool ExcludeRoot;
  
    public:
      SetExprTypes(Expr *expr, ConstraintSystem &cs, bool excludeRoot)
          : RootExpr(expr), CS(cs), ExcludeRoot(excludeRoot) {}
  
      Expr *walkToExprPost(Expr *expr) override {
        if (ExcludeRoot && expr == RootExpr)
          return expr;
  
        //assert((!expr->getType() || CS.getType(expr)->isEqual(expr->getType()))
        //       && "Mismatched types!");
        assert(!CS.getType(expr)->hasTypeVariable() &&
               "Should not write type variable into expression!");
        expr->setType(CS.getType(expr));
  
        return expr;
      }
  
      /// Ignore statements.
      std::pair<bool, Stmt *> walkToStmtPre(Stmt *stmt) override {
        return { false, stmt };
      }
  
      /// Ignore declarations.
      bool walkToDeclPre(Decl *decl) override { return false; }
    };
  
  public:
  
    void setExprTypes(Expr *expr) {
      SetExprTypes SET(expr, *this, /* excludeRoot = */ false);
      expr->walk(SET);
    }
  
    void setSubExprTypes(Expr *expr) {
      SetExprTypes SET(expr, *this, /* excludeRoot = */ true);
      expr->walk(SET);
    }
  
    /// Cache the types of the given expression and all subexpressions.
    void cacheExprTypes(Expr *expr) {
      bool excludeRoot = false;
      expr->walk(CacheExprTypes(expr, *this, excludeRoot));
    }
  
    /// Cache the types of the expressions under the given expression
    /// (but not the type of the given expression).
    void cacheSubExprTypes(Expr *expr) {
      bool excludeRoot = true;
      expr->walk(CacheExprTypes(expr, *this, excludeRoot));
    }
  
    /// The current solver state.
    ///
    /// This will be non-null when we're actively solving the constraint
    /// system, and carries temporary state related to the current path
    /// we're exploring.
    SolverState *solverState = nullptr;
  
    struct ArgumentLabelState {
      ArrayRef<Identifier> Labels;
      bool HasTrailingClosure;
    };
  
    /// A mapping from the constraint locators for references to various
    /// names (e.g., member references, normal name references, possible
    /// constructions) to the argument labels provided in the call to
    /// that locator.
    llvm::DenseMap<ConstraintLocator *, ArgumentLabelState> ArgumentLabels;
  
    ResolvedOverloadSetListItem *getResolvedOverloadSets() const {
      return resolvedOverloadSets;
    }
  
    ResolvedOverloadSetListItem *findSelectedOverloadFor(Expr *expr) const {
      auto resolvedOverload = getResolvedOverloadSets();
      while (resolvedOverload) {
        if (resolvedOverload->Locator->getAnchor() == expr)
          return resolvedOverload;
        resolvedOverload = resolvedOverload->Previous;
      }
      return nullptr;
    }
  
  private:
    unsigned assignTypeVariableID() {
      return TypeCounter++;
    }
  
    void incrementScopeCounter();
    void incrementLeafScopes();
  
  public:
    /// Introduces a new solver scope, which any changes to the
    /// solver state or constraint system are temporary and will be undone when
    /// this object is destroyed.
    ///
    ///
    class SolverScope {
      ConstraintSystem &cs;
  
      /// The current resolved overload set list.
      ResolvedOverloadSetListItem *resolvedOverloadSets;
  
      /// The length of \c TypeVariables.
      unsigned numTypeVariables;
  
      /// The length of \c SavedBindings.
      unsigned numSavedBindings;
  
      /// The length of \c ConstraintRestrictions.
      unsigned numConstraintRestrictions;
  
      /// The length of \c Fixes.
      unsigned numFixes;
  
      /// The length of \c DisjunctionChoices.
      unsigned numDisjunctionChoices;
  
      /// The length of \c OpenedTypes.
      unsigned numOpenedTypes;
  
      /// The length of \c OpenedExistentialTypes.
      unsigned numOpenedExistentialTypes;
  
      /// The length of \c DefaultedConstraints.
      unsigned numDefaultedConstraints;
  
      unsigned numAddedNodeTypes;
  
      unsigned numCheckedConformances;
  
      unsigned numMissingMembers;
  
      unsigned numDisabledConstraints;
  
      unsigned numFavoredConstraints;
  
      unsigned numBuilderTransformedClosures;
  
      /// The previous score.
      Score PreviousScore;
  
      /// The scope number of this scope. Set when the scope is registered.
      unsigned scopeNumber = 0;
  
      /// Constraint graph scope associated with this solver scope.
      ConstraintGraphScope CGScope;
  
      SolverScope(const SolverScope &) = delete;
      SolverScope &operator=(const SolverScope &) = delete;
  
      friend class ConstraintSystem;
  
    public:
      explicit SolverScope(ConstraintSystem &cs);
      ~SolverScope();
    };
  
    ConstraintSystem(TypeChecker &tc, DeclContext *dc,
                     ConstraintSystemOptions options,
                     Expr *expr = nullptr);
    ~ConstraintSystem();
  
    /// Retrieve the type checker associated with this constraint system.
    TypeChecker &getTypeChecker() const { return TC; }
  
    /// Retrieve the constraint graph associated with this constraint system.
    ConstraintGraph &getConstraintGraph() const { return CG; }
  
    /// Retrieve the AST context.
    ASTContext &getASTContext() const { return TC.Context; }
  
    /// Determine whether this constraint system has any free type
    /// variables.
    bool hasFreeTypeVariables();
  
  private:
    /// Indicates if the constraint system should retain all of the
    /// solutions it has deduced regardless of their score.
    bool retainAllSolutions() const {
      return Options.contains(
          ConstraintSystemFlags::ReturnAllDiscoveredSolutions);
    }
  
    /// Finalize this constraint system; we're done attempting to solve
    /// it.
    ///
    /// \returns the solution.
    Solution finalize();
  
    /// Apply the given solution to the current constraint system.
    ///
    /// This operation is used to take a solution computed based on some
    /// subset of the constraints and then apply it back to the
    /// constraint system for further exploration.
    void applySolution(const Solution &solution);
  
    /// Emit the fixes computed as part of the solution, returning true if we were
    /// able to emit an error message, or false if none of the fixits worked out.
    bool applySolutionFixes(Expr *E, const Solution &solution);
  
    /// If there is more than one viable solution,
    /// attempt to pick the best solution and remove all of the rest.
    ///
    /// \param solutions The set of solutions to filter.
    ///
    /// \param minimize The flag which idicates if the
    /// set of solutions should be filtered even if there is
    /// no single best solution, see `findBestSolution` for
    /// more details.
    void
    filterSolutions(SmallVectorImpl<Solution> &solutions,
                    bool minimize = false) {
      if (solutions.size() < 2)
        return;
  
      if (auto best = findBestSolution(solutions, minimize)) {
        if (*best != 0)
          solutions[0] = std::move(solutions[*best]);
        solutions.erase(solutions.begin() + 1, solutions.end());
      }
    }
  
    /// Restore the type variable bindings to what they were before
    /// we attempted to solve this constraint system.
    ///
    /// \param numBindings The number of bindings to restore, from the end of
    /// the saved-binding stack.
    void restoreTypeVariableBindings(unsigned numBindings);
  
    /// Retrieve the set of saved type variable bindings, if available.
    ///
    /// \returns null when we aren't currently solving the system.
    SavedTypeVariableBindings *getSavedBindings() const {
      return solverState ? &solverState->savedBindings : nullptr;
    }
  
    /// Add a new type variable that was already created.
    void addTypeVariable(TypeVariableType *typeVar);
    
    /// Add a constraint from the subscript base to the root of the key
    /// path literal to the constraint system.
    void addKeyPathApplicationRootConstraint(Type root, ConstraintLocatorBuilder locator);
  
  public:
    /// Lookup for a member with the given name in the given base type.
    ///
    /// This routine caches the results of member lookups in the top constraint
    /// system, to avoid.
    ///
    /// FIXME: This caching should almost certainly be performed at the
    /// module level, since type checking occurs after name binding,
    /// and no new names are introduced after name binding.
    ///
    /// \returns A reference to the member-lookup result.
    LookupResult &lookupMember(Type base, DeclName name);
  
    /// Retrieve the set of "alternative" literal types that we'll explore
    /// for a given literal protocol kind.
    ArrayRef<Type> getAlternativeLiteralTypes(KnownProtocolKind kind);
  
    /// Create a new type variable.
    TypeVariableType *createTypeVariable(ConstraintLocator *locator,
                                         unsigned options = 0);
  
    /// Retrieve the set of active type variables.
    ArrayRef<TypeVariableType *> getTypeVariables() const {
      return TypeVariables.getArrayRef();
    }
  
    /// Whether the given type variable is active in the constraint system at
    /// the moment.
    bool isActiveTypeVariable(TypeVariableType *typeVar) const {
      return TypeVariables.count(typeVar) > 0;
    }
  
    TypeBase* getFavoredType(Expr *E) {
      assert(E != nullptr);
      return this->FavoredTypes[E];
    }
    void setFavoredType(Expr *E, TypeBase *T) {
      assert(E != nullptr);
      this->FavoredTypes[E] = T;
    }
  
    /// Set the type in our type map for the given node.
    ///
    /// The side tables are used through the expression type checker to avoid mutating nodes until
    /// we know we have successfully type-checked them.
    void setType(TypedNode node, Type type) {
      assert(!node.isNull() && "Cannot set type information on null node");
      assert(type && "Expected non-null type");
  
      // Record the type.
      if (auto expr = node.dyn_cast<const Expr *>()) {
        ExprTypes[expr] = type.getPointer();
      } else if (auto typeLoc = node.dyn_cast<const TypeLoc *>()) {
        TypeLocTypes[typeLoc] = type.getPointer();
      } else {
        auto param = node.get<const ParamDecl *>();
        ParamTypes[param] = type.getPointer();
      }
  
      // Record the fact that we ascribed a type to this node.
      if (solverState && solverState->depth > 0) {
        addedNodeTypes.push_back({node, type});
      }
    }
  
    /// Set the type in our type map for a given expression. The side
    /// map is used throughout the expression type checker in order to
    /// avoid mutating expressions until we know we have successfully
    /// type-checked them.
    void setType(TypeLoc &L, Type T) {
      setType(TypedNode(&L), T);
    }
  
    /// Erase the type for the given node.
    void eraseType(TypedNode node) {
      if (auto expr = node.dyn_cast<const Expr *>()) {
        ExprTypes.erase(expr);
      } else if (auto typeLoc = node.dyn_cast<const TypeLoc *>()) {
        TypeLocTypes.erase(typeLoc);
      } else {
        auto param = node.get<const ParamDecl *>();
        ParamTypes.erase(param);
      }
    }
  
    /// Check to see if we have a type for an expression.
    bool hasType(const Expr *E) const {
      assert(E != nullptr && "Expected non-null expression!");
      return ExprTypes.find(E) != ExprTypes.end();
    }
  
    bool hasType(const TypeLoc &L) const {
      return hasType(TypedNode(&L));
    }
  
    /// Check to see if we have a type for a node.
    bool hasType(TypedNode node) const {
      assert(!node.isNull() && "Expected non-null node");
      if (auto expr = node.dyn_cast<const Expr *>()) {
        return ExprTypes.find(expr) != ExprTypes.end();
      } else if (auto typeLoc = node.dyn_cast<const TypeLoc *>()) {
        return TypeLocTypes.find(typeLoc) != TypeLocTypes.end();
      } else {
        auto param = node.get<const ParamDecl *>();
        return ParamTypes.find(param) != ParamTypes.end();
      }
    }
  
    /// Get the type for an expression.
    Type getType(const Expr *E) const {
      assert(hasType(E) && "Expected type to have been set!");
      // FIXME: lvalue differences
      //    assert((!E->getType() ||
      //            E->getType()->isEqual(ExprTypes.find(E)->second)) &&
      //           "Mismatched types!");
      return ExprTypes.find(E)->second;
    }
  
    Type getType(const TypeLoc &L) const {
      assert(hasType(L) && "Expected type to have been set!");
      return TypeLocTypes.find(&L)->second;
    }
  
    Type getType(const ParamDecl *P) const {
      assert(hasType(P) && "Expected type to have been set!");
      return ParamTypes.find(P)->second;
    }
  
    Type getType(const VarDecl *D, bool wantInterfaceType = true) const {
      if (auto *P = dyn_cast<ParamDecl>(D))
        return getType(P);
  
      assert(D->hasValidSignature());
      return wantInterfaceType ? D->getInterfaceType() : D->getType();
    }
  
    /// Cache the type of the expression argument and return that same
    /// argument.
    template <typename T>
    T *cacheType(T *E) {
      assert(E->getType() && "Expected a type!");
      setType(E, E->getType());
      return E;
    }
  
    void setContextualType(Expr *E, TypeLoc T, ContextualTypePurpose purpose) {
      assert(E != nullptr && "Expected non-null expression!");
      contextualTypeNode = E;
      contextualType = T;
      contextualTypePurpose = purpose;
    }
  
    Type getContextualType(Expr *E) const {
      assert(E != nullptr && "Expected non-null expression!");
      return E == contextualTypeNode ? contextualType.getType() : Type();
    }
    Type getContextualType() const {
      return contextualType.getType();
    }
  
    TypeLoc getContextualTypeLoc() const {
      return contextualType;
    }
  
    const Expr *getContextualTypeNode() const {
      return contextualTypeNode;
    }
  
    ContextualTypePurpose getContextualTypePurpose() const {
      return contextualTypePurpose;
    }
    
    /// Retrieve the constraint locator for the given anchor and
    /// path, uniqued.
    ConstraintLocator *
    getConstraintLocator(Expr *anchor,
                         ArrayRef<ConstraintLocator::PathElement> path,
                         unsigned summaryFlags);
  
    /// Retrive the constraint locator for the given anchor and
    /// path, uniqued and automatically infer the summary flags
    ConstraintLocator *
    getConstraintLocator(Expr *anchor,
                         ArrayRef<ConstraintLocator::PathElement> path);
  
    /// Retrieve the constraint locator for the given anchor and
    /// an empty path, uniqued.
    ConstraintLocator *getConstraintLocator(Expr *anchor) {
      return getConstraintLocator(anchor, {}, 0);
    }
  
    /// Retrieve the constraint locator for the given anchor and
    /// path element.
    ConstraintLocator *
    getConstraintLocator(Expr *anchor, ConstraintLocator::PathElement pathElt) {
      return getConstraintLocator(anchor, llvm::makeArrayRef(pathElt),
                                  pathElt.getNewSummaryFlags());
    }
  
    ConstraintLocator *
    getConstraintLocator(const Expr *anchor,
                         ConstraintLocator::PathElement pathElt) {
      return getConstraintLocator(const_cast<Expr *>(anchor), pathElt);
    }
  
    /// Extend the given constraint locator with a path element.
    ConstraintLocator *
    getConstraintLocator(ConstraintLocator *locator,
                         ConstraintLocator::PathElement pathElt) {
      return getConstraintLocator(ConstraintLocatorBuilder(locator)
                                    .withPathElement(pathElt));
    }
  
    /// Retrieve the constraint locator described by the given
    /// builder.
    ConstraintLocator *
    getConstraintLocator(const ConstraintLocatorBuilder &builder);
  
    /// Lookup and return parent associated with given expression.
    Expr *getParentExpr(Expr *expr) const {
      auto e = ExprWeights.find(expr);
      return e != ExprWeights.end() ? e->second.second : nullptr;
    }
  
    /// Returns a locator describing the callee for a given expression. For
    /// a function application, this is a locator describing the function expr.
    /// For an unresolved dot/member, this is a locator to the member.
    ConstraintLocator *getCalleeLocator(Expr *expr);
  
  public:
  
    /// Whether we should attempt to fix problems.
    bool shouldAttemptFixes() const {
      if (!(Options & ConstraintSystemFlags::AllowFixes))
        return false;
  
      return !solverState || solverState->recordFixes;
    }
  
    ArrayRef<ConstraintFix *> getFixes() const { return Fixes; }
  
    bool shouldSuppressDiagnostics() const {
      return Options.contains(ConstraintSystemFlags::SuppressDiagnostics);
    }
  
    bool shouldReusePrecheckedType() const {
      return Options.contains(ConstraintSystemFlags::ReusePrecheckedType);
    }
  
    /// Log and record the application of the fix. Return true iff any
    /// subsequent solution would be worse than the best known solution.
    bool recordFix(ConstraintFix *fix);
  
    /// Determine whether constraint system already has a fix recorded
    /// for a particular location.
    bool hasFixFor(ConstraintLocator *locator) const {
      return llvm::any_of(Fixes, [&locator](const ConstraintFix *fix) {
        return fix->getLocator() == locator;
      });
    }
  
    /// If an UnresolvedDotExpr, SubscriptMember, etc has been resolved by the
    /// constraint system, return the decl that it references.
    ValueDecl *findResolvedMemberRef(ConstraintLocator *locator);
  
    /// Try to salvage the constraint system by applying (speculative)
    /// fixes to the underlying expression.
    ///
    /// \param viable the set of viable solutions produced by the initial
    /// solution attempt.
    ///
    /// \param expr the expression we're trying to salvage.
    ///
    /// \returns false if we were able to salvage the system, in which case
    /// \c viable[0] contains the resulting solution. Otherwise, emits a
    /// diagnostic and returns true.
    bool salvage(SmallVectorImpl<Solution> &viable, Expr *expr);
    
    /// Mine the active and inactive constraints in the constraint
    /// system to generate a plausible diagnosis of why the system could not be
    /// solved.
    ///
    /// \param expr The expression whose constraints we're investigating for a
    /// better diagnostic.
    ///
    /// Assuming that this constraint system is actually erroneous, this *always*
    /// emits an error message.
    void diagnoseFailureForExpr(Expr *expr);
  
    bool diagnoseAmbiguity(Expr *expr, ArrayRef<Solution> solutions);
    bool diagnoseAmbiguityWithFixes(Expr *expr, ArrayRef<Solution> solutions);
  
    /// Give the deprecation warning for referring to a global function
    /// when there's a method from a conditional conformance in a smaller/closer
    /// scope.
    void
    diagnoseDeprecatedConditionalConformanceOuterAccess(UnresolvedDotExpr *UDE,
                                                        ValueDecl *choice);
  
    /// Add a constraint to the constraint system.
    void addConstraint(ConstraintKind kind, Type first, Type second,
                       ConstraintLocatorBuilder locator,
                       bool isFavored = false);
  
    /// Add a requirement as a constraint to the constraint system.
    void addConstraint(Requirement req, ConstraintLocatorBuilder locator,
                       bool isFavored = false);
  
    /// Add a key path application constraint to the constraint system.
    void addKeyPathApplicationConstraint(Type keypath, Type root, Type value,
                                         ConstraintLocatorBuilder locator,
                                         bool isFavored = false);
  
    /// Add a key path constraint to the constraint system.
    void addKeyPathConstraint(Type keypath, Type root, Type value,
                              ConstraintLocatorBuilder locator,
                              bool isFavored = false);
  
    /// Add a new constraint with a restriction on its application.
    void addRestrictedConstraint(ConstraintKind kind,
                                 ConversionRestrictionKind restriction,
                                 Type first, Type second,
                                 ConstraintLocatorBuilder locator);
  
    /// Add a constraint that binds an overload set to a specific choice.
    void addBindOverloadConstraint(Type boundTy, OverloadChoice choice,
                                   ConstraintLocator *locator,
                                   DeclContext *useDC) {
      resolveOverload(locator, boundTy, choice, useDC);
    }
  
    /// Add a value member constraint to the constraint system.
    void addValueMemberConstraint(Type baseTy, DeclName name, Type memberTy,
                                  DeclContext *useDC,
                                  FunctionRefKind functionRefKind,
                                  ArrayRef<OverloadChoice> outerAlternatives,
                                  ConstraintLocatorBuilder locator) {
      assert(baseTy);
      assert(memberTy);
      assert(name);
      assert(useDC);
      switch (simplifyMemberConstraint(
          ConstraintKind::ValueMember, baseTy, name, memberTy, useDC,
          functionRefKind, outerAlternatives, TMF_GenerateConstraints, locator)) {
      case SolutionKind::Unsolved:
        llvm_unreachable("Unsolved result when generating constraints!");
  
      case SolutionKind::Solved:
        break;
  
      case SolutionKind::Error:
        if (shouldAddNewFailingConstraint()) {
          addNewFailingConstraint(Constraint::createMemberOrOuterDisjunction(
              *this, ConstraintKind::ValueMember, baseTy, memberTy, name, useDC,
              functionRefKind, outerAlternatives, getConstraintLocator(locator)));
        }
        break;
      }
    }
  
    /// Add a value member constraint for an UnresolvedMemberRef
    /// to the constraint system.
    void addUnresolvedValueMemberConstraint(Type baseTy, DeclName name,
                                            Type memberTy, DeclContext *useDC,
                                            FunctionRefKind functionRefKind,
                                            ConstraintLocatorBuilder locator) {
      assert(baseTy);
      assert(memberTy);
      assert(name);
      assert(useDC);
      switch (simplifyMemberConstraint(ConstraintKind::UnresolvedValueMember,
                                       baseTy, name, memberTy,
                                       useDC, functionRefKind,
                                       /*outerAlternatives=*/{},
                                       TMF_GenerateConstraints, locator)) {
      case SolutionKind::Unsolved:
        llvm_unreachable("Unsolved result when generating constraints!");
  
      case SolutionKind::Solved:
        break;
  
      case SolutionKind::Error:
        if (shouldAddNewFailingConstraint()) {
          addNewFailingConstraint(
            Constraint::createMember(*this, ConstraintKind::UnresolvedValueMember,
                                     baseTy, memberTy, name,
                                     useDC, functionRefKind,
                                     getConstraintLocator(locator)));
        }
        break;
      }
    }
  
    /// Add an explicit conversion constraint (e.g., \c 'x as T').
    void addExplicitConversionConstraint(Type fromType, Type toType,
                                         bool allowFixes,
                                         ConstraintLocatorBuilder locator);
  
    /// Add a disjunction constraint.
    void
    addDisjunctionConstraint(ArrayRef<Constraint *> constraints,
                             ConstraintLocatorBuilder locator,
                             RememberChoice_t rememberChoice = ForgetChoice) {
      auto constraint =
        Constraint::createDisjunction(*this, constraints,
                                      getConstraintLocator(locator),
                                      rememberChoice);
  
      addUnsolvedConstraint(constraint);
    }
  
    /// Whether we should add a new constraint to capture a failure.
    bool shouldAddNewFailingConstraint() const {
      // Only do this at the top level.
      return !failedConstraint;
    }
  
    /// Add a new constraint that we know fails.
    void addNewFailingConstraint(Constraint *constraint) {
      assert(shouldAddNewFailingConstraint());
      failedConstraint = constraint;
      failedConstraint->setActive(false);
  
      // Record this as a newly-generated constraint.
      if (solverState) {
        solverState->addGeneratedConstraint(constraint);
        solverState->retireConstraint(constraint);
      }
    }
  
    /// Add a newly-generated constraint that is known not to be solvable
    /// right now.
    void addUnsolvedConstraint(Constraint *constraint) {
      // We couldn't solve this constraint; add it to the pile.
      InactiveConstraints.push_back(constraint);
  
      // Add this constraint to the constraint graph.
      CG.addConstraint(constraint);
  
      // Record this as a newly-generated constraint.
      if (solverState)
        solverState->addGeneratedConstraint(constraint);
    }
  
    /// Remove an inactive constraint from the current constraint graph.
    void removeInactiveConstraint(Constraint *constraint) {
      CG.removeConstraint(constraint);
      InactiveConstraints.erase(constraint);
  
      if (solverState)
        solverState->retireConstraint(constraint);
    }
  
    /// Transfer given constraint from to active list
    /// for solver to attempt its simplification.
    void activateConstraint(Constraint *constraint) {
      assert(!constraint->isActive() && "Constraint is already active");
      ActiveConstraints.splice(ActiveConstraints.end(), InactiveConstraints,
                               constraint);
      constraint->setActive(true);
    }
  
    void deactivateConstraint(Constraint *constraint) {
      assert(constraint->isActive() && "Constraint is already inactive");
      InactiveConstraints.splice(InactiveConstraints.end(),
                                 ActiveConstraints, constraint);
      constraint->setActive(false);
    }
  
    void retireConstraint(Constraint *constraint) {
      if (constraint->isActive())
        deactivateConstraint(constraint);
      removeInactiveConstraint(constraint);
    }
  
    /// Note that this constraint is "favored" within its disjunction, and
    /// should be tried first to the exclusion of non-favored constraints in
    /// the same disjunction.
    void favorConstraint(Constraint *constraint) {
      if (constraint->isFavored())
        return;
  
      if (solverState) {
        solverState->favorConstraint(constraint);
      } else {
        constraint->setFavored();
      }
    }
  
    /// Retrieve the list of inactive constraints.
    ConstraintList &getConstraints() { return InactiveConstraints; }
  
    /// The worklist of "active" constraints that should be revisited
    /// due to a change.
    ConstraintList &getActiveConstraints() { return ActiveConstraints; }
  
    void findConstraints(SmallVectorImpl<Constraint *> &found,
                         llvm::function_ref<bool(const Constraint &)> pred) {
      filterConstraints(ActiveConstraints, pred, found);
      filterConstraints(InactiveConstraints, pred, found);
    }
  
    /// Retrieve the representative of the equivalence class containing
    /// this type variable.
    TypeVariableType *getRepresentative(TypeVariableType *typeVar) {
      return typeVar->getImpl().getRepresentative(getSavedBindings());
    }
  
    /// Gets the VarDecl associateed with resolvedOverload, and the type of the
    /// storage wrapper if the decl has an associated storage wrapper.
    Optional<std::pair<VarDecl *, Type>>
    getStorageWrapperInformation(ResolvedOverloadSetListItem *resolvedOverload) {
      assert(resolvedOverload);
      if (resolvedOverload->Choice.isDecl()) {
        if (auto *decl = dyn_cast<VarDecl>(resolvedOverload->Choice.getDecl())) {
          if (decl->hasAttachedPropertyWrapper()) {
            if (auto storageWrapper = decl->getPropertyWrapperStorageWrapper()) {
              Type type = storageWrapper->getInterfaceType();
              if (Type baseType = resolvedOverload->Choice.getBaseType()) {
                type = baseType->getTypeOfMember(DC->getParentModule(),
                                                 storageWrapper, type);
              }
              return std::make_pair(decl, type);
            }
          }
        }
      }
      return None;
    }
  
    /// Gets the VarDecl associateed with resolvedOverload, and the type of the
    /// backing storage if the decl has an associated property wrapper.
    Optional<std::pair<VarDecl *, Type>>
    getPropertyWrapperInformation(ResolvedOverloadSetListItem *resolvedOverload) {
      assert(resolvedOverload);
      if (resolvedOverload->Choice.isDecl()) {
        if (auto *decl = dyn_cast<VarDecl>(resolvedOverload->Choice.getDecl())) {
          if (decl->hasAttachedPropertyWrapper()) {
            auto wrapperTy = decl->getPropertyWrapperBackingPropertyType();
            if (Type baseType = resolvedOverload->Choice.getBaseType()) {
              wrapperTy = baseType->getTypeOfMember(DC->getParentModule(),
                                                    decl, wrapperTy);
            }
            return std::make_pair(decl, wrapperTy);
          }
        }
      }
      return None;
    }
  
    /// Gets the VarDecl, and the type of the type property that it wraps if
    /// resolved overload has a decl which is the backing storage for a
    /// property wrapper.
    Optional<std::pair<VarDecl *, Type>>
    getWrappedPropertyInformation(ResolvedOverloadSetListItem *resolvedOverload) {
      assert(resolvedOverload);
      if (resolvedOverload->Choice.isDecl()) {
        if (auto *decl = dyn_cast<VarDecl>(resolvedOverload->Choice.getDecl())) {
          if (auto wrapped = decl->getOriginalWrappedProperty()) {
            Type type = wrapped->getInterfaceType();
            if (Type baseType = resolvedOverload->Choice.getBaseType()) {
              type = baseType->getTypeOfMember(DC->getParentModule(),
                                               wrapped, type);
            }
            return std::make_pair(decl, type);
          }
        }
      }
      return None;
    }
  
    /// Merge the equivalence sets of the two type variables.
    ///
    /// Note that both \c typeVar1 and \c typeVar2 must be the
    /// representatives of their equivalence classes, and must be
    /// distinct.
    void mergeEquivalenceClasses(TypeVariableType *typeVar1,
                                 TypeVariableType *typeVar2,
                                 bool updateWorkList = true);
  
    /// Flags that direct type matching.
    enum TypeMatchFlags {
      /// Indicates that we are in a context where we should be
      /// generating constraints for any unsolvable problems.
      ///
      /// This flag is automatically introduced when type matching destructures
      /// a type constructor (tuple, function type, etc.), solving that
      /// constraint while potentially generating others.
      TMF_GenerateConstraints = 0x01,
  
      /// Indicates that we are applying a fix.
      TMF_ApplyingFix = 0x02,
    };
  
    /// Options that govern how type matching should proceed.
    using TypeMatchOptions = OptionSet<TypeMatchFlags>;
  
    /// Retrieve the fixed type corresponding to the given type variable,
    /// or a null type if there is no fixed type.
    Type getFixedType(TypeVariableType *typeVar) {
      return typeVar->getImpl().getFixedType(getSavedBindings());
    }
  
    /// Retrieve the fixed type corresponding to a given type variable,
    /// recursively, until we hit something that isn't a type variable
    /// or a type variable that doesn't have a fixed type.
    ///
    /// \param type The type to simplify.
    ///
    /// \param wantRValue Whether this routine should look through
    /// lvalues at each step.
    Type getFixedTypeRecursive(Type type, bool wantRValue) {
      TypeMatchOptions flags = None;
      return getFixedTypeRecursive(type, flags, wantRValue);
    }
  
    /// Retrieve the fixed type corresponding to a given type variable,
    /// recursively, until we hit something that isn't a type variable
    /// or a type variable that doesn't have a fixed type.
    ///
    /// \param type The type to simplify.
    ///
    /// \param flags When simplifying one of the types that is part of a
    /// constraint we are examining, the set of flags that governs the
    /// simplification. The set of flags may be both queried and mutated.
    ///
    /// \param wantRValue Whether this routine should look through
    /// lvalues at each step.
    Type getFixedTypeRecursive(Type type, TypeMatchOptions &flags,
                               bool wantRValue);
  
    /// Determine whether the given type variable occurs within the given type.
    ///
    /// This routine assumes that the type has already been fully simplified.
    ///
    /// \param involvesOtherTypeVariables if non-null, records whether any other
    /// type variables are present in the type.
    static bool typeVarOccursInType(TypeVariableType *typeVar, Type type,
                                    bool *involvesOtherTypeVariables = nullptr);
  
    /// Assign a fixed type to the given type variable.
    ///
    /// \param typeVar The type variable to bind.
    ///
    /// \param type The fixed type to which the type variable will be bound.
    ///
    /// \param updateState Whether to update the state based on this binding.
    /// False when we're only assigning a type as part of reconstructing 
    /// a complete solution from partial solutions.
    void assignFixedType(TypeVariableType *typeVar, Type type,
                         bool updateState = true);
    
    /// Determine if the type in question is an Array<T> and, if so, provide the
    /// element type of the array.
    static Optional<Type> isArrayType(Type type);
  
    /// Determine whether the given type is a dictionary and, if so, provide the
    /// key and value types for the dictionary.
    static Optional<std::pair<Type, Type>> isDictionaryType(Type type);
  
    /// Determine if the type in question is a Set<T> and, if so, provide the
    /// element type of the set.
    static Optional<Type> isSetType(Type t);
  
    /// Determine if the type in question is one of the known collection types.
    static bool isCollectionType(Type t);
  
    /// Determine if the type in question is AnyHashable.
    bool isAnyHashableType(Type t);
  
    /// Call Expr::isTypeReference on the given expression, using a
    /// custom accessor for the type on the expression that reads the
    /// type from the ConstraintSystem expression type map.
    bool isTypeReference(const Expr *E);
  
    /// Call Expr::isIsStaticallyDerivedMetatype on the given
    /// expression, using a custom accessor for the type on the
    /// expression that reads the type from the ConstraintSystem
    /// expression type map.
    bool isStaticallyDerivedMetatype(const Expr *E);
  
    /// Call TypeExpr::getInstanceType on the given expression, using a
    /// custom accessor for the type on the expression that reads the
    /// type from the ConstraintSystem expression type map.
    Type getInstanceType(const TypeExpr *E);
  
    /// Call AbstractClosureExpr::getResultType on the given expression,
    /// using a custom accessor for the type on the expression that
    /// reads the type from the ConstraintSystem expression type map.
    Type getResultType(const AbstractClosureExpr *E);
  
  private:
    /// Introduce the constraints associated with the given type variable
    /// into the worklist.
    void addTypeVariableConstraintsToWorkList(TypeVariableType *typeVar);
  
    static void
    filterConstraints(ConstraintList &constraints,
                      llvm::function_ref<bool(const Constraint &)> pred,
                      SmallVectorImpl<Constraint *> &found) {
      for (auto &constraint : constraints) {
        if (pred(constraint))
          found.push_back(&constraint);
      }
    }
  
  public:
  
    /// Coerce the given expression to an rvalue, if it isn't already.
    Expr *coerceToRValue(Expr *expr);
  
    /// "Open" the given unbound type by introducing fresh type
    /// variables for generic parameters and constructing a bound generic
    /// type from these type variables.
    ///
    /// \param unbound The type to open.
    ///
    /// \returns The opened type.
    Type openUnboundGenericType(UnboundGenericType *unbound,
                                ConstraintLocatorBuilder locator,
                                OpenedTypeMap &replacements);
  
    /// "Open" the given type by replacing any occurrences of unbound
    /// generic types with bound generic types with fresh type variables as
    /// generic arguments.
    ///
    /// \param type The type to open.
    ///
    /// \returns The opened type.
    Type openUnboundGenericType(Type type, ConstraintLocatorBuilder locator);
  
    /// "Open" the given type by replacing any occurrences of generic
    /// parameter types and dependent member types with fresh type variables.
    ///
    /// \param type The type to open.
    ///
    /// \returns The opened type, or \c type if there are no archetypes in it.
    Type openType(Type type, OpenedTypeMap &replacements);
  
    /// "Open" the given function type.
    ///
    /// If the function type is non-generic, this is equivalent to calling
    /// openType(). Otherwise, it calls openGeneric() on the generic
    /// function's signature first.
    ///
    /// \param funcType The function type to open.
    ///
    /// \param replacements The mapping from opened types to the type
    /// variables to which they were opened.
    ///
    /// \param innerDC The generic context from which the type originates.
    ///
    /// \param outerDC The generic context containing the declaration.
    ///
    /// \param skipProtocolSelfConstraint Whether to skip the constraint on a
    /// protocol's 'Self' type.
    ///
    /// \param skipGenericRequirements Whether to skip opening generic
    /// requirements asscoiated with given function type.
    ///
    /// \returns The opened type, or \c type if there are no archetypes in it.
    Type openFunctionType(
        AnyFunctionType *funcType,
        unsigned numArgumentLabelsToRemove,
        ConstraintLocatorBuilder locator,
        OpenedTypeMap &replacements,
        DeclContext *innerDC,
        DeclContext *outerDC,
        bool skipProtocolSelfConstraint,
        bool skipGenericRequirements = false);
  
    /// Open the generic parameter list and (if requested) its requirements,
    /// creating type variables for each of the type parameters.
    void openGeneric(DeclContext *innerDC,
                     DeclContext *outerDC,
                     GenericSignature *signature,
                     bool skipProtocolSelfConstraint,
                     ConstraintLocatorBuilder locator,
                     OpenedTypeMap &replacements,
                     bool skipGenericRequirements = false);
  
    /// Given generic signature open its generic requirements,
    /// using substitution function, and record them in the
    /// constraint system for further processing.
    void openGenericRequirements(DeclContext *outerDC,
                                 GenericSignature *signature,
                                 bool skipProtocolSelfConstraint,
                                 ConstraintLocatorBuilder locator,
                                 llvm::function_ref<Type(Type)> subst);
  
    /// Record the set of opened types for the given locator.
    void recordOpenedTypes(
           ConstraintLocatorBuilder locator,
           const OpenedTypeMap &replacements);
  
    /// Retrieve the type of a reference to the given value declaration.
    ///
    /// For references to polymorphic function types, this routine "opens up"
    /// the type by replacing each instance of an archetype with a fresh type
    /// variable.
    ///
    /// \param decl The declarations whose type is being computed.
    ///
    /// \returns a pair containing the full opened type (if applicable) and
    /// opened type of a reference to declaration.
    std::pair<Type, Type> getTypeOfReference(
                            ValueDecl *decl,
                            FunctionRefKind functionRefKind,
                            ConstraintLocatorBuilder locator,
                            DeclContext *useDC);
  
    /// Return the type-of-reference of the given value.
    ///
    /// \param baseType if non-null, return the type of a member reference to
    ///   this value when the base has the given type
    ///
    /// \param UseDC The context of the access.  Some variables have different
    ///   types depending on where they are used.
    ///
    /// \param base The optional base expression of this value reference
    ///
    /// \param wantInterfaceType Whether we want the interface type, if available.
    Type getUnopenedTypeOfReference(VarDecl *value, Type baseType,
                                    DeclContext *UseDC,
                                    const DeclRefExpr *base = nullptr,
                                    bool wantInterfaceType = false);
  
    /// Retrieve the type of a reference to the given value declaration,
    /// as a member with a base of the given type.
    ///
    /// For references to generic function types or members of generic types,
    /// this routine "opens up" the type by replacing each instance of a generic
    /// parameter with a fresh type variable.
    ///
    /// \param isDynamicResult Indicates that this declaration was found via
    /// dynamic lookup.
    ///
    /// \returns a pair containing the full opened type (which includes the opened
    /// base) and opened type of a reference to this member.
    std::pair<Type, Type> getTypeOfMemberReference(
                            Type baseTy, ValueDecl *decl, DeclContext *useDC,
                            bool isDynamicResult,
                            FunctionRefKind functionRefKind,
                            ConstraintLocatorBuilder locator,
                            const DeclRefExpr *base = nullptr,
                            OpenedTypeMap *replacements = nullptr);
  
    /// Attempt to simplify the set of overloads corresponding to a given
    /// function application constraint.
    ///
    /// \param fnTypeVar The type variable that describes the set of
    /// overloads for the function.
    ///
    /// \param argFnType The call signature, which includes the call arguments
    /// (as the function parameters) and the expected result type of the
    /// call.
    ///
    /// \param argumentLabels The argument labels provided at the call site,
    /// if known.
    ///
    /// \returns \c fnType, or some simplified form of it if this function
    /// was able to find a single overload or derive some common structure
    /// among the overloads.
    Type simplifyAppliedOverloads(TypeVariableType *fnTypeVar,
                                  const FunctionType *argFnType,
                                  Optional<ArgumentLabelState> argumentLabels,
                                  ConstraintLocatorBuilder locator);
  
    /// Retrieve the type that will be used when matching the given overload.
    Type getEffectiveOverloadType(const OverloadChoice &overload,
                                  bool allowMembers,
                                  DeclContext *useDC);
  
    /// Add a new overload set to the list of unresolved overload
    /// sets.
    void addOverloadSet(Type boundType, ArrayRef<OverloadChoice> choices,
                        DeclContext *useDC, ConstraintLocator *locator,
                        Optional<unsigned> favoredIndex = None);
  
    void addOverloadSet(ArrayRef<Constraint *> choices,
                        ConstraintLocator *locator);
  
    /// Retrieve the allocator used by this constraint system.
    llvm::BumpPtrAllocator &getAllocator() { return Allocator; }
  
    template <typename It>
    ArrayRef<typename std::iterator_traits<It>::value_type>
    allocateCopy(It start, It end) {
      using T = typename std::iterator_traits<It>::value_type;
      T *result = (T*)getAllocator().Allocate(sizeof(T)*(end-start), alignof(T));
      unsigned i;
      for (i = 0; start != end; ++start, ++i)
        new (result+i) T(*start);
      return ArrayRef<T>(result, i);
    }
  
    template<typename T>
    ArrayRef<T> allocateCopy(ArrayRef<T> array) {
      return allocateCopy(array.begin(), array.end());
    }
  
    template<typename T>
    ArrayRef<T> allocateCopy(SmallVectorImpl<T> &vec) {
      return allocateCopy(vec.begin(), vec.end());
    }
  
    /// Generate constraints for the given (unchecked) expression.
    ///
    /// \returns a possibly-sanitized expression, or null if an error occurred.
    Expr *generateConstraints(Expr *E, DeclContext *dc = nullptr);
  
    /// Generate constraints for binding the given pattern to the
    /// value of the given expression.
    ///
    /// \returns a possibly-sanitized initializer, or null if an error occurred.
    Type generateConstraints(Pattern *P, ConstraintLocatorBuilder locator);
  
    /// Generate constraints for a given set of overload choices.
    ///
    /// \param constraints The container of generated constraint choices.
    ///
    /// \param type The type each choice should be bound to.
    ///
    /// \param choices The set of choices to convert into bind overload
    /// constraints so solver could attempt each one.
    ///
    /// \param useDC The declaration context where each choice is used.
    ///
    /// \param locator The locator to use when generating constraints.
    ///
    /// \param favoredIndex If there is a "favored" or preferred choice
    /// this is its index in the set of choices.
    ///
    /// \param requiresFix Determines whether choices require a fix to
    /// be included in the result. If the fix couldn't be provided by
    /// `getFix` for any given choice, such choice would be filtered out.
    ///
    /// \param getFix Optional callback to determine a fix for a given
    /// choice (first argument is a position of current choice,
    /// second - the choice in question).
    void generateConstraints(
        SmallVectorImpl<Constraint *> &constraints, Type type,
        ArrayRef<OverloadChoice> choices, DeclContext *useDC,
        ConstraintLocator *locator, Optional<unsigned> favoredIndex = None,
        bool requiresFix = false,
        llvm::function_ref<ConstraintFix *(unsigned, const OverloadChoice &)>
            getFix = [](unsigned, const OverloadChoice &) { return nullptr; });
  
    /// Propagate constraints in an effort to enforce local
    /// consistency to reduce the time to solve the system.
    ///
    /// \returns true if the system is known to be inconsistent (have no
    /// solutions).
    bool propagateConstraints();
  
    /// The result of attempting to resolve a constraint or set of
    /// constraints.
    enum class SolutionKind : char {
      /// The constraint has been solved completely, and provides no
      /// more information.
      Solved,
      /// The constraint could not be solved at this point.
      Unsolved,
      /// The constraint uncovers an inconsistency in the system.
      Error
    };
  
    class TypeMatchResult {
      SolutionKind Kind;
  
    public:
      inline bool isSuccess() const { return Kind == SolutionKind::Solved; }
      inline bool isFailure() const { return Kind == SolutionKind::Error; }
      inline bool isAmbiguous() const { return Kind == SolutionKind::Unsolved; }
  
      static TypeMatchResult success(ConstraintSystem &cs) {
        return {SolutionKind::Solved};
      }
  
      static TypeMatchResult failure(ConstraintSystem &cs,
                                     ConstraintLocatorBuilder location) {
        return {SolutionKind::Error};
      }
  
      static TypeMatchResult ambiguous(ConstraintSystem &cs) {
        return {SolutionKind::Unsolved};
      }
  
      operator SolutionKind() { return Kind; }
    private:
      TypeMatchResult(SolutionKind result) : Kind(result) {}
    };
  
    /// Attempt to repair typing failures and record fixes if needed.
    /// \return true if at least some of the failures has been repaired
    /// successfully, which allows type matcher to continue.
    bool repairFailures(Type lhs, Type rhs,
                        SmallVectorImpl<RestrictionOrFix> &conversionsOrFixes,
                        ConstraintLocatorBuilder locator);
  
    /// Subroutine of \c matchTypes(), which matches up two tuple types.
    ///
    /// \returns the result of performing the tuple-to-tuple conversion.
    TypeMatchResult matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
                                    ConstraintKind kind, TypeMatchOptions flags,
                                    ConstraintLocatorBuilder locator);
  
    /// Subroutine of \c matchTypes(), which matches a scalar type to
    /// a tuple type.
    ///
    /// \returns the result of performing the scalar-to-tuple conversion.
    TypeMatchResult matchScalarToTupleTypes(Type type1, TupleType *tuple2,
                                            ConstraintKind kind,
                                            TypeMatchOptions flags,
                                            ConstraintLocatorBuilder locator);
  
    /// Subroutine of \c matchTypes(), which matches up two function
    /// types.
    TypeMatchResult matchFunctionTypes(FunctionType *func1, FunctionType *func2,
                                       ConstraintKind kind, TypeMatchOptions flags,
                                       ConstraintLocatorBuilder locator);
    
    /// Subroutine of \c matchTypes(), which matches up a value to a
    /// superclass.
    TypeMatchResult matchSuperclassTypes(Type type1, Type type2,
                                         TypeMatchOptions flags,
                                         ConstraintLocatorBuilder locator);
  
    /// Subroutine of \c matchTypes(), which matches up two types that
    /// refer to the same declaration via their generic arguments.
    TypeMatchResult matchDeepEqualityTypes(Type type1, Type type2,
                                           ConstraintLocatorBuilder locator);
  
    /// Subroutine of \c matchTypes(), which matches up a value to an
    /// existential type.
    ///
    /// \param kind Either ConstraintKind::SelfObjectOfProtocol or
    /// ConstraintKind::ConformsTo. Usually this uses SelfObjectOfProtocol,
    /// but when matching the instance type of a metatype with the instance type
    /// of an existential metatype, since we want an actual conformance check.
    TypeMatchResult matchExistentialTypes(Type type1, Type type2,
                                          ConstraintKind kind,
                                          TypeMatchOptions flags,
                                          ConstraintLocatorBuilder locator);
  
    /// Subroutine of \c matchTypes(), used to bind a type to a
    /// type variable.
    TypeMatchResult matchTypesBindTypeVar(
        TypeVariableType *typeVar, Type type, ConstraintKind kind,
        TypeMatchOptions flags, ConstraintLocatorBuilder locator,
        llvm::function_ref<TypeMatchResult()> formUnsolvedResult);
  
  public: // FIXME: public due to statics in CSSimplify.cpp
    /// Attempt to match up types \c type1 and \c type2, which in effect
    /// is solving the given type constraint between these two types.
    ///
    /// \param type1 The first type, which is on the left of the type relation.
    ///
    /// \param type2 The second type, which is on the right of the type relation.
    ///
    /// \param kind The kind of type match being performed, e.g., exact match,
    /// trivial subtyping, subtyping, or conversion.
    ///
    /// \param flags A set of flags composed from the TMF_* constants, which
    /// indicates how the constraint should be simplified.
    ///
    /// \param locator The locator that will be used to track the location of
    /// the specific types being matched.
    ///
    /// \returns the result of attempting to solve this constraint.
    TypeMatchResult matchTypes(Type type1, Type type2, ConstraintKind kind,
                               TypeMatchOptions flags,
                               ConstraintLocatorBuilder locator);
  
    TypeMatchResult getTypeMatchSuccess() {
      return TypeMatchResult::success(*this);
    }
  
    TypeMatchResult getTypeMatchFailure(ConstraintLocatorBuilder locator) {
      return TypeMatchResult::failure(*this, locator);
    }
  
    TypeMatchResult getTypeMatchAmbiguous() {
      return TypeMatchResult::ambiguous(*this);
    }
  
  public:
    /// Given a function type where the eventual result type is an optional,
    /// where "eventual result type" is defined as:
    ///   1. The result type is an optional
    ///   2. The result type is a function type with an eventual result
    ///      type that is an optional.
    ///
    /// return the same function type but with the eventual result type
    /// replaced by its underlying type.
    ///
    /// i.e. return (S) -> T for (S) -> T?
    //       return (X) -> () -> Y for (X) -> () -> Y?
    Type replaceFinalResultTypeWithUnderlying(AnyFunctionType *fnTy) {
      auto resultTy = fnTy->getResult();
      if (auto *resultFnTy = resultTy->getAs<AnyFunctionType>())
        resultTy = replaceFinalResultTypeWithUnderlying(resultFnTy);
      else
        resultTy = resultTy->getWithoutSpecifierType()->getOptionalObjectType();
  
      assert(resultTy);
  
      if (auto *genericFn = fnTy->getAs<GenericFunctionType>()) {
        return GenericFunctionType::get(genericFn->getGenericSignature(),
                                        genericFn->getParams(), resultTy,
                                        genericFn->getExtInfo());
      }
  
      return FunctionType::get(fnTy->getParams(), resultTy, fnTy->getExtInfo());
    }
  
    // Build a disjunction that attempts both T? and T for a particular
    // type binding. The choice of T? is preferred, and we will not
    // attempt T if we can type check with T?
    void
    buildDisjunctionForOptionalVsUnderlying(Type boundTy, Type type,
                                            ConstraintLocator *locator) {
      // NOTE: If we use other locator kinds for these disjunctions, we
      // need to account for it in solution scores for forced-unwraps.
      assert(locator->getPath().back().getKind() ==
                 ConstraintLocator::ImplicitlyUnwrappedDisjunctionChoice ||
             locator->getPath().back().getKind() ==
                 ConstraintLocator::DynamicLookupResult);
  
      // Create the constraint to bind to the optional type and make it
      // the favored choice.
      auto *bindToOptional =
        Constraint::create(*this, ConstraintKind::Bind, boundTy, type, locator);
      bindToOptional->setFavored();
  
      Type underlyingType;
      if (auto *fnTy = type->getAs<AnyFunctionType>())
        underlyingType = replaceFinalResultTypeWithUnderlying(fnTy);
      else
        underlyingType = type->getWithoutSpecifierType()->getOptionalObjectType();
  
      assert(underlyingType);
  
      if (type->is<LValueType>())
        underlyingType = LValueType::get(underlyingType);
      assert(!type->is<InOutType>());
  
      auto *bindToUnderlying = Constraint::create(
          *this, ConstraintKind::Bind, boundTy, underlyingType, locator);
  
      llvm::SmallVector<Constraint *, 2> choices = {bindToOptional,
                                                    bindToUnderlying};
  
      // Create the disjunction
      addDisjunctionConstraint(choices, locator, RememberChoice);
    }
  
    // Build a disjunction for types declared IUO.
    void
    buildDisjunctionForImplicitlyUnwrappedOptional(Type boundTy, Type type,
                                                   ConstraintLocator *locator) {
      auto *disjunctionLocator = getConstraintLocator(
          locator, ConstraintLocator::ImplicitlyUnwrappedDisjunctionChoice);
      buildDisjunctionForOptionalVsUnderlying(boundTy, type, disjunctionLocator);
    }
  
    // Build a disjunction for dynamic lookup results, which are
    // implicitly unwrapped if needed.
    void buildDisjunctionForDynamicLookupResult(Type boundTy, Type type,
                                                ConstraintLocator *locator) {
      auto *dynamicLocator =
          getConstraintLocator(locator, ConstraintLocator::DynamicLookupResult);
      buildDisjunctionForOptionalVsUnderlying(boundTy, type, dynamicLocator);
    }
  
    /// Resolve the given overload set to the given choice.
    void resolveOverload(ConstraintLocator *locator, Type boundType,
                         OverloadChoice choice, DeclContext *useDC);
  
    /// Simplify a type, by replacing type variables with either their
    /// fixed types (if available) or their representatives.
    ///
    /// The resulting types can be compared canonically, so long as additional
    /// type equivalence requirements aren't introduced between comparisons.
    Type simplifyType(Type type);
  
    /// Simplify a type, by replacing type variables with either their
    /// fixed types (if available) or their representatives.
    ///
    /// \param flags If the simplified type has changed, this will be updated
    /// to include \c TMF_GenerateConstraints.
    ///
    /// The resulting types can be compared canonically, so long as additional
    /// type equivalence requirements aren't introduced between comparisons.
    Type simplifyType(Type type, TypeMatchOptions &flags) {
      Type result = simplifyType(type);
      if (result.getPointer() != type.getPointer())
        flags |= TMF_GenerateConstraints;
      return result;
    }
  
    /// Given a ValueMember, UnresolvedValueMember, or TypeMember constraint,
    /// perform a lookup into the specified base type to find a candidate list.
    /// The list returned includes the viable candidates as well as the unviable
    /// ones (along with reasons why they aren't viable).
    ///
    /// If includeInaccessibleMembers is set to true, this burns compile time to
    /// try to identify and classify inaccessible members that may be being
    /// referenced.
    MemberLookupResult performMemberLookup(ConstraintKind constraintKind,
                                           DeclName memberName, Type baseTy,
                                           FunctionRefKind functionRefKind,
                                           ConstraintLocator *memberLocator,
                                           bool includeInaccessibleMembers);
  
  private:
    /// Attempt to simplify the given construction constraint.
    ///
    /// \param valueType The type being constructed.
    ///
    /// \param fnType The argument type that will be the input to the
    /// valueType initializer and the result type will be the result of
    /// calling that initializer.
    ///
    /// \param flags A set of flags composed from the TMF_* constants, which
    /// indicates how the constraint should be simplified.
    /// 
    /// \param locator Locator describing where this construction
    /// occurred.
    SolutionKind simplifyConstructionConstraint(Type valueType, 
                                                FunctionType *fnType,
                                                TypeMatchOptions flags,
                                                DeclContext *DC,
                                                FunctionRefKind functionRefKind,
                                                ConstraintLocator *locator);
  
    /// Attempt to simplify the given conformance constraint.
    ///
    /// \param type The type being tested.
    /// \param protocol The protocol to which the type should conform.
    /// \param kind Either ConstraintKind::SelfObjectOfProtocol or
    /// ConstraintKind::ConformsTo.
    /// \param locator Locator describing where this constraint occurred.
    SolutionKind simplifyConformsToConstraint(Type type, ProtocolDecl *protocol,
                                              ConstraintKind kind,
                                              ConstraintLocatorBuilder locator,
                                              TypeMatchOptions flags);
  
    /// Attempt to simplify the given conformance constraint.
    ///
    /// \param type The type being tested.
    /// \param protocol The protocol or protocol composition type to which the
    /// type should conform.
    /// \param locator Locator describing where this constraint occurred.
    ///
    /// \param kind If this is SelfTypeOfProtocol, we allow an existential type
    /// that contains the protocol but does not conform to it (eg, due to
    /// associated types).
    SolutionKind simplifyConformsToConstraint(Type type, Type protocol,
                                              ConstraintKind kind,
                                              ConstraintLocatorBuilder locator,
                                              TypeMatchOptions flags);
  
    /// Attempt to simplify a checked-cast constraint.
    SolutionKind simplifyCheckedCastConstraint(Type fromType, Type toType,
                                               TypeMatchOptions flags,
                                               ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given member constraint.
    SolutionKind simplifyMemberConstraint(
        ConstraintKind kind, Type baseType, DeclName member, Type memberType,
        DeclContext *useDC, FunctionRefKind functionRefKind,
        ArrayRef<OverloadChoice> outerAlternatives, TypeMatchOptions flags,
        ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the optional object constraint.
    SolutionKind simplifyOptionalObjectConstraint(
                                            Type first, Type second,
                                            TypeMatchOptions flags,
                                            ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify a function input or result constraint.
    SolutionKind simplifyFunctionComponentConstraint(
                                            ConstraintKind kind,
                                            Type first, Type second,
                                            TypeMatchOptions flags,
                                            ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify an OpaqueUnderlyingType constraint.
    SolutionKind simplifyOpaqueUnderlyingTypeConstraint(Type type1,
                                                Type type2,
                                                TypeMatchOptions flags,
                                                ConstraintLocatorBuilder locator);
    
    /// Attempt to simplify the BridgingConversion constraint.
    SolutionKind simplifyBridgingConstraint(Type type1,
                                           Type type2,
                                           TypeMatchOptions flags,
                                           ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the ApplicableFunction constraint.
    SolutionKind simplifyApplicableFnConstraint(
                                        Type type1,
                                        Type type2,
                                        TypeMatchOptions flags,
                                        ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the DynamicCallableApplicableFunction constraint.
    SolutionKind simplifyDynamicCallableApplicableFnConstraint(
                                        Type type1,
                                        Type type2,
                                        TypeMatchOptions flags,
                                        ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given DynamicTypeOf constraint.
    SolutionKind simplifyDynamicTypeOfConstraint(
                                           Type type1, Type type2,
                                           TypeMatchOptions flags,
                                           ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given EscapableFunctionOf constraint.
    SolutionKind simplifyEscapableFunctionOfConstraint(
                                           Type type1, Type type2,
                                           TypeMatchOptions flags,
                                           ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given OpenedExistentialOf constraint.
    SolutionKind simplifyOpenedExistentialOfConstraint(
                                           Type type1, Type type2,
                                           TypeMatchOptions flags,
                                           ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given KeyPathApplication constraint.
    SolutionKind simplifyKeyPathApplicationConstraint(
                                           Type keyPath,
                                           Type root,
                                           Type value,
                                           TypeMatchOptions flags,
                                           ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given KeyPath constraint.
    SolutionKind simplifyKeyPathConstraint(Type keyPath,
                                           Type root,
                                           Type value,
                                           TypeMatchOptions flags,
                                           ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify the given defaultable constraint.
    SolutionKind simplifyDefaultableConstraint(Type first, Type second,
                                               TypeMatchOptions flags,
                                               ConstraintLocatorBuilder locator);
  
    /// Attempt to simplify a one-way constraint.
    SolutionKind simplifyOneWayConstraint(ConstraintKind kind,
                                          Type first, Type second,
                                          TypeMatchOptions flags,
                                          ConstraintLocatorBuilder locator);
  
    /// Simplify a conversion constraint by applying the given
    /// reduction rule, which is known to apply at the outermost level.
    SolutionKind simplifyRestrictedConstraintImpl(
                   ConversionRestrictionKind restriction,
                   Type type1, Type type2,
                   ConstraintKind matchKind,
                   TypeMatchOptions flags,
                   ConstraintLocatorBuilder locator);
  
    /// Simplify a conversion constraint by applying the given
    /// reduction rule, which is known to apply at the outermost level.
    SolutionKind simplifyRestrictedConstraint(
                   ConversionRestrictionKind restriction,
                   Type type1, Type type2,
                   ConstraintKind matchKind,
                   TypeMatchOptions flags,
                   ConstraintLocatorBuilder locator);
  
  public: // FIXME: Public for use by static functions.
    /// Simplify a conversion constraint with a fix applied to it.
    SolutionKind simplifyFixConstraint(ConstraintFix *fix, Type type1, Type type2,
                                       ConstraintKind matchKind,
                                       TypeMatchOptions flags,
                                       ConstraintLocatorBuilder locator);
  
  public:
    /// Simplify the system of constraints, by breaking down complex
    /// constraints into simpler constraints.
    ///
    /// The result of simplification is a constraint system consisting of
    /// only simple constraints relating type variables to each other or
    /// directly to fixed types. There are no constraints that involve
    /// type constructors on both sides. The simplified constraint system may,
    /// of course, include type variables for which we have constraints but
    /// no fixed type. Such type variables are left to the solver to bind.
    ///
    /// \returns true if an error occurred, false otherwise.
    bool simplify(bool ContinueAfterFailures = false);
  
    /// Simplify the given constraint.
    SolutionKind simplifyConstraint(const Constraint &constraint);
    /// Simplify the given disjunction choice.
    void simplifyDisjunctionChoice(Constraint *choice);
  
    /// Apply the given function builder to the closure expression.
    TypeMatchResult applyFunctionBuilder(ClosureExpr *closure, Type builderType,
                                         ConstraintLocator *calleeLocator,
                                         ConstraintLocatorBuilder locator);
  
  private:
    /// The kind of bindings that are permitted.
    enum class AllowedBindingKind : uint8_t {
      /// Only the exact type.
      Exact,
      /// Supertypes of the specified type.
      Supertypes,
      /// Subtypes of the specified type.
      Subtypes
    };
  
    /// The kind of literal binding found.
    enum class LiteralBindingKind : uint8_t {
      None,
      Collection,
      Float,
      Atom,
    };
  
    /// A potential binding from the type variable to a particular type,
    /// along with information that can be used to construct related
    /// bindings, e.g., the supertypes of a given type.
    struct PotentialBinding {
      /// The type to which the type variable can be bound.
      Type BindingType;
  
      /// The kind of bindings permitted.
      AllowedBindingKind Kind;
  
      /// The kind of the constraint this binding came from.
      ConstraintKind BindingSource;
  
      /// The defaulted protocol associated with this binding.
      ProtocolDecl *DefaultedProtocol;
  
      /// If this is a binding that comes from a \c Defaultable constraint,
      /// the locator of that constraint.
      ConstraintLocator *DefaultableBinding = nullptr;
  
      PotentialBinding(Type type, AllowedBindingKind kind,
                       ConstraintKind bindingSource,
                       ProtocolDecl *defaultedProtocol = nullptr,
                       ConstraintLocator *defaultableBinding = nullptr)
          : BindingType(type->getWithoutParens()), Kind(kind),
            BindingSource(bindingSource), DefaultedProtocol(defaultedProtocol),
            DefaultableBinding(defaultableBinding) {}
  
      bool isDefaultableBinding() const { return DefaultableBinding != nullptr; }
  
      PotentialBinding withType(Type type) const {
        return {type, Kind, BindingSource, DefaultedProtocol, DefaultableBinding};
      }
    };
  
    struct PotentialBindings {
      using BindingScore = std::tuple<bool, bool, bool, bool, unsigned char, int>;
  
      TypeVariableType *TypeVar;
  
      /// The set of potential bindings.
      SmallVector<PotentialBinding, 4> Bindings;
  
      /// Whether this type variable is fully bound by one of its constraints.
      bool FullyBound = false;
  
      /// Whether the bindings of this type involve other type variables.
      bool InvolvesTypeVariables = false;
  
      /// Whether the bindings represent (potentially) incomplete set,
      /// there is no way to say with absolute certainty if that's the
      /// case, but that could happen when certain constraints like
      /// `bind param` are present in the system.
      bool PotentiallyIncomplete = false;
  
      /// Whether this type variable has literal bindings.
      LiteralBindingKind LiteralBinding = LiteralBindingKind::None;
  
      /// Whether this type variable is only bound above by existential types.
      bool SubtypeOfExistentialType = false;
  
      /// The number of defaultable bindings.
      unsigned NumDefaultableBindings = 0;
  
      /// Tracks the position of the last known supertype in the group.
      Optional<unsigned> lastSupertypeIndex;
  
      /// A set of all constraints which contribute to pontential bindings.
      llvm::SmallPtrSet<Constraint *, 8> Sources;
  
      PotentialBindings(TypeVariableType *typeVar)
          : TypeVar(typeVar), PotentiallyIncomplete(isGenericParameter()) {}
  
      /// Determine whether the set of bindings is non-empty.
      explicit operator bool() const { return !Bindings.empty(); }
  
      /// Whether there are any non-defaultable bindings.
      bool hasNonDefaultableBindings() const {
        return Bindings.size() > NumDefaultableBindings;
      }
  
      static BindingScore formBindingScore(const PotentialBindings &b) {
        return std::make_tuple(!b.hasNonDefaultableBindings(),
                               b.FullyBound,
                               b.SubtypeOfExistentialType,
                               b.InvolvesTypeVariables,
                               static_cast<unsigned char>(b.LiteralBinding),
                               -(b.Bindings.size() - b.NumDefaultableBindings));
      }
  
      /// Compare two sets of bindings, where \c x < y indicates that
      /// \c x is a better set of bindings that \c y.
      friend bool operator<(const PotentialBindings &x,
                            const PotentialBindings &y) {
        if (formBindingScore(x) < formBindingScore(y))
          return true;
  
        if (formBindingScore(y) < formBindingScore(x))
          return false;
  
        // If there is a difference in number of default types,
        // prioritize bindings with fewer of them.
        if (x.NumDefaultableBindings != y.NumDefaultableBindings)
          return x.NumDefaultableBindings < y.NumDefaultableBindings;
  
        // As a last resort, let's check if the bindings are
        // potentially incomplete, and if so, let's de-prioritize them.
        return x.PotentiallyIncomplete < y.PotentiallyIncomplete;
      }
  
      void foundLiteralBinding(ProtocolDecl *proto) {
        switch (*proto->getKnownProtocolKind()) {
        case KnownProtocolKind::ExpressibleByDictionaryLiteral:
        case KnownProtocolKind::ExpressibleByArrayLiteral:
        case KnownProtocolKind::ExpressibleByStringInterpolation:
          LiteralBinding = LiteralBindingKind::Collection;
          break;
  
        case KnownProtocolKind::ExpressibleByFloatLiteral:
          LiteralBinding = LiteralBindingKind::Float;
          break;
  
        default:
          if (LiteralBinding != LiteralBindingKind::Collection)
            LiteralBinding = LiteralBindingKind::Atom;
          break;
        }
      }
  
      /// Add a potential binding to the list of bindings,
      /// coalescing supertype bounds when we are able to compute the meet.
      void addPotentialBinding(PotentialBinding binding,
                               bool allowJoinMeet = true);
  
      /// Check if this binding is viable for inclusion in the set.
      bool isViable(PotentialBinding &binding) const;
  
      bool isGenericParameter() const {
        if (auto *locator = TypeVar->getImpl().getLocator()) {
          auto path = locator->getPath();
          return path.empty() ? false
                              : path.back().getKind() ==
                                    ConstraintLocator::GenericParameter;
        }
        return false;
      }
  
      void dump(llvm::raw_ostream &out,
                unsigned indent = 0) const LLVM_ATTRIBUTE_USED {
        out.indent(indent);
        if (PotentiallyIncomplete)
          out << "potentially_incomplete ";
        if (FullyBound)
          out << "fully_bound ";
        if (SubtypeOfExistentialType)
          out << "subtype_of_existential ";
        if (LiteralBinding != LiteralBindingKind::None)
          out << "literal=" << static_cast<int>(LiteralBinding) << " ";
        if (InvolvesTypeVariables)
          out << "involves_type_vars ";
        if (NumDefaultableBindings > 0)
          out << "#defaultable_bindings=" << NumDefaultableBindings << " ";
  
        out << "bindings={";
        interleave(Bindings,
                   [&](const PotentialBinding &binding) {
                     auto type = binding.BindingType;
                     auto &ctx = type->getASTContext();
                     llvm::SaveAndRestore<bool> debugConstraints(
                         ctx.LangOpts.DebugConstraintSolver, true);
                     switch (binding.Kind) {
                     case AllowedBindingKind::Exact:
                       break;
  
                     case AllowedBindingKind::Subtypes:
                       out << "(subtypes of) ";
                       break;
  
                     case AllowedBindingKind::Supertypes:
                       out << "(supertypes of) ";
                       break;
                     }
                     if (binding.DefaultedProtocol)
                       out << "(default from "
                           << binding.DefaultedProtocol->getName() << ") ";
                     out << type.getString();
                   },
                   [&]() { out << "; "; });
        out << "}";
      }
  
      void dump(ConstraintSystem *cs,
                unsigned indent = 0) const LLVM_ATTRIBUTE_USED {
        dump(cs->getASTContext().TypeCheckerDebug->getStream());
      }
  
      void dump(TypeVariableType *typeVar, llvm::raw_ostream &out,
                unsigned indent = 0) const LLVM_ATTRIBUTE_USED {
        out.indent(indent);
        out << "(";
        if (typeVar)
          out << "$T" << typeVar->getImpl().getID();
        dump(out, 1);
        out << ")\n";
      }
    };
  
    Optional<Type> checkTypeOfBinding(TypeVariableType *typeVar, Type type);
    Optional<PotentialBindings> determineBestBindings();
    Optional<ConstraintSystem::PotentialBinding>
    getPotentialBindingForRelationalConstraint(
        PotentialBindings &result, Constraint *constraint,
        bool &hasDependentMemberRelationalConstraints,
        bool &hasNonDependentMemberRelationalConstraints,
        bool &addOptionalSupertypeBindings);
    PotentialBindings getPotentialBindings(TypeVariableType *typeVar);
  
  private:
    /// Add a constraint to the constraint system.
    SolutionKind addConstraintImpl(ConstraintKind kind, Type first, Type second,
                                   ConstraintLocatorBuilder locator,
                                   bool isFavored);
  
    /// Collect the current inactive disjunction constraints.
    void collectDisjunctions(SmallVectorImpl<Constraint *> &disjunctions);
  
    /// Record a particular disjunction choice of
    void recordDisjunctionChoice(ConstraintLocator *disjunctionLocator,
                                 unsigned index) {
      DisjunctionChoices.push_back({disjunctionLocator, index});
    }
  
    /// Filter the set of disjunction terms, keeping only those where the
    /// predicate returns \c true.
    ///
    /// The terms of the disjunction that are filtered out will be marked as
    /// "disabled" so they won't be visited later. If only one term remains
    /// enabled, the disjunction itself will be returned and that term will
    /// be made active.
    ///
    /// \param restoreOnFail If true, then all of the disabled terms will
    /// be re-enabled when this function returns \c Error.
    ///
    /// \returns One of \c Solved (only a single term remained),
    /// \c Unsolved (more than one disjunction terms remain), or
    /// \c Error (all terms were filtered out).
    SolutionKind filterDisjunction(Constraint *disjunction,
                                    bool restoreOnFail,
                                    llvm::function_ref<bool(Constraint *)> pred);
  
    bool isReadOnlyKeyPathComponent(const AbstractStorageDecl *storage) {
      // See whether key paths can store to this component. (Key paths don't
      // get any special power from being formed in certain contexts, such
      // as the ability to assign to `let`s in initialization contexts, so
      // we pass null for the DC to `isSettable` here.)
      if (!getASTContext().isSwiftVersionAtLeast(5)) {
        // As a source-compatibility measure, continue to allow
        // WritableKeyPaths to be formed in the same conditions we did
        // in previous releases even if we should not be able to set
        // the value in this context.
        if (!storage->isSettable(DC)) {
          // A non-settable component makes the key path read-only, unless
          // a reference-writable component shows up later.
          return true;
        }
      } else if (!storage->isSettable(nullptr) ||
                 !storage->isSetterAccessibleFrom(DC)) {
        // A non-settable component makes the key path read-only, unless
        // a reference-writable component shows up later.
        return true;
      }
  
      return false;
    }
  
  public:
    // Given a type variable, attempt to find the disjunction of
    // bind overloads associated with it. This may return null in cases where
    // the disjunction has either not been created or binds the type variable
    // in some manner other than by binding overloads.
    ///
    /// \param numOptionalUnwraps If non-null, this will receive the number
    /// of "optional object of" constraints that this function looked through
    /// to uncover the disjunction. The actual overloads will have this number
    /// of optionals wrapping the type.
    Constraint *getUnboundBindOverloadDisjunction(
      TypeVariableType *tyvar,
      unsigned *numOptionalUnwraps = nullptr);
  
  private:
    /// Solve the system of constraints after it has already been
    /// simplified.
    ///
    /// \param solutions The set of solutions to this system of constraints.
    ///
    /// \returns true if an error occurred, false otherwise.
    bool solveSimplified(SmallVectorImpl<Solution> &solutions);
  
    /// Find reduced domains of disjunction constraints for given
    /// expression, this is achieved to solving individual sub-expressions
    /// and combining resolving types. Such algorithm is called directional
    /// path consistency because it goes from children to parents for all
    /// related sub-expressions taking union of their domains.
    ///
    /// \param expr The expression to find reductions for.
    void shrink(Expr *expr);
  
    /// Pick a disjunction from the InactiveConstraints list.
    ///
    /// \returns The selected disjunction.
    Constraint *selectDisjunction();
  
    Constraint *selectApplyDisjunction();
  
    /// Solve the system of constraints generated from provided expression.
    ///
    /// \param expr The expression to generate constraints from.
    /// \param convertType The expected type of the expression.
    /// \param listener The callback to check solving progress.
    /// \param solutions The set of solutions to the system of constraints.
    /// \param allowFreeTypeVariables How to bind free type variables in
    /// the solution.
    ///
    /// \returns Error is an error occurred, Solved is system is consistent
    /// and solutions were found, Unsolved otherwise.
    SolutionKind solveImpl(Expr *&expr,
                           Type convertType,
                           ExprTypeCheckListener *listener,
                           SmallVectorImpl<Solution> &solutions,
                           FreeTypeVariableBinding allowFreeTypeVariables
                            = FreeTypeVariableBinding::Disallow);
  
  public:
    /// Solve the system of constraints generated from provided expression.
    ///
    /// The expression should have already been pre-checked with
    /// preCheckExpression().
    ///
    /// \param expr The expression to generate constraints from.
    /// \param convertType The expected type of the expression.
    /// \param listener The callback to check solving progress.
    /// \param solutions The set of solutions to the system of constraints.
    /// \param allowFreeTypeVariables How to bind free type variables in
    /// the solution.
    ///
    /// \returns true is an error occurred, false is system is consistent
    /// and solutions were found.
    bool solve(Expr *&expr,
               Type convertType,
               ExprTypeCheckListener *listener,
               SmallVectorImpl<Solution> &solutions,
               FreeTypeVariableBinding allowFreeTypeVariables
               = FreeTypeVariableBinding::Disallow);
  
    /// Solve the system of constraints.
    ///
    /// \param solutions The set of solutions to this system of constraints.
    ///
    /// \param allowFreeTypeVariables How to bind free type variables in
    /// the solution.
    ///
    /// \returns true if an error occurred, false otherwise.  Note that multiple
    /// ambiguous solutions for the same constraint system are considered to be
    /// success by this API.
    bool solve(Expr *const expr, SmallVectorImpl<Solution> &solutions,
               FreeTypeVariableBinding allowFreeTypeVariables =
                   FreeTypeVariableBinding::Disallow);
  
    /// Solve the system of constraints.
    ///
    /// \param allowFreeTypeVariables How to bind free type variables in
    /// the solution.
    ///
    /// \param allowFixes Whether to allow fixes in the solution.
    ///
    /// \returns a solution if a single unambiguous one could be found, or None if
    /// ambiguous or unsolvable.
    Optional<Solution> solveSingle(FreeTypeVariableBinding allowFreeTypeVariables
                                   = FreeTypeVariableBinding::Disallow,
                                   bool allowFixes = false);
  
  private:
    /// Solve the system of constraints.
    ///
    /// This method responsible for running search/solver algorithm.
    /// It doesn't filter solutions, that's the job of top-level `solve` methods.
    ///
    /// \param solutions The set of solutions to this system of constraints.
    void solve(SmallVectorImpl<Solution> &solutions);
  
    /// Compare two solutions to the same set of constraints.
    ///
    /// \param cs The constraint system.
    /// \param solutions All of the solutions to the system.
    /// \param diff The differences among the solutions.
    /// \param idx1 The index of the first solution.
    /// \param idx2 The index of the second solution.
    static SolutionCompareResult
    compareSolutions(ConstraintSystem &cs, ArrayRef<Solution> solutions,
                     const SolutionDiff &diff, unsigned idx1, unsigned idx2,
                     llvm::DenseMap<Expr *, std::pair<unsigned, Expr *>> &weights);
  
  public:
    /// Increase the score of the given kind for the current (partial) solution
    /// along the.
    void increaseScore(ScoreKind kind, unsigned value = 1);
  
    /// Determine whether this solution is guaranteed to be worse than the best
    /// solution found so far.
    bool worseThanBestSolution() const;
  
    /// Given a set of viable solutions, find the best
    /// solution.
    ///
    /// \param solutions The set of viable solutions to consider.
    ///
    /// \param minimize If true, then in the case where there is no single
    /// best solution, minimize the set of solutions by removing any solutions
    /// that are identical to or worse than some other solution. This operation
    /// is quadratic.
    ///
    /// \returns The index of the best solution, or nothing if there was no
    /// best solution.
    Optional<unsigned>
    findBestSolution(SmallVectorImpl<Solution> &solutions,
                     bool minimize);
  
    /// Apply a given solution to the expression, producing a fully
    /// type-checked expression.
    ///
    /// \param convertType the contextual type to which the
    /// expression should be converted, if any.
    /// \param discardedExpr if true, the result of the expression
    /// is contextually ignored.
    /// \param skipClosures if true, don't descend into bodies of
    /// non-single expression closures.
    Expr *applySolution(Solution &solution, Expr *expr,
                        Type convertType, bool discardedExpr,
                        bool skipClosures);
  
    /// Reorder the disjunctive clauses for a given expression to
    /// increase the likelihood that a favored constraint will be successfully
    /// resolved before any others.
    void optimizeConstraints(Expr *e);
    
    /// Determine if we've already explored too many paths in an
    /// attempt to solve this expression.
    bool isExpressionAlreadyTooComplex = false;
    bool getExpressionTooComplex(SmallVectorImpl<Solution> const &solutions) {
      if (isExpressionAlreadyTooComplex)
        return true;
  
      auto used = TC.Context.getSolverMemory();
      for (auto const& s : solutions) {
        used += s.getTotalMemory();
      }
      MaxMemory = std::max(used, MaxMemory);
      auto threshold = TC.Context.LangOpts.SolverMemoryThreshold;
      if (MaxMemory > threshold) {
        return isExpressionAlreadyTooComplex= true;
      }
  
      auto timeoutThresholdInMillis = TC.getExpressionTimeoutThresholdInSeconds();
      if (Timer && Timer->isExpired(timeoutThresholdInMillis)) {
        // Disable warnings about expressions that go over the warning
        // threshold since we're arbitrarily ending evaluation and
        // emitting an error.
        Timer->disableWarning();
  
        return isExpressionAlreadyTooComplex = true;
      }
  
      // Bail out once we've looked at a really large number of
      // choices.
      if (CountScopes > TC.Context.LangOpts.SolverBindingThreshold) {
        return isExpressionAlreadyTooComplex = true;
      }
  
      return false;
    }
  
    // Utility class that can collect information about the type of an
    // argument in an apply.
    //
    // For example, when given a type variable type that represents the
    // argument of a function call, it will walk the constraint graph
    // finding any concrete types that are reachable through various
    // subtype constraints and will also collect all the literal types
    // conformed to by the types it finds on the walk.
    //
    // This makes it possible to get an idea of the kinds of literals
    // and types of arguments that are used in the subexpression rooted
    // in this argument, which we can then use to make better choices
    // for how we partition the operators in a disjunction (in order to
    // avoid visiting all the options).
    class ArgumentInfoCollector {
      ConstraintSystem &CS;
      llvm::SetVector<Type> Types;
      llvm::SetVector<ProtocolDecl *> LiteralProtocols;
  
      void addType(Type ty) {
        assert(!ty->is<TypeVariableType>());
        Types.insert(ty);
      }
  
      void addLiteralProtocol(ProtocolDecl *proto) {
        LiteralProtocols.insert(proto);
      }
  
      void walk(Type argType);
      void minimizeLiteralProtocols();
  
    public:
      ArgumentInfoCollector(ConstraintSystem &cs, FunctionType *fnTy) : CS(cs) {
        for (auto &param : fnTy->getParams())
          walk(param.getPlainType());
  
        minimizeLiteralProtocols();
      }
  
      ArgumentInfoCollector(ConstraintSystem &cs, AnyFunctionType::Param param)
          : CS(cs) {
        walk(param.getPlainType());
        minimizeLiteralProtocols();
      }
  
      const llvm::SetVector<Type> &getTypes() const { return Types; }
      const llvm::SetVector<ProtocolDecl *> &getLiteralProtocols() const {
        return LiteralProtocols;
      }
  
      LLVM_ATTRIBUTE_DEPRECATED(void dump() const LLVM_ATTRIBUTE_USED,
                                "only for use within the debugger");
    };
  
    bool haveTypeInformationForAllArguments(FunctionType *fnType);
  
    typedef std::function<bool(unsigned index, Constraint *)> ConstraintMatcher;
    typedef std::function<void(ArrayRef<Constraint *>, ConstraintMatcher)>
        ConstraintMatchLoop;
    typedef std::function<void(SmallVectorImpl<unsigned> &options)>
        PartitionAppendCallback;
  
    // Attempt to sort nominalTypes based on what we can discover about
    // calls into the overloads in the disjunction that bindOverload is
    // a part of.
    void sortDesignatedTypes(SmallVectorImpl<NominalTypeDecl *> &nominalTypes,
                             Constraint *bindOverload);
  
    // Partition the choices in a disjunction based on those that match
    // the designated types for the operator that the disjunction was
    // formed for.
    void partitionForDesignatedTypes(ArrayRef<Constraint *> Choices,
                                     ConstraintMatchLoop forEachChoice,
                                     PartitionAppendCallback appendPartition);
  
    // Partition the choices in the disjunction into groups that we will
    // iterate over in an order appropriate to attempt to stop before we
    // have to visit all of the options.
    void partitionDisjunction(ArrayRef<Constraint *> Choices,
                              SmallVectorImpl<unsigned> &Ordering,
                              SmallVectorImpl<unsigned> &PartitionBeginning);
  
    LLVM_ATTRIBUTE_DEPRECATED(
        void dump() LLVM_ATTRIBUTE_USED,
        "only for use within the debugger");
    LLVM_ATTRIBUTE_DEPRECATED(void dump(Expr *) LLVM_ATTRIBUTE_USED,
                              "only for use within the debugger");
  
    void print(raw_ostream &out);
  };
  
  /// Compute the shuffle required to map from a given tuple type to
  /// another tuple type.
  ///
  /// \param fromTuple The tuple type we're converting from, as represented by its
  /// TupleTypeElt members.
  ///
  /// \param toTuple The tuple type we're converting to, as represented by its
  /// TupleTypeElt members.
  ///
  /// \param sources Will be populated with information about the source of each
  /// of the elements for the result tuple. The indices into this array are the
  /// indices of the tuple type we're converting to, while the values are
  /// an index into the source tuple.
  ///
  /// \returns true if no tuple conversion is possible, false otherwise.
  bool computeTupleShuffle(ArrayRef<TupleTypeElt> fromTuple,
                           ArrayRef<TupleTypeElt> toTuple,
                           SmallVectorImpl<unsigned> &sources);
  static inline bool computeTupleShuffle(TupleType *fromTuple,
                                         TupleType *toTuple,
                                         SmallVectorImpl<unsigned> &sources){
    return computeTupleShuffle(fromTuple->getElements(), toTuple->getElements(),
                               sources);
  }
  
  /// Describes the arguments to which a parameter binds.
  /// FIXME: This is an awful data structure. We want the equivalent of a
  /// TinyPtrVector for unsigned values.
  using ParamBinding = SmallVector<unsigned, 1>;
  
  /// Class used as the base for listeners to the \c matchCallArguments process.
  ///
  /// By default, none of the callbacks do anything.
  class MatchCallArgumentListener {
  public:
    virtual ~MatchCallArgumentListener();
  
    /// Indicates that the argument at the given index does not match any
    /// parameter.
    ///
    /// \param argIdx The index of the extra argument.
    virtual void extraArgument(unsigned argIdx);
  
    /// Indicates that no argument was provided for the parameter at the given
    /// indices.
    ///
    /// \param paramIdx The index of the parameter that is missing an argument.
    virtual void missingArgument(unsigned paramIdx);
  
    /// Indicate that there was no label given when one was expected by parameter.
    ///
    /// \param paramIndex The index of the parameter that is missing a label.
    ///
    /// \returns true to indicate that this should cause a failure, false
    /// otherwise.
    virtual bool missingLabel(unsigned paramIndex);
  
    /// Indicate that there was label given when none was expected by parameter.
    ///
    /// \param paramIndex The index of the parameter that wasn't expecting a label.
    ///
    /// \returns true to indicate that this should cause a failure, false
    /// otherwise.
    virtual bool extraneousLabel(unsigned paramIndex);
  
    /// Indicate that there was a label given with a typo(s) in it.
    ///
    /// \param paramIndex The index of the parameter with misspelled label.
    ///
    /// \returns true to indicate that this should cause a failure, false
    /// otherwise.
    virtual bool incorrectLabel(unsigned paramIndex);
  
    /// Indicates that an argument is out-of-order with respect to a previously-
    /// seen argument.
    ///
    /// \param argIdx The argument that came too late in the argument list.
    /// \param prevArgIdx The argument that the \c argIdx should have preceded.
    ///
    /// \returns true to indicate that this should cause a failure, false
    /// otherwise.
    virtual bool outOfOrderArgument(unsigned argIdx, unsigned prevArgIdx);
  
    /// Indicates that the arguments need to be relabeled to match the parameters.
    ///
    /// \returns true to indicate that this should cause a failure, false
    /// otherwise.
    virtual bool relabelArguments(ArrayRef<Identifier> newNames);
  };
  
  /// Match the call arguments (as described by the given argument type) to
  /// the parameters (as described by the given parameter type).
  ///
  /// \param args The arguments.
  /// \param params The parameters.
  /// \param paramInfo Declaration-level information about the parameters.
  /// \param hasTrailingClosure Whether the last argument is a trailing closure.
  /// \param allowFixes Whether to allow fixes when matching arguments.
  ///
  /// \param listener Listener that will be notified when certain problems occur,
  /// e.g., to produce a diagnostic.
  ///
  /// \param parameterBindings Will be populated with the arguments that are
  /// bound to each of the parameters.
  /// \returns true if the call arguments could not be matched to the parameters.
  bool matchCallArguments(ArrayRef<AnyFunctionType::Param> args,
                          ArrayRef<AnyFunctionType::Param> params,
                          const ParameterListInfo &paramInfo,
                          bool hasTrailingClosure,
                          bool allowFixes,
                          MatchCallArgumentListener &listener,
                          SmallVectorImpl<ParamBinding> &parameterBindings);
  
  ConstraintSystem::TypeMatchResult
  matchCallArguments(ConstraintSystem &cs,
                     ArrayRef<AnyFunctionType::Param> args,
                     ArrayRef<AnyFunctionType::Param> params,
                     ConstraintLocatorBuilder locator);
  
  /// Given an expression that is the target of argument labels (for a call,
  /// subscript, etc.), find the underlying target expression.
  Expr *getArgumentLabelTargetExpr(Expr *fn);
  
  /// Attempt to prove that arguments with the given labels at the
  /// given parameter depth cannot be used with the given value.
  /// If this cannot be proven, conservatively returns true.
  bool areConservativelyCompatibleArgumentLabels(OverloadChoice choice,
                                                 ArrayRef<Identifier> labels,
                                                 bool hasTrailingClosure);
  
  /// Simplify the given locator by zeroing in on the most specific
  /// subexpression described by the locator.
  ///
  /// This routine can also find the corresponding "target" locator, which
  /// typically provides the other end of a relational constraint. For example,
  /// if the primary locator refers to a function argument, the target locator
  /// will be set to refer to the corresponding function parameter.
  ///
  /// \param cs The constraint system in which the locator will be simplified.
  ///
  /// \param locator The locator to simplify.
  ///
  /// \param range Will be populated with an "interesting" range.
  ///
  /// \param targetLocator If non-null, will be set to a locator that describes
  /// the target of the input locator.
  ///
  /// \return the simplified locator.
  ConstraintLocator *simplifyLocator(ConstraintSystem &cs,
                                     ConstraintLocator *locator,
                                     SourceRange &range,
                                     ConstraintLocator **targetLocator = nullptr);
  
  void simplifyLocator(Expr *&anchor,
                       ArrayRef<LocatorPathElt> &path,
                       Expr *&targetAnchor,
                       SmallVectorImpl<LocatorPathElt> &targetPath,
                       SourceRange &range);
  
  /// Simplify the given locator down to a specific anchor expression,
  /// if possible.
  ///
  /// \returns the anchor expression if it fully describes the locator, or
  /// null otherwise.
  Expr *simplifyLocatorToAnchor(ConstraintSystem &cs, ConstraintLocator *locator);
  
  /// Retrieve argument at specified index from given expression.
  /// The expression could be "application", "subscript" or "member" call.
  ///
  /// \returns argument expression or `nullptr` if given "base" expression
  /// wasn't of one of the kinds listed above.
  Expr *getArgumentExpr(Expr *expr, unsigned index);
  
  // Check whether argument of the call at given position refers to
  // parameter marked as `@autoclosure`. This function is used to
  // maintain source compatibility with Swift versions < 5,
  // previously examples like following used to type-check:
  //
  // func foo(_ x: @autoclosure () -> Int) {}
  // func bar(_ y: @autoclosure () -> Int) {
  //   foo(y)
  // }
  bool isAutoClosureArgument(Expr *argExpr);
  
  class DisjunctionChoice {
    unsigned Index;
    Constraint *Choice;
    bool ExplicitConversion;
    bool IsBeginningOfPartition;
  
  public:
    DisjunctionChoice(unsigned index, Constraint *choice, bool explicitConversion,
                      bool isBeginningOfPartition)
        : Index(index), Choice(choice), ExplicitConversion(explicitConversion),
          IsBeginningOfPartition(isBeginningOfPartition) {}
  
    unsigned getIndex() const { return Index; }
  
    bool attempt(ConstraintSystem &cs) const;
  
    bool isDisabled() const { return Choice->isDisabled(); }
  
    bool hasFix() const {
      return bool(Choice->getFix());
    }
  
    bool isUnavailable() const {
      if (auto *decl = getDecl(Choice))
        return decl->getAttrs().isUnavailable(decl->getASTContext());
      return false;
    }
  
    bool isBeginningOfPartition() const { return IsBeginningOfPartition; }
  
    // FIXME: Both of the accessors below are required to support
    //        performance optimization hacks in constraint solver.
  
    bool isGenericOperator() const;
    bool isSymmetricOperator() const;
  
    void print(llvm::raw_ostream &Out, SourceManager *SM) const {
      Out << "disjunction choice ";
      Choice->print(Out, SM);
    }
  
    operator Constraint *() { return Choice; }
    operator Constraint *() const { return Choice; }
  
  private:
    /// If associated disjunction is an explicit conversion,
    /// let's try to propagate its type early to prune search space.
    void propagateConversionInfo(ConstraintSystem &cs) const;
  
    static ValueDecl *getOperatorDecl(Constraint *choice) {
      auto *decl = getDecl(choice);
      if (!decl)
        return nullptr;
  
      return decl->isOperator() ? decl : nullptr;
    }
  
    static ValueDecl *getDecl(Constraint *constraint) {
      if (constraint->getKind() != ConstraintKind::BindOverload)
        return nullptr;
  
      auto choice = constraint->getOverloadChoice();
      if (choice.getKind() != OverloadChoiceKind::Decl)
        return nullptr;
  
      return choice.getDecl();
    }
  };
  
  class TypeVariableBinding {
    TypeVariableType *TypeVar;
    ConstraintSystem::PotentialBinding Binding;
  
  public:
    TypeVariableBinding(TypeVariableType *typeVar,
                        ConstraintSystem::PotentialBinding &binding)
        : TypeVar(typeVar), Binding(binding) {}
  
    bool isDefaultable() const { return Binding.isDefaultableBinding(); }
  
    bool hasDefaultedProtocol() const { return Binding.DefaultedProtocol; }
  
    bool attempt(ConstraintSystem &cs) const;
  
    void print(llvm::raw_ostream &Out, SourceManager *) const {
      Out << "type variable " << TypeVar->getString()
          << " := " << Binding.BindingType->getString();
    }
  };
  
  template<typename Choice>
  class BindingProducer {
    ConstraintLocator *Locator;
  
  protected:
    ConstraintSystem &CS;
  
  public:
    BindingProducer(ConstraintSystem &cs, ConstraintLocator *locator)
        : Locator(locator), CS(cs) {}
  
    virtual ~BindingProducer() {}
    virtual Optional<Choice> operator()() = 0;
  
    ConstraintLocator *getLocator() const { return Locator; }
  
    /// Check whether generator would have to compute next
    /// batch of bindings because it freshly ran out of current one.
    /// This is useful to be able to exhaustively attempt bindings
    /// for type variables found at one level, before proceeding to
    /// supertypes or literal defaults etc.
    virtual bool needsToComputeNext() const { return false; }
  };
  
  class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
    using BindingKind = ConstraintSystem::AllowedBindingKind;
    using Binding = ConstraintSystem::PotentialBinding;
  
    TypeVariableType *TypeVar;
    llvm::SmallVector<Binding, 8> Bindings;
  
    // The index pointing to the offset in the bindings
    // generator is currently at, `numTries` represents
    // the number of times bindings have been recomputed.
    unsigned Index = 0, NumTries = 0;
  
    llvm::SmallPtrSet<CanType, 4> ExploredTypes;
    llvm::SmallPtrSet<TypeBase *, 4> BoundTypes;
  
  public:
    using Element = TypeVariableBinding;
  
    TypeVarBindingProducer(ConstraintSystem &cs,
                           ConstraintSystem::PotentialBindings &bindings)
        : BindingProducer(cs, bindings.TypeVar->getImpl().getLocator()),
          TypeVar(bindings.TypeVar),
          Bindings(bindings.Bindings.begin(), bindings.Bindings.end()) {}
  
    Optional<Element> operator()() override {
      // Once we reach the end of the current bindings
      // let's try to compute new ones, e.g. supertypes,
      // literal defaults, if that fails, we are done.
      if (needsToComputeNext() && !computeNext())
        return None;
  
      return TypeVariableBinding(TypeVar, Bindings[Index++]);
    }
  
    bool needsToComputeNext() const override { return Index >= Bindings.size(); }
  
  private:
    /// Compute next batch of bindings if possible, this could
    /// be supertypes extracted from one of the current bindings
    /// or default literal types etc.
    ///
    /// \returns true if some new bindings were sucessfully computed,
    /// false otherwise.
    bool computeNext();
  };
  
  /// Iterator over disjunction choices, makes it
  /// easy to work with disjunction and encapsulates
  /// some other important information such as locator.
  class DisjunctionChoiceProducer : public BindingProducer<DisjunctionChoice> {
    // The disjunction choices that this producer will iterate through.
    ArrayRef<Constraint *> Choices;
  
    // The ordering of disjunction choices. We index into Choices
    // through this vector in order to visit the disjunction choices in
    // the order we want to visit them.
    SmallVector<unsigned, 8> Ordering;
  
    // The index of the first element in a partition of the disjunction
    // choices. The choices are split into partitions where we will
    // visit all elements within a single partition before moving to the
    // elements of the next partition. If we visit all choices within a
    // single partition and have found a successful solution with one of
    // the choices in that partition, we stop looking for other
    // solutions.
    SmallVector<unsigned, 4> PartitionBeginning;
  
    // The index in the current partition of disjunction choices that we
    // are iterating over.
    unsigned PartitionIndex = 0;
  
    bool IsExplicitConversion;
  
    unsigned Index = 0;
  
  public:
    using Element = DisjunctionChoice;
  
    DisjunctionChoiceProducer(ConstraintSystem &cs, Constraint *disjunction)
        : BindingProducer(cs, disjunction->shouldRememberChoice()
                                  ? disjunction->getLocator()
                                  : nullptr),
          Choices(disjunction->getNestedConstraints()),
          IsExplicitConversion(disjunction->isExplicitConversion()) {
      assert(disjunction->getKind() == ConstraintKind::Disjunction);
      assert(!disjunction->shouldRememberChoice() || disjunction->getLocator());
  
      // Order and partition the disjunction choices.
      CS.partitionDisjunction(Choices, Ordering, PartitionBeginning);
    }
  
    DisjunctionChoiceProducer(ConstraintSystem &cs,
                              ArrayRef<Constraint *> choices,
                              ConstraintLocator *locator, bool explicitConversion)
        : BindingProducer(cs, locator), Choices(choices),
          IsExplicitConversion(explicitConversion) {
  
      // Order and partition the disjunction choices.
      CS.partitionDisjunction(Choices, Ordering, PartitionBeginning);
    }
  
    Optional<Element> operator()() override {
      unsigned currIndex = Index;
      if (currIndex >= Choices.size())
        return None;
  
      bool isBeginningOfPartition = PartitionIndex < PartitionBeginning.size() &&
                                    PartitionBeginning[PartitionIndex] == Index;
      if (isBeginningOfPartition)
        ++PartitionIndex;
  
      ++Index;
  
      return DisjunctionChoice(currIndex, Choices[Ordering[currIndex]],
                               IsExplicitConversion, isBeginningOfPartition);
    }
  };
  
  /// Determine whether given type is a known one
  /// for a key path `{Writable, ReferenceWritable}KeyPath`.
  bool isKnownKeyPathType(Type type);
  
  /// Determine whether given declaration is one for a key path
  /// `{Writable, ReferenceWritable}KeyPath`.
  bool isKnownKeyPathDecl(ASTContext &ctx, ValueDecl *decl);
  } // end namespace constraints
  
  template<typename ...Args>
  TypeVariableType *TypeVariableType::getNew(const ASTContext &C, unsigned ID,
                                             Args &&...args) {
    // Allocate memory
    void *mem = C.Allocate(sizeof(TypeVariableType) + sizeof(Implementation),
                           alignof(TypeVariableType),
                           AllocationArena::ConstraintSolver);
  
    // Construct the type variable.
    auto *result = ::new (mem) TypeVariableType(C, ID);
  
    // Construct the implementation object.
    new (result+1) TypeVariableType::Implementation(std::forward<Args>(args)...);
  
    return result;
  }
  
  /// If the expression has the effect of a forced downcast, find the
  /// underlying forced downcast expression.
  ForcedCheckedCastExpr *findForcedDowncast(ASTContext &ctx, Expr *expr);
  
  
  // Erases any opened existentials from the given expression.
  // Note: this may update the provided expr pointer.
  void eraseOpenedExistentials(constraints::ConstraintSystem &CS, Expr *&expr);
  
  // Count the number of overload sets present
  // in the expression and all of the children.
  class OverloadSetCounter : public ASTWalker {
    unsigned &NumOverloads;
  
  public:
    OverloadSetCounter(unsigned &overloads)
    : NumOverloads(overloads)
    {}
  
    std::pair<bool, Expr *> walkToExprPre(Expr *expr) override {
      if (auto applyExpr = dyn_cast<ApplyExpr>(expr)) {
        // If we've found function application and it's
        // function is an overload set, count it.
        if (isa<OverloadSetRefExpr>(applyExpr->getFn()))
          ++NumOverloads;
      }
  
      // Always recur into the children.
      return { true, expr };
    }
  };
  
  /// Matches array of function parameters to candidate inputs,
  /// which can be anything suitable (e.g., parameters, arguments).
  ///
  /// It claims inputs sequentially and tries to pair between an input
  /// and the next appropriate parameter. The detailed matching behavior
  /// of each pair is specified by a custom function (i.e., pairMatcher).
  /// It considers variadic and defaulted arguments when forming proper
  /// input-parameter pairs; however, other information like label and
  /// type information is not directly used here. It can be implemented
  /// in the custom function when necessary.
  class InputMatcher {
    size_t NumSkippedParameters;
    const ParameterListInfo &ParamInfo;
    const ArrayRef<AnyFunctionType::Param> Params;
  
  public:
    enum Result {
      /// The specified inputs are successfully matched.
      IM_Succeeded,
      /// There are input(s) left unclaimed while all parameters are matched.
      IM_HasUnclaimedInput,
      /// There are parateter(s) left unmatched while all inputs are claimed.
      IM_HasUnmatchedParam,
      /// Custom pair matcher function failure.
      IM_CustomPairMatcherFailed,
    };
  
    InputMatcher(const ArrayRef<AnyFunctionType::Param> params,
                 const ParameterListInfo &paramInfo);
  
    /// Matching a given array of inputs.
    ///
    /// \param numInputs The number of inputs.
    /// \param pairMatcher Custom matching behavior of an input-parameter pair.
    /// \return the matching result.
    Result
    match(int numInputs,
          std::function<bool(unsigned inputIdx, unsigned paramIdx)> pairMatcher);
  
    size_t getNumSkippedParameters() const { return NumSkippedParameters; }
  };
  
  /// Diagnose an attempt to recover from a member access into a value of
  /// optional type which needed to be unwrapped for the member to be found.
  ///
  /// \returns true if a diagnostic was produced.
  bool diagnoseBaseUnwrapForMemberAccess(Expr *baseExpr, Type baseType,
                                         DeclName memberName, bool resultOptional,
                                         SourceRange memberRange);
  
  // Return true if, when replacing "<expr>" with "<expr> ?? T", parentheses need
  // to be added around <expr> first in order to maintain the correct precedence.
  bool exprNeedsParensBeforeAddingNilCoalescing(TypeChecker &TC,
                                                DeclContext *DC,
                                                Expr *expr);
  
  // Return true if, when replacing "<expr>" with "<expr> as T", parentheses need
  // to be added around the new expression in order to maintain the correct
  // precedence.
  bool exprNeedsParensAfterAddingNilCoalescing(TypeChecker &TC,
                                               DeclContext *DC,
                                               Expr *expr,
                                               Expr *rootExpr);
  
  /// Return true if, when replacing "<expr>" with "<expr> op <something>",
  /// parentheses must be added around "<expr>" to allow the new operator
  /// to bind correctly.
  bool exprNeedsParensInsideFollowingOperator(TypeChecker &TC, DeclContext *DC,
                                              Expr *expr,
                                              PrecedenceGroupDecl *followingPG);
  
  /// Return true if, when replacing "<expr>" with "<expr> op <something>"
  /// within the given root expression, parentheses must be added around
  /// the new operator to prevent it from binding incorrectly in the
  /// surrounding context.
  bool exprNeedsParensOutsideFollowingOperator(
      TypeChecker &TC, DeclContext *DC, Expr *expr, Expr *rootExpr,
      PrecedenceGroupDecl *followingPG);
  
  /// Determine whether this is a SIMD operator.
  bool isSIMDOperator(ValueDecl *value);
  
  } // end namespace swift
  
  #endif // LLVM_SWIFT_SEMA_CONSTRAINT_SYSTEM_H
  