//===- Overload.h - C++ Overloading -----------------------------*- C++ -*-===//

//

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

// See https://llvm.org/LICENSE.txt for license information.

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

//

//===----------------------------------------------------------------------===//

//

// This file defines the data structures and types used in C++

// overload resolution.

//

//===----------------------------------------------------------------------===//

#ifndef LLVM_CLANG_SEMA_OVERLOAD_H

#define LLVM_CLANG_SEMA_OVERLOAD_H

#include "clang/AST/Decl.h"

#include "clang/AST/DeclAccessPair.h"

#include "clang/AST/DeclBase.h"

#include "clang/AST/DeclCXX.h"

#include "clang/AST/DeclTemplate.h"

#include "clang/AST/Expr.h"

#include "clang/AST/Type.h"

#include "clang/Basic/LLVM.h"

#include "clang/Basic/SourceLocation.h"

#include "clang/Sema/SemaFixItUtils.h"

#include "clang/Sema/TemplateDeduction.h"

#include "llvm/ADT/ArrayRef.h"

#include "llvm/ADT/None.h"

#include "llvm/ADT/STLExtras.h"

#include "llvm/ADT/SmallPtrSet.h"

#include "llvm/ADT/SmallVector.h"

#include "llvm/ADT/StringRef.h"

#include "llvm/Support/AlignOf.h"

#include "llvm/Support/Allocator.h"

#include "llvm/Support/Casting.h"

#include "llvm/Support/ErrorHandling.h"

#include <cassert>

#include <cstddef>

#include <cstdint>

#include <utility>

namespace clang {

class APValue;

class ASTContext;

class Sema;

  /// OverloadingResult - Capture the result of performing overload

  /// resolution.

  enum OverloadingResult {

    /// Overload resolution succeeded.

    OR_Success,

    /// No viable function found.

    OR_No_Viable_Function,

    /// Ambiguous candidates found.

    OR_Ambiguous,

    /// Succeeded, but refers to a deleted function.

    OR_Deleted

  };

  enum OverloadCandidateDisplayKind {

    /// Requests that all candidates be shown.  Viable candidates will

    /// be printed first.

    OCD_AllCandidates,

    /// Requests that only viable candidates be shown.

    OCD_ViableCandidates,

    /// Requests that only tied-for-best candidates be shown.

    OCD_AmbiguousCandidates

  };

  /// The parameter ordering that will be used for the candidate. This is

  /// used to represent C++20 binary operator rewrites that reverse the order

  /// of the arguments. If the parameter ordering is Reversed, the Args list is

  /// reversed (but obviously the ParamDecls for the function are not).

  ///

  /// After forming an OverloadCandidate with reversed parameters, the list

  /// of conversions will (as always) be indexed by argument, so will be

  /// in reverse parameter order.

  enum class OverloadCandidateParamOrder : char { Normal, Reversed };

  /// The kinds of rewrite we perform on overload candidates. Note that the

  /// values here are chosen to serve as both bitflags and as a rank (lower

  /// values are preferred by overload resolution).

  enum OverloadCandidateRewriteKind : unsigned {

    /// Candidate is not a rewritten candidate.

    CRK_None = 0x0,

    /// Candidate is a rewritten candidate with a different operator name.

    CRK_DifferentOperator = 0x1,

    /// Candidate is a rewritten candidate with a reversed order of parameters.

    CRK_Reversed = 0x2,

  };

  /// ImplicitConversionKind - The kind of implicit conversion used to

  /// convert an argument to a parameter's type. The enumerator values

  /// match with the table titled 'Conversions' in [over.ics.scs] and are listed

  /// such that better conversion kinds have smaller values.

  enum ImplicitConversionKind {

    /// Identity conversion (no conversion)

    ICK_Identity = 0,

    /// Lvalue-to-rvalue conversion (C++ [conv.lval])

    ICK_Lvalue_To_Rvalue,

    /// Array-to-pointer conversion (C++ [conv.array])

    ICK_Array_To_Pointer,

    /// Function-to-pointer (C++ [conv.array])

    ICK_Function_To_Pointer,

    /// Function pointer conversion (C++17 [conv.fctptr])

    ICK_Function_Conversion,

    /// Qualification conversions (C++ [conv.qual])

    ICK_Qualification,

    /// Integral promotions (C++ [conv.prom])

    ICK_Integral_Promotion,

    /// Floating point promotions (C++ [conv.fpprom])

    ICK_Floating_Promotion,

    /// Complex promotions (Clang extension)

    ICK_Complex_Promotion,

    /// Integral conversions (C++ [conv.integral])

    ICK_Integral_Conversion,

    /// Floating point conversions (C++ [conv.double]

    ICK_Floating_Conversion,

    /// Complex conversions (C99 6.3.1.6)

    ICK_Complex_Conversion,

    /// Floating-integral conversions (C++ [conv.fpint])

    ICK_Floating_Integral,

    /// Pointer conversions (C++ [conv.ptr])

    ICK_Pointer_Conversion,

    /// Pointer-to-member conversions (C++ [conv.mem])

    ICK_Pointer_Member,

    /// Boolean conversions (C++ [conv.bool])

    ICK_Boolean_Conversion,

    /// Conversions between compatible types in C99

    ICK_Compatible_Conversion,

    /// Derived-to-base (C++ [over.best.ics])

    ICK_Derived_To_Base,

    /// Vector conversions

    ICK_Vector_Conversion,

    /// Arm SVE Vector conversions

    ICK_SVE_Vector_Conversion,

    /// A vector splat from an arithmetic type

    ICK_Vector_Splat,

    /// Complex-real conversions (C99 6.3.1.7)

    ICK_Complex_Real,

    /// Block Pointer conversions

    ICK_Block_Pointer_Conversion,

    /// Transparent Union Conversions

    ICK_TransparentUnionConversion,

    /// Objective-C ARC writeback conversion

    ICK_Writeback_Conversion,

    /// Zero constant to event (OpenCL1.2 6.12.10)

    ICK_Zero_Event_Conversion,

    /// Zero constant to queue

    ICK_Zero_Queue_Conversion,

    /// Conversions allowed in C, but not C++

    ICK_C_Only_Conversion,

    /// C-only conversion between pointers with incompatible types

    ICK_Incompatible_Pointer_Conversion,

    /// The number of conversion kinds

    ICK_Num_Conversion_Kinds,

  };

  /// ImplicitConversionRank - The rank of an implicit conversion

  /// kind. The enumerator values match with Table 9 of (C++

  /// 13.3.3.1.1) and are listed such that better conversion ranks

  /// have smaller values.

  enum ImplicitConversionRank {

    /// Exact Match

    ICR_Exact_Match = 0,

    /// Promotion

    ICR_Promotion,

    /// Conversion

    ICR_Conversion,

    /// OpenCL Scalar Widening

    ICR_OCL_Scalar_Widening,

    /// Complex <-> Real conversion

    ICR_Complex_Real_Conversion,

    /// ObjC ARC writeback conversion

    ICR_Writeback_Conversion,

    /// Conversion only allowed in the C standard (e.g. void* to char*).

    ICR_C_Conversion,

    /// Conversion not allowed by the C standard, but that we accept as an

    /// extension anyway.

    ICR_C_Conversion_Extension

  };

  ImplicitConversionRank GetConversionRank(ImplicitConversionKind Kind);

  /// NarrowingKind - The kind of narrowing conversion being performed by a

  /// standard conversion sequence according to C++11 [dcl.init.list]p7.

  enum NarrowingKind {

    /// Not a narrowing conversion.

    NK_Not_Narrowing,

    /// A narrowing conversion by virtue of the source and destination types.

    NK_Type_Narrowing,

    /// A narrowing conversion, because a constant expression got narrowed.

    NK_Constant_Narrowing,

    /// A narrowing conversion, because a non-constant-expression variable might

    /// have got narrowed.

    NK_Variable_Narrowing,

    /// Cannot tell whether this is a narrowing conversion because the

    /// expression is value-dependent.

    NK_Dependent_Narrowing,

  };

  /// StandardConversionSequence - represents a standard conversion

  /// sequence (C++ 13.3.3.1.1). A standard conversion sequence

  /// contains between zero and three conversions. If a particular

  /// conversion is not needed, it will be set to the identity conversion

  /// (ICK_Identity). Note that the three conversions are

  /// specified as separate members (rather than in an array) so that

  /// we can keep the size of a standard conversion sequence to a

  /// single word.

  class StandardConversionSequence {

  public:

    /// First -- The first conversion can be an lvalue-to-rvalue

    /// conversion, array-to-pointer conversion, or

    /// function-to-pointer conversion.

    ImplicitConversionKind First : 8;

    /// Second - The second conversion can be an integral promotion,

    /// floating point promotion, integral conversion, floating point

    /// conversion, floating-integral conversion, pointer conversion,

    /// pointer-to-member conversion, or boolean conversion.

    ImplicitConversionKind Second : 8;

    /// Third - The third conversion can be a qualification conversion

    /// or a function conversion.

    ImplicitConversionKind Third : 8;

    /// Whether this is the deprecated conversion of a

    /// string literal to a pointer to non-const character data

    /// (C++ 4.2p2).

    unsigned DeprecatedStringLiteralToCharPtr : 1;

    /// Whether the qualification conversion involves a change in the

    /// Objective-C lifetime (for automatic reference counting).

    unsigned QualificationIncludesObjCLifetime : 1;

    /// IncompatibleObjC - Whether this is an Objective-C conversion

    /// that we should warn about (if we actually use it).

    unsigned IncompatibleObjC : 1;

    /// ReferenceBinding - True when this is a reference binding

    /// (C++ [over.ics.ref]).

    unsigned ReferenceBinding : 1;

    /// DirectBinding - True when this is a reference binding that is a

    /// direct binding (C++ [dcl.init.ref]).

    unsigned DirectBinding : 1;

    /// Whether this is an lvalue reference binding (otherwise, it's

    /// an rvalue reference binding).

    unsigned IsLvalueReference : 1;

    /// Whether we're binding to a function lvalue.

    unsigned BindsToFunctionLvalue : 1;

    /// Whether we're binding to an rvalue.

    unsigned BindsToRvalue : 1;

    /// Whether this binds an implicit object argument to a

    /// non-static member function without a ref-qualifier.

    unsigned BindsImplicitObjectArgumentWithoutRefQualifier : 1;

    /// Whether this binds a reference to an object with a different

    /// Objective-C lifetime qualifier.

    unsigned ObjCLifetimeConversionBinding : 1;

    /// FromType - The type that this conversion is converting

    /// from. This is an opaque pointer that can be translated into a

    /// QualType.

    void *FromTypePtr;

    /// ToType - The types that this conversion is converting to in

    /// each step. This is an opaque pointer that can be translated

    /// into a QualType.

    void *ToTypePtrs[3];

    /// CopyConstructor - The copy constructor that is used to perform

    /// this conversion, when the conversion is actually just the

    /// initialization of an object via copy constructor. Such

    /// conversions are either identity conversions or derived-to-base

    /// conversions.

    CXXConstructorDecl *CopyConstructor;

    DeclAccessPair FoundCopyConstructor;

    void setFromType(QualType T) { FromTypePtr = T.getAsOpaquePtr(); }

    void setToType(unsigned Idx, QualType T) {

      assert(Idx < 3 && "To type index is out of range");

      ToTypePtrs[Idx] = T.getAsOpaquePtr();

    }

    void setAllToTypes(QualType T) {

      ToTypePtrs[0] = T.getAsOpaquePtr();

      ToTypePtrs[1] = ToTypePtrs[0];

      ToTypePtrs[2] = ToTypePtrs[0];

    }

    QualType getFromType() const {

      return QualType::getFromOpaquePtr(FromTypePtr);

    }

    QualType getToType(unsigned Idx) const {

      assert(Idx < 3 && "To type index is out of range");

      return QualType::getFromOpaquePtr(ToTypePtrs[Idx]);

    }

    void setAsIdentityConversion();

    bool isIdentityConversion() const {

      return Second == ICK_Identity && 
Third == ICK_Identity15.2M
;

    }

    ImplicitConversionRank getRank() const;

    NarrowingKind

    getNarrowingKind(ASTContext &Context, const Expr *Converted,

                     APValue &ConstantValue, QualType &ConstantType,

                     bool IgnoreFloatToIntegralConversion = false) const;

    bool isPointerConversionToBool() const;

    bool isPointerConversionToVoidPointer(ASTContext& Context) const;

    void dump() const;

  };

  /// UserDefinedConversionSequence - Represents a user-defined

  /// conversion sequence (C++ 13.3.3.1.2).

  struct UserDefinedConversionSequence {

    /// Represents the standard conversion that occurs before

    /// the actual user-defined conversion.

    ///

    /// C++11 13.3.3.1.2p1:

    ///   If the user-defined conversion is specified by a constructor

    ///   (12.3.1), the initial standard conversion sequence converts

    ///   the source type to the type required by the argument of the

    ///   constructor. If the user-defined conversion is specified by

    ///   a conversion function (12.3.2), the initial standard

    ///   conversion sequence converts the source type to the implicit

    ///   object parameter of the conversion function.

    StandardConversionSequence Before;

    /// EllipsisConversion - When this is true, it means user-defined

    /// conversion sequence starts with a ... (ellipsis) conversion, instead of

    /// a standard conversion. In this case, 'Before' field must be ignored.

    // FIXME. I much rather put this as the first field. But there seems to be

    // a gcc code gen. bug which causes a crash in a test. Putting it here seems

    // to work around the crash.

    bool EllipsisConversion : 1;

    /// HadMultipleCandidates - When this is true, it means that the

    /// conversion function was resolved from an overloaded set having

    /// size greater than 1.

    bool HadMultipleCandidates : 1;

    /// After - Represents the standard conversion that occurs after

    /// the actual user-defined conversion.

    StandardConversionSequence After;

    /// ConversionFunction - The function that will perform the

    /// user-defined conversion. Null if the conversion is an

    /// aggregate initialization from an initializer list.

    FunctionDecl* ConversionFunction;

    /// The declaration that we found via name lookup, which might be

    /// the same as \c ConversionFunction or it might be a using declaration

    /// that refers to \c ConversionFunction.

    DeclAccessPair FoundConversionFunction;

    void dump() const;

  };

  /// Represents an ambiguous user-defined conversion sequence.

  struct AmbiguousConversionSequence {

    using ConversionSet =

        SmallVector<std::pair<NamedDecl *, FunctionDecl *>, 4>;

    void *FromTypePtr;

    void *ToTypePtr;

    char Buffer[sizeof(ConversionSet)];

    QualType getFromType() const {

      return QualType::getFromOpaquePtr(FromTypePtr);

    }

    QualType getToType() const {

      return QualType::getFromOpaquePtr(ToTypePtr);

    }

    void setFromType(QualType T) { FromTypePtr = T.getAsOpaquePtr(); }

    void setToType(QualType T) { ToTypePtr = T.getAsOpaquePtr(); }

    ConversionSet &conversions() {

      return *reinterpret_cast<ConversionSet*>(Buffer);

    }

    const ConversionSet &conversions() const {

      return *reinterpret_cast<const ConversionSet*>(Buffer);

    }

    void addConversion(NamedDecl *Found, FunctionDecl *D) {

      conversions().push_back(std::make_pair(Found, D));

    }

    using iterator = ConversionSet::iterator;

    iterator begin() { return conversions().begin(); }

    iterator end() { return conversions().end(); }

    using const_iterator = ConversionSet::const_iterator;

    const_iterator begin() const { return conversions().begin(); }

    const_iterator end() const { return conversions().end(); }

    void construct();

    void destruct();

    void copyFrom(const AmbiguousConversionSequence &);

  };

  /// BadConversionSequence - Records information about an invalid

  /// conversion sequence.

  struct BadConversionSequence {

    enum FailureKind {

      no_conversion,

      unrelated_class,

      bad_qualifiers,

      lvalue_ref_to_rvalue,

      rvalue_ref_to_lvalue

    };

    // This can be null, e.g. for implicit object arguments.

    Expr *FromExpr;

    FailureKind Kind;

  private:

    // The type we're converting from (an opaque QualType).

    void *FromTy;

    // The type we're converting to (an opaque QualType).

    void *ToTy;

  public:

    void init(FailureKind K, Expr *From, QualType To) {

      init(K, From->getType(), To);

      FromExpr = From;

    }

    void init(FailureKind K, QualType From, QualType To) {

      Kind = K;

      FromExpr = nullptr;

      setFromType(From);

      setToType(To);

    }

    QualType getFromType() const { return QualType::getFromOpaquePtr(FromTy); }

    QualType getToType() const { return QualType::getFromOpaquePtr(ToTy); }

    void setFromExpr(Expr *E) {

      FromExpr = E;

      setFromType(E->getType());

    }

    void setFromType(QualType T) { FromTy = T.getAsOpaquePtr(); }

    void setToType(QualType T) { ToTy = T.getAsOpaquePtr(); }

  };

  /// ImplicitConversionSequence - Represents an implicit conversion

  /// sequence, which may be a standard conversion sequence

  /// (C++ 13.3.3.1.1), user-defined conversion sequence (C++ 13.3.3.1.2),

  /// or an ellipsis conversion sequence (C++ 13.3.3.1.3).

  class ImplicitConversionSequence {

  public:

    /// Kind - The kind of implicit conversion sequence. BadConversion

    /// specifies that there is no conversion from the source type to

    /// the target type.  AmbiguousConversion represents the unique

    /// ambiguous conversion (C++0x [over.best.ics]p10).

    enum Kind {

      StandardConversion = 0,

      UserDefinedConversion,

      AmbiguousConversion,

      EllipsisConversion,

      BadConversion

    };

  private:

    enum {

      Uninitialized = BadConversion + 1

    };

    /// ConversionKind - The kind of implicit conversion sequence.

    unsigned ConversionKind : 30;

    /// Whether the target is really a std::initializer_list, and the

    /// sequence only represents the worst element conversion.

    unsigned StdInitializerListElement : 1;

    void setKind(Kind K) {

      destruct();

      ConversionKind = K;

    }

    void destruct() {

      if (ConversionKind == AmbiguousConversion) 
Ambiguous.destruct()16.0k
;

    }

  public:

    union {

      /// When ConversionKind == StandardConversion, provides the

      /// details of the standard conversion sequence.

      StandardConversionSequence Standard;

      /// When ConversionKind == UserDefinedConversion, provides the

      /// details of the user-defined conversion sequence.

      UserDefinedConversionSequence UserDefined;

      /// When ConversionKind == AmbiguousConversion, provides the

      /// details of the ambiguous conversion.

      AmbiguousConversionSequence Ambiguous;

      /// When ConversionKind == BadConversion, provides the details

      /// of the bad conversion.

      BadConversionSequence Bad;

    };

    ImplicitConversionSequence()

        : ConversionKind(Uninitialized), StdInitializerListElement(false) {

      Standard.setAsIdentityConversion();

    }

    ImplicitConversionSequence(const ImplicitConversionSequence &Other)

        : ConversionKind(Other.ConversionKind),

          StdInitializerListElement(Other.StdInitializerListElement) {

      switch (ConversionKind) {

      case Uninitialized: break;

      case StandardConversion: Standard = Other.Standard; break;

      case UserDefinedConversion: UserDefined = Other.UserDefined; break;

      case AmbiguousConversion: Ambiguous.copyFrom(Other.Ambiguous); break;

      case EllipsisConversion: break;

      case BadConversion: Bad = Other.Bad; break;

      }

    }

    ImplicitConversionSequence &

    operator=(const ImplicitConversionSequence &Other) {

      destruct();

      new (this) ImplicitConversionSequence(Other);

      return *this;

    }

    ~ImplicitConversionSequence() {

      destruct();

    }

    Kind getKind() const {

      assert(isInitialized() && "querying uninitialized conversion");

      return Kind(ConversionKind);

    }

    /// Return a ranking of the implicit conversion sequence

    /// kind, where smaller ranks represent better conversion

    /// sequences.

    ///

    /// In particular, this routine gives user-defined conversion

    /// sequences and ambiguous conversion sequences the same rank,

    /// per C++ [over.best.ics]p10.

    unsigned getKindRank() const {

      switch (getKind()) {

      case StandardConversion:

        return 0;

      case UserDefinedConversion:

      case AmbiguousConversion:

        return 1;

      case EllipsisConversion:

        return 2;

      case BadConversion:

        return 3;

      }

      llvm_unreachable("Invalid ImplicitConversionSequence::Kind!");

    }

    bool isBad() const { return getKind() == BadConversion; }

    bool isStandard() const { return getKind() == StandardConversion; }

    bool isEllipsis() const { return getKind() == EllipsisConversion; }

    bool isAmbiguous() const { return getKind() == AmbiguousConversion; }

    bool isUserDefined() const { return getKind() == UserDefinedConversion; }

    bool isFailure() const { return isBad() || 
isAmbiguous()937
; }

    /// Determines whether this conversion sequence has been

    /// initialized.  Most operations should never need to query

    /// uninitialized conversions and should assert as above.

    bool isInitialized() const { return ConversionKind != Uninitialized; }

    /// Sets this sequence as a bad conversion for an explicit argument.

    void setBad(BadConversionSequence::FailureKind Failure,

                Expr *FromExpr, QualType ToType) {

      setKind(BadConversion);

      Bad.init(Failure, FromExpr, ToType);

    }

    /// Sets this sequence as a bad conversion for an implicit argument.

    void setBad(BadConversionSequence::FailureKind Failure,

                QualType FromType, QualType ToType) {

      setKind(BadConversion);

      Bad.init(Failure, FromType, ToType);

    }

    void setStandard() { setKind(StandardConversion); }

    void setEllipsis() { setKind(EllipsisConversion); }

    void setUserDefined() { setKind(UserDefinedConversion); }

    void setAmbiguous() {

      if (ConversionKind == AmbiguousConversion) 
return0
;

      ConversionKind = AmbiguousConversion;

      Ambiguous.construct();

    }

    void setAsIdentityConversion(QualType T) {

      setStandard();

      Standard.setAsIdentityConversion();

      Standard.setFromType(T);

      Standard.setAllToTypes(T);

    }

    /// Whether the target is really a std::initializer_list, and the

    /// sequence only represents the worst element conversion.

    bool isStdInitializerListElement() const {

      return StdInitializerListElement;

    }

    void setStdInitializerListElement(bool V = true) {

      StdInitializerListElement = V;

    }

    /// Form an "implicit" conversion sequence from nullptr_t to bool, for a

    /// direct-initialization of a bool object from nullptr_t.

    static ImplicitConversionSequence getNullptrToBool(QualType SourceType,

                                                       QualType DestType,

                                                       bool NeedLValToRVal) {

      ImplicitConversionSequence ICS;

      ICS.setStandard();

      ICS.Standard.setAsIdentityConversion();

      ICS.Standard.setFromType(SourceType);

      if (NeedLValToRVal)

        ICS.Standard.First = ICK_Lvalue_To_Rvalue;

      ICS.Standard.setToType(0, SourceType);

      ICS.Standard.Second = ICK_Boolean_Conversion;

      ICS.Standard.setToType(1, DestType);

      ICS.Standard.setToType(2, DestType);

      return ICS;

    }

    // The result of a comparison between implicit conversion

    // sequences. Use Sema::CompareImplicitConversionSequences to

    // actually perform the comparison.

    enum CompareKind {

      Better = -1,

      Indistinguishable = 0,

      Worse = 1

    };

    void DiagnoseAmbiguousConversion(Sema &S,

                                     SourceLocation CaretLoc,

                                     const PartialDiagnostic &PDiag) const;

    void dump() const;

  };

  enum OverloadFailureKind {

    ovl_fail_too_many_arguments,

    ovl_fail_too_few_arguments,

    ovl_fail_bad_conversion,

    ovl_fail_bad_deduction,

    /// This conversion candidate was not considered because it

    /// duplicates the work of a trivial or derived-to-base

    /// conversion.

    ovl_fail_trivial_conversion,

    /// This conversion candidate was not considered because it is

    /// an illegal instantiation of a constructor temploid: it is

    /// callable with one argument, we only have one argument, and

    /// its first parameter type is exactly the type of the class.

    ///

    /// Defining such a constructor directly is illegal, and

    /// template-argument deduction is supposed to ignore such

    /// instantiations, but we can still get one with the right

    /// kind of implicit instantiation.

    ovl_fail_illegal_constructor,

    /// This conversion candidate is not viable because its result

    /// type is not implicitly convertible to the desired type.

    ovl_fail_bad_final_conversion,

    /// This conversion function template specialization candidate is not

    /// viable because the final conversion was not an exact match.

    ovl_fail_final_conversion_not_exact,

    /// (CUDA) This candidate was not viable because the callee

    /// was not accessible from the caller's target (i.e. host->device,

    /// global->host, device->host).

    ovl_fail_bad_target,

    /// This candidate function was not viable because an enable_if

    /// attribute disabled it.

    ovl_fail_enable_if,

    /// This candidate constructor or conversion function is explicit but

    /// the context doesn't permit explicit functions.

    ovl_fail_explicit,

    /// This candidate was not viable because its address could not be taken.

    ovl_fail_addr_not_available,

    /// This candidate was not viable because its OpenCL extension is disabled.

    ovl_fail_ext_disabled,

    /// This inherited constructor is not viable because it would slice the

    /// argument.

    ovl_fail_inhctor_slice,

    /// This candidate was not viable because it is a non-default multiversioned

    /// function.

    ovl_non_default_multiversion_function,

    /// This constructor/conversion candidate fail due to an address space

    /// mismatch between the object being constructed and the overload

    /// candidate.

    ovl_fail_object_addrspace_mismatch,

    /// This candidate was not viable because its associated constraints were

    /// not satisfied.

    ovl_fail_constraints_not_satisfied,

  };

  /// A list of implicit conversion sequences for the arguments of an

  /// OverloadCandidate.

  using ConversionSequenceList =

      llvm::MutableArrayRef<ImplicitConversionSequence>;

  /// OverloadCandidate - A single candidate in an overload set (C++ 13.3).

  struct OverloadCandidate {

    /// Function - The actual function that this candidate

    /// represents. When NULL, this is a built-in candidate

    /// (C++ [over.oper]) or a surrogate for a conversion to a

    /// function pointer or reference (C++ [over.call.object]).

    FunctionDecl *Function;

    /// FoundDecl - The original declaration that was looked up /

    /// invented / otherwise found, together with its access.

    /// Might be a UsingShadowDecl or a FunctionTemplateDecl.

    DeclAccessPair FoundDecl;

    /// BuiltinParamTypes - Provides the parameter types of a built-in overload

    /// candidate. Only valid when Function is NULL.

    QualType BuiltinParamTypes[3];

    /// Surrogate - The conversion function for which this candidate

    /// is a surrogate, but only if IsSurrogate is true.

    CXXConversionDecl *Surrogate;

    /// The conversion sequences used to convert the function arguments

    /// to the function parameters. Note that these are indexed by argument,

    /// so may not match the parameter order of Function.

    ConversionSequenceList Conversions;

    /// The FixIt hints which can be used to fix the Bad candidate.

    ConversionFixItGenerator Fix;

    /// Viable - True to indicate that this overload candidate is viable.

    bool Viable : 1;

    /// Whether this candidate is the best viable function, or tied for being

    /// the best viable function.

    ///

    /// For an ambiguous overload resolution, indicates whether this candidate

    /// was part of the ambiguity kernel: the minimal non-empty set of viable

    /// candidates such that all elements of the ambiguity kernel are better

    /// than all viable candidates not in the ambiguity kernel.

    bool Best : 1;

    /// IsSurrogate - True to indicate that this candidate is a

    /// surrogate for a conversion to a function pointer or reference

    /// (C++ [over.call.object]).

    bool IsSurrogate : 1;

    /// IgnoreObjectArgument - True to indicate that the first

    /// argument's conversion, which for this function represents the

    /// implicit object argument, should be ignored. This will be true

    /// when the candidate is a static member function (where the

    /// implicit object argument is just a placeholder) or a

    /// non-static member function when the call doesn't have an

    /// object argument.

    bool IgnoreObjectArgument : 1;

    /// True if the candidate was found using ADL.

    CallExpr::ADLCallKind IsADLCandidate : 1;

    /// Whether this is a rewritten candidate, and if so, of what kind?

    unsigned RewriteKind : 2;

    /// FailureKind - The reason why this candidate is not viable.

    /// Actually an OverloadFailureKind.

    unsigned char FailureKind;

    /// The number of call arguments that were explicitly provided,

    /// to be used while performing partial ordering of function templates.

    unsigned ExplicitCallArguments;

    union {

      DeductionFailureInfo DeductionFailure;

      /// FinalConversion - For a conversion function (where Function is

      /// a CXXConversionDecl), the standard conversion that occurs

      /// after the call to the overload candidate to convert the result

      /// of calling the conversion function to the required type.

      StandardConversionSequence FinalConversion;

    };

    /// Get RewriteKind value in OverloadCandidateRewriteKind type (This

    /// function is to workaround the spurious GCC bitfield enum warning)

    OverloadCandidateRewriteKind getRewriteKind() const {

      return static_cast<OverloadCandidateRewriteKind>(RewriteKind);

    }

    bool isReversed() const { return getRewriteKind() & CRK_Reversed; }

    /// hasAmbiguousConversion - Returns whether this overload

    /// candidate requires an ambiguous conversion or not.

    bool hasAmbiguousConversion() const {

      for (auto &C : Conversions) {

        if (!C.isInitialized()) return false;

        if (C.isAmbiguous()) return true;

      }

      return false;

    }

    bool TryToFixBadConversion(unsigned Idx, Sema &S) {

      bool CanFix = Fix.tryToFixConversion(

                      Conversions[Idx].Bad.FromExpr,

                      Conversions[Idx].Bad.getFromType(),

                      Conversions[Idx].Bad.getToType(), S);

      // If at least one conversion fails, the candidate cannot be fixed.

      if (!CanFix)

        Fix.clear();

      return CanFix;

    }

    unsigned getNumParams() const {

      if (IsSurrogate) {

        QualType STy = Surrogate->getConversionType();

        while (STy->isPointerType() || 
STy->isReferenceType()10
)

          STy = STy->getPointeeType();

        return STy->castAs<FunctionProtoType>()->getNumParams();

      }

      if (Function)

        return Function->getNumParams();

      return ExplicitCallArguments;

    }

  private:

    friend class OverloadCandidateSet;

    OverloadCandidate()

        : IsSurrogate(false), IsADLCandidate(CallExpr::NotADL), RewriteKind(CRK_None) {}

  };

  /// OverloadCandidateSet - A set of overload candidates, used in C++

  /// overload resolution (C++ 13.3).

  class OverloadCandidateSet {

  public:

    enum CandidateSetKind {

      /// Normal lookup.

      CSK_Normal,

      /// C++ [over.match.oper]:

      /// Lookup of operator function candidates in a call using operator

      /// syntax. Candidates that have no parameters of class type will be

      /// skipped unless there is a parameter of (reference to) enum type and

      /// the corresponding argument is of the same enum type.

      CSK_Operator,

      /// C++ [over.match.copy]:

      /// Copy-initialization of an object of class type by user-defined

      /// conversion.

      CSK_InitByUserDefinedConversion,

      /// C++ [over.match.ctor], [over.match.list]

      /// Initialization of an object of class type by constructor,

      /// using either a parenthesized or braced list of arguments.

      CSK_InitByConstructor,

    };

    /// Information about operator rewrites to consider when adding operator

    /// functions to a candidate set.

    struct OperatorRewriteInfo {

      OperatorRewriteInfo()

          : OriginalOperator(OO_None), AllowRewrittenCandidates(false) {}

      OperatorRewriteInfo(OverloadedOperatorKind Op, bool AllowRewritten)

          : OriginalOperator(Op), AllowRewrittenCandidates(AllowRewritten) {}

      /// The original operator as written in the source.

      OverloadedOperatorKind OriginalOperator;

      /// Whether we should include rewritten candidates in the overload set.

      bool AllowRewrittenCandidates;

      /// Would use of this function result in a rewrite using a different

      /// operator?

      bool isRewrittenOperator(const FunctionDecl *FD) {

        return OriginalOperator &&

               FD->getDeclName().getCXXOverloadedOperator() != OriginalOperator;

      }

      bool isAcceptableCandidate(const FunctionDecl *FD) {

        if (!OriginalOperator)

          return true;

        // For an overloaded operator, we can have candidates with a different

        // name in our unqualified lookup set. Make sure we only consider the

        // ones we're supposed to.

        OverloadedOperatorKind OO =

            FD->getDeclName().getCXXOverloadedOperator();

        return OO && (OO == OriginalOperator ||

                      (AllowRewrittenCandidates &&

                       OO == getRewrittenOverloadedOperator(OriginalOperator)));

      }

      /// Determine the kind of rewrite that should be performed for this

      /// candidate.

      OverloadCandidateRewriteKind

      getRewriteKind(const FunctionDecl *FD, OverloadCandidateParamOrder PO) {

        OverloadCandidateRewriteKind CRK = CRK_None;

        if (isRewrittenOperator(FD))

          CRK = OverloadCandidateRewriteKind(CRK | CRK_DifferentOperator);

        if (PO == OverloadCandidateParamOrder::Reversed)

          CRK = OverloadCandidateRewriteKind(CRK | CRK_Reversed);

        return CRK;

      }

      /// Determines whether this operator could be implemented by a function

      /// with reversed parameter order.

      bool isReversible() {

        return AllowRewrittenCandidates && OriginalOperator &&

               (getRewrittenOverloadedOperator(OriginalOperator) != OO_None ||

                shouldAddReversed(OriginalOperator));

      }

      /// Determine whether we should consider looking for and adding reversed

      /// candidates for operator Op.

      bool shouldAddReversed(OverloadedOperatorKind Op);

      /// Determine whether we should add a rewritten candidate for \p FD with

      /// reversed parameter order.

      bool shouldAddReversed(ASTContext &Ctx, const FunctionDecl *FD);

    };

  private:

    SmallVector<OverloadCandidate, 16> Candidates;

    llvm::SmallPtrSet<uintptr_t, 16> Functions;

    // Allocator for ConversionSequenceLists. We store the first few of these

    // inline to avoid allocation for small sets.

    llvm::BumpPtrAllocator SlabAllocator;

    SourceLocation Loc;

    CandidateSetKind Kind;

    OperatorRewriteInfo RewriteInfo;

    constexpr static unsigned NumInlineBytes =

        24 * sizeof(ImplicitConversionSequence);

    unsigned NumInlineBytesUsed = 0;

    alignas(void *) char InlineSpace[NumInlineBytes];

    // Address space of the object being constructed.

    LangAS DestAS = LangAS::Default;

    /// If we have space, allocates from inline storage. Otherwise, allocates

    /// from the slab allocator.

    /// FIXME: It would probably be nice to have a SmallBumpPtrAllocator

    /// instead.

    /// FIXME: Now that this only allocates ImplicitConversionSequences, do we

    /// want to un-generalize this?

    template <typename T>

    T *slabAllocate(unsigned N) {

      // It's simpler if this doesn't need to consider alignment.

      static_assert(alignof(T) == alignof(void *),

                    "Only works for pointer-aligned types.");

      static_assert(std::is_trivial<T>::value ||

                        std::is_same<ImplicitConversionSequence, T>::value,

                    "Add destruction logic to OverloadCandidateSet::clear().");

      unsigned NBytes = sizeof(T) * N;

      if (NBytes > NumInlineBytes - NumInlineBytesUsed)

        return SlabAllocator.Allocate<T>(N);

      char *FreeSpaceStart = InlineSpace + NumInlineBytesUsed;

      assert(uintptr_t(FreeSpaceStart) % alignof(void *) == 0 &&

             "Misaligned storage!");

      NumInlineBytesUsed += NBytes;

      return reinterpret_cast<T *>(FreeSpaceStart);

    }

    void destroyCandidates();

    /// Whether diagnostics should be deferred.

    bool shouldDeferDiags(Sema &S, ArrayRef<Expr *> Args, SourceLocation OpLoc);

  public:

    OverloadCandidateSet(SourceLocation Loc, CandidateSetKind CSK,

                         OperatorRewriteInfo RewriteInfo = {})

        : Loc(Loc), Kind(CSK), RewriteInfo(RewriteInfo) {}

    OverloadCandidateSet(const OverloadCandidateSet &) = delete;

    OverloadCandidateSet &operator=(const OverloadCandidateSet &) = delete;

    ~OverloadCandidateSet() { destroyCandidates(); }

    SourceLocation getLocation() const { return Loc; }

    CandidateSetKind getKind() const { return Kind; }

    OperatorRewriteInfo getRewriteInfo() const { return RewriteInfo; }

    /// Determine when this overload candidate will be new to the

    /// overload set.

    bool isNewCandidate(Decl *F, OverloadCandidateParamOrder PO =

                                     OverloadCandidateParamOrder::Normal) {

      uintptr_t Key = reinterpret_cast<uintptr_t>(F->getCanonicalDecl());

      Key |= static_cast<uintptr_t>(PO);

      return Functions.insert(Key).second;

    }

    /// Exclude a function from being considered by overload resolution.

    void exclude(Decl *F) {

      isNewCandidate(F, OverloadCandidateParamOrder::Normal);

      isNewCandidate(F, OverloadCandidateParamOrder::Reversed);

    }

    /// Clear out all of the candidates.

    void clear(CandidateSetKind CSK);

    using iterator = SmallVectorImpl<OverloadCandidate>::iterator;

    iterator begin() { return Candidates.begin(); }

    iterator end() { return Candidates.end(); }

    size_t size() const { return Candidates.size(); }

    bool empty() const { return Candidates.empty(); }

    /// Allocate storage for conversion sequences for NumConversions

    /// conversions.

    ConversionSequenceList

    allocateConversionSequences(unsigned NumConversions) {

      ImplicitConversionSequence *Conversions =

          slabAllocate<ImplicitConversionSequence>(NumConversions);