Overload.h

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/===- 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/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,
 
    / RISC-V RVV Vector conversions
    ICK_RVV_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,
 
    / Fixed point type conversions according to N1169.
    ICK_Fixed_Point_Conversion,
 
    / HLSL vector truncation.
    ICK_HLSL_Vector_Truncation,
 
    / HLSL Matrix truncation.
    ICK_HLSL_Matrix_Truncation,
 
    / HLSL non-decaying array rvalue cast.
    ICK_HLSL_Array_RValue,
 
    / HLSL vector splat from scalar or boolean type.
    ICK_HLSL_Vector_Splat,
 
    / HLSL matrix splat from scalar or boolean type.
    ICK_HLSL_Matrix_Splat,
 
    / 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,
 
    / HLSL Scalar Widening
    ICR_HLSL_Scalar_Widening,
 
    / Promotion
    ICR_Promotion,
 
    / HLSL Scalar Widening with promotion
    ICR_HLSL_Scalar_Widening_Promotion,
 
    / Conversion
    ICR_Conversion,
 
    / OpenCL Scalar Widening
    ICR_OCL_Scalar_Widening,
 
    / HLSL Scalar Widening with conversion
    ICR_HLSL_Scalar_Widening_Conversion,
 
    / 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,
 
    / HLSL Matching Dimension Reduction
    ICR_HLSL_Dimension_Reduction,
 
    / HLSL Dimension reduction with promotion
    ICR_HLSL_Dimension_Reduction_Promotion,
 
    / HLSL Dimension reduction with conversion
    ICR_HLSL_Dimension_Reduction_Conversion,
  };
 
  ImplicitConversionRank GetConversionRank(ImplicitConversionKind Kind);
 
  ImplicitConversionRank
  GetDimensionConversionRank(ImplicitConversionRank Base,
                             ImplicitConversionKind Dimension);
 
  / 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).
  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;
 
    / Dimension - Between the second and third conversion a vector or matrix
    / dimension conversion may occur. If this is not ICK_Identity this
    / conversion truncates the vector or matrix, or extends a scalar.
    ImplicitConversionKind Dimension : 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).
    LLVM_PREFERRED_TYPE(bool)
    unsigned DeprecatedStringLiteralToCharPtr : 1;
 
    / Whether the qualification conversion involves a change in the
    / Objective-C lifetime (for automatic reference counting).
    LLVM_PREFERRED_TYPE(bool)
    unsigned QualificationIncludesObjCLifetime : 1;
 
    / IncompatibleObjC - Whether this is an Objective-C conversion
    / that we should warn about (if we actually use it).
    LLVM_PREFERRED_TYPE(bool)
    unsigned IncompatibleObjC : 1;
 
    / ReferenceBinding - True when this is a reference binding
    / (C++ [over.ics.ref]).
    LLVM_PREFERRED_TYPE(bool)
    unsigned ReferenceBinding : 1;
 
    / DirectBinding - True when this is a reference binding that is a
    / direct binding (C++ [dcl.init.ref]).
    LLVM_PREFERRED_TYPE(bool)
    unsigned DirectBinding : 1;
 
    / Whether this is an lvalue reference binding (otherwise, it's
    / an rvalue reference binding).
    LLVM_PREFERRED_TYPE(bool)
    unsigned IsLvalueReference : 1;
 
    / Whether we're binding to a function lvalue.
    LLVM_PREFERRED_TYPE(bool)
    unsigned BindsToFunctionLvalue : 1;
 
    / Whether we're binding to an rvalue.
    LLVM_PREFERRED_TYPE(bool)
    unsigned BindsToRvalue : 1;
 
    / Whether this binds an implicit object argument to a
    / non-static member function without a ref-qualifier.
    LLVM_PREFERRED_TYPE(bool)
    unsigned BindsImplicitObjectArgumentWithoutRefQualifier : 1;
 
    / Whether this binds a reference to an object with a different
    / Objective-C lifetime qualifier.
    LLVM_PREFERRED_TYPE(bool)
    unsigned ObjCLifetimeConversionBinding : 1;
 
    / Whether the source expression was originally a single element
    / braced-init-list. Such a conversion is not a perfect match,
    / as we prefer a std::initializer_list constructor over an exact match
    / constructor.
    LLVM_PREFERRED_TYPE(bool)
    unsigned FromBracedInitList : 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 && Dimension == ICK_Identity &&
             Third == ICK_Identity;
    }
 
    / A conversion sequence is perfect if it is an identity conversion and
    / the type of the source is the same as the type of the target.
    bool isPerfect(const ASTContext &C) const {
      if (!isIdentityConversion())
        return false;
 
      / We might prefer a std::initializer_list constructor,
      / so this sequence cannot be perfect
      if (FromBracedInitList)
        return false;
 
      / If we are not performing a reference binding, we can skip comparing
      / the types, which has a noticeable performance impact.
      if (!ReferenceBinding) {
#ifndef NDEBUG
        auto Decay = [&](QualType T) {
          if (T->isArrayType() || T->isFunctionType())
            T = C.getDecayedType(T);
 
          / A function pointer type can be resolved to a member function type,
          / which is still an identity conversion.
          if (auto *N = T->getAs<MemberPointerType>();
              N && N->isMemberFunctionPointer())
            T = C.getDecayedType(N->getPointeeType());
          return T;
        };
        / The types might differ if there is an array-to-pointer conversion
        / an function-to-pointer conversion, or lvalue-to-rvalue conversion.
        / In some cases, this may happen even if First is not set.
        assert(C.hasSameUnqualifiedType(Decay(getFromType()),
                                        Decay(getToType(2))));
#endif
        return true;
      }
      if (!C.hasSameType(getFromType(), getToType(2)))
        return false;
      if (BindsToRvalue && IsLvalueReference)
        return false;
      return true;
    }
 
    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,
      too_few_initializers,
      too_many_initializers,
    };
 
    / 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).
    / StaticObjectArgumentConversion represents the conversion rules for
    / the synthesized first argument of calls to static member functions
    / ([over.best.ics.general]p8).
    enum Kind {
      StandardConversion = 0,
      StaticObjectArgumentConversion,
      UserDefinedConversion,
      AmbiguousConversion,
      EllipsisConversion,
      BadConversion
    };
 
  private:
    enum {
      Uninitialized = BadConversion + 1
    };
 
    / ConversionKind - The kind of implicit conversion sequence.
    LLVM_PREFERRED_TYPE(Kind)
    unsigned ConversionKind : 31;
 
    / Whether the initializer list was of an incomplete array.
    LLVM_PREFERRED_TYPE(bool)
    unsigned InitializerListOfIncompleteArray : 1;
 
    / When initializing an array or std::initializer_list from an
    / initializer-list, this is the array or std::initializer_list type being
    / initialized. The remainder of the conversion sequence, including ToType,
    / describe the worst conversion of an initializer to an element of the
    / array or std::initializer_list. (Note, 'worst' is not well defined.)
    QualType InitializerListContainerType;
 
    void setKind(Kind K) {
      destruct();
      ConversionKind = K;
    }
 
    void destruct() {
      if (ConversionKind == AmbiguousConversion) Ambiguous.destruct();
    }
 
  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),
          InitializerListOfIncompleteArray(false) {
      Standard.setAsIdentityConversion();
    }
 
    ImplicitConversionSequence(const ImplicitConversionSequence &Other)
        : ConversionKind(Other.ConversionKind),
          InitializerListOfIncompleteArray(
              Other.InitializerListOfIncompleteArray),
          InitializerListContainerType(Other.InitializerListContainerType) {
      switch (ConversionKind) {
      case Uninitialized: break;
      case StandardConversion: Standard = Other.Standard; break;
      case StaticObjectArgumentConversion:
        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:
      case StaticObjectArgumentConversion:
        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 isStaticObjectArgument() const {
      return getKind() == StaticObjectArgumentConversion;
    }
    bool isEllipsis() const { return getKind() == EllipsisConversion; }
    bool isAmbiguous() const { return getKind() == AmbiguousConversion; }
    bool isUserDefined() const { return getKind() == UserDefinedConversion; }
    bool isFailure() const { return isBad() || isAmbiguous(); }
 
    / 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 setStaticObjectArgument() { setKind(StaticObjectArgumentConversion); }
    void setEllipsis() { setKind(EllipsisConversion); }
    void setUserDefined() { setKind(UserDefinedConversion); }
 
    void setAmbiguous() {
      if (ConversionKind == AmbiguousConversion) return;
      ConversionKind = AmbiguousConversion;
      Ambiguous.construct();
    }
 
    void setAsIdentityConversion(QualType T) {
      setStandard();
      Standard.setAsIdentityConversion();
      Standard.setFromType(T);
      Standard.setAllToTypes(T);
    }
 
    / A conversion sequence is perfect if it is an identity conversion and
    / the type of the source is the same as the type of the target.
    bool isPerfect(const ASTContext &C) const {
      return isStandard() && Standard.isPerfect(C);
    }
 
    / True iff this is a conversion sequence from an initializer list to an
    / array or std::initializer.
    bool hasInitializerListContainerType() const {
      return !InitializerListContainerType.isNull();
    }
    void setInitializerListContainerType(QualType T, bool IA) {
      InitializerListContainerType = T;
      InitializerListOfIncompleteArray = IA;
    }
    bool isInitializerListOfIncompleteArray() const {
      return InitializerListOfIncompleteArray;
    }
    QualType getInitializerListContainerType() const {
      assert(hasInitializerListContainerType() &&
             "not initializer list container");
      return InitializerListContainerType;
    }
 
    / 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 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,
 
    / This candidate was not viable because it has internal linkage and is
    / from a different module unit than the use.
    ovl_fail_module_mismatched,
  };
 
  / 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.
    LLVM_PREFERRED_TYPE(bool)
    unsigned 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.
    LLVM_PREFERRED_TYPE(bool)
    unsigned 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]).
    LLVM_PREFERRED_TYPE(bool)
    unsigned 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.
    LLVM_PREFERRED_TYPE(bool)
    unsigned IgnoreObjectArgument : 1;
 
    LLVM_PREFERRED_TYPE(bool)
    unsigned TookAddressOfOverload : 1;
 
    / Have we matched any packs on the parameter side, versus any non-packs on
    / the argument side, in a context where the opposite matching is also
    / allowed?
    LLVM_PREFERRED_TYPE(bool)
    unsigned StrictPackMatch : 1;
 
    / True if the candidate was found using ADL.
    LLVM_PREFERRED_TYPE(CallExpr::ADLCallKind)
    unsigned IsADLCandidate : 1;
 
    / Whether FinalConversion has been set.
    LLVM_PREFERRED_TYPE(bool)
    unsigned HasFinalConversion : 1;
 
    / Whether this is a rewritten candidate, and if so, of what kind?
    LLVM_PREFERRED_TYPE(OverloadCandidateRewriteKind)
    unsigned RewriteKind : 2;
 
    / FailureKind - The reason why this candidate is not viable.
    / Actually an OverloadFailureKind.
    LLVM_PREFERRED_TYPE(OverloadFailureKind)
    unsigned FailureKind : 8;
 
    / 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;
    }
 
    / An overload is a perfect match if the conversion
    / sequences for each argument are perfect.
    bool isPerfectMatch(const ASTContext &Ctx) const {
      if (!Viable)
        return false;
      for (const auto &C : Conversions) {
        if (!C.isInitialized() || !C.isPerfect(Ctx))
          return false;
      }
      if (HasFinalConversion)
        return FinalConversion.isPerfect(Ctx);
      return true;
    }
 
    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->isPointerOrReferenceType())
          STy = STy->getPointeeType();
        return STy->castAs<FunctionProtoType>()->getNumParams();
      }
      if (Function)
        return Function->getNumParams();
      return ExplicitCallArguments;
    }
 
    bool NotValidBecauseConstraintExprHasError() const;
 
  private:
    friend class OverloadCandidateSet;
    OverloadCandidate()
        : IsSurrogate(false), IgnoreObjectArgument(false),
          TookAddressOfOverload(false), StrictPackMatch(false),
          IsADLCandidate(llvm::to_underlying(CallExpr::NotADL)),
          HasFinalConversion(false), RewriteKind(CRK_None) {}
  };
 
  struct DeferredTemplateOverloadCandidate {
 
    / intrusive linked list support for allocateDeferredCandidate
    DeferredTemplateOverloadCandidate *Next = nullptr;
 
    enum Kind { Function, Method, Conversion };
 
    LLVM_PREFERRED_TYPE(Kind)
    unsigned Kind : 2;
    LLVM_PREFERRED_TYPE(bool)
    unsigned AllowObjCConversionOnExplicit : 1;
    LLVM_PREFERRED_TYPE(bool)
    unsigned AllowResultConversion : 1;
    LLVM_PREFERRED_TYPE(bool)
    unsigned AllowExplicit : 1;
    LLVM_PREFERRED_TYPE(bool)
    unsigned SuppressUserConversions : 1;
    LLVM_PREFERRED_TYPE(bool)
    unsigned PartialOverloading : 1;
    LLVM_PREFERRED_TYPE(bool)
    unsigned AggregateCandidateDeduction : 1;
  };
 
  struct DeferredFunctionTemplateOverloadCandidate
      : public DeferredTemplateOverloadCandidate {
    FunctionTemplateDecl *FunctionTemplate;
    DeclAccessPair FoundDecl;
    ArrayRef<Expr *> Args;
    CallExpr::ADLCallKind IsADLCandidate;
    OverloadCandidateParamOrder PO;
  };
  static_assert(std::is_trivially_destructible_v<
                DeferredFunctionTemplateOverloadCandidate>);
 
  struct DeferredMethodTemplateOverloadCandidate
      : public DeferredTemplateOverloadCandidate {
    FunctionTemplateDecl *FunctionTemplate;
    DeclAccessPair FoundDecl;
    ArrayRef<Expr *> Args;
    CXXRecordDecl *ActingContext;
    Expr::Classification ObjectClassification;
    QualType ObjectType;
    OverloadCandidateParamOrder PO;
  };
  static_assert(std::is_trivially_destructible_v<
                DeferredMethodTemplateOverloadCandidate>);
 
  struct DeferredConversionTemplateOverloadCandidate
      : public DeferredTemplateOverloadCandidate {
    FunctionTemplateDecl *FunctionTemplate;
    DeclAccessPair FoundDecl;
    CXXRecordDecl *ActingContext;
    Expr *From;
    QualType ToType;
  };
 
  static_assert(std::is_trivially_destructible_v<
                DeferredConversionTemplateOverloadCandidate>);
 
  / 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,
 
      / C++ [over.match.call.general]
      / Resolve a call through the address of an overload set.
      CSK_AddressOfOverloadSet,
 
      / When doing overload resolution during code completion,
      / we want to show all viable candidates, including otherwise
      / deferred template candidates.
      CSK_CodeCompletion,
    };
 
    / Information about operator rewrites to consider when adding operator
    / functions to a candidate set.
    struct OperatorRewriteInfo {
      OperatorRewriteInfo()
          : OriginalOperator(OO_None), OpLoc(), AllowRewrittenCandidates(false) {}
      OperatorRewriteInfo(OverloadedOperatorKind Op, SourceLocation OpLoc,
                          bool AllowRewritten)
          : OriginalOperator(Op), OpLoc(OpLoc),
            AllowRewrittenCandidates(AllowRewritten) {}
 
      / The original operator as written in the source.
      OverloadedOperatorKind OriginalOperator;
      / The source location of the operator.
      SourceLocation OpLoc;
      / 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) const {
        return OriginalOperator &&
               FD->getDeclName().getCXXOverloadedOperator() != OriginalOperator;
      }
 
      bool isAcceptableCandidate(const FunctionDecl *FD) const {
        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() const {
        return AllowRewrittenCandidates && OriginalOperator &&
               (getRewrittenOverloadedOperator(OriginalOperator) != OO_None ||
                allowsReversed(OriginalOperator));
      }
 
      / Determine whether reversing parameter order is allowed for operator
      / Op.
      bool allowsReversed(OverloadedOperatorKind Op) const;
 
      / Determine whether we should add a rewritten candidate for \p FD with
      / reversed parameter order.
      / \param OriginalArgs are the original non reversed arguments.
      bool shouldAddReversed(Sema &S, ArrayRef<Expr *> OriginalArgs,
                             FunctionDecl *FD) const;
    };
 
  private:
    SmallVector<OverloadCandidate, 16> Candidates;
    llvm::SmallPtrSet<uintptr_t, 16> Functions;
 
    DeferredTemplateOverloadCandidate *FirstDeferredCandidate = nullptr;
    unsigned DeferredCandidatesCount : 8 * sizeof(unsigned) - 2;
    LLVM_PREFERRED_TYPE(bool)
    unsigned HasDeferredTemplateConstructors : 1;
    LLVM_PREFERRED_TYPE(bool)
    unsigned ResolutionByPerfectCandidateIsDisabled : 1;
 
    / Allocator for ConversionSequenceLists and deferred candidate args.
    / We store the first few of these
    / inline to avoid allocation for small sets.
    llvm::BumpPtrAllocator SlabAllocator;
 
    SourceLocation Loc;
    CandidateSetKind Kind;
    OperatorRewriteInfo RewriteInfo;
 
    / Small storage size for ImplicitConversionSequences
    / and the persisted arguments of deferred candidates.
    constexpr static unsigned NumInlineBytes =
        32 * 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.
    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_trivially_destructible_v<T> ||
                        (std::is_same_v<ImplicitConversionSequence, T>),
                    "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);
    }
 
    / Because the size of OverloadCandidateSet has a noticeable impact on
    / performance, we store each deferred template candidate in the slab
    / allocator such that deferred candidates are ultimately a singly-linked
    / intrusive linked list. This ends up being much more efficient than a
    / SmallVector that is empty in the common case.
    template <typename T> T *allocateDeferredCandidate() {
      T *C = slabAllocate<T>(1);
      if (!FirstDeferredCandidate)
        FirstDeferredCandidate = C;
      else {
        auto *F = FirstDeferredCandidate;
        while (F->Next)
          F = F->Next;
        F->Next = C;
      }
      DeferredCandidatesCount++;
      return C;
    }
 
    void destroyCandidates();
 
  public:
    OverloadCandidateSet(SourceLocation Loc, CandidateSetKind CSK,
                         OperatorRewriteInfo RewriteInfo = {})
        : FirstDeferredCandidate(nullptr), DeferredCandidatesCount(0),
          HasDeferredTemplateConstructors(false),
          ResolutionByPerfectCandidateIsDisabled(false), 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; }
 
    / Whether diagnostics should be deferred.
    bool shouldDeferDiags(Sema &S, ArrayRef<Expr *> Args, SourceLocation OpLoc);
 
    / Whether the resolution of template candidates should be deferred
    bool shouldDeferTemplateArgumentDeduction(const LangOptions &Opts) const;
 
    / 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() + DeferredCandidatesCount; }
 
    size_t nonDeferredCandidatesCount() const { return Candidates.size(); }
 
    bool empty() const {
      return Candidates.empty() && DeferredCandidatesCount == 0;
    }
 
    / Allocate storage for conversion sequences for NumConversions
    / conversions.
    ConversionSequenceList
    allocateConversionSequences(unsigned NumConversions) {
      ImplicitConversionSequence *Conversions =
          slabAllocate<ImplicitConversionSequence>(NumConversions);
 
      / Construct the new objects.
      for (unsigned I = 0; I != NumConversions; ++I)
        new (&Conversions[I]) ImplicitConversionSequence();
 
      return ConversionSequenceList(Conversions, NumConversions);
    }
 
    / Provide storage for any Expr* arg that must be preserved
    / until deferred template candidates are deduced.
    / Typically this should be used for reversed operator arguments
    / and any time the argument array is transformed while adding
    / a template candidate.
    llvm::MutableArrayRef<Expr *> getPersistentArgsArray(unsigned N) {
      Expr **Exprs = slabAllocate<Expr *>(N);
      return llvm::MutableArrayRef<Expr *>(Exprs, N);
    }
 
    template <typename... T>
    llvm::MutableArrayRef<Expr *> getPersistentArgsArray(T *...Exprs) {
      llvm::MutableArrayRef<Expr *> Arr =
          getPersistentArgsArray(sizeof...(Exprs));
      llvm::copy(std::initializer_list<Expr *>{Exprs...}, Arr.data());
      return Arr;
    }
 
    / Add a new candidate with NumConversions conversion sequence slots
    / to the overload set.
    OverloadCandidate &addCandidate(unsigned NumConversions = 0,
                                    ConversionSequenceList Conversions = {}) {
      assert((Conversions.empty() || Conversions.size() == NumConversions) &&
             "preallocated conversion sequence has wrong length");
 
      Candidates.push_back(OverloadCandidate());
      OverloadCandidate &C = Candidates.back();
      C.Conversions = Conversions.empty()
                          ? allocateConversionSequences(NumConversions)
                          : Conversions;
      return C;
    }
 
    void AddDeferredTemplateCandidate(
        FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
        ArrayRef<Expr *> Args, bool SuppressUserConversions,
        bool PartialOverloading, bool AllowExplicit,
        CallExpr::ADLCallKind IsADLCandidate, OverloadCandidateParamOrder PO,
        bool AggregateCandidateDeduction);
 
    void AddDeferredMethodTemplateCandidate(
        FunctionTemplateDecl *MethodTmpl, DeclAccessPair FoundDecl,
        CXXRecordDecl *ActingContext, QualType ObjectType,
        Expr::Classification ObjectClassification, ArrayRef<Expr *> Args,
        bool SuppressUserConversions, bool PartialOverloading,
        OverloadCandidateParamOrder PO);
 
    void AddDeferredConversionTemplateCandidate(
        FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
        CXXRecordDecl *ActingContext, Expr *From, QualType ToType,
        bool AllowObjCConversionOnExplicit, bool AllowExplicit,
        bool AllowResultConversion);
 
    void InjectNonDeducedTemplateCandidates(Sema &S);
 
    void DisableResolutionByPerfectCandidate() {
      ResolutionByPerfectCandidateIsDisabled = true;
    }
 
    / Find the best viable function on this overload set, if it exists.
    OverloadingResult BestViableFunction(Sema &S, SourceLocation Loc,
                                         OverloadCandidateSet::iterator& Best);
 
    SmallVector<OverloadCandidate *, 32> CompleteCandidates(
        Sema &S, OverloadCandidateDisplayKind OCD, ArrayRef<Expr *> Args,
        SourceLocation OpLoc = SourceLocation(),
        llvm::function_ref<bool(OverloadCandidate &)> Filter =
            [](OverloadCandidate &) { return true; });
 
    void NoteCandidates(
        PartialDiagnosticAt PA, Sema &S, OverloadCandidateDisplayKind OCD,
        ArrayRef<Expr *> Args, StringRef Opc = "",
        SourceLocation Loc = SourceLocation(),
        llvm::function_ref<bool(OverloadCandidate &)> Filter =
            [](OverloadCandidate &) { return true; });
 
    void NoteCandidates(Sema &S, ArrayRef<Expr *> Args,
                        ArrayRef<OverloadCandidate *> Cands,
                        StringRef Opc = "",
                        SourceLocation OpLoc = SourceLocation());
 
    LangAS getDestAS() { return DestAS; }
 
    void setDestAS(LangAS AS) {
      assert((Kind == CSK_InitByConstructor ||
              Kind == CSK_InitByUserDefinedConversion) &&
             "can't set the destination address space when not constructing an "
             "object");
      DestAS = AS;
    }
 
  private:
    OverloadingResult ResultForBestCandidate(const iterator &Best);
    void CudaExcludeWrongSideCandidates(
        Sema &S, SmallVectorImpl<OverloadCandidate *> &Candidates);
    OverloadingResult
    BestViableFunctionImpl(Sema &S, SourceLocation Loc,
                           OverloadCandidateSet::iterator &Best);
  };
 
  bool isBetterOverloadCandidate(Sema &S, const OverloadCandidate &Cand1,
                                 const OverloadCandidate &Cand2,
                                 SourceLocation Loc,
                                 OverloadCandidateSet::CandidateSetKind Kind,
                                 bool PartialOverloading = false);
 
  struct ConstructorInfo {
    DeclAccessPair FoundDecl;
    CXXConstructorDecl *Constructor;
    FunctionTemplateDecl *ConstructorTmpl;
 
    explicit operator bool() const { return Constructor; }
  };
 
  / FIXME: Add an AddOverloadCandidate / AddTemplateOverloadCandidate overload
  / that takes one of these.
  inline ConstructorInfo getConstructorInfo(NamedDecl *ND) {
    if (isa<UsingDecl>(ND))
      return ConstructorInfo{};
 
    / For constructors, the access check is performed against the underlying
    / declaration, not the found declaration.
    auto *D = ND->getUnderlyingDecl();
    ConstructorInfo Info = {DeclAccessPair::make(ND, D->getAccess()), nullptr,
                            nullptr};
    Info.ConstructorTmpl = dyn_cast<FunctionTemplateDecl>(D);
    if (Info.ConstructorTmpl)
      D = Info.ConstructorTmpl->getTemplatedDecl();
    Info.Constructor = dyn_cast<CXXConstructorDecl>(D);
    return Info;
  }
 
  / Returns false if signature help is relevant despite number of arguments
  / exceeding parameters. Specifically, it returns false when
  / PartialOverloading is true and one of the following:
  / * Function is variadic
  / * Function is template variadic
  / * Function is an instantiation of template variadic function
  / The last case may seem strange. The idea is that if we added one more
  / argument, we'd end up with a function similar to Function. Since, in the
  / context of signature help and/or code completion, we do not know what the
  / type of the next argument (that the user is typing) will be, this is as
  / good candidate as we can get, despite the fact that it takes one less
  / parameter.
  bool shouldEnforceArgLimit(bool PartialOverloading, FunctionDecl *Function);
 
  inline bool OverloadCandidateSet::shouldDeferTemplateArgumentDeduction(
      const LangOptions &Opts) const {
    return
        / For user defined conversion we need to check against different
        / combination of CV qualifiers and look at any explicit specifier, so
        / always deduce template candidates.
        Kind != CSK_InitByUserDefinedConversion
        / When doing code completion, we want to see all the
        / viable candidates.
        && Kind != CSK_CodeCompletion
        / CUDA may prefer template candidates even when a non-candidate
        / is a perfect match
        && !Opts.CUDA;
  }
 
} / namespace clang
 
#endif / LLVM_CLANG_SEMA_OVERLOAD_H