SemaOverload.cpp

Go to the documentation of this file.

//===--- SemaOverload.cpp - C++ Overloading -------------------------------===//
//
// 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 provides Sema routines for C++ overloading.
//
//===----------------------------------------------------------------------===//
 
#include "CheckExprLifetime.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/ExprObjC.h"
#include "clang/AST/Type.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/Basic/DiagnosticOptions.h"
#include "clang/Basic/OperatorKinds.h"
#include "clang/Basic/PartialDiagnostic.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/EnterExpressionEvaluationContext.h"
#include "clang/Sema/Initialization.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/Overload.h"
#include "clang/Sema/SemaCUDA.h"
#include "clang/Sema/SemaObjC.h"
#include "clang/Sema/Template.h"
#include "clang/Sema/TemplateDeduction.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/STLForwardCompat.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdlib>
#include <optional>
 
using namespace clang;
using namespace sema;
 
using AllowedExplicit = Sema::AllowedExplicit;
 
static bool functionHasPassObjectSizeParams(const FunctionDecl *FD) {
  return llvm::any_of(FD->parameters(), [](const ParmVarDecl *P) {
    return P->hasAttr<PassObjectSizeAttr>();
  });
}
 
/// A convenience routine for creating a decayed reference to a function.
static ExprResult CreateFunctionRefExpr(
    Sema &S, FunctionDecl *Fn, NamedDecl *FoundDecl, const Expr *Base,
    bool HadMultipleCandidates, SourceLocation Loc = SourceLocation(),
    const DeclarationNameLoc &LocInfo = DeclarationNameLoc()) {
  if (S.DiagnoseUseOfDecl(FoundDecl, Loc))
    return ExprError();
  // If FoundDecl is different from Fn (such as if one is a template
  // and the other a specialization), make sure DiagnoseUseOfDecl is
  // called on both.
  // FIXME: This would be more comprehensively addressed by modifying
  // DiagnoseUseOfDecl to accept both the FoundDecl and the decl
  // being used.
  if (FoundDecl != Fn && S.DiagnoseUseOfDecl(Fn, Loc))
    return ExprError();
  DeclRefExpr *DRE = new (S.Context)
      DeclRefExpr(S.Context, Fn, false, Fn->getType(), VK_LValue, Loc, LocInfo);
  if (HadMultipleCandidates)
    DRE->setHadMultipleCandidates(true);
 
  S.MarkDeclRefReferenced(DRE, Base);
  if (auto *FPT = DRE->getType()->getAs<FunctionProtoType>()) {
    if (isUnresolvedExceptionSpec(FPT->getExceptionSpecType())) {
      S.ResolveExceptionSpec(Loc, FPT);
      DRE->setType(Fn->getType());
    }
  }
  return S.ImpCastExprToType(DRE, S.Context.getPointerType(DRE->getType()),
                             CK_FunctionToPointerDecay);
}
 
static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
                                 bool InOverloadResolution,
                                 StandardConversionSequence &SCS,
                                 bool CStyle,
                                 bool AllowObjCWritebackConversion);
 
static bool IsTransparentUnionStandardConversion(Sema &S, Expr* From,
                                                 QualType &ToType,
                                                 bool InOverloadResolution,
                                                 StandardConversionSequence &SCS,
                                                 bool CStyle);
static OverloadingResult
IsUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
                        UserDefinedConversionSequence& User,
                        OverloadCandidateSet& Conversions,
                        AllowedExplicit AllowExplicit,
                        bool AllowObjCConversionOnExplicit);
 
static ImplicitConversionSequence::CompareKind
CompareStandardConversionSequences(Sema &S, SourceLocation Loc,
                                   const StandardConversionSequence& SCS1,
                                   const StandardConversionSequence& SCS2);
 
static ImplicitConversionSequence::CompareKind
CompareQualificationConversions(Sema &S,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2);
 
static ImplicitConversionSequence::CompareKind
CompareDerivedToBaseConversions(Sema &S, SourceLocation Loc,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2);
 
/// GetConversionRank - Retrieve the implicit conversion rank
/// corresponding to the given implicit conversion kind.
ImplicitConversionRank clang::GetConversionRank(ImplicitConversionKind Kind) {
  static const ImplicitConversionRank Rank[] = {
      ICR_Exact_Match,
      ICR_Exact_Match,
      ICR_Exact_Match,
      ICR_Exact_Match,
      ICR_Exact_Match,
      ICR_Exact_Match,
      ICR_Promotion,
      ICR_Promotion,
      ICR_Promotion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_OCL_Scalar_Widening,
      ICR_Complex_Real_Conversion,
      ICR_Conversion,
      ICR_Conversion,
      ICR_Writeback_Conversion,
      ICR_Exact_Match, // NOTE(gbiv): This may not be completely right --
                       // it was omitted by the patch that added
                       // ICK_Zero_Event_Conversion
      ICR_Exact_Match, // NOTE(ctopper): This may not be completely right --
                       // it was omitted by the patch that added
                       // ICK_Zero_Queue_Conversion
      ICR_C_Conversion,
      ICR_C_Conversion_Extension,
      ICR_Conversion,
      ICR_HLSL_Dimension_Reduction,
      ICR_Conversion,
      ICR_HLSL_Scalar_Widening,
  };
  static_assert(std::size(Rank) == (int)ICK_Num_Conversion_Kinds);
  return Rank[(int)Kind];
}
 
ImplicitConversionRank
clang::GetDimensionConversionRank(ImplicitConversionRank Base,
                                  ImplicitConversionKind Dimension) {
  ImplicitConversionRank Rank = GetConversionRank(Dimension);
  if (Rank == ICR_HLSL_Scalar_Widening) {
    if (Base == ICR_Promotion)
      return ICR_HLSL_Scalar_Widening_Promotion;
    if (Base == ICR_Conversion)
      return ICR_HLSL_Scalar_Widening_Conversion;
  }
  if (Rank == ICR_HLSL_Dimension_Reduction) {
    if (Base == ICR_Promotion)
      return ICR_HLSL_Dimension_Reduction_Promotion;
    if (Base == ICR_Conversion)
      return ICR_HLSL_Dimension_Reduction_Conversion;
  }
  return Rank;
}
 
/// GetImplicitConversionName - Return the name of this kind of
/// implicit conversion.
static const char *GetImplicitConversionName(ImplicitConversionKind Kind) {
  static const char *const Name[] = {
      "No conversion",
      "Lvalue-to-rvalue",
      "Array-to-pointer",
      "Function-to-pointer",
      "Function pointer conversion",
      "Qualification",
      "Integral promotion",
      "Floating point promotion",
      "Complex promotion",
      "Integral conversion",
      "Floating conversion",
      "Complex conversion",
      "Floating-integral conversion",
      "Pointer conversion",
      "Pointer-to-member conversion",
      "Boolean conversion",
      "Compatible-types conversion",
      "Derived-to-base conversion",
      "Vector conversion",
      "SVE Vector conversion",
      "RVV Vector conversion",
      "Vector splat",
      "Complex-real conversion",
      "Block Pointer conversion",
      "Transparent Union Conversion",
      "Writeback conversion",
      "OpenCL Zero Event Conversion",
      "OpenCL Zero Queue Conversion",
      "C specific type conversion",
      "Incompatible pointer conversion",
      "Fixed point conversion",
      "HLSL vector truncation",
      "Non-decaying array conversion",
      "HLSL vector splat",
  };
  static_assert(std::size(Name) == (int)ICK_Num_Conversion_Kinds);
  return Name[Kind];
}
 
/// StandardConversionSequence - Set the standard conversion
/// sequence to the identity conversion.
void StandardConversionSequence::setAsIdentityConversion() {
  First = ICK_Identity;
  Second = ICK_Identity;
  Dimension = ICK_Identity;
  Third = ICK_Identity;
  DeprecatedStringLiteralToCharPtr = false;
  QualificationIncludesObjCLifetime = false;
  ReferenceBinding = false;
  DirectBinding = false;
  IsLvalueReference = true;
  BindsToFunctionLvalue = false;
  BindsToRvalue = false;
  BindsImplicitObjectArgumentWithoutRefQualifier = false;
  ObjCLifetimeConversionBinding = false;
  CopyConstructor = nullptr;
}
 
/// getRank - Retrieve the rank of this standard conversion sequence
/// (C++ 13.3.3.1.1p3). The rank is the largest rank of each of the
/// implicit conversions.
ImplicitConversionRank StandardConversionSequence::getRank() const {
  ImplicitConversionRank Rank = ICR_Exact_Match;
  if (GetConversionRank(First) > Rank)
    Rank = GetConversionRank(First);
  if (GetConversionRank(Second) > Rank)
    Rank = GetConversionRank(Second);
  if (GetDimensionConversionRank(Rank, Dimension) > Rank)
    Rank = GetDimensionConversionRank(Rank, Dimension);
  if (GetConversionRank(Third) > Rank)
    Rank = GetConversionRank(Third);
  return Rank;
}
 
/// isPointerConversionToBool - Determines whether this conversion is
/// a conversion of a pointer or pointer-to-member to bool. This is
/// used as part of the ranking of standard conversion sequences
/// (C++ 13.3.3.2p4).
bool StandardConversionSequence::isPointerConversionToBool() const {
  // Note that FromType has not necessarily been transformed by the
  // array-to-pointer or function-to-pointer implicit conversions, so
  // check for their presence as well as checking whether FromType is
  // a pointer.
  if (getToType(1)->isBooleanType() &&
      (getFromType()->isPointerType() ||
       getFromType()->isMemberPointerType() ||
       getFromType()->isObjCObjectPointerType() ||
       getFromType()->isBlockPointerType() ||
       First == ICK_Array_To_Pointer || First == ICK_Function_To_Pointer))
    return true;
 
  return false;
}
 
/// isPointerConversionToVoidPointer - Determines whether this
/// conversion is a conversion of a pointer to a void pointer. This is
/// used as part of the ranking of standard conversion sequences (C++
/// 13.3.3.2p4).
bool
StandardConversionSequence::
isPointerConversionToVoidPointer(ASTContext& Context) const {
  QualType FromType = getFromType();
  QualType ToType = getToType(1);
 
  // Note that FromType has not necessarily been transformed by the
  // array-to-pointer implicit conversion, so check for its presence
  // and redo the conversion to get a pointer.
  if (First == ICK_Array_To_Pointer)
    FromType = Context.getArrayDecayedType(FromType);
 
  if (Second == ICK_Pointer_Conversion && FromType->isAnyPointerType())
    if (const PointerType* ToPtrType = ToType->getAs<PointerType>())
      return ToPtrType->getPointeeType()->isVoidType();
 
  return false;
}
 
/// Skip any implicit casts which could be either part of a narrowing conversion
/// or after one in an implicit conversion.
static const Expr *IgnoreNarrowingConversion(ASTContext &Ctx,
                                             const Expr *Converted) {
  // We can have cleanups wrapping the converted expression; these need to be
  // preserved so that destructors run if necessary.
  if (auto *EWC = dyn_cast<ExprWithCleanups>(Converted)) {
    Expr *Inner =
        const_cast<Expr *>(IgnoreNarrowingConversion(Ctx, EWC->getSubExpr()));
    return ExprWithCleanups::Create(Ctx, Inner, EWC->cleanupsHaveSideEffects(),
                                    EWC->getObjects());
  }
 
  while (auto *ICE = dyn_cast<ImplicitCastExpr>(Converted)) {
    switch (ICE->getCastKind()) {
    case CK_NoOp:
    case CK_IntegralCast:
    case CK_IntegralToBoolean:
    case CK_IntegralToFloating:
    case CK_BooleanToSignedIntegral:
    case CK_FloatingToIntegral:
    case CK_FloatingToBoolean:
    case CK_FloatingCast:
      Converted = ICE->getSubExpr();
      continue;
 
    default:
      return Converted;
    }
  }
 
  return Converted;
}
 
/// Check if this standard conversion sequence represents a narrowing
/// conversion, according to C++11 [dcl.init.list]p7.
///
/// \param Ctx  The AST context.
/// \param Converted  The result of applying this standard conversion sequence.
/// \param ConstantValue  If this is an NK_Constant_Narrowing conversion, the
///        value of the expression prior to the narrowing conversion.
/// \param ConstantType  If this is an NK_Constant_Narrowing conversion, the
///        type of the expression prior to the narrowing conversion.
/// \param IgnoreFloatToIntegralConversion If true type-narrowing conversions
///        from floating point types to integral types should be ignored.
NarrowingKind StandardConversionSequence::getNarrowingKind(
    ASTContext &Ctx, const Expr *Converted, APValue &ConstantValue,
    QualType &ConstantType, bool IgnoreFloatToIntegralConversion) const {
  assert((Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23) &&
         "narrowing check outside C++");
 
  // C++11 [dcl.init.list]p7:
  //   A narrowing conversion is an implicit conversion ...
  QualType FromType = getToType(0);
  QualType ToType = getToType(1);
 
  // A conversion to an enumeration type is narrowing if the conversion to
  // the underlying type is narrowing. This only arises for expressions of
  // the form 'Enum{init}'.
  if (auto *ET = ToType->getAs<EnumType>())
    ToType = ET->getDecl()->getIntegerType();
 
  switch (Second) {
  // 'bool' is an integral type; dispatch to the right place to handle it.
  case ICK_Boolean_Conversion:
    if (FromType->isRealFloatingType())
      goto FloatingIntegralConversion;
    if (FromType->isIntegralOrUnscopedEnumerationType())
      goto IntegralConversion;
    // -- from a pointer type or pointer-to-member type to bool, or
    return NK_Type_Narrowing;
 
  // -- from a floating-point type to an integer type, or
  //
  // -- from an integer type or unscoped enumeration type to a floating-point
  //    type, except where the source is a constant expression and the actual
  //    value after conversion will fit into the target type and will produce
  //    the original value when converted back to the original type, or
  case ICK_Floating_Integral:
  FloatingIntegralConversion:
    if (FromType->isRealFloatingType() && ToType->isIntegralType(Ctx)) {
      return NK_Type_Narrowing;
    } else if (FromType->isIntegralOrUnscopedEnumerationType() &&
               ToType->isRealFloatingType()) {
      if (IgnoreFloatToIntegralConversion)
        return NK_Not_Narrowing;
      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
      assert(Initializer && "Unknown conversion expression");
 
      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;
 
      if (std::optional<llvm::APSInt> IntConstantValue =
              Initializer->getIntegerConstantExpr(Ctx)) {
        // Convert the integer to the floating type.
        llvm::APFloat Result(Ctx.getFloatTypeSemantics(ToType));
        Result.convertFromAPInt(*IntConstantValue, IntConstantValue->isSigned(),
                                llvm::APFloat::rmNearestTiesToEven);
        // And back.
        llvm::APSInt ConvertedValue = *IntConstantValue;
        bool ignored;
        Result.convertToInteger(ConvertedValue,
                                llvm::APFloat::rmTowardZero, &ignored);
        // If the resulting value is different, this was a narrowing conversion.
        if (*IntConstantValue != ConvertedValue) {
          ConstantValue = APValue(*IntConstantValue);
          ConstantType = Initializer->getType();
          return NK_Constant_Narrowing;
        }
      } else {
        // Variables are always narrowings.
        return NK_Variable_Narrowing;
      }
    }
    return NK_Not_Narrowing;
 
  // -- from long double to double or float, or from double to float, except
  //    where the source is a constant expression and the actual value after
  //    conversion is within the range of values that can be represented (even
  //    if it cannot be represented exactly), or
  case ICK_Floating_Conversion:
    if (FromType->isRealFloatingType() && ToType->isRealFloatingType() &&
        Ctx.getFloatingTypeOrder(FromType, ToType) == 1) {
      // FromType is larger than ToType.
      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
 
      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;
 
      Expr::EvalResult R;
      if ((Ctx.getLangOpts().C23 && Initializer->EvaluateAsRValue(R, Ctx)) ||
          Initializer->isCXX11ConstantExpr(Ctx, &ConstantValue)) {
        // Constant!
        if (Ctx.getLangOpts().C23)
          ConstantValue = R.Val;
        assert(ConstantValue.isFloat());
        llvm::APFloat FloatVal = ConstantValue.getFloat();
        // Convert the source value into the target type.
        bool ignored;
        llvm::APFloat Converted = FloatVal;
        llvm::APFloat::opStatus ConvertStatus =
            Converted.convert(Ctx.getFloatTypeSemantics(ToType),
                              llvm::APFloat::rmNearestTiesToEven, &ignored);
        Converted.convert(Ctx.getFloatTypeSemantics(FromType),
                          llvm::APFloat::rmNearestTiesToEven, &ignored);
        if (Ctx.getLangOpts().C23) {
          if (FloatVal.isNaN() && Converted.isNaN() &&
              !FloatVal.isSignaling() && !Converted.isSignaling()) {
            // Quiet NaNs are considered the same value, regardless of
            // payloads.
            return NK_Not_Narrowing;
          }
          // For normal values, check exact equality.
          if (!Converted.bitwiseIsEqual(FloatVal)) {
            ConstantType = Initializer->getType();
            return NK_Constant_Narrowing;
          }
        } else {
          // If there was no overflow, the source value is within the range of
          // values that can be represented.
          if (ConvertStatus & llvm::APFloat::opOverflow) {
            ConstantType = Initializer->getType();
            return NK_Constant_Narrowing;
          }
        }
      } else {
        return NK_Variable_Narrowing;
      }
    }
    return NK_Not_Narrowing;
 
  // -- from an integer type or unscoped enumeration type to an integer type
  //    that cannot represent all the values of the original type, except where
  //    (CWG2627) -- the source is a bit-field whose width w is less than that
  //    of its type (or, for an enumeration type, its underlying type) and the
  //    target type can represent all the values of a hypothetical extended
  //    integer type with width w and with the same signedness as the original
  //    type or
  //    -- the source is a constant expression and the actual value after
  //    conversion will fit into the target type and will produce the original
  //    value when converted back to the original type.
  case ICK_Integral_Conversion:
  IntegralConversion: {
    assert(FromType->isIntegralOrUnscopedEnumerationType());
    assert(ToType->isIntegralOrUnscopedEnumerationType());
    const bool FromSigned = FromType->isSignedIntegerOrEnumerationType();
    unsigned FromWidth = Ctx.getIntWidth(FromType);
    const bool ToSigned = ToType->isSignedIntegerOrEnumerationType();
    const unsigned ToWidth = Ctx.getIntWidth(ToType);
 
    constexpr auto CanRepresentAll = [](bool FromSigned, unsigned FromWidth,
                                        bool ToSigned, unsigned ToWidth) {
      return (FromWidth < ToWidth + (FromSigned == ToSigned)) &&
             !(FromSigned && !ToSigned);
    };
 
    if (CanRepresentAll(FromSigned, FromWidth, ToSigned, ToWidth))
      return NK_Not_Narrowing;
 
    // Not all values of FromType can be represented in ToType.
    const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
 
    bool DependentBitField = false;
    if (const FieldDecl *BitField = Initializer->getSourceBitField()) {
      if (BitField->getBitWidth()->isValueDependent())
        DependentBitField = true;
      else if (unsigned BitFieldWidth = BitField->getBitWidthValue(Ctx);
               BitFieldWidth < FromWidth) {
        if (CanRepresentAll(FromSigned, BitFieldWidth, ToSigned, ToWidth))
          return NK_Not_Narrowing;
 
        // The initializer will be truncated to the bit-field width
        FromWidth = BitFieldWidth;
      }
    }
 
    // If it's value-dependent, we can't tell whether it's narrowing.
    if (Initializer->isValueDependent())
      return NK_Dependent_Narrowing;
 
    std::optional<llvm::APSInt> OptInitializerValue =
        Initializer->getIntegerConstantExpr(Ctx);
    if (!OptInitializerValue) {
      // If the bit-field width was dependent, it might end up being small
      // enough to fit in the target type (unless the target type is unsigned
      // and the source type is signed, in which case it will never fit)
      if (DependentBitField && !(FromSigned && !ToSigned))
        return NK_Dependent_Narrowing;
 
      // Otherwise, such a conversion is always narrowing
      return NK_Variable_Narrowing;
    }
    llvm::APSInt &InitializerValue = *OptInitializerValue;
    bool Narrowing = false;
    if (FromWidth < ToWidth) {
      // Negative -> unsigned is narrowing. Otherwise, more bits is never
      // narrowing.
      if (InitializerValue.isSigned() && InitializerValue.isNegative())
        Narrowing = true;
    } else {
      // Add a bit to the InitializerValue so we don't have to worry about
      // signed vs. unsigned comparisons.
      InitializerValue =
          InitializerValue.extend(InitializerValue.getBitWidth() + 1);
      // Convert the initializer to and from the target width and signed-ness.
      llvm::APSInt ConvertedValue = InitializerValue;
      ConvertedValue = ConvertedValue.trunc(ToWidth);
      ConvertedValue.setIsSigned(ToSigned);
      ConvertedValue = ConvertedValue.extend(InitializerValue.getBitWidth());
      ConvertedValue.setIsSigned(InitializerValue.isSigned());
      // If the result is different, this was a narrowing conversion.
      if (ConvertedValue != InitializerValue)
        Narrowing = true;
    }
    if (Narrowing) {
      ConstantType = Initializer->getType();
      ConstantValue = APValue(InitializerValue);
      return NK_Constant_Narrowing;
    }
 
    return NK_Not_Narrowing;
  }
  case ICK_Complex_Real:
    if (FromType->isComplexType() && !ToType->isComplexType())
      return NK_Type_Narrowing;
    return NK_Not_Narrowing;
 
  case ICK_Floating_Promotion:
    if (Ctx.getLangOpts().C23) {
      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
      Expr::EvalResult R;
      if (Initializer->EvaluateAsRValue(R, Ctx)) {
        ConstantValue = R.Val;
        assert(ConstantValue.isFloat());
        llvm::APFloat FloatVal = ConstantValue.getFloat();
        // C23 6.7.3p6 If the initializer has real type and a signaling NaN
        // value, the unqualified versions of the type of the initializer and
        // the corresponding real type of the object declared shall be
        // compatible.
        if (FloatVal.isNaN() && FloatVal.isSignaling()) {
          ConstantType = Initializer->getType();
          return NK_Constant_Narrowing;
        }
      }
    }
    return NK_Not_Narrowing;
  default:
    // Other kinds of conversions are not narrowings.
    return NK_Not_Narrowing;
  }
}
 
/// dump - Print this standard conversion sequence to standard
/// error. Useful for debugging overloading issues.
LLVM_DUMP_METHOD void StandardConversionSequence::dump() const {
  raw_ostream &OS = llvm::errs();
  bool PrintedSomething = false;
  if (First != ICK_Identity) {
    OS << GetImplicitConversionName(First);
    PrintedSomething = true;
  }
 
  if (Second != ICK_Identity) {
    if (PrintedSomething) {
      OS << " -> ";
    }
    OS << GetImplicitConversionName(Second);
 
    if (CopyConstructor) {
      OS << " (by copy constructor)";
    } else if (DirectBinding) {
      OS << " (direct reference binding)";
    } else if (ReferenceBinding) {
      OS << " (reference binding)";
    }
    PrintedSomething = true;
  }
 
  if (Third != ICK_Identity) {
    if (PrintedSomething) {
      OS << " -> ";
    }
    OS << GetImplicitConversionName(Third);
    PrintedSomething = true;
  }
 
  if (!PrintedSomething) {
    OS << "No conversions required";
  }
}
 
/// dump - Print this user-defined conversion sequence to standard
/// error. Useful for debugging overloading issues.
void UserDefinedConversionSequence::dump() const {
  raw_ostream &OS = llvm::errs();
  if (Before.First || Before.Second || Before.Third) {
    Before.dump();
    OS << " -> ";
  }
  if (ConversionFunction)
    OS << '\'' << *ConversionFunction << '\'';
  else
    OS << "aggregate initialization";
  if (After.First || After.Second || After.Third) {
    OS << " -> ";
    After.dump();
  }
}
 
/// dump - Print this implicit conversion sequence to standard
/// error. Useful for debugging overloading issues.
void ImplicitConversionSequence::dump() const {
  raw_ostream &OS = llvm::errs();
  if (hasInitializerListContainerType())
    OS << "Worst list element conversion: ";
  switch (ConversionKind) {
  case StandardConversion:
    OS << "Standard conversion: ";
    Standard.dump();
    break;
  case UserDefinedConversion:
    OS << "User-defined conversion: ";
    UserDefined.dump();
    break;
  case EllipsisConversion:
    OS << "Ellipsis conversion";
    break;
  case AmbiguousConversion:
    OS << "Ambiguous conversion";
    break;
  case BadConversion:
    OS << "Bad conversion";
    break;
  }
 
  OS << "\n";
}
 
void AmbiguousConversionSequence::construct() {
  new (&conversions()) ConversionSet();
}
 
void AmbiguousConversionSequence::destruct() {
  conversions().~ConversionSet();
}
 
void
AmbiguousConversionSequence::copyFrom(const AmbiguousConversionSequence &O) {
  FromTypePtr = O.FromTypePtr;
  ToTypePtr = O.ToTypePtr;
  new (&conversions()) ConversionSet(O.conversions());
}
 
namespace {
  // Structure used by DeductionFailureInfo to store
  // template argument information.
  struct DFIArguments {
    TemplateArgument FirstArg;
    TemplateArgument SecondArg;
  };
  // Structure used by DeductionFailureInfo to store
  // template parameter and template argument information.
  struct DFIParamWithArguments : DFIArguments {
    TemplateParameter Param;
  };
  // Structure used by DeductionFailureInfo to store template argument
  // information and the index of the problematic call argument.
  struct DFIDeducedMismatchArgs : DFIArguments {
    TemplateArgumentList *TemplateArgs;
    unsigned CallArgIndex;
  };
  // Structure used by DeductionFailureInfo to store information about
  // unsatisfied constraints.
  struct CNSInfo {
    TemplateArgumentList *TemplateArgs;
    ConstraintSatisfaction Satisfaction;
  };
}
 
/// Convert from Sema's representation of template deduction information
/// to the form used in overload-candidate information.
DeductionFailureInfo
clang::MakeDeductionFailureInfo(ASTContext &Context,
                                TemplateDeductionResult TDK,
                                TemplateDeductionInfo &Info) {
  DeductionFailureInfo Result;
  Result.Result = static_cast<unsigned>(TDK);
  Result.HasDiagnostic = false;
  switch (TDK) {
  case TemplateDeductionResult::Invalid:
  case TemplateDeductionResult::InstantiationDepth:
  case TemplateDeductionResult::TooManyArguments:
  case TemplateDeductionResult::TooFewArguments:
  case TemplateDeductionResult::MiscellaneousDeductionFailure:
  case TemplateDeductionResult::CUDATargetMismatch:
    Result.Data = nullptr;
    break;
 
  case TemplateDeductionResult::Incomplete:
  case TemplateDeductionResult::InvalidExplicitArguments:
    Result.Data = Info.Param.getOpaqueValue();
    break;
 
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested: {
    // FIXME: Should allocate from normal heap so that we can free this later.
    auto *Saved = new (Context) DFIDeducedMismatchArgs;
    Saved->FirstArg = Info.FirstArg;
    Saved->SecondArg = Info.SecondArg;
    Saved->TemplateArgs = Info.takeSugared();
    Saved->CallArgIndex = Info.CallArgIndex;
    Result.Data = Saved;
    break;
  }
 
  case TemplateDeductionResult::NonDeducedMismatch: {
    // FIXME: Should allocate from normal heap so that we can free this later.
    DFIArguments *Saved = new (Context) DFIArguments;
    Saved->FirstArg = Info.FirstArg;
    Saved->SecondArg = Info.SecondArg;
    Result.Data = Saved;
    break;
  }
 
  case TemplateDeductionResult::IncompletePack:
    // FIXME: It's slightly wasteful to allocate two TemplateArguments for this.
  case TemplateDeductionResult::Inconsistent:
  case TemplateDeductionResult::Underqualified: {
    // FIXME: Should allocate from normal heap so that we can free this later.
    DFIParamWithArguments *Saved = new (Context) DFIParamWithArguments;
    Saved->Param = Info.Param;
    Saved->FirstArg = Info.FirstArg;
    Saved->SecondArg = Info.SecondArg;
    Result.Data = Saved;
    break;
  }
 
  case TemplateDeductionResult::SubstitutionFailure:
    Result.Data = Info.takeSugared();
    if (Info.hasSFINAEDiagnostic()) {
      PartialDiagnosticAt *Diag = new (Result.Diagnostic) PartialDiagnosticAt(
          SourceLocation(), PartialDiagnostic::NullDiagnostic());
      Info.takeSFINAEDiagnostic(*Diag);
      Result.HasDiagnostic = true;
    }
    break;
 
  case TemplateDeductionResult::ConstraintsNotSatisfied: {
    CNSInfo *Saved = new (Context) CNSInfo;
    Saved->TemplateArgs = Info.takeSugared();
    Saved->Satisfaction = Info.AssociatedConstraintsSatisfaction;
    Result.Data = Saved;
    break;
  }
 
  case TemplateDeductionResult::Success:
  case TemplateDeductionResult::NonDependentConversionFailure:
  case TemplateDeductionResult::AlreadyDiagnosed:
    llvm_unreachable("not a deduction failure");
  }
 
  return Result;
}
 
void DeductionFailureInfo::Destroy() {
  switch (static_cast<TemplateDeductionResult>(Result)) {
  case TemplateDeductionResult::Success:
  case TemplateDeductionResult::Invalid:
  case TemplateDeductionResult::InstantiationDepth:
  case TemplateDeductionResult::Incomplete:
  case TemplateDeductionResult::TooManyArguments:
  case TemplateDeductionResult::TooFewArguments:
  case TemplateDeductionResult::InvalidExplicitArguments:
  case TemplateDeductionResult::CUDATargetMismatch:
  case TemplateDeductionResult::NonDependentConversionFailure:
    break;
 
  case TemplateDeductionResult::IncompletePack:
  case TemplateDeductionResult::Inconsistent:
  case TemplateDeductionResult::Underqualified:
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested:
  case TemplateDeductionResult::NonDeducedMismatch:
    // FIXME: Destroy the data?
    Data = nullptr;
    break;
 
  case TemplateDeductionResult::SubstitutionFailure:
    // FIXME: Destroy the template argument list?
    Data = nullptr;
    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
      Diag->~PartialDiagnosticAt();
      HasDiagnostic = false;
    }
    break;
 
  case TemplateDeductionResult::ConstraintsNotSatisfied:
    // FIXME: Destroy the template argument list?
    Data = nullptr;
    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
      Diag->~PartialDiagnosticAt();
      HasDiagnostic = false;
    }
    break;
 
  // Unhandled
  case TemplateDeductionResult::MiscellaneousDeductionFailure:
  case TemplateDeductionResult::AlreadyDiagnosed:
    break;
  }
}
 
PartialDiagnosticAt *DeductionFailureInfo::getSFINAEDiagnostic() {
  if (HasDiagnostic)
    return static_cast<PartialDiagnosticAt*>(static_cast<void*>(Diagnostic));
  return nullptr;
}
 
TemplateParameter DeductionFailureInfo::getTemplateParameter() {
  switch (static_cast<TemplateDeductionResult>(Result)) {
  case TemplateDeductionResult::Success:
  case TemplateDeductionResult::Invalid:
  case TemplateDeductionResult::InstantiationDepth:
  case TemplateDeductionResult::TooManyArguments:
  case TemplateDeductionResult::TooFewArguments:
  case TemplateDeductionResult::SubstitutionFailure:
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested:
  case TemplateDeductionResult::NonDeducedMismatch:
  case TemplateDeductionResult::CUDATargetMismatch:
  case TemplateDeductionResult::NonDependentConversionFailure:
  case TemplateDeductionResult::ConstraintsNotSatisfied:
    return TemplateParameter();
 
  case TemplateDeductionResult::Incomplete:
  case TemplateDeductionResult::InvalidExplicitArguments:
    return TemplateParameter::getFromOpaqueValue(Data);
 
  case TemplateDeductionResult::IncompletePack:
  case TemplateDeductionResult::Inconsistent:
  case TemplateDeductionResult::Underqualified:
    return static_cast<DFIParamWithArguments*>(Data)->Param;
 
  // Unhandled
  case TemplateDeductionResult::MiscellaneousDeductionFailure:
  case TemplateDeductionResult::AlreadyDiagnosed:
    break;
  }
 
  return TemplateParameter();
}
 
TemplateArgumentList *DeductionFailureInfo::getTemplateArgumentList() {
  switch (static_cast<TemplateDeductionResult>(Result)) {
  case TemplateDeductionResult::Success:
  case TemplateDeductionResult::Invalid:
  case TemplateDeductionResult::InstantiationDepth:
  case TemplateDeductionResult::TooManyArguments:
  case TemplateDeductionResult::TooFewArguments:
  case TemplateDeductionResult::Incomplete:
  case TemplateDeductionResult::IncompletePack:
  case TemplateDeductionResult::InvalidExplicitArguments:
  case TemplateDeductionResult::Inconsistent:
  case TemplateDeductionResult::Underqualified:
  case TemplateDeductionResult::NonDeducedMismatch:
  case TemplateDeductionResult::CUDATargetMismatch:
  case TemplateDeductionResult::NonDependentConversionFailure:
    return nullptr;
 
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested:
    return static_cast<DFIDeducedMismatchArgs*>(Data)->TemplateArgs;
 
  case TemplateDeductionResult::SubstitutionFailure:
    return static_cast<TemplateArgumentList*>(Data);
 
  case TemplateDeductionResult::ConstraintsNotSatisfied:
    return static_cast<CNSInfo*>(Data)->TemplateArgs;
 
  // Unhandled
  case TemplateDeductionResult::MiscellaneousDeductionFailure:
  case TemplateDeductionResult::AlreadyDiagnosed:
    break;
  }
 
  return nullptr;
}
 
const TemplateArgument *DeductionFailureInfo::getFirstArg() {
  switch (static_cast<TemplateDeductionResult>(Result)) {
  case TemplateDeductionResult::Success:
  case TemplateDeductionResult::Invalid:
  case TemplateDeductionResult::InstantiationDepth:
  case TemplateDeductionResult::Incomplete:
  case TemplateDeductionResult::TooManyArguments:
  case TemplateDeductionResult::TooFewArguments:
  case TemplateDeductionResult::InvalidExplicitArguments:
  case TemplateDeductionResult::SubstitutionFailure:
  case TemplateDeductionResult::CUDATargetMismatch:
  case TemplateDeductionResult::NonDependentConversionFailure:
  case TemplateDeductionResult::ConstraintsNotSatisfied:
    return nullptr;
 
  case TemplateDeductionResult::IncompletePack:
  case TemplateDeductionResult::Inconsistent:
  case TemplateDeductionResult::Underqualified:
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested:
  case TemplateDeductionResult::NonDeducedMismatch:
    return &static_cast<DFIArguments*>(Data)->FirstArg;
 
  // Unhandled
  case TemplateDeductionResult::MiscellaneousDeductionFailure:
  case TemplateDeductionResult::AlreadyDiagnosed:
    break;
  }
 
  return nullptr;
}
 
const TemplateArgument *DeductionFailureInfo::getSecondArg() {
  switch (static_cast<TemplateDeductionResult>(Result)) {
  case TemplateDeductionResult::Success:
  case TemplateDeductionResult::Invalid:
  case TemplateDeductionResult::InstantiationDepth:
  case TemplateDeductionResult::Incomplete:
  case TemplateDeductionResult::IncompletePack:
  case TemplateDeductionResult::TooManyArguments:
  case TemplateDeductionResult::TooFewArguments:
  case TemplateDeductionResult::InvalidExplicitArguments:
  case TemplateDeductionResult::SubstitutionFailure:
  case TemplateDeductionResult::CUDATargetMismatch:
  case TemplateDeductionResult::NonDependentConversionFailure:
  case TemplateDeductionResult::ConstraintsNotSatisfied:
    return nullptr;
 
  case TemplateDeductionResult::Inconsistent:
  case TemplateDeductionResult::Underqualified:
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested:
  case TemplateDeductionResult::NonDeducedMismatch:
    return &static_cast<DFIArguments*>(Data)->SecondArg;
 
  // Unhandled
  case TemplateDeductionResult::MiscellaneousDeductionFailure:
  case TemplateDeductionResult::AlreadyDiagnosed:
    break;
  }
 
  return nullptr;
}
 
std::optional<unsigned> DeductionFailureInfo::getCallArgIndex() {
  switch (static_cast<TemplateDeductionResult>(Result)) {
  case TemplateDeductionResult::DeducedMismatch:
  case TemplateDeductionResult::DeducedMismatchNested:
    return static_cast<DFIDeducedMismatchArgs*>(Data)->CallArgIndex;
 
  default:
    return std::nullopt;
  }
}
 
static bool FunctionsCorrespond(ASTContext &Ctx, const FunctionDecl *X,
                                const FunctionDecl *Y) {
  if (!X || !Y)
    return false;
  if (X->getNumParams() != Y->getNumParams())
    return false;
  // FIXME: when do rewritten comparison operators
  // with explicit object parameters correspond?
  // https://cplusplus.github.io/CWG/issues/2797.html
  for (unsigned I = 0; I < X->getNumParams(); ++I)
    if (!Ctx.hasSameUnqualifiedType(X->getParamDecl(I)->getType(),
                                    Y->getParamDecl(I)->getType()))
      return false;
  if (auto *FTX = X->getDescribedFunctionTemplate()) {
    auto *FTY = Y->getDescribedFunctionTemplate();
    if (!FTY)
      return false;
    if (!Ctx.isSameTemplateParameterList(FTX->getTemplateParameters(),
                                         FTY->getTemplateParameters()))
      return false;
  }
  return true;
}
 
static bool shouldAddReversedEqEq(Sema &S, SourceLocation OpLoc,
                                  Expr *FirstOperand, FunctionDecl *EqFD) {
  assert(EqFD->getOverloadedOperator() ==
         OverloadedOperatorKind::OO_EqualEqual);
  // C++2a [over.match.oper]p4:
  // A non-template function or function template F named operator== is a
  // rewrite target with first operand o unless a search for the name operator!=
  // in the scope S from the instantiation context of the operator expression
  // finds a function or function template that would correspond
  // ([basic.scope.scope]) to F if its name were operator==, where S is the
  // scope of the class type of o if F is a class member, and the namespace
  // scope of which F is a member otherwise. A function template specialization
  // named operator== is a rewrite target if its function template is a rewrite
  // target.
  DeclarationName NotEqOp = S.Context.DeclarationNames.getCXXOperatorName(
      OverloadedOperatorKind::OO_ExclaimEqual);
  if (isa<CXXMethodDecl>(EqFD)) {
    // If F is a class member, search scope is class type of first operand.
    QualType RHS = FirstOperand->getType();
    auto *RHSRec = RHS->getAs<RecordType>();
    if (!RHSRec)
      return true;
    LookupResult Members(S, NotEqOp, OpLoc,
                         Sema::LookupNameKind::LookupMemberName);
    S.LookupQualifiedName(Members, RHSRec->getDecl());
    Members.suppressAccessDiagnostics();
    for (NamedDecl *Op : Members)
      if (FunctionsCorrespond(S.Context, EqFD, Op->getAsFunction()))
        return false;
    return true;
  }
  // Otherwise the search scope is the namespace scope of which F is a member.
  for (NamedDecl *Op : EqFD->getEnclosingNamespaceContext()->lookup(NotEqOp)) {
    auto *NotEqFD = Op->getAsFunction();
    if (auto *UD = dyn_cast<UsingShadowDecl>(Op))
      NotEqFD = UD->getUnderlyingDecl()->getAsFunction();
    if (FunctionsCorrespond(S.Context, EqFD, NotEqFD) && S.isVisible(NotEqFD) &&
        declaresSameEntity(cast<Decl>(EqFD->getEnclosingNamespaceContext()),
                           cast<Decl>(Op->getLexicalDeclContext())))
      return false;
  }
  return true;
}
 
bool OverloadCandidateSet::OperatorRewriteInfo::allowsReversed(
    OverloadedOperatorKind Op) {
  if (!AllowRewrittenCandidates)
    return false;
  return Op == OO_EqualEqual || Op == OO_Spaceship;
}
 
bool OverloadCandidateSet::OperatorRewriteInfo::shouldAddReversed(
    Sema &S, ArrayRef<Expr *> OriginalArgs, FunctionDecl *FD) {
  auto Op = FD->getOverloadedOperator();
  if (!allowsReversed(Op))
    return false;
  if (Op == OverloadedOperatorKind::OO_EqualEqual) {
    assert(OriginalArgs.size() == 2);
    if (!shouldAddReversedEqEq(
            S, OpLoc, /*FirstOperand in reversed args*/ OriginalArgs[1], FD))
      return false;
  }
  // Don't bother adding a reversed candidate that can never be a better
  // match than the non-reversed version.
  return FD->getNumNonObjectParams() != 2 ||
         !S.Context.hasSameUnqualifiedType(FD->getParamDecl(0)->getType(),
                                           FD->getParamDecl(1)->getType()) ||
         FD->hasAttr<EnableIfAttr>();
}
 
void OverloadCandidateSet::destroyCandidates() {
  for (iterator i = begin(), e = end(); i != e; ++i) {
    for (auto &C : i->Conversions)
      C.~ImplicitConversionSequence();
    if (!i->Viable && i->FailureKind == ovl_fail_bad_deduction)
      i->DeductionFailure.Destroy();
  }
}
 
void OverloadCandidateSet::clear(CandidateSetKind CSK) {
  destroyCandidates();
  SlabAllocator.Reset();
  NumInlineBytesUsed = 0;
  Candidates.clear();
  Functions.clear();
  Kind = CSK;
}
 
namespace {
  class UnbridgedCastsSet {
    struct Entry {
      Expr **Addr;
      Expr *Saved;
    };
    SmallVector<Entry, 2> Entries;
 
  public:
    void save(Sema &S, Expr *&E) {
      assert(E->hasPlaceholderType(BuiltinType::ARCUnbridgedCast));
      Entry entry = { &E, E };
      Entries.push_back(entry);
      E = S.ObjC().stripARCUnbridgedCast(E);
    }
 
    void restore() {
      for (SmallVectorImpl<Entry>::iterator
             i = Entries.begin(), e = Entries.end(); i != e; ++i)
        *i->Addr = i->Saved;
    }
  };
}
 
/// checkPlaceholderForOverload - Do any interesting placeholder-like
/// preprocessing on the given expression.
///
/// \param unbridgedCasts a collection to which to add unbridged casts;
///   without this, they will be immediately diagnosed as errors
///
/// Return true on unrecoverable error.
static bool
checkPlaceholderForOverload(Sema &S, Expr *&E,
                            UnbridgedCastsSet *unbridgedCasts = nullptr) {
  if (const BuiltinType *placeholder =  E->getType()->getAsPlaceholderType()) {
    // We can't handle overloaded expressions here because overload
    // resolution might reasonably tweak them.
    if (placeholder->getKind() == BuiltinType::Overload) return false;
 
    // If the context potentially accepts unbridged ARC casts, strip
    // the unbridged cast and add it to the collection for later restoration.
    if (placeholder->getKind() == BuiltinType::ARCUnbridgedCast &&
        unbridgedCasts) {
      unbridgedCasts->save(S, E);
      return false;
    }
 
    // Go ahead and check everything else.
    ExprResult result = S.CheckPlaceholderExpr(E);
    if (result.isInvalid())
      return true;
 
    E = result.get();
    return false;
  }
 
  // Nothing to do.
  return false;
}
 
/// checkArgPlaceholdersForOverload - Check a set of call operands for
/// placeholders.
static bool checkArgPlaceholdersForOverload(Sema &S, MultiExprArg Args,
                                            UnbridgedCastsSet &unbridged) {
  for (unsigned i = 0, e = Args.size(); i != e; ++i)
    if (checkPlaceholderForOverload(S, Args[i], &unbridged))
      return true;
 
  return false;
}
 
Sema::OverloadKind
Sema::CheckOverload(Scope *S, FunctionDecl *New, const LookupResult &Old,
                    NamedDecl *&Match, bool NewIsUsingDecl) {
  for (LookupResult::iterator I = Old.begin(), E = Old.end();
         I != E; ++I) {
    NamedDecl *OldD = *I;
 
    bool OldIsUsingDecl = false;
    if (isa<UsingShadowDecl>(OldD)) {
      OldIsUsingDecl = true;
 
      // We can always introduce two using declarations into the same
      // context, even if they have identical signatures.
      if (NewIsUsingDecl) continue;
 
      OldD = cast<UsingShadowDecl>(OldD)->getTargetDecl();
    }
 
    // A using-declaration does not conflict with another declaration
    // if one of them is hidden.
    if ((OldIsUsingDecl || NewIsUsingDecl) && !isVisible(*I))
      continue;
 
    // If either declaration was introduced by a using declaration,
    // we'll need to use slightly different rules for matching.
    // Essentially, these rules are the normal rules, except that
    // function templates hide function templates with different
    // return types or template parameter lists.
    bool UseMemberUsingDeclRules =
      (OldIsUsingDecl || NewIsUsingDecl) && CurContext->isRecord() &&
      !New->getFriendObjectKind();
 
    if (FunctionDecl *OldF = OldD->getAsFunction()) {
      if (!IsOverload(New, OldF, UseMemberUsingDeclRules)) {
        if (UseMemberUsingDeclRules && OldIsUsingDecl) {
          HideUsingShadowDecl(S, cast<UsingShadowDecl>(*I));
          continue;
        }
 
        if (!isa<FunctionTemplateDecl>(OldD) &&
            !shouldLinkPossiblyHiddenDecl(*I, New))
          continue;
 
        Match = *I;
        return Ovl_Match;
      }
 
      // Builtins that have custom typechecking or have a reference should
      // not be overloadable or redeclarable.
      if (!getASTContext().canBuiltinBeRedeclared(OldF)) {
        Match = *I;
        return Ovl_NonFunction;
      }
    } else if (isa<UsingDecl>(OldD) || isa<UsingPackDecl>(OldD)) {
      // We can overload with these, which can show up when doing
      // redeclaration checks for UsingDecls.
      assert(Old.getLookupKind() == LookupUsingDeclName);
    } else if (isa<TagDecl>(OldD)) {
      // We can always overload with tags by hiding them.
    } else if (auto *UUD = dyn_cast<UnresolvedUsingValueDecl>(OldD)) {
      // Optimistically assume that an unresolved using decl will
      // overload; if it doesn't, we'll have to diagnose during
      // template instantiation.
      //
      // Exception: if the scope is dependent and this is not a class
      // member, the using declaration can only introduce an enumerator.
      if (UUD->getQualifier()->isDependent() && !UUD->isCXXClassMember()) {
        Match = *I;
        return Ovl_NonFunction;
      }
    } else {
      // (C++ 13p1):
      //   Only function declarations can be overloaded; object and type
      //   declarations cannot be overloaded.
      Match = *I;
      return Ovl_NonFunction;
    }
  }
 
  // C++ [temp.friend]p1:
  //   For a friend function declaration that is not a template declaration:
  //    -- if the name of the friend is a qualified or unqualified template-id,
  //       [...], otherwise
  //    -- if the name of the friend is a qualified-id and a matching
  //       non-template function is found in the specified class or namespace,
  //       the friend declaration refers to that function, otherwise,
  //    -- if the name of the friend is a qualified-id and a matching function
  //       template is found in the specified class or namespace, the friend
  //       declaration refers to the deduced specialization of that function
  //       template, otherwise
  //    -- the name shall be an unqualified-id [...]
  // If we get here for a qualified friend declaration, we've just reached the
  // third bullet. If the type of the friend is dependent, skip this lookup
  // until instantiation.
  if (New->getFriendObjectKind() && New->getQualifier() &&
      !New->getDescribedFunctionTemplate() &&
      !New->getDependentSpecializationInfo() &&
      !New->getType()->isDependentType()) {
    LookupResult TemplateSpecResult(LookupResult::Temporary, Old);
    TemplateSpecResult.addAllDecls(Old);
    if (CheckFunctionTemplateSpecialization(New, nullptr, TemplateSpecResult,
                                            /*QualifiedFriend*/true)) {
      New->setInvalidDecl();
      return Ovl_Overload;
    }
 
    Match = TemplateSpecResult.getAsSingle<FunctionDecl>();
    return Ovl_Match;
  }
 
  return Ovl_Overload;
}
 
static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
                                     FunctionDecl *Old,
                                     bool UseMemberUsingDeclRules,
                                     bool ConsiderCudaAttrs,
                                     bool UseOverrideRules = false) {
  // C++ [basic.start.main]p2: This function shall not be overloaded.
  if (New->isMain())
    return false;
 
  // MSVCRT user defined entry points cannot be overloaded.
  if (New->isMSVCRTEntryPoint())
    return false;
 
  NamedDecl *OldDecl = Old;
  NamedDecl *NewDecl = New;
  FunctionTemplateDecl *OldTemplate = Old->getDescribedFunctionTemplate();
  FunctionTemplateDecl *NewTemplate = New->getDescribedFunctionTemplate();
 
  // C++ [temp.fct]p2:
  //   A function template can be overloaded with other function templates
  //   and with normal (non-template) functions.
  if ((OldTemplate == nullptr) != (NewTemplate == nullptr))
    return true;
 
  // Is the function New an overload of the function Old?
  QualType OldQType = SemaRef.Context.getCanonicalType(Old->getType());
  QualType NewQType = SemaRef.Context.getCanonicalType(New->getType());
 
  // Compare the signatures (C++ 1.3.10) of the two functions to
  // determine whether they are overloads. If we find any mismatch
  // in the signature, they are overloads.
 
  // If either of these functions is a K&R-style function (no
  // prototype), then we consider them to have matching signatures.
  if (isa<FunctionNoProtoType>(OldQType.getTypePtr()) ||
      isa<FunctionNoProtoType>(NewQType.getTypePtr()))
    return false;
 
  const auto *OldType = cast<FunctionProtoType>(OldQType);
  const auto *NewType = cast<FunctionProtoType>(NewQType);
 
  // The signature of a function includes the types of its
  // parameters (C++ 1.3.10), which includes the presence or absence
  // of the ellipsis; see C++ DR 357).
  if (OldQType != NewQType && OldType->isVariadic() != NewType->isVariadic())
    return true;
 
  // For member-like friends, the enclosing class is part of the signature.
  if ((New->isMemberLikeConstrainedFriend() ||
       Old->isMemberLikeConstrainedFriend()) &&
      !New->getLexicalDeclContext()->Equals(Old->getLexicalDeclContext()))
    return true;
 
  // Compare the parameter lists.
  // This can only be done once we have establish that friend functions
  // inhabit the same context, otherwise we might tried to instantiate
  // references to non-instantiated entities during constraint substitution.
  // GH78101.
  if (NewTemplate) {
    OldDecl = OldTemplate;
    NewDecl = NewTemplate;
    // C++ [temp.over.link]p4:
    //   The signature of a function template consists of its function
    //   signature, its return type and its template parameter list. The names
    //   of the template parameters are significant only for establishing the
    //   relationship between the template parameters and the rest of the
    //   signature.
    //
    // We check the return type and template parameter lists for function
    // templates first; the remaining checks follow.
    bool SameTemplateParameterList = SemaRef.TemplateParameterListsAreEqual(
        NewTemplate, NewTemplate->getTemplateParameters(), OldTemplate,
        OldTemplate->getTemplateParameters(), false, Sema::TPL_TemplateMatch);
    bool SameReturnType = SemaRef.Context.hasSameType(
        Old->getDeclaredReturnType(), New->getDeclaredReturnType());
    // FIXME(GH58571): Match template parameter list even for non-constrained
    // template heads. This currently ensures that the code prior to C++20 is
    // not newly broken.
    bool ConstraintsInTemplateHead =
        NewTemplate->getTemplateParameters()->hasAssociatedConstraints() ||
        OldTemplate->getTemplateParameters()->hasAssociatedConstraints();
    // C++ [namespace.udecl]p11:
    //   The set of declarations named by a using-declarator that inhabits a
    //   class C does not include member functions and member function
    //   templates of a base class that "correspond" to (and thus would
    //   conflict with) a declaration of a function or function template in
    //   C.
    // Comparing return types is not required for the "correspond" check to
    // decide whether a member introduced by a shadow declaration is hidden.
    if (UseMemberUsingDeclRules && ConstraintsInTemplateHead &&
        !SameTemplateParameterList)
      return true;
    if (!UseMemberUsingDeclRules &&
        (!SameTemplateParameterList || !SameReturnType))
      return true;
  }
 
  const auto *OldMethod = dyn_cast<CXXMethodDecl>(Old);
  const auto *NewMethod = dyn_cast<CXXMethodDecl>(New);
 
  int OldParamsOffset = 0;
  int NewParamsOffset = 0;
 
  // When determining if a method is an overload from a base class, act as if
  // the implicit object parameter are of the same type.
 
  auto NormalizeQualifiers = [&](const CXXMethodDecl *M, Qualifiers Q) {
    if (M->isExplicitObjectMemberFunction()) {
      auto ThisType = M->getFunctionObjectParameterReferenceType();
      if (ThisType.isConstQualified())
        Q.removeConst();
      return Q;
    }
 
    // We do not allow overloading based off of '__restrict'.
    Q.removeRestrict();
 
    // We may not have applied the implicit const for a constexpr member
    // function yet (because we haven't yet resolved whether this is a static
    // or non-static member function). Add it now, on the assumption that this
    // is a redeclaration of OldMethod.
    if (!SemaRef.getLangOpts().CPlusPlus14 &&
        (M->isConstexpr() || M->isConsteval()) &&
        !isa<CXXConstructorDecl>(NewMethod))
      Q.addConst();
    return Q;
  };
 
  auto AreQualifiersEqual = [&](SplitQualType BS, SplitQualType DS) {
    BS.Quals = NormalizeQualifiers(OldMethod, BS.Quals);
    DS.Quals = NormalizeQualifiers(NewMethod, DS.Quals);
 
    if (OldMethod->isExplicitObjectMemberFunction()) {
      BS.Quals.removeVolatile();
      DS.Quals.removeVolatile();
    }
 
    return BS.Quals == DS.Quals;
  };
 
  auto CompareType = [&](QualType Base, QualType D) {
    auto BS = Base.getNonReferenceType().getCanonicalType().split();
    auto DS = D.getNonReferenceType().getCanonicalType().split();
 
    if (!AreQualifiersEqual(BS, DS))
      return false;
 
    if (OldMethod->isImplicitObjectMemberFunction() &&
        OldMethod->getParent() != NewMethod->getParent()) {
      QualType ParentType =
          SemaRef.Context.getTypeDeclType(OldMethod->getParent())
              .getCanonicalType();
      if (ParentType.getTypePtr() != BS.Ty)
        return false;
      BS.Ty = DS.Ty;
    }
 
    // FIXME: should we ignore some type attributes here?
    if (BS.Ty != DS.Ty)
      return false;
 
    if (Base->isLValueReferenceType())
      return D->isLValueReferenceType();
    return Base->isRValueReferenceType() == D->isRValueReferenceType();
  };
 
  // If the function is a class member, its signature includes the
  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
  auto DiagnoseInconsistentRefQualifiers = [&]() {
    if (SemaRef.LangOpts.CPlusPlus23)
      return false;
    if (OldMethod->getRefQualifier() == NewMethod->getRefQualifier())
      return false;
    if (OldMethod->isExplicitObjectMemberFunction() ||
        NewMethod->isExplicitObjectMemberFunction())
      return false;
    if (!UseMemberUsingDeclRules && (OldMethod->getRefQualifier() == RQ_None ||
                                     NewMethod->getRefQualifier() == RQ_None)) {
      SemaRef.Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
          << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
      SemaRef.Diag(OldMethod->getLocation(), diag::note_previous_declaration);
      return true;
    }
    return false;
  };
 
  if (OldMethod && OldMethod->isExplicitObjectMemberFunction())
    OldParamsOffset++;
  if (NewMethod && NewMethod->isExplicitObjectMemberFunction())
    NewParamsOffset++;
 
  if (OldType->getNumParams() - OldParamsOffset !=
          NewType->getNumParams() - NewParamsOffset ||
      !SemaRef.FunctionParamTypesAreEqual(
          {OldType->param_type_begin() + OldParamsOffset,
           OldType->param_type_end()},
          {NewType->param_type_begin() + NewParamsOffset,
           NewType->param_type_end()},
          nullptr)) {
    return true;
  }
 
  if (OldMethod && NewMethod && !OldMethod->isStatic() &&
      !NewMethod->isStatic()) {
    bool HaveCorrespondingObjectParameters = [&](const CXXMethodDecl *Old,
                                                 const CXXMethodDecl *New) {
      auto NewObjectType = New->getFunctionObjectParameterReferenceType();
      auto OldObjectType = Old->getFunctionObjectParameterReferenceType();
 
      auto IsImplicitWithNoRefQual = [](const CXXMethodDecl *F) {
        return F->getRefQualifier() == RQ_None &&
               !F->isExplicitObjectMemberFunction();
      };
 
      if (IsImplicitWithNoRefQual(Old) != IsImplicitWithNoRefQual(New) &&
          CompareType(OldObjectType.getNonReferenceType(),
                      NewObjectType.getNonReferenceType()))
        return true;
      return CompareType(OldObjectType, NewObjectType);
    }(OldMethod, NewMethod);
 
    if (!HaveCorrespondingObjectParameters) {
      if (DiagnoseInconsistentRefQualifiers())
        return true;
      // CWG2554
      // and, if at least one is an explicit object member function, ignoring
      // object parameters
      if (!UseOverrideRules || (!NewMethod->isExplicitObjectMemberFunction() &&
                                !OldMethod->isExplicitObjectMemberFunction()))
        return true;
    }
  }
 
  if (!UseOverrideRules &&
      New->getTemplateSpecializationKind() != TSK_ExplicitSpecialization) {
    Expr *NewRC = New->getTrailingRequiresClause(),
         *OldRC = Old->getTrailingRequiresClause();
    if ((NewRC != nullptr) != (OldRC != nullptr))
      return true;
    if (NewRC &&
        !SemaRef.AreConstraintExpressionsEqual(OldDecl, OldRC, NewDecl, NewRC))
      return true;
  }
 
  if (NewMethod && OldMethod && OldMethod->isImplicitObjectMemberFunction() &&
      NewMethod->isImplicitObjectMemberFunction()) {
    if (DiagnoseInconsistentRefQualifiers())
      return true;
  }
 
  // Though pass_object_size is placed on parameters and takes an argument, we
  // consider it to be a function-level modifier for the sake of function
  // identity. Either the function has one or more parameters with
  // pass_object_size or it doesn't.
  if (functionHasPassObjectSizeParams(New) !=
      functionHasPassObjectSizeParams(Old))
    return true;
 
  // enable_if attributes are an order-sensitive part of the signature.
  for (specific_attr_iterator<EnableIfAttr>
         NewI = New->specific_attr_begin<EnableIfAttr>(),
         NewE = New->specific_attr_end<EnableIfAttr>(),
         OldI = Old->specific_attr_begin<EnableIfAttr>(),
         OldE = Old->specific_attr_end<EnableIfAttr>();
       NewI != NewE || OldI != OldE; ++NewI, ++OldI) {
    if (NewI == NewE || OldI == OldE)
      return true;
    llvm::FoldingSetNodeID NewID, OldID;
    NewI->getCond()->Profile(NewID, SemaRef.Context, true);
    OldI->getCond()->Profile(OldID, SemaRef.Context, true);
    if (NewID != OldID)
      return true;
  }
 
  if (SemaRef.getLangOpts().CUDA && ConsiderCudaAttrs) {
    // Don't allow overloading of destructors.  (In theory we could, but it
    // would be a giant change to clang.)
    if (!isa<CXXDestructorDecl>(New)) {
      CUDAFunctionTarget NewTarget = SemaRef.CUDA().IdentifyTarget(New),
                         OldTarget = SemaRef.CUDA().IdentifyTarget(Old);
      if (NewTarget != CUDAFunctionTarget::InvalidTarget) {
        assert((OldTarget != CUDAFunctionTarget::InvalidTarget) &&
               "Unexpected invalid target.");
 
        // Allow overloading of functions with same signature and different CUDA
        // target attributes.
        if (NewTarget != OldTarget)
          return true;
      }
    }
  }
 
  // The signatures match; this is not an overload.
  return false;
}
 
bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
                      bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs) {
  return IsOverloadOrOverrideImpl(*this, New, Old, UseMemberUsingDeclRules,
                                  ConsiderCudaAttrs);
}
 
bool Sema::IsOverride(FunctionDecl *MD, FunctionDecl *BaseMD,
                      bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs) {
  return IsOverloadOrOverrideImpl(*this, MD, BaseMD,
                                  /*UseMemberUsingDeclRules=*/false,
                                  /*ConsiderCudaAttrs=*/true,
                                  /*UseOverrideRules=*/true);
}
 
/// Tries a user-defined conversion from From to ToType.
///
/// Produces an implicit conversion sequence for when a standard conversion
/// is not an option. See TryImplicitConversion for more information.
static ImplicitConversionSequence
TryUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
                         bool SuppressUserConversions,
                         AllowedExplicit AllowExplicit,
                         bool InOverloadResolution,
                         bool CStyle,
                         bool AllowObjCWritebackConversion,
                         bool AllowObjCConversionOnExplicit) {
  ImplicitConversionSequence ICS;
 
  if (SuppressUserConversions) {
    // We're not in the case above, so there is no conversion that
    // we can perform.
    ICS.setBad(BadConversionSequence::no_conversion, From, ToType);
    return ICS;
  }
 
  // Attempt user-defined conversion.
  OverloadCandidateSet Conversions(From->getExprLoc(),
                                   OverloadCandidateSet::CSK_Normal);
  switch (IsUserDefinedConversion(S, From, ToType, ICS.UserDefined,
                                  Conversions, AllowExplicit,
                                  AllowObjCConversionOnExplicit)) {
  case OR_Success:
  case OR_Deleted:
    ICS.setUserDefined();
    // C++ [over.ics.user]p4:
    //   A conversion of an expression of class type to the same class
    //   type is given Exact Match rank, and a conversion of an
    //   expression of class type to a base class of that type is
    //   given Conversion rank, in spite of the fact that a copy
    //   constructor (i.e., a user-defined conversion function) is
    //   called for those cases.
    if (CXXConstructorDecl *Constructor
          = dyn_cast<CXXConstructorDecl>(ICS.UserDefined.ConversionFunction)) {
      QualType FromType;
      SourceLocation FromLoc;
      // C++11 [over.ics.list]p6, per DR2137:
      // C++17 [over.ics.list]p6:
      //   If C is not an initializer-list constructor and the initializer list
      //   has a single element of type cv U, where U is X or a class derived
      //   from X, the implicit conversion sequence has Exact Match rank if U is
      //   X, or Conversion rank if U is derived from X.
      if (const auto *InitList = dyn_cast<InitListExpr>(From);
          InitList && InitList->getNumInits() == 1 &&
          !S.isInitListConstructor(Constructor)) {
        const Expr *SingleInit = InitList->getInit(0);
        FromType = SingleInit->getType();
        FromLoc = SingleInit->getBeginLoc();
      } else {
        FromType = From->getType();
        FromLoc = From->getBeginLoc();
      }
      QualType FromCanon =
          S.Context.getCanonicalType(FromType.getUnqualifiedType());
      QualType ToCanon
        = S.Context.getCanonicalType(ToType).getUnqualifiedType();
      if ((FromCanon == ToCanon ||
           S.IsDerivedFrom(FromLoc, FromCanon, ToCanon))) {
        // Turn this into a "standard" conversion sequence, so that it
        // gets ranked with standard conversion sequences.
        DeclAccessPair Found = ICS.UserDefined.FoundConversionFunction;
        ICS.setStandard();
        ICS.Standard.setAsIdentityConversion();
        ICS.Standard.setFromType(FromType);
        ICS.Standard.setAllToTypes(ToType);
        ICS.Standard.CopyConstructor = Constructor;
        ICS.Standard.FoundCopyConstructor = Found;
        if (ToCanon != FromCanon)
          ICS.Standard.Second = ICK_Derived_To_Base;
      }
    }
    break;
 
  case OR_Ambiguous:
    ICS.setAmbiguous();
    ICS.Ambiguous.setFromType(From->getType());
    ICS.Ambiguous.setToType(ToType);
    for (OverloadCandidateSet::iterator Cand = Conversions.begin();
         Cand != Conversions.end(); ++Cand)
      if (Cand->Best)
        ICS.Ambiguous.addConversion(Cand->FoundDecl, Cand->Function);
    break;
 
    // Fall through.
  case OR_No_Viable_Function:
    ICS.setBad(BadConversionSequence::no_conversion, From, ToType);
    break;
  }
 
  return ICS;
}
 
/// TryImplicitConversion - Attempt to perform an implicit conversion
/// from the given expression (Expr) to the given type (ToType). This
/// function returns an implicit conversion sequence that can be used
/// to perform the initialization. Given
///
///   void f(float f);
///   void g(int i) { f(i); }
///
/// this routine would produce an implicit conversion sequence to
/// describe the initialization of f from i, which will be a standard
/// conversion sequence containing an lvalue-to-rvalue conversion (C++
/// 4.1) followed by a floating-integral conversion (C++ 4.9).
//
/// Note that this routine only determines how the conversion can be
/// performed; it does not actually perform the conversion. As such,
/// it will not produce any diagnostics if no conversion is available,
/// but will instead return an implicit conversion sequence of kind
/// "BadConversion".
///
/// If @p SuppressUserConversions, then user-defined conversions are
/// not permitted.
/// If @p AllowExplicit, then explicit user-defined conversions are
/// permitted.
///
/// \param AllowObjCWritebackConversion Whether we allow the Objective-C
/// writeback conversion, which allows __autoreleasing id* parameters to
/// be initialized with __strong id* or __weak id* arguments.
static ImplicitConversionSequence
TryImplicitConversion(Sema &S, Expr *From, QualType ToType,
                      bool SuppressUserConversions,
                      AllowedExplicit AllowExplicit,
                      bool InOverloadResolution,
                      bool CStyle,
                      bool AllowObjCWritebackConversion,
                      bool AllowObjCConversionOnExplicit) {
  ImplicitConversionSequence ICS;
  if (IsStandardConversion(S, From, ToType, InOverloadResolution,
                           ICS.Standard, CStyle, AllowObjCWritebackConversion)){
    ICS.setStandard();
    return ICS;
  }
 
  if (!S.getLangOpts().CPlusPlus) {
    ICS.setBad(BadConversionSequence::no_conversion, From, ToType);
    return ICS;
  }
 
  // C++ [over.ics.user]p4:
  //   A conversion of an expression of class type to the same class
  //   type is given Exact Match rank, and a conversion of an
  //   expression of class type to a base class of that type is
  //   given Conversion rank, in spite of the fact that a copy/move
  //   constructor (i.e., a user-defined conversion function) is
  //   called for those cases.
  QualType FromType = From->getType();
  if (ToType->getAs<RecordType>() && FromType->getAs<RecordType>() &&
      (S.Context.hasSameUnqualifiedType(FromType, ToType) ||
       S.IsDerivedFrom(From->getBeginLoc(), FromType, ToType))) {
    ICS.setStandard();
    ICS.Standard.setAsIdentityConversion();
    ICS.Standard.setFromType(FromType);
    ICS.Standard.setAllToTypes(ToType);
 
    // We don't actually check at this point whether there is a valid
    // copy/move constructor, since overloading just assumes that it
    // exists. When we actually perform initialization, we'll find the
    // appropriate constructor to copy the returned object, if needed.
    ICS.Standard.CopyConstructor = nullptr;
 
    // Determine whether this is considered a derived-to-base conversion.
    if (!S.Context.hasSameUnqualifiedType(FromType, ToType))
      ICS.Standard.Second = ICK_Derived_To_Base;
 
    return ICS;
  }
 
  if (S.getLangOpts().HLSL && ToType->isHLSLAttributedResourceType() &&
      FromType->isHLSLAttributedResourceType()) {
    auto *ToResType = cast<HLSLAttributedResourceType>(ToType);
    auto *FromResType = cast<HLSLAttributedResourceType>(FromType);
    if (S.Context.hasSameUnqualifiedType(ToResType->getWrappedType(),
                                         FromResType->getWrappedType()) &&
        S.Context.hasSameUnqualifiedType(ToResType->getContainedType(),
                                         FromResType->getContainedType()) &&
        ToResType->getAttrs() == FromResType->getAttrs()) {
      ICS.setStandard();
      ICS.Standard.setAsIdentityConversion();
      ICS.Standard.setFromType(FromType);
      ICS.Standard.setAllToTypes(ToType);
      return ICS;
    }
  }
 
  return TryUserDefinedConversion(S, From, ToType, SuppressUserConversions,
                                  AllowExplicit, InOverloadResolution, CStyle,
                                  AllowObjCWritebackConversion,
                                  AllowObjCConversionOnExplicit);
}
 
ImplicitConversionSequence
Sema::TryImplicitConversion(Expr *From, QualType ToType,
                            bool SuppressUserConversions,
                            AllowedExplicit AllowExplicit,
                            bool InOverloadResolution,
                            bool CStyle,
                            bool AllowObjCWritebackConversion) {
  return ::TryImplicitConversion(*this, From, ToType, SuppressUserConversions,
                                 AllowExplicit, InOverloadResolution, CStyle,
                                 AllowObjCWritebackConversion,
                                 /*AllowObjCConversionOnExplicit=*/false);
}
 
ExprResult Sema::PerformImplicitConversion(Expr *From, QualType ToType,
                                           AssignmentAction Action,
                                           bool AllowExplicit) {
  if (checkPlaceholderForOverload(*this, From))
    return ExprError();
 
  // Objective-C ARC: Determine whether we will allow the writeback conversion.
  bool AllowObjCWritebackConversion =
      getLangOpts().ObjCAutoRefCount && (Action == AssignmentAction::Passing ||
                                         Action == AssignmentAction::Sending);
  if (getLangOpts().ObjC)
    ObjC().CheckObjCBridgeRelatedConversions(From->getBeginLoc(), ToType,
                                             From->getType(), From);
  ImplicitConversionSequence ICS = ::TryImplicitConversion(
      *this, From, ToType,
      /*SuppressUserConversions=*/false,
      AllowExplicit ? AllowedExplicit::All : AllowedExplicit::None,
      /*InOverloadResolution=*/false,
      /*CStyle=*/false, AllowObjCWritebackConversion,
      /*AllowObjCConversionOnExplicit=*/false);
  return PerformImplicitConversion(From, ToType, ICS, Action);
}
 
bool Sema::IsFunctionConversion(QualType FromType, QualType ToType,
                                QualType &ResultTy) {
  if (Context.hasSameUnqualifiedType(FromType, ToType))
    return false;
 
  // Permit the conversion F(t __attribute__((noreturn))) -> F(t)
  //                    or F(t noexcept) -> F(t)
  // where F adds one of the following at most once:
  //   - a pointer
  //   - a member pointer
  //   - a block pointer
  // Changes here need matching changes in FindCompositePointerType.
  CanQualType CanTo = Context.getCanonicalType(ToType);
  CanQualType CanFrom = Context.getCanonicalType(FromType);
  Type::TypeClass TyClass = CanTo->getTypeClass();
  if (TyClass != CanFrom->getTypeClass()) return false;
  if (TyClass != Type::FunctionProto && TyClass != Type::FunctionNoProto) {
    if (TyClass == Type::Pointer) {
      CanTo = CanTo.castAs<PointerType>()->getPointeeType();
      CanFrom = CanFrom.castAs<PointerType>()->getPointeeType();
    } else if (TyClass == Type::BlockPointer) {
      CanTo = CanTo.castAs<BlockPointerType>()->getPointeeType();
      CanFrom = CanFrom.castAs<BlockPointerType>()->getPointeeType();
    } else if (TyClass == Type::MemberPointer) {
      auto ToMPT = CanTo.castAs<MemberPointerType>();
      auto FromMPT = CanFrom.castAs<MemberPointerType>();
      // A function pointer conversion cannot change the class of the function.
      if (ToMPT->getClass() != FromMPT->getClass())
        return false;
      CanTo = ToMPT->getPointeeType();
      CanFrom = FromMPT->getPointeeType();
    } else {
      return false;
    }
 
    TyClass = CanTo->getTypeClass();
    if (TyClass != CanFrom->getTypeClass()) return false;
    if (TyClass != Type::FunctionProto && TyClass != Type::FunctionNoProto)
      return false;
  }
 
  const auto *FromFn = cast<FunctionType>(CanFrom);
  FunctionType::ExtInfo FromEInfo = FromFn->getExtInfo();
 
  const auto *ToFn = cast<FunctionType>(CanTo);
  FunctionType::ExtInfo ToEInfo = ToFn->getExtInfo();
 
  bool Changed = false;
 
  // Drop 'noreturn' if not present in target type.
  if (FromEInfo.getNoReturn() && !ToEInfo.getNoReturn()) {
    FromFn = Context.adjustFunctionType(FromFn, FromEInfo.withNoReturn(false));
    Changed = true;
  }
 
  // Drop 'noexcept' if not present in target type.
  if (const auto *FromFPT = dyn_cast<FunctionProtoType>(FromFn)) {
    const auto *ToFPT = cast<FunctionProtoType>(ToFn);
    if (FromFPT->isNothrow() && !ToFPT->isNothrow()) {
      FromFn = cast<FunctionType>(
          Context.getFunctionTypeWithExceptionSpec(QualType(FromFPT, 0),
                                                   EST_None)
                 .getTypePtr());
      Changed = true;
    }
 
    // Convert FromFPT's ExtParameterInfo if necessary. The conversion is valid
    // only if the ExtParameterInfo lists of the two function prototypes can be
    // merged and the merged list is identical to ToFPT's ExtParameterInfo list.
    SmallVector<FunctionProtoType::ExtParameterInfo, 4> NewParamInfos;
    bool CanUseToFPT, CanUseFromFPT;
    if (Context.mergeExtParameterInfo(ToFPT, FromFPT, CanUseToFPT,
                                      CanUseFromFPT, NewParamInfos) &&
        CanUseToFPT && !CanUseFromFPT) {
      FunctionProtoType::ExtProtoInfo ExtInfo = FromFPT->getExtProtoInfo();
      ExtInfo.ExtParameterInfos =
          NewParamInfos.empty() ? nullptr : NewParamInfos.data();
      QualType QT = Context.getFunctionType(FromFPT->getReturnType(),
                                            FromFPT->getParamTypes(), ExtInfo);
      FromFn = QT->getAs<FunctionType>();
      Changed = true;
    }
 
    // For C, when called from checkPointerTypesForAssignment,
    // we need to not alter FromFn, or else even an innocuous cast
    // like dropping effects will fail. In C++ however we do want to
    // alter FromFn (because of the way PerformImplicitConversion works).
    if (Context.hasAnyFunctionEffects() && getLangOpts().CPlusPlus) {
      FromFPT = cast<FunctionProtoType>(FromFn); // in case FromFn changed above
 
      // Transparently add/drop effects; here we are concerned with
      // language rules/canonicalization. Adding/dropping effects is a warning.
      const auto FromFX = FromFPT->getFunctionEffects();
      const auto ToFX = ToFPT->getFunctionEffects();
      if (FromFX != ToFX) {
        FunctionProtoType::ExtProtoInfo ExtInfo = FromFPT->getExtProtoInfo();
        ExtInfo.FunctionEffects = ToFX;
        QualType QT = Context.getFunctionType(
            FromFPT->getReturnType(), FromFPT->getParamTypes(), ExtInfo);
        FromFn = QT->getAs<FunctionType>();
        Changed = true;
      }
    }
  }
 
  if (!Changed)
    return false;
 
  assert(QualType(FromFn, 0).isCanonical());
  if (QualType(FromFn, 0) != CanTo) return false;
 
  ResultTy = ToType;
  return true;
}
 
/// Determine whether the conversion from FromType to ToType is a valid
/// floating point conversion.
///
static bool IsFloatingPointConversion(Sema &S, QualType FromType,
                                      QualType ToType) {
  if (!FromType->isRealFloatingType() || !ToType->isRealFloatingType())
    return false;
  // FIXME: disable conversions between long double, __ibm128 and __float128
  // if their representation is different until there is back end support
  // We of course allow this conversion if long double is really double.
 
  // Conversions between bfloat16 and float16 are currently not supported.
  if ((FromType->isBFloat16Type() &&
       (ToType->isFloat16Type() || ToType->isHalfType())) ||
      (ToType->isBFloat16Type() &&
       (FromType->isFloat16Type() || FromType->isHalfType())))
    return false;
 
  // Conversions between IEEE-quad and IBM-extended semantics are not
  // permitted.
  const llvm::fltSemantics &FromSem = S.Context.getFloatTypeSemantics(FromType);
  const llvm::fltSemantics &ToSem = S.Context.getFloatTypeSemantics(ToType);
  if ((&FromSem == &llvm::APFloat::PPCDoubleDouble() &&
       &ToSem == &llvm::APFloat::IEEEquad()) ||
      (&FromSem == &llvm::APFloat::IEEEquad() &&
       &ToSem == &llvm::APFloat::PPCDoubleDouble()))
    return false;
  return true;
}
 
static bool IsVectorElementConversion(Sema &S, QualType FromType,
                                      QualType ToType,
                                      ImplicitConversionKind &ICK, Expr *From) {
  if (S.Context.hasSameUnqualifiedType(FromType, ToType))
    return true;
 
  if (S.IsFloatingPointPromotion(FromType, ToType)) {
    ICK = ICK_Floating_Promotion;
    return true;
  }
 
  if (IsFloatingPointConversion(S, FromType, ToType)) {
    ICK = ICK_Floating_Conversion;
    return true;
  }
 
  if (ToType->isBooleanType() && FromType->isArithmeticType()) {
    ICK = ICK_Boolean_Conversion;
    return true;
  }
 
  if ((FromType->isRealFloatingType() && ToType->isIntegralType(S.Context)) ||
      (FromType->isIntegralOrUnscopedEnumerationType() &&
       ToType->isRealFloatingType())) {
    ICK = ICK_Floating_Integral;
    return true;
  }
 
  if (S.IsIntegralPromotion(From, FromType, ToType)) {
    ICK = ICK_Integral_Promotion;
    return true;
  }
 
  if (FromType->isIntegralOrUnscopedEnumerationType() &&
      ToType->isIntegralType(S.Context)) {
    ICK = ICK_Integral_Conversion;
    return true;
  }
 
  return false;
}
 
/// Determine whether the conversion from FromType to ToType is a valid
/// vector conversion.
///
/// \param ICK Will be set to the vector conversion kind, if this is a vector
/// conversion.
static bool IsVectorConversion(Sema &S, QualType FromType, QualType ToType,
                               ImplicitConversionKind &ICK,
                               ImplicitConversionKind &ElConv, Expr *From,
                               bool InOverloadResolution, bool CStyle) {
  // We need at least one of these types to be a vector type to have a vector
  // conversion.
  if (!ToType->isVectorType() && !FromType->isVectorType())
    return false;
 
  // Identical types require no conversions.
  if (S.Context.hasSameUnqualifiedType(FromType, ToType))
    return false;
 
  // HLSL allows implicit truncation of vector types.
  if (S.getLangOpts().HLSL) {
    auto *ToExtType = ToType->getAs<ExtVectorType>();
    auto *FromExtType = FromType->getAs<ExtVectorType>();
 
    // If both arguments are vectors, handle possible vector truncation and
    // element conversion.
    if (ToExtType && FromExtType) {
      unsigned FromElts = FromExtType->getNumElements();
      unsigned ToElts = ToExtType->getNumElements();
      if (FromElts < ToElts)
        return false;
      if (FromElts == ToElts)
        ElConv = ICK_Identity;
      else
        ElConv = ICK_HLSL_Vector_Truncation;
 
      QualType FromElTy = FromExtType->getElementType();
      QualType ToElTy = ToExtType->getElementType();
      if (S.Context.hasSameUnqualifiedType(FromElTy, ToElTy))
        return true;
      return IsVectorElementConversion(S, FromElTy, ToElTy, ICK, From);
    }
    if (FromExtType && !ToExtType) {
      ElConv = ICK_HLSL_Vector_Truncation;
      QualType FromElTy = FromExtType->getElementType();
      if (S.Context.hasSameUnqualifiedType(FromElTy, ToType))
        return true;
      return IsVectorElementConversion(S, FromElTy, ToType, ICK, From);
    }
    // Fallthrough for the case where ToType is a vector and FromType is not.
  }
 
  // There are no conversions between extended vector types, only identity.
  if (auto *ToExtType = ToType->getAs<ExtVectorType>()) {
    if (FromType->getAs<ExtVectorType>()) {
      // There are no conversions between extended vector types other than the
      // identity conversion.
      return false;
    }
 
    // Vector splat from any arithmetic type to a vector.
    if (FromType->isArithmeticType()) {
      if (S.getLangOpts().HLSL) {
        ElConv = ICK_HLSL_Vector_Splat;
        QualType ToElTy = ToExtType->getElementType();
        return IsVectorElementConversion(S, FromType, ToElTy, ICK, From);
      }
      ICK = ICK_Vector_Splat;
      return true;
    }
  }
 
  if (ToType->isSVESizelessBuiltinType() ||
      FromType->isSVESizelessBuiltinType())
    if (S.Context.areCompatibleSveTypes(FromType, ToType) ||
        S.Context.areLaxCompatibleSveTypes(FromType, ToType)) {
      ICK = ICK_SVE_Vector_Conversion;
      return true;
    }
 
  if (ToType->isRVVSizelessBuiltinType() ||
      FromType->isRVVSizelessBuiltinType())
    if (S.Context.areCompatibleRVVTypes(FromType, ToType) ||
        S.Context.areLaxCompatibleRVVTypes(FromType, ToType)) {
      ICK = ICK_RVV_Vector_Conversion;
      return true;
    }
 
  // We can perform the conversion between vector types in the following cases:
  // 1)vector types are equivalent AltiVec and GCC vector types
  // 2)lax vector conversions are permitted and the vector types are of the
  //   same size
  // 3)the destination type does not have the ARM MVE strict-polymorphism
  //   attribute, which inhibits lax vector conversion for overload resolution
  //   only
  if (ToType->isVectorType() && FromType->isVectorType()) {
    if (S.Context.areCompatibleVectorTypes(FromType, ToType) ||
        (S.isLaxVectorConversion(FromType, ToType) &&
         !ToType->hasAttr(attr::ArmMveStrictPolymorphism))) {
      if (S.getASTContext().getTargetInfo().getTriple().isPPC() &&
          S.isLaxVectorConversion(FromType, ToType) &&
          S.anyAltivecTypes(FromType, ToType) &&
          !S.Context.areCompatibleVectorTypes(FromType, ToType) &&
          !InOverloadResolution && !CStyle) {
        S.Diag(From->getBeginLoc(), diag::warn_deprecated_lax_vec_conv_all)
            << FromType << ToType;
      }
      ICK = ICK_Vector_Conversion;
      return true;
    }
  }
 
  return false;
}
 
static bool tryAtomicConversion(Sema &S, Expr *From, QualType ToType,
                                bool InOverloadResolution,
                                StandardConversionSequence &SCS,
                                bool CStyle);
 
/// IsStandardConversion - Determines whether there is a standard
/// conversion sequence (C++ [conv], C++ [over.ics.scs]) from the
/// expression From to the type ToType. Standard conversion sequences
/// only consider non-class types; for conversions that involve class
/// types, use TryImplicitConversion. If a conversion exists, SCS will
/// contain the standard conversion sequence required to perform this
/// conversion and this routine will return true. Otherwise, this
/// routine will return false and the value of SCS is unspecified.
static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
                                 bool InOverloadResolution,
                                 StandardConversionSequence &SCS,
                                 bool CStyle,
                                 bool AllowObjCWritebackConversion) {
  QualType FromType = From->getType();
 
  // Standard conversions (C++ [conv])
  SCS.setAsIdentityConversion();
  SCS.IncompatibleObjC = false;
  SCS.setFromType(FromType);
  SCS.CopyConstructor = nullptr;
 
  // There are no standard conversions for class types in C++, so
  // abort early. When overloading in C, however, we do permit them.
  if (S.getLangOpts().CPlusPlus &&
      (FromType->isRecordType() || ToType->isRecordType()))
    return false;
 
  // The first conversion can be an lvalue-to-rvalue conversion,
  // array-to-pointer conversion, or function-to-pointer conversion
  // (C++ 4p1).
 
  if (FromType == S.Context.OverloadTy) {
    DeclAccessPair AccessPair;
    if (FunctionDecl *Fn
          = S.ResolveAddressOfOverloadedFunction(From, ToType, false,
                                                 AccessPair)) {
      // We were able to resolve the address of the overloaded function,
      // so we can convert to the type of that function.
      FromType = Fn->getType();
      SCS.setFromType(FromType);
 
      // we can sometimes resolve &foo<int> regardless of ToType, so check
      // if the type matches (identity) or we are converting to bool
      if (!S.Context.hasSameUnqualifiedType(
                      S.ExtractUnqualifiedFunctionType(ToType), FromType)) {
        QualType resultTy;
        // if the function type matches except for [[noreturn]], it's ok
        if (!S.IsFunctionConversion(FromType,
              S.ExtractUnqualifiedFunctionType(ToType), resultTy))
          // otherwise, only a boolean conversion is standard
          if (!ToType->isBooleanType())
            return false;
      }
 
      // Check if the "from" expression is taking the address of an overloaded
      // function and recompute the FromType accordingly. Take advantage of the
      // fact that non-static member functions *must* have such an address-of
      // expression.
      CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Fn);
      if (Method && !Method->isStatic() &&
          !Method->isExplicitObjectMemberFunction()) {
        assert(isa<UnaryOperator>(From->IgnoreParens()) &&
               "Non-unary operator on non-static member address");
        assert(cast<UnaryOperator>(From->IgnoreParens())->getOpcode()
               == UO_AddrOf &&
               "Non-address-of operator on non-static member address");
        const Type *ClassType
          = S.Context.getTypeDeclType(Method->getParent()).getTypePtr();
        FromType = S.Context.getMemberPointerType(FromType, ClassType);
      } else if (isa<UnaryOperator>(From->IgnoreParens())) {
        assert(cast<UnaryOperator>(From->IgnoreParens())->getOpcode() ==
               UO_AddrOf &&
               "Non-address-of operator for overloaded function expression");
        FromType = S.Context.getPointerType(FromType);
      }
    } else {
      return false;
    }
  }
 
  bool argIsLValue = From->isGLValue();
  // To handle conversion from ArrayParameterType to ConstantArrayType
  // this block must be above the one below because Array parameters
  // do not decay and when handling HLSLOutArgExprs and
  // the From expression is an LValue.
  if (S.getLangOpts().HLSL && FromType->isConstantArrayType() &&
      ToType->isConstantArrayType()) {
    // HLSL constant array parameters do not decay, so if the argument is a
    // constant array and the parameter is an ArrayParameterType we have special
    // handling here.
    if (ToType->isArrayParameterType()) {
      FromType = S.Context.getArrayParameterType(FromType);
      SCS.First = ICK_HLSL_Array_RValue;
    } else if (FromType->isArrayParameterType()) {
      const ArrayParameterType *APT = cast<ArrayParameterType>(FromType);
      FromType = APT->getConstantArrayType(S.Context);
      SCS.First = ICK_HLSL_Array_RValue;
    } else {
      SCS.First = ICK_Identity;
    }
 
    if (S.Context.getCanonicalType(FromType) !=
        S.Context.getCanonicalType(ToType))
      return false;
 
    SCS.setAllToTypes(ToType);
    return true;
  } else if (argIsLValue && !FromType->canDecayToPointerType() &&
             S.Context.getCanonicalType(FromType) != S.Context.OverloadTy) {
    // Lvalue-to-rvalue conversion (C++11 4.1):
    //   A glvalue (3.10) of a non-function, non-array type T can
    //   be converted to a prvalue.
 
    SCS.First = ICK_Lvalue_To_Rvalue;
 
    // C11 6.3.2.1p2:
    //   ... if the lvalue has atomic type, the value has the non-atomic version
    //   of the type of the lvalue ...
    if (const AtomicType *Atomic = FromType->getAs<AtomicType>())
      FromType = Atomic->getValueType();
 
    // If T is a non-class type, the type of the rvalue is the
    // cv-unqualified version of T. Otherwise, the type of the rvalue
    // is T (C++ 4.1p1). C++ can't get here with class types; in C, we
    // just strip the qualifiers because they don't matter.
    FromType = FromType.getUnqualifiedType();
  } else if (FromType->isArrayType()) {
    // Array-to-pointer conversion (C++ 4.2)
    SCS.First = ICK_Array_To_Pointer;
 
    // An lvalue or rvalue of type "array of N T" or "array of unknown
    // bound of T" can be converted to an rvalue of type "pointer to
    // T" (C++ 4.2p1).
    FromType = S.Context.getArrayDecayedType(FromType);
 
    if (S.IsStringLiteralToNonConstPointerConversion(From, ToType)) {
      // This conversion is deprecated in C++03 (D.4)
      SCS.DeprecatedStringLiteralToCharPtr = true;
 
      // For the purpose of ranking in overload resolution
      // (13.3.3.1.1), this conversion is considered an
      // array-to-pointer conversion followed by a qualification
      // conversion (4.4). (C++ 4.2p2)
      SCS.Second = ICK_Identity;
      SCS.Third = ICK_Qualification;
      SCS.QualificationIncludesObjCLifetime = false;
      SCS.setAllToTypes(FromType);
      return true;
    }
  } else if (FromType->isFunctionType() && argIsLValue) {
    // Function-to-pointer conversion (C++ 4.3).
    SCS.First = ICK_Function_To_Pointer;
 
    if (auto *DRE = dyn_cast<DeclRefExpr>(From->IgnoreParenCasts()))
      if (auto *FD = dyn_cast<FunctionDecl>(DRE->getDecl()))
        if (!S.checkAddressOfFunctionIsAvailable(FD))
          return false;
 
    // An lvalue of function type T can be converted to an rvalue of
    // type "pointer to T." The result is a pointer to the
    // function. (C++ 4.3p1).
    FromType = S.Context.getPointerType(FromType);
  } else {
    // We don't require any conversions for the first step.
    SCS.First = ICK_Identity;
  }
  SCS.setToType(0, FromType);
 
  // 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 (C++ 4p1).
  // For overloading in C, this can also be a "compatible-type"
  // conversion.
  bool IncompatibleObjC = false;
  ImplicitConversionKind SecondICK = ICK_Identity;
  ImplicitConversionKind DimensionICK = ICK_Identity;
  if (S.Context.hasSameUnqualifiedType(FromType, ToType)) {
    // The unqualified versions of the types are the same: there's no
    // conversion to do.
    SCS.Second = ICK_Identity;
  } else if (S.IsIntegralPromotion(From, FromType, ToType)) {
    // Integral promotion (C++ 4.5).
    SCS.Second = ICK_Integral_Promotion;
    FromType = ToType.getUnqualifiedType();
  } else if (S.IsFloatingPointPromotion(FromType, ToType)) {
    // Floating point promotion (C++ 4.6).
    SCS.Second = ICK_Floating_Promotion;
    FromType = ToType.getUnqualifiedType();
  } else if (S.IsComplexPromotion(FromType, ToType)) {
    // Complex promotion (Clang extension)
    SCS.Second = ICK_Complex_Promotion;
    FromType = ToType.getUnqualifiedType();
  } else if (ToType->isBooleanType() &&
             (FromType->isArithmeticType() ||
              FromType->isAnyPointerType() ||
              FromType->isBlockPointerType() ||
              FromType->isMemberPointerType())) {
    // Boolean conversions (C++ 4.12).
    SCS.Second = ICK_Boolean_Conversion;
    FromType = S.Context.BoolTy;
  } else if (FromType->isIntegralOrUnscopedEnumerationType() &&
             ToType->isIntegralType(S.Context)) {
    // Integral conversions (C++ 4.7).
    SCS.Second = ICK_Integral_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if (FromType->isAnyComplexType() && ToType->isAnyComplexType()) {
    // Complex conversions (C99 6.3.1.6)
    SCS.Second = ICK_Complex_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if ((FromType->isAnyComplexType() && ToType->isArithmeticType()) ||
             (ToType->isAnyComplexType() && FromType->isArithmeticType())) {
    // Complex-real conversions (C99 6.3.1.7)
    SCS.Second = ICK_Complex_Real;
    FromType = ToType.getUnqualifiedType();
  } else if (IsFloatingPointConversion(S, FromType, ToType)) {
    // Floating point conversions (C++ 4.8).
    SCS.Second = ICK_Floating_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if ((FromType->isRealFloatingType() &&
              ToType->isIntegralType(S.Context)) ||
             (FromType->isIntegralOrUnscopedEnumerationType() &&
              ToType->isRealFloatingType())) {
 
    // Floating-integral conversions (C++ 4.9).
    SCS.Second = ICK_Floating_Integral;
    FromType = ToType.getUnqualifiedType();
  } else if (S.IsBlockPointerConversion(FromType, ToType, FromType)) {
    SCS.Second = ICK_Block_Pointer_Conversion;
  } else if (AllowObjCWritebackConversion &&
             S.ObjC().isObjCWritebackConversion(FromType, ToType, FromType)) {
    SCS.Second = ICK_Writeback_Conversion;
  } else if (S.IsPointerConversion(From, FromType, ToType, InOverloadResolution,
                                   FromType, IncompatibleObjC)) {
    // Pointer conversions (C++ 4.10).
    SCS.Second = ICK_Pointer_Conversion;
    SCS.IncompatibleObjC = IncompatibleObjC;
    FromType = FromType.getUnqualifiedType();
  } else if (S.IsMemberPointerConversion(From, FromType, ToType,
                                         InOverloadResolution, FromType)) {
    // Pointer to member conversions (4.11).
    SCS.Second = ICK_Pointer_Member;
  } else if (IsVectorConversion(S, FromType, ToType, SecondICK, DimensionICK,
                                From, InOverloadResolution, CStyle)) {
    SCS.Second = SecondICK;
    SCS.Dimension = DimensionICK;
    FromType = ToType.getUnqualifiedType();
  } else if (!S.getLangOpts().CPlusPlus &&
             S.Context.typesAreCompatible(ToType, FromType)) {
    // Compatible conversions (Clang extension for C function overloading)
    SCS.Second = ICK_Compatible_Conversion;
    FromType = ToType.getUnqualifiedType();
  } else if (IsTransparentUnionStandardConversion(
                 S, From, ToType, InOverloadResolution, SCS, CStyle)) {
    SCS.Second = ICK_TransparentUnionConversion;
    FromType = ToType;
  } else if (tryAtomicConversion(S, From, ToType, InOverloadResolution, SCS,
                                 CStyle)) {
    // tryAtomicConversion has updated the standard conversion sequence
    // appropriately.
    return true;
  } else if (ToType->isEventT() &&
             From->isIntegerConstantExpr(S.getASTContext()) &&
             From->EvaluateKnownConstInt(S.getASTContext()) == 0) {
    SCS.Second = ICK_Zero_Event_Conversion;
    FromType = ToType;
  } else if (ToType->isQueueT() &&
             From->isIntegerConstantExpr(S.getASTContext()) &&
             (From->EvaluateKnownConstInt(S.getASTContext()) == 0)) {
    SCS.Second = ICK_Zero_Queue_Conversion;
    FromType = ToType;
  } else if (ToType->isSamplerT() &&
             From->isIntegerConstantExpr(S.getASTContext())) {
    SCS.Second = ICK_Compatible_Conversion;
    FromType = ToType;
  } else if ((ToType->isFixedPointType() &&
              FromType->isConvertibleToFixedPointType()) ||
             (FromType->isFixedPointType() &&
              ToType->isConvertibleToFixedPointType())) {
    SCS.Second = ICK_Fixed_Point_Conversion;
    FromType = ToType;
  } else {
    // No second conversion required.
    SCS.Second = ICK_Identity;
  }
  SCS.setToType(1, FromType);
 
  // The third conversion can be a function pointer conversion or a
  // qualification conversion (C++ [conv.fctptr], [conv.qual]).
  bool ObjCLifetimeConversion;
  if (S.IsFunctionConversion(FromType, ToType, FromType)) {
    // Function pointer conversions (removing 'noexcept') including removal of
    // 'noreturn' (Clang extension).
    SCS.Third = ICK_Function_Conversion;
  } else if (S.IsQualificationConversion(FromType, ToType, CStyle,
                                         ObjCLifetimeConversion)) {
    SCS.Third = ICK_Qualification;
    SCS.QualificationIncludesObjCLifetime = ObjCLifetimeConversion;
    FromType = ToType;
  } else {
    // No conversion required
    SCS.Third = ICK_Identity;
  }
 
  // C++ [over.best.ics]p6:
  //   [...] Any difference in top-level cv-qualification is
  //   subsumed by the initialization itself and does not constitute
  //   a conversion. [...]
  QualType CanonFrom = S.Context.getCanonicalType(FromType);
  QualType CanonTo = S.Context.getCanonicalType(ToType);
  if (CanonFrom.getLocalUnqualifiedType()
                                     == CanonTo.getLocalUnqualifiedType() &&
      CanonFrom.getLocalQualifiers() != CanonTo.getLocalQualifiers()) {
    FromType = ToType;
    CanonFrom = CanonTo;
  }
 
  SCS.setToType(2, FromType);
 
  if (CanonFrom == CanonTo)
    return true;
 
  // If we have not converted the argument type to the parameter type,
  // this is a bad conversion sequence, unless we're resolving an overload in C.
  if (S.getLangOpts().CPlusPlus || !InOverloadResolution)
    return false;
 
  ExprResult ER = ExprResult{From};
  Sema::AssignConvertType Conv =
      S.CheckSingleAssignmentConstraints(ToType, ER,
                                         /*Diagnose=*/false,
                                         /*DiagnoseCFAudited=*/false,
                                         /*ConvertRHS=*/false);
  ImplicitConversionKind SecondConv;
  switch (Conv) {
  case Sema::Compatible:
    SecondConv = ICK_C_Only_Conversion;
    break;
  // For our purposes, discarding qualifiers is just as bad as using an
  // incompatible pointer. Note that an IncompatiblePointer conversion can drop
  // qualifiers, as well.
  case Sema::CompatiblePointerDiscardsQualifiers:
  case Sema::IncompatiblePointer:
  case Sema::IncompatiblePointerSign:
    SecondConv = ICK_Incompatible_Pointer_Conversion;
    break;
  default:
    return false;
  }
 
  // First can only be an lvalue conversion, so we pretend that this was the
  // second conversion. First should already be valid from earlier in the
  // function.
  SCS.Second = SecondConv;
  SCS.setToType(1, ToType);
 
  // Third is Identity, because Second should rank us worse than any other
  // conversion. This could also be ICK_Qualification, but it's simpler to just
  // lump everything in with the second conversion, and we don't gain anything
  // from making this ICK_Qualification.
  SCS.Third = ICK_Identity;
  SCS.setToType(2, ToType);
  return true;
}
 
static bool
IsTransparentUnionStandardConversion(Sema &S, Expr* From,
                                     QualType &ToType,
                                     bool InOverloadResolution,
                                     StandardConversionSequence &SCS,
                                     bool CStyle) {
 
  const RecordType *UT = ToType->getAsUnionType();
  if (!UT || !UT->getDecl()->hasAttr<TransparentUnionAttr>())
    return false;
  // The field to initialize within the transparent union.
  RecordDecl *UD = UT->getDecl();
  // It's compatible if the expression matches any of the fields.
  for (const auto *it : UD->fields()) {
    if (IsStandardConversion(S, From, it->getType(), InOverloadResolution, SCS,
                             CStyle, /*AllowObjCWritebackConversion=*/false)) {
      ToType = it->getType();
      return true;
    }
  }
  return false;
}
 
bool Sema::IsIntegralPromotion(Expr *From, QualType FromType, QualType ToType) {
  const BuiltinType *To = ToType->getAs<BuiltinType>();
  // All integers are built-in.
  if (!To) {
    return false;
  }
 
  // An rvalue of type char, signed char, unsigned char, short int, or
  // unsigned short int can be converted to an rvalue of type int if
  // int can represent all the values of the source type; otherwise,
  // the source rvalue can be converted to an rvalue of type unsigned
  // int (C++ 4.5p1).
  if (Context.isPromotableIntegerType(FromType) && !FromType->isBooleanType() &&
      !FromType->isEnumeralType()) {
    if ( // We can promote any signed, promotable integer type to an int
        (FromType->isSignedIntegerType() ||
         // We can promote any unsigned integer type whose size is
         // less than int to an int.
         Context.getTypeSize(FromType) < Context.getTypeSize(ToType))) {
      return To->getKind() == BuiltinType::Int;
    }
 
    return To->getKind() == BuiltinType::UInt;
  }
 
  // C++11 [conv.prom]p3:
  //   A prvalue of an unscoped enumeration type whose underlying type is not
  //   fixed (7.2) can be converted to an rvalue a prvalue of the first of the
  //   following types that can represent all the values of the enumeration
  //   (i.e., the values in the range bmin to bmax as described in 7.2): int,
  //   unsigned int, long int, unsigned long int, long long int, or unsigned
  //   long long int. If none of the types in that list can represent all the
  //   values of the enumeration, an rvalue a prvalue of an unscoped enumeration
  //   type can be converted to an rvalue a prvalue of the extended integer type
  //   with lowest integer conversion rank (4.13) greater than the rank of long
  //   long in which all the values of the enumeration can be represented. If
  //   there are two such extended types, the signed one is chosen.
  // C++11 [conv.prom]p4:
  //   A prvalue of an unscoped enumeration type whose underlying type is fixed
  //   can be converted to a prvalue of its underlying type. Moreover, if
  //   integral promotion can be applied to its underlying type, a prvalue of an
  //   unscoped enumeration type whose underlying type is fixed can also be
  //   converted to a prvalue of the promoted underlying type.
  if (const EnumType *FromEnumType = FromType->getAs<EnumType>()) {
    // C++0x 7.2p9: Note that this implicit enum to int conversion is not
    // provided for a scoped enumeration.
    if (FromEnumType->getDecl()->isScoped())
      return false;
 
    // We can perform an integral promotion to the underlying type of the enum,
    // even if that's not the promoted type. Note that the check for promoting
    // the underlying type is based on the type alone, and does not consider
    // the bitfield-ness of the actual source expression.
    if (FromEnumType->getDecl()->isFixed()) {
      QualType Underlying = FromEnumType->getDecl()->getIntegerType();
      return Context.hasSameUnqualifiedType(Underlying, ToType) ||
             IsIntegralPromotion(nullptr, Underlying, ToType);
    }
 
    // We have already pre-calculated the promotion type, so this is trivial.
    if (ToType->isIntegerType() &&
        isCompleteType(From->getBeginLoc(), FromType))
      return Context.hasSameUnqualifiedType(
          ToType, FromEnumType->getDecl()->getPromotionType());
 
    // C++ [conv.prom]p5:
    //   If the bit-field has an enumerated type, it is treated as any other
    //   value of that type for promotion purposes.
    //
    // ... so do not fall through into the bit-field checks below in C++.
    if (getLangOpts().CPlusPlus)
      return false;
  }
 
  // C++0x [conv.prom]p2:
  //   A prvalue of type char16_t, char32_t, or wchar_t (3.9.1) can be converted
  //   to an rvalue a prvalue of the first of the following types that can
  //   represent all the values of its underlying type: int, unsigned int,
  //   long int, unsigned long int, long long int, or unsigned long long int.
  //   If none of the types in that list can represent all the values of its
  //   underlying type, an rvalue a prvalue of type char16_t, char32_t,
  //   or wchar_t can be converted to an rvalue a prvalue of its underlying
  //   type.
  if (FromType->isAnyCharacterType() && !FromType->isCharType() &&
      ToType->isIntegerType()) {
    // Determine whether the type we're converting from is signed or
    // unsigned.
    bool FromIsSigned = FromType->isSignedIntegerType();
    uint64_t FromSize = Context.getTypeSize(FromType);
 
    // The types we'll try to promote to, in the appropriate
    // order. Try each of these types.
    QualType PromoteTypes[6] = {
      Context.IntTy, Context.UnsignedIntTy,
      Context.LongTy, Context.UnsignedLongTy ,
      Context.LongLongTy, Context.UnsignedLongLongTy
    };
    for (int Idx = 0; Idx < 6; ++Idx) {
      uint64_t ToSize = Context.getTypeSize(PromoteTypes[Idx]);
      if (FromSize < ToSize ||
          (FromSize == ToSize &&
           FromIsSigned == PromoteTypes[Idx]->isSignedIntegerType())) {
        // We found the type that we can promote to. If this is the
        // type we wanted, we have a promotion. Otherwise, no
        // promotion.
        return Context.hasSameUnqualifiedType(ToType, PromoteTypes[Idx]);
      }
    }
  }
 
  // An rvalue for an integral bit-field (9.6) can be converted to an
  // rvalue of type int if int can represent all the values of the
  // bit-field; otherwise, it can be converted to unsigned int if
  // unsigned int can represent all the values of the bit-field. If
  // the bit-field is larger yet, no integral promotion applies to
  // it. If the bit-field has an enumerated type, it is treated as any
  // other value of that type for promotion purposes (C++ 4.5p3).
  // FIXME: We should delay checking of bit-fields until we actually perform the
  // conversion.
  //
  // FIXME: In C, only bit-fields of types _Bool, int, or unsigned int may be
  // promoted, per C11 6.3.1.1/2. We promote all bit-fields (including enum
  // bit-fields and those whose underlying type is larger than int) for GCC
  // compatibility.
  if (From) {
    if (FieldDecl *MemberDecl = From->getSourceBitField()) {
      std::optional<llvm::APSInt> BitWidth;
      if (FromType->isIntegralType(Context) &&
          (BitWidth =
               MemberDecl->getBitWidth()->getIntegerConstantExpr(Context))) {
        llvm::APSInt ToSize(BitWidth->getBitWidth(), BitWidth->isUnsigned());
        ToSize = Context.getTypeSize(ToType);
 
        // Are we promoting to an int from a bitfield that fits in an int?
        if (*BitWidth < ToSize ||
            (FromType->isSignedIntegerType() && *BitWidth <= ToSize)) {
          return To->getKind() == BuiltinType::Int;
        }
 
        // Are we promoting to an unsigned int from an unsigned bitfield
        // that fits into an unsigned int?
        if (FromType->isUnsignedIntegerType() && *BitWidth <= ToSize) {
          return To->getKind() == BuiltinType::UInt;
        }
 
        return false;
      }
    }
  }
 
  // An rvalue of type bool can be converted to an rvalue of type int,
  // with false becoming zero and true becoming one (C++ 4.5p4).
  if (FromType->isBooleanType() && To->getKind() == BuiltinType::Int) {
    return true;
  }
 
  // In HLSL an rvalue of integral type can be promoted to an rvalue of a larger
  // integral type.
  if (Context.getLangOpts().HLSL && FromType->isIntegerType() &&
      ToType->isIntegerType())
    return Context.getTypeSize(FromType) < Context.getTypeSize(ToType);
 
  return false;
}
 
bool Sema::IsFloatingPointPromotion(QualType FromType, QualType ToType) {
  if (const BuiltinType *FromBuiltin = FromType->getAs<BuiltinType>())
    if (const BuiltinType *ToBuiltin = ToType->getAs<BuiltinType>()) {
      /// An rvalue of type float can be converted to an rvalue of type
      /// double. (C++ 4.6p1).
      if (FromBuiltin->getKind() == BuiltinType::Float &&
          ToBuiltin->getKind() == BuiltinType::Double)
        return true;
 
      // C99 6.3.1.5p1:
      //   When a float is promoted to double or long double, or a
      //   double is promoted to long double [...].
      if (!getLangOpts().CPlusPlus &&
          (FromBuiltin->getKind() == BuiltinType::Float ||
           FromBuiltin->getKind() == BuiltinType::Double) &&
          (ToBuiltin->getKind() == BuiltinType::LongDouble ||
           ToBuiltin->getKind() == BuiltinType::Float128 ||
           ToBuiltin->getKind() == BuiltinType::Ibm128))
        return true;
 
      // In HLSL, `half` promotes to `float` or `double`, regardless of whether
      // or not native half types are enabled.
      if (getLangOpts().HLSL && FromBuiltin->getKind() == BuiltinType::Half &&
          (ToBuiltin->getKind() == BuiltinType::Float ||
           ToBuiltin->getKind() == BuiltinType::Double))
        return true;
 
      // Half can be promoted to float.
      if (!getLangOpts().NativeHalfType &&
           FromBuiltin->getKind() == BuiltinType::Half &&
          ToBuiltin->getKind() == BuiltinType::Float)
        return true;
    }
 
  return false;
}
 
bool Sema::IsComplexPromotion(QualType FromType, QualType ToType) {
  const ComplexType *FromComplex = FromType->getAs<ComplexType>();
  if (!FromComplex)
    return false;
 
  const ComplexType *ToComplex = ToType->getAs<ComplexType>();
  if (!ToComplex)
    return false;
 
  return IsFloatingPointPromotion(FromComplex->getElementType(),
                                  ToComplex->getElementType()) ||
    IsIntegralPromotion(nullptr, FromComplex->getElementType(),
                        ToComplex->getElementType());
}
 
/// BuildSimilarlyQualifiedPointerType - In a pointer conversion from
/// the pointer type FromPtr to a pointer to type ToPointee, with the
/// same type qualifiers as FromPtr has on its pointee type. ToType,
/// if non-empty, will be a pointer to ToType that may or may not have
/// the right set of qualifiers on its pointee.
///
static QualType
BuildSimilarlyQualifiedPointerType(const Type *FromPtr,
                                   QualType ToPointee, QualType ToType,
                                   ASTContext &Context,
                                   bool StripObjCLifetime = false) {
  assert((FromPtr->getTypeClass() == Type::Pointer ||
          FromPtr->getTypeClass() == Type::ObjCObjectPointer) &&
         "Invalid similarly-qualified pointer type");
 
  /// Conversions to 'id' subsume cv-qualifier conversions.
  if (ToType->isObjCIdType() || ToType->isObjCQualifiedIdType())
    return ToType.getUnqualifiedType();
 
  QualType CanonFromPointee
    = Context.getCanonicalType(FromPtr->getPointeeType());
  QualType CanonToPointee = Context.getCanonicalType(ToPointee);
  Qualifiers Quals = CanonFromPointee.getQualifiers();
 
  if (StripObjCLifetime)
    Quals.removeObjCLifetime();
 
  // Exact qualifier match -> return the pointer type we're converting to.
  if (CanonToPointee.getLocalQualifiers() == Quals) {
    // ToType is exactly what we need. Return it.
    if (!ToType.isNull())
      return ToType.getUnqualifiedType();
 
    // Build a pointer to ToPointee. It has the right qualifiers
    // already.
    if (isa<ObjCObjectPointerType>(ToType))
      return Context.getObjCObjectPointerType(ToPointee);
    return Context.getPointerType(ToPointee);
  }
 
  // Just build a canonical type that has the right qualifiers.
  QualType QualifiedCanonToPointee
    = Context.getQualifiedType(CanonToPointee.getLocalUnqualifiedType(), Quals);
 
  if (isa<ObjCObjectPointerType>(ToType))
    return Context.getObjCObjectPointerType(QualifiedCanonToPointee);
  return Context.getPointerType(QualifiedCanonToPointee);
}
 
static bool isNullPointerConstantForConversion(Expr *Expr,
                                               bool InOverloadResolution,
                                               ASTContext &Context) {
  // Handle value-dependent integral null pointer constants correctly.
  // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#903
  if (Expr->isValueDependent() && !Expr->isTypeDependent() &&
      Expr->getType()->isIntegerType() && !Expr->getType()->isEnumeralType())
    return !InOverloadResolution;
 
  return Expr->isNullPointerConstant(Context,
                    InOverloadResolution? Expr::NPC_ValueDependentIsNotNull
                                        : Expr::NPC_ValueDependentIsNull);
}
 
bool Sema::IsPointerConversion(Expr *From, QualType FromType, QualType ToType,
                               bool InOverloadResolution,
                               QualType& ConvertedType,
                               bool &IncompatibleObjC) {
  IncompatibleObjC = false;
  if (isObjCPointerConversion(FromType, ToType, ConvertedType,
                              IncompatibleObjC))
    return true;
 
  // Conversion from a null pointer constant to any Objective-C pointer type.
  if (ToType->isObjCObjectPointerType() &&
      isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }
 
  // Blocks: Block pointers can be converted to void*.
  if (FromType->isBlockPointerType() && ToType->isPointerType() &&
      ToType->castAs<PointerType>()->getPointeeType()->isVoidType()) {
    ConvertedType = ToType;
    return true;
  }
  // Blocks: A null pointer constant can be converted to a block
  // pointer type.
  if (ToType->isBlockPointerType() &&
      isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }
 
  // If the left-hand-side is nullptr_t, the right side can be a null
  // pointer constant.
  if (ToType->isNullPtrType() &&
      isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }
 
  const PointerType* ToTypePtr = ToType->getAs<PointerType>();
  if (!ToTypePtr)
    return false;
 
  // A null pointer constant can be converted to a pointer type (C++ 4.10p1).
  if (isNullPointerConstantForConversion(From, InOverloadResolution, Context)) {
    ConvertedType = ToType;
    return true;
  }
 
  // Beyond this point, both types need to be pointers
  // , including objective-c pointers.
  QualType ToPointeeType = ToTypePtr->getPointeeType();
  if (FromType->isObjCObjectPointerType() && ToPointeeType->isVoidType() &&
      !getLangOpts().ObjCAutoRefCount) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(
        FromType->castAs<ObjCObjectPointerType>(), ToPointeeType, ToType,
        Context);
    return true;
  }
  const PointerType *FromTypePtr = FromType->getAs<PointerType>();
  if (!FromTypePtr)
    return false;
 
  QualType FromPointeeType = FromTypePtr->getPointeeType();
 
  // If the unqualified pointee types are the same, this can't be a
  // pointer conversion, so don't do all of the work below.
  if (Context.hasSameUnqualifiedType(FromPointeeType, ToPointeeType))
    return false;
 
  // An rvalue of type "pointer to cv T," where T is an object type,
  // can be converted to an rvalue of type "pointer to cv void" (C++
  // 4.10p2).
  if (FromPointeeType->isIncompleteOrObjectType() &&
      ToPointeeType->isVoidType()) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context,
                                                   /*StripObjCLifetime=*/true);
    return true;
  }
 
  // MSVC allows implicit function to void* type conversion.
  if (getLangOpts().MSVCCompat && FromPointeeType->isFunctionType() &&
      ToPointeeType->isVoidType()) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }
 
  // When we're overloading in C, we allow a special kind of pointer
  // conversion for compatible-but-not-identical pointee types.
  if (!getLangOpts().CPlusPlus &&
      Context.typesAreCompatible(FromPointeeType, ToPointeeType)) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }
 
  // C++ [conv.ptr]p3:
  //
  //   An rvalue of type "pointer to cv D," where D is a class type,
  //   can be converted to an rvalue of type "pointer to cv B," where
  //   B is a base class (clause 10) of D. If B is an inaccessible
  //   (clause 11) or ambiguous (10.2) base class of D, a program that
  //   necessitates this conversion is ill-formed. The result of the
  //   conversion is a pointer to the base class sub-object of the
  //   derived class object. The null pointer value is converted to
  //   the null pointer value of the destination type.
  //
  // Note that we do not check for ambiguity or inaccessibility
  // here. That is handled by CheckPointerConversion.
  if (getLangOpts().CPlusPlus && FromPointeeType->isRecordType() &&
      ToPointeeType->isRecordType() &&
      !Context.hasSameUnqualifiedType(FromPointeeType, ToPointeeType) &&
      IsDerivedFrom(From->getBeginLoc(), FromPointeeType, ToPointeeType)) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }
 
  if (FromPointeeType->isVectorType() && ToPointeeType->isVectorType() &&
      Context.areCompatibleVectorTypes(FromPointeeType, ToPointeeType)) {
    ConvertedType = BuildSimilarlyQualifiedPointerType(FromTypePtr,
                                                       ToPointeeType,
                                                       ToType, Context);
    return true;
  }
 
  return false;
}
 
/// Adopt the given qualifiers for the given type.
static QualType AdoptQualifiers(ASTContext &Context, QualType T, Qualifiers Qs){
  Qualifiers TQs = T.getQualifiers();
 
  // Check whether qualifiers already match.
  if (TQs == Qs)
    return T;
 
  if (Qs.compatiblyIncludes(TQs, Context))
    return Context.getQualifiedType(T, Qs);
 
  return Context.getQualifiedType(T.getUnqualifiedType(), Qs);
}
 
bool Sema::isObjCPointerConversion(QualType FromType, QualType ToType,
                                   QualType& ConvertedType,
                                   bool &IncompatibleObjC) {
  if (!getLangOpts().ObjC)
    return false;
 
  // The set of qualifiers on the type we're converting from.
  Qualifiers FromQualifiers = FromType.getQualifiers();
 
  // First, we handle all conversions on ObjC object pointer types.
  const ObjCObjectPointerType* ToObjCPtr =
    ToType->getAs<ObjCObjectPointerType>();
  const ObjCObjectPointerType *FromObjCPtr =
    FromType->getAs<ObjCObjectPointerType>();
 
  if (ToObjCPtr && FromObjCPtr) {
    // If the pointee types are the same (ignoring qualifications),
    // then this is not a pointer conversion.
    if (Context.hasSameUnqualifiedType(ToObjCPtr->getPointeeType(),
                                       FromObjCPtr->getPointeeType()))
      return false;
 
    // Conversion between Objective-C pointers.
    if (Context.canAssignObjCInterfaces(ToObjCPtr, FromObjCPtr)) {
      const ObjCInterfaceType* LHS = ToObjCPtr->getInterfaceType();
      const ObjCInterfaceType* RHS = FromObjCPtr->getInterfaceType();
      if (getLangOpts().CPlusPlus && LHS && RHS &&
          !ToObjCPtr->getPointeeType().isAtLeastAsQualifiedAs(
              FromObjCPtr->getPointeeType(), getASTContext()))
        return false;
      ConvertedType = BuildSimilarlyQualifiedPointerType(FromObjCPtr,
                                                   ToObjCPtr->getPointeeType(),
                                                         ToType, Context);
      ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
      return true;
    }
 
    if (Context.canAssignObjCInterfaces(FromObjCPtr, ToObjCPtr)) {
      // Okay: this is some kind of implicit downcast of Objective-C
      // interfaces, which is permitted. However, we're going to
      // complain about it.
      IncompatibleObjC = true;
      ConvertedType = BuildSimilarlyQualifiedPointerType(FromObjCPtr,
                                                   ToObjCPtr->getPointeeType(),
                                                         ToType, Context);
      ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
      return true;
    }
  }
  // Beyond this point, both types need to be C pointers or block pointers.
  QualType ToPointeeType;
  if (const PointerType *ToCPtr = ToType->getAs<PointerType>())
    ToPointeeType = ToCPtr->getPointeeType();
  else if (const BlockPointerType *ToBlockPtr =
            ToType->getAs<BlockPointerType>()) {
    // Objective C++: We're able to convert from a pointer to any object
    // to a block pointer type.
    if (FromObjCPtr && FromObjCPtr->isObjCBuiltinType()) {
      ConvertedType = AdoptQualifiers(Context, ToType, FromQualifiers);
      return true;
    }
    ToPointeeType = ToBlockPtr->getPointeeType();
  }
  else if (FromType->getAs<BlockPointerType>() &&
           ToObjCPtr && ToObjCPtr->isObjCBuiltinType()) {
    // Objective C++: We're able to convert from a block pointer type to a
    // pointer to any object.
    ConvertedType = AdoptQualifiers(Context, ToType, FromQualifiers);
    return true;
  }
  else
    return false;
 
  QualType FromPointeeType;
  if (const PointerType *FromCPtr = FromType->getAs<PointerType>())
    FromPointeeType = FromCPtr->getPointeeType();
  else if (const BlockPointerType *FromBlockPtr =
           FromType->getAs<BlockPointerType>())
    FromPointeeType = FromBlockPtr->getPointeeType();
  else
    return false;
 
  // If we have pointers to pointers, recursively check whether this
  // is an Objective-C conversion.
  if (FromPointeeType->isPointerType() && ToPointeeType->isPointerType() &&
      isObjCPointerConversion(FromPointeeType, ToPointeeType, ConvertedType,
                              IncompatibleObjC)) {
    // We always complain about this conversion.
    IncompatibleObjC = true;
    ConvertedType = Context.getPointerType(ConvertedType);
    ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
    return true;
  }
  // Allow conversion of pointee being objective-c pointer to another one;
  // as in I* to id.
  if (FromPointeeType->getAs<ObjCObjectPointerType>() &&
      ToPointeeType->getAs<ObjCObjectPointerType>() &&
      isObjCPointerConversion(FromPointeeType, ToPointeeType, ConvertedType,
                              IncompatibleObjC)) {
 
    ConvertedType = Context.getPointerType(ConvertedType);
    ConvertedType = AdoptQualifiers(Context, ConvertedType, FromQualifiers);
    return true;
  }
 
  // If we have pointers to functions or blocks, check whether the only
  // differences in the argument and result types are in Objective-C
  // pointer conversions. If so, we permit the conversion (but
  // complain about it).
  const FunctionProtoType *FromFunctionType
    = FromPointeeType->getAs<FunctionProtoType>();
  const FunctionProtoType *ToFunctionType
    = ToPointeeType->getAs<FunctionProtoType>();
  if (FromFunctionType && ToFunctionType) {
    // If the function types are exactly the same, this isn't an
    // Objective-C pointer conversion.
    if (Context.getCanonicalType(FromPointeeType)
          == Context.getCanonicalType(ToPointeeType))
      return false;
 
    // Perform the quick checks that will tell us whether these
    // function types are obviously different.
    if (FromFunctionType->getNumParams() != ToFunctionType->getNumParams() ||
        FromFunctionType->isVariadic() != ToFunctionType->isVariadic() ||
        FromFunctionType->getMethodQuals() != ToFunctionType->getMethodQuals())
      return false;
 
    bool HasObjCConversion = false;
    if (Context.getCanonicalType(FromFunctionType->getReturnType()) ==
        Context.getCanonicalType(ToFunctionType->getReturnType())) {
      // Okay, the types match exactly. Nothing to do.
    } else if (isObjCPointerConversion(FromFunctionType->getReturnType(),
                                       ToFunctionType->getReturnType(),
                                       ConvertedType, IncompatibleObjC)) {
      // Okay, we have an Objective-C pointer conversion.
      HasObjCConversion = true;
    } else {
      // Function types are too different. Abort.
      return false;
    }
 
    // Check argument types.
    for (unsigned ArgIdx = 0, NumArgs = FromFunctionType->getNumParams();
         ArgIdx != NumArgs; ++ArgIdx) {
      QualType FromArgType = FromFunctionType->getParamType(ArgIdx);
      QualType ToArgType = ToFunctionType->getParamType(ArgIdx);
      if (Context.getCanonicalType(FromArgType)
            == Context.getCanonicalType(ToArgType)) {
        // Okay, the types match exactly. Nothing to do.
      } else if (isObjCPointerConversion(FromArgType, ToArgType,
                                         ConvertedType, IncompatibleObjC)) {
        // Okay, we have an Objective-C pointer conversion.
        HasObjCConversion = true;
      } else {
        // Argument types are too different. Abort.
        return false;
      }
    }
 
    if (HasObjCConversion) {
      // We had an Objective-C conversion. Allow this pointer
      // conversion, but complain about it.
      ConvertedType = AdoptQualifiers(Context, ToType, FromQualifiers);
      IncompatibleObjC = true;
      return true;
    }
  }
 
  return false;
}
 
bool Sema::IsBlockPointerConversion(QualType FromType, QualType ToType,
                                    QualType& ConvertedType) {
  QualType ToPointeeType;
  if (const BlockPointerType *ToBlockPtr =
        ToType->getAs<BlockPointerType>())
    ToPointeeType = ToBlockPtr->getPointeeType();
  else
    return false;
 
  QualType FromPointeeType;
  if (const BlockPointerType *FromBlockPtr =
      FromType->getAs<BlockPointerType>())
    FromPointeeType = FromBlockPtr->getPointeeType();
  else
    return false;
  // We have pointer to blocks, check whether the only
  // differences in the argument and result types are in Objective-C
  // pointer conversions. If so, we permit the conversion.
 
  const FunctionProtoType *FromFunctionType
    = FromPointeeType->getAs<FunctionProtoType>();
  const FunctionProtoType *ToFunctionType
    = ToPointeeType->getAs<FunctionProtoType>();
 
  if (!FromFunctionType || !ToFunctionType)
    return false;
 
  if (Context.hasSameType(FromPointeeType, ToPointeeType))
    return true;
 
  // Perform the quick checks that will tell us whether these
  // function types are obviously different.
  if (FromFunctionType->getNumParams() != ToFunctionType->getNumParams() ||
      FromFunctionType->isVariadic() != ToFunctionType->isVariadic())
    return false;
 
  FunctionType::ExtInfo FromEInfo = FromFunctionType->getExtInfo();
  FunctionType::ExtInfo ToEInfo = ToFunctionType->getExtInfo();
  if (FromEInfo != ToEInfo)
    return false;
 
  bool IncompatibleObjC = false;
  if (Context.hasSameType(FromFunctionType->getReturnType(),
                          ToFunctionType->getReturnType())) {
    // Okay, the types match exactly. Nothing to do.
  } else {
    QualType RHS = FromFunctionType->getReturnType();
    QualType LHS = ToFunctionType->getReturnType();
    if ((!getLangOpts().CPlusPlus || !RHS->isRecordType()) &&
        !RHS.hasQualifiers() && LHS.hasQualifiers())
       LHS = LHS.getUnqualifiedType();
 
     if (Context.hasSameType(RHS,LHS)) {
       // OK exact match.
     } else if (isObjCPointerConversion(RHS, LHS,
                                        ConvertedType, IncompatibleObjC)) {
     if (IncompatibleObjC)
       return false;
     // Okay, we have an Objective-C pointer conversion.
     }
     else
       return false;
   }
 
   // Check argument types.
   for (unsigned ArgIdx = 0, NumArgs = FromFunctionType->getNumParams();
        ArgIdx != NumArgs; ++ArgIdx) {
     IncompatibleObjC = false;
     QualType FromArgType = FromFunctionType->getParamType(ArgIdx);
     QualType ToArgType = ToFunctionType->getParamType(ArgIdx);
     if (Context.hasSameType(FromArgType, ToArgType)) {
       // Okay, the types match exactly. Nothing to do.
     } else if (isObjCPointerConversion(ToArgType, FromArgType,
                                        ConvertedType, IncompatibleObjC)) {
       if (IncompatibleObjC)
         return false;
       // Okay, we have an Objective-C pointer conversion.
     } else
       // Argument types are too different. Abort.
       return false;
   }
 
   SmallVector<FunctionProtoType::ExtParameterInfo, 4> NewParamInfos;
   bool CanUseToFPT, CanUseFromFPT;
   if (!Context.mergeExtParameterInfo(ToFunctionType, FromFunctionType,
                                      CanUseToFPT, CanUseFromFPT,
                                      NewParamInfos))
     return false;
 
   ConvertedType = ToType;
   return true;
}
 
enum {
  ft_default,
  ft_different_class,
  ft_parameter_arity,
  ft_parameter_mismatch,
  ft_return_type,
  ft_qualifer_mismatch,
  ft_noexcept
};
 
/// Attempts to get the FunctionProtoType from a Type. Handles
/// MemberFunctionPointers properly.
static const FunctionProtoType *tryGetFunctionProtoType(QualType FromType) {
  if (auto *FPT = FromType->getAs<FunctionProtoType>())
    return FPT;
 
  if (auto *MPT = FromType->getAs<MemberPointerType>())
    return MPT->getPointeeType()->getAs<FunctionProtoType>();
 
  return nullptr;
}
 
void Sema::HandleFunctionTypeMismatch(PartialDiagnostic &PDiag,
                                      QualType FromType, QualType ToType) {
  // If either type is not valid, include no extra info.
  if (FromType.isNull() || ToType.isNull()) {
    PDiag << ft_default;
    return;
  }
 
  // Get the function type from the pointers.
  if (FromType->isMemberPointerType() && ToType->isMemberPointerType()) {
    const auto *FromMember = FromType->castAs<MemberPointerType>(),
               *ToMember = ToType->castAs<MemberPointerType>();
    if (!Context.hasSameType(FromMember->getClass(), ToMember->getClass())) {
      PDiag << ft_different_class << QualType(ToMember->getClass(), 0)
            << QualType(FromMember->getClass(), 0);
      return;
    }
    FromType = FromMember->getPointeeType();
    ToType = ToMember->getPointeeType();
  }
 
  if (FromType->isPointerType())
    FromType = FromType->getPointeeType();
  if (ToType->isPointerType())
    ToType = ToType->getPointeeType();
 
  // Remove references.
  FromType = FromType.getNonReferenceType();
  ToType = ToType.getNonReferenceType();
 
  // Don't print extra info for non-specialized template functions.
  if (FromType->isInstantiationDependentType() &&
      !FromType->getAs<TemplateSpecializationType>()) {
    PDiag << ft_default;
    return;
  }
 
  // No extra info for same types.
  if (Context.hasSameType(FromType, ToType)) {
    PDiag << ft_default;
    return;
  }
 
  const FunctionProtoType *FromFunction = tryGetFunctionProtoType(FromType),
                          *ToFunction = tryGetFunctionProtoType(ToType);
 
  // Both types need to be function types.
  if (!FromFunction || !ToFunction) {
    PDiag << ft_default;
    return;
  }
 
  if (FromFunction->getNumParams() != ToFunction->getNumParams()) {
    PDiag << ft_parameter_arity << ToFunction->getNumParams()
          << FromFunction->getNumParams();
    return;
  }
 
  // Handle different parameter types.
  unsigned ArgPos;
  if (!FunctionParamTypesAreEqual(FromFunction, ToFunction, &ArgPos)) {
    PDiag << ft_parameter_mismatch << ArgPos + 1
          << ToFunction->getParamType(ArgPos)
          << FromFunction->getParamType(ArgPos);
    return;
  }
 
  // Handle different return type.
  if (!Context.hasSameType(FromFunction->getReturnType(),
                           ToFunction->getReturnType())) {
    PDiag << ft_return_type << ToFunction->getReturnType()
          << FromFunction->getReturnType();
    return;
  }
 
  if (FromFunction->getMethodQuals() != ToFunction->getMethodQuals()) {
    PDiag << ft_qualifer_mismatch << ToFunction->getMethodQuals()
          << FromFunction->getMethodQuals();
    return;
  }
 
  // Handle exception specification differences on canonical type (in C++17
  // onwards).
  if (cast<FunctionProtoType>(FromFunction->getCanonicalTypeUnqualified())
          ->isNothrow() !=
      cast<FunctionProtoType>(ToFunction->getCanonicalTypeUnqualified())
          ->isNothrow()) {
    PDiag << ft_noexcept;
    return;
  }
 
  // Unable to find a difference, so add no extra info.
  PDiag << ft_default;
}
 
bool Sema::FunctionParamTypesAreEqual(ArrayRef<QualType> Old,
                                      ArrayRef<QualType> New, unsigned *ArgPos,
                                      bool Reversed) {
  assert(llvm::size(Old) == llvm::size(New) &&
         "Can't compare parameters of functions with different number of "
         "parameters!");
 
  for (auto &&[Idx, Type] : llvm::enumerate(Old)) {
    // Reverse iterate over the parameters of `OldType` if `Reversed` is true.
    size_t J = Reversed ? (llvm::size(New) - Idx - 1) : Idx;
 
    // Ignore address spaces in pointee type. This is to disallow overloading
    // on __ptr32/__ptr64 address spaces.
    QualType OldType =
        Context.removePtrSizeAddrSpace(Type.getUnqualifiedType());
    QualType NewType =
        Context.removePtrSizeAddrSpace((New.begin() + J)->getUnqualifiedType());
 
    if (!Context.hasSameType(OldType, NewType)) {
      if (ArgPos)
        *ArgPos = Idx;
      return false;
    }
  }
  return true;
}
 
bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
                                      const FunctionProtoType *NewType,
                                      unsigned *ArgPos, bool Reversed) {
  return FunctionParamTypesAreEqual(OldType->param_types(),
                                    NewType->param_types(), ArgPos, Reversed);
}
 
bool Sema::FunctionNonObjectParamTypesAreEqual(const FunctionDecl *OldFunction,
                                               const FunctionDecl *NewFunction,
                                               unsigned *ArgPos,
                                               bool Reversed) {
 
  if (OldFunction->getNumNonObjectParams() !=
      NewFunction->getNumNonObjectParams())
    return false;
 
  unsigned OldIgnore =
      unsigned(OldFunction->hasCXXExplicitFunctionObjectParameter());
  unsigned NewIgnore =
      unsigned(NewFunction->hasCXXExplicitFunctionObjectParameter());
 
  auto *OldPT = cast<FunctionProtoType>(OldFunction->getFunctionType());
  auto *NewPT = cast<FunctionProtoType>(NewFunction->getFunctionType());
 
  return FunctionParamTypesAreEqual(OldPT->param_types().slice(OldIgnore),
                                    NewPT->param_types().slice(NewIgnore),
                                    ArgPos, Reversed);
}
 
bool Sema::CheckPointerConversion(Expr *From, QualType ToType,
                                  CastKind &Kind,
                                  CXXCastPath& BasePath,
                                  bool IgnoreBaseAccess,
                                  bool Diagnose) {
  QualType FromType = From->getType();
  bool IsCStyleOrFunctionalCast = IgnoreBaseAccess;
 
  Kind = CK_BitCast;
 
  if (Diagnose && !IsCStyleOrFunctionalCast && !FromType->isAnyPointerType() &&
      From->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNotNull) ==
          Expr::NPCK_ZeroExpression) {
    if (Context.hasSameUnqualifiedType(From->getType(), Context.BoolTy))
      DiagRuntimeBehavior(From->getExprLoc(), From,
                          PDiag(diag::warn_impcast_bool_to_null_pointer)
                            << ToType << From->getSourceRange());
    else if (!isUnevaluatedContext())
      Diag(From->getExprLoc(), diag::warn_non_literal_null_pointer)
        << ToType << From->getSourceRange();
  }
  if (const PointerType *ToPtrType = ToType->getAs<PointerType>()) {
    if (const PointerType *FromPtrType = FromType->getAs<PointerType>()) {
      QualType FromPointeeType = FromPtrType->getPointeeType(),
               ToPointeeType   = ToPtrType->getPointeeType();
 
      if (FromPointeeType->isRecordType() && ToPointeeType->isRecordType() &&
          !Context.hasSameUnqualifiedType(FromPointeeType, ToPointeeType)) {
        // We must have a derived-to-base conversion. Check an
        // ambiguous or inaccessible conversion.
        unsigned InaccessibleID = 0;
        unsigned AmbiguousID = 0;
        if (Diagnose) {
          InaccessibleID = diag::err_upcast_to_inaccessible_base;
          AmbiguousID = diag::err_ambiguous_derived_to_base_conv;
        }
        if (CheckDerivedToBaseConversion(
                FromPointeeType, ToPointeeType, InaccessibleID, AmbiguousID,
                From->getExprLoc(), From->getSourceRange(), DeclarationName(),
                &BasePath, IgnoreBaseAccess))
          return true;
 
        // The conversion was successful.
        Kind = CK_DerivedToBase;
      }
 
      if (Diagnose && !IsCStyleOrFunctionalCast &&
          FromPointeeType->isFunctionType() && ToPointeeType->isVoidType()) {
        assert(getLangOpts().MSVCCompat &&
               "this should only be possible with MSVCCompat!");
        Diag(From->getExprLoc(), diag::ext_ms_impcast_fn_obj)
            << From->getSourceRange();
      }
    }
  } else if (const ObjCObjectPointerType *ToPtrType =
               ToType->getAs<ObjCObjectPointerType>()) {
    if (const ObjCObjectPointerType *FromPtrType =
          FromType->getAs<ObjCObjectPointerType>()) {
      // Objective-C++ conversions are always okay.
      // FIXME: We should have a different class of conversions for the
      // Objective-C++ implicit conversions.
      if (FromPtrType->isObjCBuiltinType() || ToPtrType->isObjCBuiltinType())
        return false;
    } else if (FromType->isBlockPointerType()) {
      Kind = CK_BlockPointerToObjCPointerCast;
    } else {
      Kind = CK_CPointerToObjCPointerCast;
    }
  } else if (ToType->isBlockPointerType()) {
    if (!FromType->isBlockPointerType())
      Kind = CK_AnyPointerToBlockPointerCast;
  }
 
  // We shouldn't fall into this case unless it's valid for other
  // reasons.
  if (From->isNullPointerConstant(Context, Expr::NPC_ValueDependentIsNull))
    Kind = CK_NullToPointer;
 
  return false;
}
 
bool Sema::IsMemberPointerConversion(Expr *From, QualType FromType,
                                     QualType ToType,
                                     bool InOverloadResolution,
                                     QualType &ConvertedType) {
  const MemberPointerType *ToTypePtr = ToType->getAs<MemberPointerType>();
  if (!ToTypePtr)
    return false;
 
  // A null pointer constant can be converted to a member pointer (C++ 4.11p1)
  if (From->isNullPointerConstant(Context,
                    InOverloadResolution? Expr::NPC_ValueDependentIsNotNull
                                        : Expr::NPC_ValueDependentIsNull)) {
    ConvertedType = ToType;
    return true;
  }
 
  // Otherwise, both types have to be member pointers.
  const MemberPointerType *FromTypePtr = FromType->getAs<MemberPointerType>();
  if (!FromTypePtr)
    return false;
 
  // A pointer to member of B can be converted to a pointer to member of D,
  // where D is derived from B (C++ 4.11p2).
  QualType FromClass(FromTypePtr->getClass(), 0);
  QualType ToClass(ToTypePtr->getClass(), 0);
 
  if (!Context.hasSameUnqualifiedType(FromClass, ToClass) &&
      IsDerivedFrom(From->getBeginLoc(), ToClass, FromClass)) {
    ConvertedType = Context.getMemberPointerType(FromTypePtr->getPointeeType(),
                                                 ToClass.getTypePtr());
    return true;
  }
 
  return false;
}
 
bool Sema::CheckMemberPointerConversion(Expr *From, QualType ToType,
                                        CastKind &Kind,
                                        CXXCastPath &BasePath,
                                        bool IgnoreBaseAccess) {
  QualType FromType = From->getType();
  const MemberPointerType *FromPtrType = FromType->getAs<MemberPointerType>();
  if (!FromPtrType) {
    // This must be a null pointer to member pointer conversion
    assert(From->isNullPointerConstant(Context,
                                       Expr::NPC_ValueDependentIsNull) &&
           "Expr must be null pointer constant!");
    Kind = CK_NullToMemberPointer;
    return false;
  }
 
  const MemberPointerType *ToPtrType = ToType->getAs<MemberPointerType>();
  assert(ToPtrType && "No member pointer cast has a target type "
                      "that is not a member pointer.");
 
  QualType FromClass = QualType(FromPtrType->getClass(), 0);
  QualType ToClass   = QualType(ToPtrType->getClass(), 0);
 
  // FIXME: What about dependent types?
  assert(FromClass->isRecordType() && "Pointer into non-class.");
  assert(ToClass->isRecordType() && "Pointer into non-class.");
 
  CXXBasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
                     /*DetectVirtual=*/true);
  bool DerivationOkay =
      IsDerivedFrom(From->getBeginLoc(), ToClass, FromClass, Paths);
  assert(DerivationOkay &&
         "Should not have been called if derivation isn't OK.");
  (void)DerivationOkay;
 
  if (Paths.isAmbiguous(Context.getCanonicalType(FromClass).
                                  getUnqualifiedType())) {
    std::string PathDisplayStr = getAmbiguousPathsDisplayString(Paths);
    Diag(From->getExprLoc(), diag::err_ambiguous_memptr_conv)
      << 0 << FromClass << ToClass << PathDisplayStr << From->getSourceRange();
    return true;
  }
 
  if (const RecordType *VBase = Paths.getDetectedVirtual()) {
    Diag(From->getExprLoc(), diag::err_memptr_conv_via_virtual)
      << FromClass << ToClass << QualType(VBase, 0)
      << From->getSourceRange();
    return true;
  }
 
  if (!IgnoreBaseAccess)
    CheckBaseClassAccess(From->getExprLoc(), FromClass, ToClass,
                         Paths.front(),
                         diag::err_downcast_from_inaccessible_base);
 
  // Must be a base to derived member conversion.
  BuildBasePathArray(Paths, BasePath);
  Kind = CK_BaseToDerivedMemberPointer;
  return false;
}
 
/// Determine whether the lifetime conversion between the two given
/// qualifiers sets is nontrivial.
static bool isNonTrivialObjCLifetimeConversion(Qualifiers FromQuals,
                                               Qualifiers ToQuals) {
  // Converting anything to const __unsafe_unretained is trivial.
  if (ToQuals.hasConst() &&
      ToQuals.getObjCLifetime() == Qualifiers::OCL_ExplicitNone)
    return false;
 
  return true;
}
 
/// Perform a single iteration of the loop for checking if a qualification
/// conversion is valid.
///
/// Specifically, check whether any change between the qualifiers of \p
/// FromType and \p ToType is permissible, given knowledge about whether every
/// outer layer is const-qualified.
static bool isQualificationConversionStep(QualType FromType, QualType ToType,
                                          bool CStyle, bool IsTopLevel,
                                          bool &PreviousToQualsIncludeConst,
                                          bool &ObjCLifetimeConversion,
                                          const ASTContext &Ctx) {
  Qualifiers FromQuals = FromType.getQualifiers();
  Qualifiers ToQuals = ToType.getQualifiers();
 
  // Ignore __unaligned qualifier.
  FromQuals.removeUnaligned();
 
  // Objective-C ARC:
  //   Check Objective-C lifetime conversions.
  if (FromQuals.getObjCLifetime() != ToQuals.getObjCLifetime()) {
    if (ToQuals.compatiblyIncludesObjCLifetime(FromQuals)) {
      if (isNonTrivialObjCLifetimeConversion(FromQuals, ToQuals))
        ObjCLifetimeConversion = true;
      FromQuals.removeObjCLifetime();
      ToQuals.removeObjCLifetime();
    } else {
      // Qualification conversions cannot cast between different
      // Objective-C lifetime qualifiers.
      return false;
    }
  }
 
  // Allow addition/removal of GC attributes but not changing GC attributes.
  if (FromQuals.getObjCGCAttr() != ToQuals.getObjCGCAttr() &&
      (!FromQuals.hasObjCGCAttr() || !ToQuals.hasObjCGCAttr())) {
    FromQuals.removeObjCGCAttr();
    ToQuals.removeObjCGCAttr();
  }
 
  //   -- for every j > 0, if const is in cv 1,j then const is in cv
  //      2,j, and similarly for volatile.
  if (!CStyle && !ToQuals.compatiblyIncludes(FromQuals, Ctx))
    return false;
 
  // If address spaces mismatch:
  //  - in top level it is only valid to convert to addr space that is a
  //    superset in all cases apart from C-style casts where we allow
  //    conversions between overlapping address spaces.
  //  - in non-top levels it is not a valid conversion.
  if (ToQuals.getAddressSpace() != FromQuals.getAddressSpace() &&
      (!IsTopLevel ||
       !(ToQuals.isAddressSpaceSupersetOf(FromQuals, Ctx) ||
         (CStyle && FromQuals.isAddressSpaceSupersetOf(ToQuals, Ctx)))))
    return false;
 
  //   -- if the cv 1,j and cv 2,j are different, then const is in
  //      every cv for 0 < k < j.
  if (!CStyle && FromQuals.getCVRQualifiers() != ToQuals.getCVRQualifiers() &&
      !PreviousToQualsIncludeConst)
    return false;
 
  // The following wording is from C++20, where the result of the conversion
  // is T3, not T2.
  //   -- if [...] P1,i [...] is "array of unknown bound of", P3,i is
  //      "array of unknown bound of"
  if (FromType->isIncompleteArrayType() && !ToType->isIncompleteArrayType())
    return false;
 
  //   -- if the resulting P3,i is different from P1,i [...], then const is
  //      added to every cv 3_k for 0 < k < i.
  if (!CStyle && FromType->isConstantArrayType() &&
      ToType->isIncompleteArrayType() && !PreviousToQualsIncludeConst)
    return false;
 
  // Keep track of whether all prior cv-qualifiers in the "to" type
  // include const.
  PreviousToQualsIncludeConst =
      PreviousToQualsIncludeConst && ToQuals.hasConst();
  return true;
}
 
bool
Sema::IsQualificationConversion(QualType FromType, QualType ToType,
                                bool CStyle, bool &ObjCLifetimeConversion) {
  FromType = Context.getCanonicalType(FromType);
  ToType = Context.getCanonicalType(ToType);
  ObjCLifetimeConversion = false;
 
  // If FromType and ToType are the same type, this is not a
  // qualification conversion.
  if (FromType.getUnqualifiedType() == ToType.getUnqualifiedType())
    return false;
 
  // (C++ 4.4p4):
  //   A conversion can add cv-qualifiers at levels other than the first
  //   in multi-level pointers, subject to the following rules: [...]
  bool PreviousToQualsIncludeConst = true;
  bool UnwrappedAnyPointer = false;
  while (Context.UnwrapSimilarTypes(FromType, ToType)) {
    if (!isQualificationConversionStep(FromType, ToType, CStyle,
                                       !UnwrappedAnyPointer,
                                       PreviousToQualsIncludeConst,
                                       ObjCLifetimeConversion, getASTContext()))
      return false;
    UnwrappedAnyPointer = true;
  }
 
  // We are left with FromType and ToType being the pointee types
  // after unwrapping the original FromType and ToType the same number
  // of times. If we unwrapped any pointers, and if FromType and
  // ToType have the same unqualified type (since we checked
  // qualifiers above), then this is a qualification conversion.
  return UnwrappedAnyPointer && Context.hasSameUnqualifiedType(FromType,ToType);
}
 
/// - Determine whether this is a conversion from a scalar type to an
/// atomic type.
///
/// If successful, updates \c SCS's second and third steps in the conversion
/// sequence to finish the conversion.
static bool tryAtomicConversion(Sema &S, Expr *From, QualType ToType,
                                bool InOverloadResolution,
                                StandardConversionSequence &SCS,
                                bool CStyle) {
  const AtomicType *ToAtomic = ToType->getAs<AtomicType>();
  if (!ToAtomic)
    return false;
 
  StandardConversionSequence InnerSCS;
  if (!IsStandardConversion(S, From, ToAtomic->getValueType(),
                            InOverloadResolution, InnerSCS,
                            CStyle, /*AllowObjCWritebackConversion=*/false))
    return false;
 
  SCS.Second = InnerSCS.Second;
  SCS.setToType(1, InnerSCS.getToType(1));
  SCS.Third = InnerSCS.Third;
  SCS.QualificationIncludesObjCLifetime
    = InnerSCS.QualificationIncludesObjCLifetime;
  SCS.setToType(2, InnerSCS.getToType(2));
  return true;
}
 
static bool isFirstArgumentCompatibleWithType(ASTContext &Context,
                                              CXXConstructorDecl *Constructor,
                                              QualType Type) {
  const auto *CtorType = Constructor->getType()->castAs<FunctionProtoType>();
  if (CtorType->getNumParams() > 0) {
    QualType FirstArg = CtorType->getParamType(0);
    if (Context.hasSameUnqualifiedType(Type, FirstArg.getNonReferenceType()))
      return true;
  }
  return false;
}
 
static OverloadingResult
IsInitializerListConstructorConversion(Sema &S, Expr *From, QualType ToType,
                                       CXXRecordDecl *To,
                                       UserDefinedConversionSequence &User,
                                       OverloadCandidateSet &CandidateSet,
                                       bool AllowExplicit) {
  CandidateSet.clear(OverloadCandidateSet::CSK_InitByUserDefinedConversion);
  for (auto *D : S.LookupConstructors(To)) {
    auto Info = getConstructorInfo(D);
    if (!Info)
      continue;
 
    bool Usable = !Info.Constructor->isInvalidDecl() &&
                  S.isInitListConstructor(Info.Constructor);
    if (Usable) {
      bool SuppressUserConversions = false;
      if (Info.ConstructorTmpl)
        S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
                                       /*ExplicitArgs*/ nullptr, From,
                                       CandidateSet, SuppressUserConversions,
                                       /*PartialOverloading*/ false,
                                       AllowExplicit);
      else
        S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl, From,
                               CandidateSet, SuppressUserConversions,
                               /*PartialOverloading*/ false, AllowExplicit);
    }
  }
 
  bool HadMultipleCandidates = (CandidateSet.size() > 1);
 
  OverloadCandidateSet::iterator Best;
  switch (auto Result =
              CandidateSet.BestViableFunction(S, From->getBeginLoc(), Best)) {
  case OR_Deleted:
  case OR_Success: {
    // Record the standard conversion we used and the conversion function.
    CXXConstructorDecl *Constructor = cast<CXXConstructorDecl>(Best->Function);
    QualType ThisType = Constructor->getFunctionObjectParameterType();
    // Initializer lists don't have conversions as such.
    User.Before.setAsIdentityConversion();
    User.HadMultipleCandidates = HadMultipleCandidates;
    User.ConversionFunction = Constructor;
    User.FoundConversionFunction = Best->FoundDecl;
    User.After.setAsIdentityConversion();
    User.After.setFromType(ThisType);
    User.After.setAllToTypes(ToType);
    return Result;
  }
 
  case OR_No_Viable_Function:
    return OR_No_Viable_Function;
  case OR_Ambiguous:
    return OR_Ambiguous;
  }
 
  llvm_unreachable("Invalid OverloadResult!");
}
 
/// Determines whether there is a user-defined conversion sequence
/// (C++ [over.ics.user]) that converts expression From to the type
/// ToType. If such a conversion exists, User will contain the
/// user-defined conversion sequence that performs such a conversion
/// and this routine will return true. Otherwise, this routine returns
/// false and User is unspecified.
///
/// \param AllowExplicit  true if the conversion should consider C++0x
/// "explicit" conversion functions as well as non-explicit conversion
/// functions (C++0x [class.conv.fct]p2).
///
/// \param AllowObjCConversionOnExplicit true if the conversion should
/// allow an extra Objective-C pointer conversion on uses of explicit
/// constructors. Requires \c AllowExplicit to also be set.
static OverloadingResult
IsUserDefinedConversion(Sema &S, Expr *From, QualType ToType,
                        UserDefinedConversionSequence &User,
                        OverloadCandidateSet &CandidateSet,
                        AllowedExplicit AllowExplicit,
                        bool AllowObjCConversionOnExplicit) {
  assert(AllowExplicit != AllowedExplicit::None ||
         !AllowObjCConversionOnExplicit);
  CandidateSet.clear(OverloadCandidateSet::CSK_InitByUserDefinedConversion);
 
  // Whether we will only visit constructors.
  bool ConstructorsOnly = false;
 
  // If the type we are conversion to is a class type, enumerate its
  // constructors.
  if (const RecordType *ToRecordType = ToType->getAs<RecordType>()) {
    // C++ [over.match.ctor]p1:
    //   When objects of class type are direct-initialized (8.5), or
    //   copy-initialized from an expression of the same or a
    //   derived class type (8.5), overload resolution selects the
    //   constructor. [...] For copy-initialization, the candidate
    //   functions are all the converting constructors (12.3.1) of
    //   that class. The argument list is the expression-list within
    //   the parentheses of the initializer.
    if (S.Context.hasSameUnqualifiedType(ToType, From->getType()) ||
        (From->getType()->getAs<RecordType>() &&
         S.IsDerivedFrom(From->getBeginLoc(), From->getType(), ToType)))
      ConstructorsOnly = true;
 
    if (!S.isCompleteType(From->getExprLoc(), ToType)) {
      // We're not going to find any constructors.
    } else if (CXXRecordDecl *ToRecordDecl
                 = dyn_cast<CXXRecordDecl>(ToRecordType->getDecl())) {
 
      Expr **Args = &From;
      unsigned NumArgs = 1;
      bool ListInitializing = false;
      if (InitListExpr *InitList = dyn_cast<InitListExpr>(From)) {
        // But first, see if there is an init-list-constructor that will work.
        OverloadingResult Result = IsInitializerListConstructorConversion(
            S, From, ToType, ToRecordDecl, User, CandidateSet,
            AllowExplicit == AllowedExplicit::All);
        if (Result != OR_No_Viable_Function)
          return Result;
        // Never mind.
        CandidateSet.clear(
            OverloadCandidateSet::CSK_InitByUserDefinedConversion);
 
        // If we're list-initializing, we pass the individual elements as
        // arguments, not the entire list.
        Args = InitList->getInits();
        NumArgs = InitList->getNumInits();
        ListInitializing = true;
      }
 
      for (auto *D : S.LookupConstructors(ToRecordDecl)) {
        auto Info = getConstructorInfo(D);
        if (!Info)
          continue;
 
        bool Usable = !Info.Constructor->isInvalidDecl();
        if (!ListInitializing)
          Usable = Usable && Info.Constructor->isConvertingConstructor(
                                 /*AllowExplicit*/ true);
        if (Usable) {
          bool SuppressUserConversions = !ConstructorsOnly;
          // C++20 [over.best.ics.general]/4.5:
          //   if the target is the first parameter of a constructor [of class
          //   X] and the constructor [...] is a candidate by [...] the second
          //   phase of [over.match.list] when the initializer list has exactly
          //   one element that is itself an initializer list, [...] and the
          //   conversion is to X or reference to cv X, user-defined conversion
          //   sequences are not cnosidered.
          if (SuppressUserConversions && ListInitializing) {
            SuppressUserConversions =
                NumArgs == 1 && isa<InitListExpr>(Args[0]) &&
                isFirstArgumentCompatibleWithType(S.Context, Info.Constructor,
                                                  ToType);
          }
          if (Info.ConstructorTmpl)
            S.AddTemplateOverloadCandidate(
                Info.ConstructorTmpl, Info.FoundDecl,
                /*ExplicitArgs*/ nullptr, llvm::ArrayRef(Args, NumArgs),
                CandidateSet, SuppressUserConversions,
                /*PartialOverloading*/ false,
                AllowExplicit == AllowedExplicit::All);
          else
            // Allow one user-defined conversion when user specifies a
            // From->ToType conversion via an static cast (c-style, etc).
            S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl,
                                   llvm::ArrayRef(Args, NumArgs), CandidateSet,
                                   SuppressUserConversions,
                                   /*PartialOverloading*/ false,
                                   AllowExplicit == AllowedExplicit::All);
        }
      }
    }
  }
 
  // Enumerate conversion functions, if we're allowed to.
  if (ConstructorsOnly || isa<InitListExpr>(From)) {
  } else if (!S.isCompleteType(From->getBeginLoc(), From->getType())) {
    // No conversion functions from incomplete types.
  } else if (const RecordType *FromRecordType =
                 From->getType()->getAs<RecordType>()) {
    if (CXXRecordDecl *FromRecordDecl
         = dyn_cast<CXXRecordDecl>(FromRecordType->getDecl())) {
      // Add all of the conversion functions as candidates.
      const auto &Conversions = FromRecordDecl->getVisibleConversionFunctions();
      for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
        DeclAccessPair FoundDecl = I.getPair();
        NamedDecl *D = FoundDecl.getDecl();
        CXXRecordDecl *ActingContext = cast<CXXRecordDecl>(D->getDeclContext());
        if (isa<UsingShadowDecl>(D))
          D = cast<UsingShadowDecl>(D)->getTargetDecl();
 
        CXXConversionDecl *Conv;
        FunctionTemplateDecl *ConvTemplate;
        if ((ConvTemplate = dyn_cast<FunctionTemplateDecl>(D)))
          Conv = cast<CXXConversionDecl>(ConvTemplate->getTemplatedDecl());
        else
          Conv = cast<CXXConversionDecl>(D);
 
        if (ConvTemplate)
          S.AddTemplateConversionCandidate(
              ConvTemplate, FoundDecl, ActingContext, From, ToType,
              CandidateSet, AllowObjCConversionOnExplicit,
              AllowExplicit != AllowedExplicit::None);
        else
          S.AddConversionCandidate(Conv, FoundDecl, ActingContext, From, ToType,
                                   CandidateSet, AllowObjCConversionOnExplicit,
                                   AllowExplicit != AllowedExplicit::None);
      }
    }
  }
 
  bool HadMultipleCandidates = (CandidateSet.size() > 1);
 
  OverloadCandidateSet::iterator Best;
  switch (auto Result =
              CandidateSet.BestViableFunction(S, From->getBeginLoc(), Best)) {
  case OR_Success:
  case OR_Deleted:
    // Record the standard conversion we used and the conversion function.
    if (CXXConstructorDecl *Constructor
          = dyn_cast<CXXConstructorDecl>(Best->Function)) {
      // C++ [over.ics.user]p1:
      //   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 (isa<InitListExpr>(From)) {
        // Initializer lists don't have conversions as such.
        User.Before.setAsIdentityConversion();
      } else {
        if (Best->Conversions[0].isEllipsis())
          User.EllipsisConversion = true;
        else {
          User.Before = Best->Conversions[0].Standard;
          User.EllipsisConversion = false;
        }
      }
      User.HadMultipleCandidates = HadMultipleCandidates;
      User.ConversionFunction = Constructor;
      User.FoundConversionFunction = Best->FoundDecl;
      User.After.setAsIdentityConversion();
      User.After.setFromType(Constructor->getFunctionObjectParameterType());
      User.After.setAllToTypes(ToType);
      return Result;
    }
    if (CXXConversionDecl *Conversion
                 = dyn_cast<CXXConversionDecl>(Best->Function)) {
      // C++ [over.ics.user]p1:
      //
      //   [...] 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.
      User.Before = Best->Conversions[0].Standard;
      User.HadMultipleCandidates = HadMultipleCandidates;
      User.ConversionFunction = Conversion;
      User.FoundConversionFunction = Best->FoundDecl;
      User.EllipsisConversion = false;
 
      // C++ [over.ics.user]p2:
      //   The second standard conversion sequence converts the
      //   result of the user-defined conversion to the target type
      //   for the sequence. Since an implicit conversion sequence
      //   is an initialization, the special rules for
      //   initialization by user-defined conversion apply when
      //   selecting the best user-defined conversion for a
      //   user-defined conversion sequence (see 13.3.3 and
      //   13.3.3.1).
      User.After = Best->FinalConversion;
      return Result;
    }
    llvm_unreachable("Not a constructor or conversion function?");
 
  case OR_No_Viable_Function:
    return OR_No_Viable_Function;
 
  case OR_Ambiguous:
    return OR_Ambiguous;
  }
 
  llvm_unreachable("Invalid OverloadResult!");
}
 
bool
Sema::DiagnoseMultipleUserDefinedConversion(Expr *From, QualType ToType) {
  ImplicitConversionSequence ICS;
  OverloadCandidateSet CandidateSet(From->getExprLoc(),
                                    OverloadCandidateSet::CSK_Normal);
  OverloadingResult OvResult =
    IsUserDefinedConversion(*this, From, ToType, ICS.UserDefined,
                            CandidateSet, AllowedExplicit::None, false);
 
  if (!(OvResult == OR_Ambiguous ||
        (OvResult == OR_No_Viable_Function && !CandidateSet.empty())))
    return false;
 
  auto Cands = CandidateSet.CompleteCandidates(
      *this,
      OvResult == OR_Ambiguous ? OCD_AmbiguousCandidates : OCD_AllCandidates,
      From);
  if (OvResult == OR_Ambiguous)
    Diag(From->getBeginLoc(), diag::err_typecheck_ambiguous_condition)
        << From->getType() << ToType << From->getSourceRange();
  else { // OR_No_Viable_Function && !CandidateSet.empty()
    if (!RequireCompleteType(From->getBeginLoc(), ToType,
                             diag::err_typecheck_nonviable_condition_incomplete,
                             From->getType(), From->getSourceRange()))
      Diag(From->getBeginLoc(), diag::err_typecheck_nonviable_condition)
          << false << From->getType() << From->getSourceRange() << ToType;
  }
 
  CandidateSet.NoteCandidates(
                              *this, From, Cands);
  return true;
}
 
// Helper for compareConversionFunctions that gets the FunctionType that the
// conversion-operator return  value 'points' to, or nullptr.
static const FunctionType *
getConversionOpReturnTyAsFunction(CXXConversionDecl *Conv) {
  const FunctionType *ConvFuncTy = Conv->getType()->castAs<FunctionType>();
  const PointerType *RetPtrTy =
      ConvFuncTy->getReturnType()->getAs<PointerType>();
 
  if (!RetPtrTy)
    return nullptr;
 
  return RetPtrTy->getPointeeType()->getAs<FunctionType>();
}
 
/// Compare the user-defined conversion functions or constructors
/// of two user-defined conversion sequences to determine whether any ordering
/// is possible.
static ImplicitConversionSequence::CompareKind
compareConversionFunctions(Sema &S, FunctionDecl *Function1,
                           FunctionDecl *Function2) {
  CXXConversionDecl *Conv1 = dyn_cast_or_null<CXXConversionDecl>(Function1);
  CXXConversionDecl *Conv2 = dyn_cast_or_null<CXXConversionDecl>(Function2);
  if (!Conv1 || !Conv2)
    return ImplicitConversionSequence::Indistinguishable;
 
  if (!Conv1->getParent()->isLambda() || !Conv2->getParent()->isLambda())
    return ImplicitConversionSequence::Indistinguishable;
 
  // Objective-C++:
  //   If both conversion functions are implicitly-declared conversions from
  //   a lambda closure type to a function pointer and a block pointer,
  //   respectively, always prefer the conversion to a function pointer,
  //   because the function pointer is more lightweight and is more likely
  //   to keep code working.
  if (S.getLangOpts().ObjC && S.getLangOpts().CPlusPlus11) {
    bool Block1 = Conv1->getConversionType()->isBlockPointerType();
    bool Block2 = Conv2->getConversionType()->isBlockPointerType();
    if (Block1 != Block2)
      return Block1 ? ImplicitConversionSequence::Worse
                    : ImplicitConversionSequence::Better;
  }
 
  // In order to support multiple calling conventions for the lambda conversion
  // operator (such as when the free and member function calling convention is
  // different), prefer the 'free' mechanism, followed by the calling-convention
  // of operator(). The latter is in place to support the MSVC-like solution of
  // defining ALL of the possible conversions in regards to calling-convention.
  const FunctionType *Conv1FuncRet = getConversionOpReturnTyAsFunction(Conv1);
  const FunctionType *Conv2FuncRet = getConversionOpReturnTyAsFunction(Conv2);
 
  if (Conv1FuncRet && Conv2FuncRet &&
      Conv1FuncRet->getCallConv() != Conv2FuncRet->getCallConv()) {
    CallingConv Conv1CC = Conv1FuncRet->getCallConv();
    CallingConv Conv2CC = Conv2FuncRet->getCallConv();
 
    CXXMethodDecl *CallOp = Conv2->getParent()->getLambdaCallOperator();
    const auto *CallOpProto = CallOp->getType()->castAs<FunctionProtoType>();
 
    CallingConv CallOpCC =
        CallOp->getType()->castAs<FunctionType>()->getCallConv();
    CallingConv DefaultFree = S.Context.getDefaultCallingConvention(
        CallOpProto->isVariadic(), /*IsCXXMethod=*/false);
    CallingConv DefaultMember = S.Context.getDefaultCallingConvention(
        CallOpProto->isVariadic(), /*IsCXXMethod=*/true);
 
    CallingConv PrefOrder[] = {DefaultFree, DefaultMember, CallOpCC};
    for (CallingConv CC : PrefOrder) {
      if (Conv1CC == CC)
        return ImplicitConversionSequence::Better;
      if (Conv2CC == CC)
        return ImplicitConversionSequence::Worse;
    }
  }
 
  return ImplicitConversionSequence::Indistinguishable;
}
 
static bool hasDeprecatedStringLiteralToCharPtrConversion(
    const ImplicitConversionSequence &ICS) {
  return (ICS.isStandard() && ICS.Standard.DeprecatedStringLiteralToCharPtr) ||
         (ICS.isUserDefined() &&
          ICS.UserDefined.Before.DeprecatedStringLiteralToCharPtr);
}
 
/// CompareImplicitConversionSequences - Compare two implicit
/// conversion sequences to determine whether one is better than the
/// other or if they are indistinguishable (C++ 13.3.3.2).
static ImplicitConversionSequence::CompareKind
CompareImplicitConversionSequences(Sema &S, SourceLocation Loc,
                                   const ImplicitConversionSequence& ICS1,
                                   const ImplicitConversionSequence& ICS2)
{
  // (C++ 13.3.3.2p2): When comparing the basic forms of implicit
  // conversion sequences (as defined in 13.3.3.1)
  //   -- a standard conversion sequence (13.3.3.1.1) is a better
  //      conversion sequence than a user-defined conversion sequence or
  //      an ellipsis conversion sequence, and
  //   -- a user-defined conversion sequence (13.3.3.1.2) is a better
  //      conversion sequence than an ellipsis conversion sequence
  //      (13.3.3.1.3).
  //
  // C++0x [over.best.ics]p10:
  //   For the purpose of ranking implicit conversion sequences as
  //   described in 13.3.3.2, the ambiguous conversion sequence is
  //   treated as a user-defined sequence that is indistinguishable
  //   from any other user-defined conversion sequence.
 
  // String literal to 'char *' conversion has been deprecated in C++03. It has
  // been removed from C++11. We still accept this conversion, if it happens at
  // the best viable function. Otherwise, this conversion is considered worse
  // than ellipsis conversion. Consider this as an extension; this is not in the
  // standard. For example:
  //
  // int &f(...);    // #1
  // void f(char*);  // #2
  // void g() { int &r = f("foo"); }
  //
  // In C++03, we pick #2 as the best viable function.
  // In C++11, we pick #1 as the best viable function, because ellipsis
  // conversion is better than string-literal to char* conversion (since there
  // is no such conversion in C++11). If there was no #1 at all or #1 couldn't
  // convert arguments, #2 would be the best viable function in C++11.
  // If the best viable function has this conversion, a warning will be issued
  // in C++03, or an ExtWarn (+SFINAE failure) will be issued in C++11.
 
  if (S.getLangOpts().CPlusPlus11 && !S.getLangOpts().WritableStrings &&
      hasDeprecatedStringLiteralToCharPtrConversion(ICS1) !=
          hasDeprecatedStringLiteralToCharPtrConversion(ICS2) &&
      // Ill-formedness must not differ
      ICS1.isBad() == ICS2.isBad())
    return hasDeprecatedStringLiteralToCharPtrConversion(ICS1)
               ? ImplicitConversionSequence::Worse
               : ImplicitConversionSequence::Better;
 
  if (ICS1.getKindRank() < ICS2.getKindRank())
    return ImplicitConversionSequence::Better;
  if (ICS2.getKindRank() < ICS1.getKindRank())
    return ImplicitConversionSequence::Worse;
 
  // The following checks require both conversion sequences to be of
  // the same kind.
  if (ICS1.getKind() != ICS2.getKind())
    return ImplicitConversionSequence::Indistinguishable;
 
  ImplicitConversionSequence::CompareKind Result =
      ImplicitConversionSequence::Indistinguishable;
 
  // Two implicit conversion sequences of the same form are
  // indistinguishable conversion sequences unless one of the
  // following rules apply: (C++ 13.3.3.2p3):
 
  // List-initialization sequence L1 is a better conversion sequence than
  // list-initialization sequence L2 if:
  // - L1 converts to std::initializer_list<X> for some X and L2 does not, or,
  //   if not that,
  // — L1 and L2 convert to arrays of the same element type, and either the
  //   number of elements n_1 initialized by L1 is less than the number of
  //   elements n_2 initialized by L2, or (C++20) n_1 = n_2 and L2 converts to
  //   an array of unknown bound and L1 does not,
  // even if one of the other rules in this paragraph would otherwise apply.
  if (!ICS1.isBad()) {
    bool StdInit1 = false, StdInit2 = false;
    if (ICS1.hasInitializerListContainerType())
      StdInit1 = S.isStdInitializerList(ICS1.getInitializerListContainerType(),
                                        nullptr);
    if (ICS2.hasInitializerListContainerType())
      StdInit2 = S.isStdInitializerList(ICS2.getInitializerListContainerType(),
                                        nullptr);
    if (StdInit1 != StdInit2)
      return StdInit1 ? ImplicitConversionSequence::Better
                      : ImplicitConversionSequence::Worse;
 
    if (ICS1.hasInitializerListContainerType() &&
        ICS2.hasInitializerListContainerType())
      if (auto *CAT1 = S.Context.getAsConstantArrayType(
              ICS1.getInitializerListContainerType()))
        if (auto *CAT2 = S.Context.getAsConstantArrayType(
                ICS2.getInitializerListContainerType())) {
          if (S.Context.hasSameUnqualifiedType(CAT1->getElementType(),
                                               CAT2->getElementType())) {
            // Both to arrays of the same element type
            if (CAT1->getSize() != CAT2->getSize())
              // Different sized, the smaller wins
              return CAT1->getSize().ult(CAT2->getSize())
                         ? ImplicitConversionSequence::Better
                         : ImplicitConversionSequence::Worse;
            if (ICS1.isInitializerListOfIncompleteArray() !=
                ICS2.isInitializerListOfIncompleteArray())
              // One is incomplete, it loses
              return ICS2.isInitializerListOfIncompleteArray()
                         ? ImplicitConversionSequence::Better
                         : ImplicitConversionSequence::Worse;
          }
        }
  }
 
  if (ICS1.isStandard())
    // Standard conversion sequence S1 is a better conversion sequence than
    // standard conversion sequence S2 if [...]
    Result = CompareStandardConversionSequences(S, Loc,
                                                ICS1.Standard, ICS2.Standard);
  else if (ICS1.isUserDefined()) {
    // User-defined conversion sequence U1 is a better conversion
    // sequence than another user-defined conversion sequence U2 if
    // they contain the same user-defined conversion function or
    // constructor and if the second standard conversion sequence of
    // U1 is better than the second standard conversion sequence of
    // U2 (C++ 13.3.3.2p3).
    if (ICS1.UserDefined.ConversionFunction ==
          ICS2.UserDefined.ConversionFunction)
      Result = CompareStandardConversionSequences(S, Loc,
                                                  ICS1.UserDefined.After,
                                                  ICS2.UserDefined.After);
    else
      Result = compareConversionFunctions(S,
                                          ICS1.UserDefined.ConversionFunction,
                                          ICS2.UserDefined.ConversionFunction);
  }
 
  return Result;
}
 
// Per 13.3.3.2p3, compare the given standard conversion sequences to
// determine if one is a proper subset of the other.
static ImplicitConversionSequence::CompareKind
compareStandardConversionSubsets(ASTContext &Context,
                                 const StandardConversionSequence& SCS1,
                                 const StandardConversionSequence& SCS2) {
  ImplicitConversionSequence::CompareKind Result
    = ImplicitConversionSequence::Indistinguishable;
 
  // the identity conversion sequence is considered to be a subsequence of
  // any non-identity conversion sequence
  if (SCS1.isIdentityConversion() && !SCS2.isIdentityConversion())
    return ImplicitConversionSequence::Better;
  else if (!SCS1.isIdentityConversion() && SCS2.isIdentityConversion())
    return ImplicitConversionSequence::Worse;
 
  if (SCS1.Second != SCS2.Second) {
    if (SCS1.Second == ICK_Identity)
      Result = ImplicitConversionSequence::Better;
    else if (SCS2.Second == ICK_Identity)
      Result = ImplicitConversionSequence::Worse;
    else
      return ImplicitConversionSequence::Indistinguishable;
  } else if (!Context.hasSimilarType(SCS1.getToType(1), SCS2.getToType(1)))
    return ImplicitConversionSequence::Indistinguishable;
 
  if (SCS1.Third == SCS2.Third) {
    return Context.hasSameType(SCS1.getToType(2), SCS2.getToType(2))? Result
                             : ImplicitConversionSequence::Indistinguishable;
  }
 
  if (SCS1.Third == ICK_Identity)
    return Result == ImplicitConversionSequence::Worse
             ? ImplicitConversionSequence::Indistinguishable
             : ImplicitConversionSequence::Better;
 
  if (SCS2.Third == ICK_Identity)
    return Result == ImplicitConversionSequence::Better
             ? ImplicitConversionSequence::Indistinguishable
             : ImplicitConversionSequence::Worse;
 
  return ImplicitConversionSequence::Indistinguishable;
}
 
/// Determine whether one of the given reference bindings is better
/// than the other based on what kind of bindings they are.
static bool
isBetterReferenceBindingKind(const StandardConversionSequence &SCS1,
                             const StandardConversionSequence &SCS2) {
  // C++0x [over.ics.rank]p3b4:
  //   -- S1 and S2 are reference bindings (8.5.3) and neither refers to an
  //      implicit object parameter of a non-static member function declared
  //      without a ref-qualifier, and *either* S1 binds an rvalue reference
  //      to an rvalue and S2 binds an lvalue reference *or S1 binds an
  //      lvalue reference to a function lvalue and S2 binds an rvalue
  //      reference*.
  //
  // FIXME: Rvalue references. We're going rogue with the above edits,
  // because the semantics in the current C++0x working paper (N3225 at the
  // time of this writing) break the standard definition of std::forward
  // and std::reference_wrapper when dealing with references to functions.
  // Proposed wording changes submitted to CWG for consideration.
  if (SCS1.BindsImplicitObjectArgumentWithoutRefQualifier ||
      SCS2.BindsImplicitObjectArgumentWithoutRefQualifier)
    return false;
 
  return (!SCS1.IsLvalueReference && SCS1.BindsToRvalue &&
          SCS2.IsLvalueReference) ||
         (SCS1.IsLvalueReference && SCS1.BindsToFunctionLvalue &&
          !SCS2.IsLvalueReference && SCS2.BindsToFunctionLvalue);
}
 
enum class FixedEnumPromotion {
  None,
  ToUnderlyingType,
  ToPromotedUnderlyingType
};
 
/// Returns kind of fixed enum promotion the \a SCS uses.
static FixedEnumPromotion
getFixedEnumPromtion(Sema &S, const StandardConversionSequence &SCS) {
 
  if (SCS.Second != ICK_Integral_Promotion)
    return FixedEnumPromotion::None;
 
  QualType FromType = SCS.getFromType();
  if (!FromType->isEnumeralType())
    return FixedEnumPromotion::None;
 
  EnumDecl *Enum = FromType->castAs<EnumType>()->getDecl();
  if (!Enum->isFixed())
    return FixedEnumPromotion::None;
 
  QualType UnderlyingType = Enum->getIntegerType();
  if (S.Context.hasSameType(SCS.getToType(1), UnderlyingType))
    return FixedEnumPromotion::ToUnderlyingType;
 
  return FixedEnumPromotion::ToPromotedUnderlyingType;
}
 
/// CompareStandardConversionSequences - Compare two standard
/// conversion sequences to determine whether one is better than the
/// other or if they are indistinguishable (C++ 13.3.3.2p3).
static ImplicitConversionSequence::CompareKind
CompareStandardConversionSequences(Sema &S, SourceLocation Loc,
                                   const StandardConversionSequence& SCS1,
                                   const StandardConversionSequence& SCS2)
{
  // Standard conversion sequence S1 is a better conversion sequence
  // than standard conversion sequence S2 if (C++ 13.3.3.2p3):
 
  //  -- S1 is a proper subsequence of S2 (comparing the conversion
  //     sequences in the canonical form defined by 13.3.3.1.1,
  //     excluding any Lvalue Transformation; the identity conversion
  //     sequence is considered to be a subsequence of any
  //     non-identity conversion sequence) or, if not that,
  if (ImplicitConversionSequence::CompareKind CK
        = compareStandardConversionSubsets(S.Context, SCS1, SCS2))
    return CK;
 
  //  -- the rank of S1 is better than the rank of S2 (by the rules
  //     defined below), or, if not that,
  ImplicitConversionRank Rank1 = SCS1.getRank();
  ImplicitConversionRank Rank2 = SCS2.getRank();
  if (Rank1 < Rank2)
    return ImplicitConversionSequence::Better;
  else if (Rank2 < Rank1)
    return ImplicitConversionSequence::Worse;
 
  // (C++ 13.3.3.2p4): Two conversion sequences with the same rank
  // are indistinguishable unless one of the following rules
  // applies:
 
  //   A conversion that is not a conversion of a pointer, or
  //   pointer to member, to bool is better than another conversion
  //   that is such a conversion.
  if (SCS1.isPointerConversionToBool() != SCS2.isPointerConversionToBool())
    return SCS2.isPointerConversionToBool()
             ? ImplicitConversionSequence::Better
             : ImplicitConversionSequence::Worse;
 
  // C++14 [over.ics.rank]p4b2:
  // This is retroactively applied to C++11 by CWG 1601.
  //
  //   A conversion that promotes an enumeration whose underlying type is fixed
  //   to its underlying type is better than one that promotes to the promoted
  //   underlying type, if the two are different.
  FixedEnumPromotion FEP1 = getFixedEnumPromtion(S, SCS1);
  FixedEnumPromotion FEP2 = getFixedEnumPromtion(S, SCS2);
  if (FEP1 != FixedEnumPromotion::None && FEP2 != FixedEnumPromotion::None &&
      FEP1 != FEP2)
    return FEP1 == FixedEnumPromotion::ToUnderlyingType
               ? ImplicitConversionSequence::Better
               : ImplicitConversionSequence::Worse;
 
  // C++ [over.ics.rank]p4b2:
  //
  //   If class B is derived directly or indirectly from class A,
  //   conversion of B* to A* is better than conversion of B* to
  //   void*, and conversion of A* to void* is better than conversion
  //   of B* to void*.
  bool SCS1ConvertsToVoid
    = SCS1.isPointerConversionToVoidPointer(S.Context);
  bool SCS2ConvertsToVoid
    = SCS2.isPointerConversionToVoidPointer(S.Context);
  if (SCS1ConvertsToVoid != SCS2ConvertsToVoid) {
    // Exactly one of the conversion sequences is a conversion to
    // a void pointer; it's the worse conversion.
    return SCS2ConvertsToVoid ? ImplicitConversionSequence::Better
                              : ImplicitConversionSequence::Worse;
  } else if (!SCS1ConvertsToVoid && !SCS2ConvertsToVoid) {
    // Neither conversion sequence converts to a void pointer; compare
    // their derived-to-base conversions.
    if (ImplicitConversionSequence::CompareKind DerivedCK
          = CompareDerivedToBaseConversions(S, Loc, SCS1, SCS2))
      return DerivedCK;
  } else if (SCS1ConvertsToVoid && SCS2ConvertsToVoid &&
             !S.Context.hasSameType(SCS1.getFromType(), SCS2.getFromType())) {
    // Both conversion sequences are conversions to void
    // pointers. Compare the source types to determine if there's an
    // inheritance relationship in their sources.
    QualType FromType1 = SCS1.getFromType();
    QualType FromType2 = SCS2.getFromType();
 
    // Adjust the types we're converting from via the array-to-pointer
    // conversion, if we need to.
    if (SCS1.First == ICK_Array_To_Pointer)
      FromType1 = S.Context.getArrayDecayedType(FromType1);
    if (SCS2.First == ICK_Array_To_Pointer)
      FromType2 = S.Context.getArrayDecayedType(FromType2);
 
    QualType FromPointee1 = FromType1->getPointeeType().getUnqualifiedType();
    QualType FromPointee2 = FromType2->getPointeeType().getUnqualifiedType();
 
    if (S.IsDerivedFrom(Loc, FromPointee2, FromPointee1))
      return ImplicitConversionSequence::Better;
    else if (S.IsDerivedFrom(Loc, FromPointee1, FromPointee2))
      return ImplicitConversionSequence::Worse;
 
    // Objective-C++: If one interface is more specific than the
    // other, it is the better one.
    const ObjCObjectPointerType* FromObjCPtr1
      = FromType1->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType* FromObjCPtr2
      = FromType2->getAs<ObjCObjectPointerType>();
    if (FromObjCPtr1 && FromObjCPtr2) {
      bool AssignLeft = S.Context.canAssignObjCInterfaces(FromObjCPtr1,
                                                          FromObjCPtr2);
      bool AssignRight = S.Context.canAssignObjCInterfaces(FromObjCPtr2,
                                                           FromObjCPtr1);
      if (AssignLeft != AssignRight) {
        return AssignLeft? ImplicitConversionSequence::Better
                         : ImplicitConversionSequence::Worse;
      }
    }
  }
 
  if (SCS1.ReferenceBinding && SCS2.ReferenceBinding) {
    // Check for a better reference binding based on the kind of bindings.
    if (isBetterReferenceBindingKind(SCS1, SCS2))
      return ImplicitConversionSequence::Better;
    else if (isBetterReferenceBindingKind(SCS2, SCS1))
      return ImplicitConversionSequence::Worse;
  }
 
  // Compare based on qualification conversions (C++ 13.3.3.2p3,
  // bullet 3).
  if (ImplicitConversionSequence::CompareKind QualCK
        = CompareQualificationConversions(S, SCS1, SCS2))
    return QualCK;
 
  if (SCS1.ReferenceBinding && SCS2.ReferenceBinding) {
    // C++ [over.ics.rank]p3b4:
    //   -- S1 and S2 are reference bindings (8.5.3), and the types to
    //      which the references refer are the same type except for
    //      top-level cv-qualifiers, and the type to which the reference
    //      initialized by S2 refers is more cv-qualified than the type
    //      to which the reference initialized by S1 refers.
    QualType T1 = SCS1.getToType(2);
    QualType T2 = SCS2.getToType(2);
    T1 = S.Context.getCanonicalType(T1);
    T2 = S.Context.getCanonicalType(T2);
    Qualifiers T1Quals, T2Quals;
    QualType UnqualT1 = S.Context.getUnqualifiedArrayType(T1, T1Quals);
    QualType UnqualT2 = S.Context.getUnqualifiedArrayType(T2, T2Quals);
    if (UnqualT1 == UnqualT2) {
      // Objective-C++ ARC: If the references refer to objects with different
      // lifetimes, prefer bindings that don't change lifetime.
      if (SCS1.ObjCLifetimeConversionBinding !=
                                          SCS2.ObjCLifetimeConversionBinding) {
        return SCS1.ObjCLifetimeConversionBinding
                                           ? ImplicitConversionSequence::Worse
                                           : ImplicitConversionSequence::Better;
      }
 
      // If the type is an array type, promote the element qualifiers to the
      // type for comparison.
      if (isa<ArrayType>(T1) && T1Quals)
        T1 = S.Context.getQualifiedType(UnqualT1, T1Quals);
      if (isa<ArrayType>(T2) && T2Quals)
        T2 = S.Context.getQualifiedType(UnqualT2, T2Quals);
      if (T2.isMoreQualifiedThan(T1, S.getASTContext()))
        return ImplicitConversionSequence::Better;
      if (T1.isMoreQualifiedThan(T2, S.getASTContext()))
        return ImplicitConversionSequence::Worse;
    }
  }
 
  // In Microsoft mode (below 19.28), prefer an integral conversion to a
  // floating-to-integral conversion if the integral conversion
  // is between types of the same size.
  // For example:
  // void f(float);
  // void f(int);
  // int main {
  //    long a;
  //    f(a);
  // }
  // Here, MSVC will call f(int) instead of generating a compile error
  // as clang will do in standard mode.
  if (S.getLangOpts().MSVCCompat &&
      !S.getLangOpts().isCompatibleWithMSVC(LangOptions::MSVC2019_8) &&
      SCS1.Second == ICK_Integral_Conversion &&
      SCS2.Second == ICK_Floating_Integral &&
      S.Context.getTypeSize(SCS1.getFromType()) ==
          S.Context.getTypeSize(SCS1.getToType(2)))
    return ImplicitConversionSequence::Better;
 
  // Prefer a compatible vector conversion over a lax vector conversion
  // For example:
  //
  // typedef float __v4sf __attribute__((__vector_size__(16)));
  // void f(vector float);
  // void f(vector signed int);
  // int main() {
  //   __v4sf a;
  //   f(a);
  // }
  // Here, we'd like to choose f(vector float) and not
  // report an ambiguous call error
  if (SCS1.Second == ICK_Vector_Conversion &&
      SCS2.Second == ICK_Vector_Conversion) {
    bool SCS1IsCompatibleVectorConversion = S.Context.areCompatibleVectorTypes(
        SCS1.getFromType(), SCS1.getToType(2));
    bool SCS2IsCompatibleVectorConversion = S.Context.areCompatibleVectorTypes(
        SCS2.getFromType(), SCS2.getToType(2));
 
    if (SCS1IsCompatibleVectorConversion != SCS2IsCompatibleVectorConversion)
      return SCS1IsCompatibleVectorConversion
                 ? ImplicitConversionSequence::Better
                 : ImplicitConversionSequence::Worse;
  }
 
  if (SCS1.Second == ICK_SVE_Vector_Conversion &&
      SCS2.Second == ICK_SVE_Vector_Conversion) {
    bool SCS1IsCompatibleSVEVectorConversion =
        S.Context.areCompatibleSveTypes(SCS1.getFromType(), SCS1.getToType(2));
    bool SCS2IsCompatibleSVEVectorConversion =
        S.Context.areCompatibleSveTypes(SCS2.getFromType(), SCS2.getToType(2));
 
    if (SCS1IsCompatibleSVEVectorConversion !=
        SCS2IsCompatibleSVEVectorConversion)
      return SCS1IsCompatibleSVEVectorConversion
                 ? ImplicitConversionSequence::Better
                 : ImplicitConversionSequence::Worse;
  }
 
  if (SCS1.Second == ICK_RVV_Vector_Conversion &&
      SCS2.Second == ICK_RVV_Vector_Conversion) {
    bool SCS1IsCompatibleRVVVectorConversion =
        S.Context.areCompatibleRVVTypes(SCS1.getFromType(), SCS1.getToType(2));
    bool SCS2IsCompatibleRVVVectorConversion =
        S.Context.areCompatibleRVVTypes(SCS2.getFromType(), SCS2.getToType(2));
 
    if (SCS1IsCompatibleRVVVectorConversion !=
        SCS2IsCompatibleRVVVectorConversion)
      return SCS1IsCompatibleRVVVectorConversion
                 ? ImplicitConversionSequence::Better
                 : ImplicitConversionSequence::Worse;
  }
  return ImplicitConversionSequence::Indistinguishable;
}
 
/// CompareQualificationConversions - Compares two standard conversion
/// sequences to determine whether they can be ranked based on their
/// qualification conversions (C++ 13.3.3.2p3 bullet 3).
static ImplicitConversionSequence::CompareKind
CompareQualificationConversions(Sema &S,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2) {
  // C++ [over.ics.rank]p3:
  //  -- S1 and S2 differ only in their qualification conversion and
  //     yield similar types T1 and T2 (C++ 4.4), respectively, [...]
  // [C++98]
  //     [...] and the cv-qualification signature of type T1 is a proper subset
  //     of the cv-qualification signature of type T2, and S1 is not the
  //     deprecated string literal array-to-pointer conversion (4.2).
  // [C++2a]
  //     [...] where T1 can be converted to T2 by a qualification conversion.
  if (SCS1.First != SCS2.First || SCS1.Second != SCS2.Second ||
      SCS1.Third != SCS2.Third || SCS1.Third != ICK_Qualification)
    return ImplicitConversionSequence::Indistinguishable;
 
  // FIXME: the example in the standard doesn't use a qualification
  // conversion (!)
  QualType T1 = SCS1.getToType(2);
  QualType T2 = SCS2.getToType(2);
  T1 = S.Context.getCanonicalType(T1);
  T2 = S.Context.getCanonicalType(T2);
  assert(!T1->isReferenceType() && !T2->isReferenceType());
  Qualifiers T1Quals, T2Quals;
  QualType UnqualT1 = S.Context.getUnqualifiedArrayType(T1, T1Quals);
  QualType UnqualT2 = S.Context.getUnqualifiedArrayType(T2, T2Quals);
 
  // If the types are the same, we won't learn anything by unwrapping
  // them.
  if (UnqualT1 == UnqualT2)
    return ImplicitConversionSequence::Indistinguishable;
 
  // Don't ever prefer a standard conversion sequence that uses the deprecated
  // string literal array to pointer conversion.
  bool CanPick1 = !SCS1.DeprecatedStringLiteralToCharPtr;
  bool CanPick2 = !SCS2.DeprecatedStringLiteralToCharPtr;
 
  // Objective-C++ ARC:
  //   Prefer qualification conversions not involving a change in lifetime
  //   to qualification conversions that do change lifetime.
  if (SCS1.QualificationIncludesObjCLifetime &&
      !SCS2.QualificationIncludesObjCLifetime)
    CanPick1 = false;
  if (SCS2.QualificationIncludesObjCLifetime &&
      !SCS1.QualificationIncludesObjCLifetime)
    CanPick2 = false;
 
  bool ObjCLifetimeConversion;
  if (CanPick1 &&
      !S.IsQualificationConversion(T1, T2, false, ObjCLifetimeConversion))
    CanPick1 = false;
  // FIXME: In Objective-C ARC, we can have qualification conversions in both
  // directions, so we can't short-cut this second check in general.
  if (CanPick2 &&
      !S.IsQualificationConversion(T2, T1, false, ObjCLifetimeConversion))
    CanPick2 = false;
 
  if (CanPick1 != CanPick2)
    return CanPick1 ? ImplicitConversionSequence::Better
                    : ImplicitConversionSequence::Worse;
  return ImplicitConversionSequence::Indistinguishable;
}
 
/// CompareDerivedToBaseConversions - Compares two standard conversion
/// sequences to determine whether they can be ranked based on their
/// various kinds of derived-to-base conversions (C++
/// [over.ics.rank]p4b3).  As part of these checks, we also look at
/// conversions between Objective-C interface types.
static ImplicitConversionSequence::CompareKind
CompareDerivedToBaseConversions(Sema &S, SourceLocation Loc,
                                const StandardConversionSequence& SCS1,
                                const StandardConversionSequence& SCS2) {
  QualType FromType1 = SCS1.getFromType();
  QualType ToType1 = SCS1.getToType(1);
  QualType FromType2 = SCS2.getFromType();
  QualType ToType2 = SCS2.getToType(1);
 
  // Adjust the types we're converting from via the array-to-pointer
  // conversion, if we need to.
  if (SCS1.First == ICK_Array_To_Pointer)
    FromType1 = S.Context.getArrayDecayedType(FromType1);
  if (SCS2.First == ICK_Array_To_Pointer)
    FromType2 = S.Context.getArrayDecayedType(FromType2);
 
  // Canonicalize all of the types.
  FromType1 = S.Context.getCanonicalType(FromType1);
  ToType1 = S.Context.getCanonicalType(ToType1);
  FromType2 = S.Context.getCanonicalType(FromType2);
  ToType2 = S.Context.getCanonicalType(ToType2);
 
  // C++ [over.ics.rank]p4b3:
  //
  //   If class B is derived directly or indirectly from class A and
  //   class C is derived directly or indirectly from B,
  //
  // Compare based on pointer conversions.
  if (SCS1.Second == ICK_Pointer_Conversion &&
      SCS2.Second == ICK_Pointer_Conversion &&
      /*FIXME: Remove if Objective-C id conversions get their own rank*/
      FromType1->isPointerType() && FromType2->isPointerType() &&
      ToType1->isPointerType() && ToType2->isPointerType()) {
    QualType FromPointee1 =
        FromType1->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType ToPointee1 =
        ToType1->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType FromPointee2 =
        FromType2->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType ToPointee2 =
        ToType2->castAs<PointerType>()->getPointeeType().getUnqualifiedType();
 
    //   -- conversion of C* to B* is better than conversion of C* to A*,
    if (FromPointee1 == FromPointee2 && ToPointee1 != ToPointee2) {
      if (S.IsDerivedFrom(Loc, ToPointee1, ToPointee2))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, ToPointee2, ToPointee1))
        return ImplicitConversionSequence::Worse;
    }
 
    //   -- conversion of B* to A* is better than conversion of C* to A*,
    if (FromPointee1 != FromPointee2 && ToPointee1 == ToPointee2) {
      if (S.IsDerivedFrom(Loc, FromPointee2, FromPointee1))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, FromPointee1, FromPointee2))
        return ImplicitConversionSequence::Worse;
    }
  } else if (SCS1.Second == ICK_Pointer_Conversion &&
             SCS2.Second == ICK_Pointer_Conversion) {
    const ObjCObjectPointerType *FromPtr1
      = FromType1->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType *FromPtr2
      = FromType2->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType *ToPtr1
      = ToType1->getAs<ObjCObjectPointerType>();
    const ObjCObjectPointerType *ToPtr2
      = ToType2->getAs<ObjCObjectPointerType>();
 
    if (FromPtr1 && FromPtr2 && ToPtr1 && ToPtr2) {
      // Apply the same conversion ranking rules for Objective-C pointer types
      // that we do for C++ pointers to class types. However, we employ the
      // Objective-C pseudo-subtyping relationship used for assignment of
      // Objective-C pointer types.
      bool FromAssignLeft
        = S.Context.canAssignObjCInterfaces(FromPtr1, FromPtr2);
      bool FromAssignRight
        = S.Context.canAssignObjCInterfaces(FromPtr2, FromPtr1);
      bool ToAssignLeft
        = S.Context.canAssignObjCInterfaces(ToPtr1, ToPtr2);
      bool ToAssignRight
        = S.Context.canAssignObjCInterfaces(ToPtr2, ToPtr1);
 
      // A conversion to an a non-id object pointer type or qualified 'id'
      // type is better than a conversion to 'id'.
      if (ToPtr1->isObjCIdType() &&
          (ToPtr2->isObjCQualifiedIdType() || ToPtr2->getInterfaceDecl()))
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCIdType() &&
          (ToPtr1->isObjCQualifiedIdType() || ToPtr1->getInterfaceDecl()))
        return ImplicitConversionSequence::Better;
 
      // A conversion to a non-id object pointer type is better than a
      // conversion to a qualified 'id' type
      if (ToPtr1->isObjCQualifiedIdType() && ToPtr2->getInterfaceDecl())
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCQualifiedIdType() && ToPtr1->getInterfaceDecl())
        return ImplicitConversionSequence::Better;
 
      // A conversion to an a non-Class object pointer type or qualified 'Class'
      // type is better than a conversion to 'Class'.
      if (ToPtr1->isObjCClassType() &&
          (ToPtr2->isObjCQualifiedClassType() || ToPtr2->getInterfaceDecl()))
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCClassType() &&
          (ToPtr1->isObjCQualifiedClassType() || ToPtr1->getInterfaceDecl()))
        return ImplicitConversionSequence::Better;
 
      // A conversion to a non-Class object pointer type is better than a
      // conversion to a qualified 'Class' type.
      if (ToPtr1->isObjCQualifiedClassType() && ToPtr2->getInterfaceDecl())
        return ImplicitConversionSequence::Worse;
      if (ToPtr2->isObjCQualifiedClassType() && ToPtr1->getInterfaceDecl())
        return ImplicitConversionSequence::Better;
 
      //   -- "conversion of C* to B* is better than conversion of C* to A*,"
      if (S.Context.hasSameType(FromType1, FromType2) &&
          !FromPtr1->isObjCIdType() && !FromPtr1->isObjCClassType() &&
          (ToAssignLeft != ToAssignRight)) {
        if (FromPtr1->isSpecialized()) {
          // "conversion of B<A> * to B * is better than conversion of B * to
          // C *.
          bool IsFirstSame =
              FromPtr1->getInterfaceDecl() == ToPtr1->getInterfaceDecl();
          bool IsSecondSame =
              FromPtr1->getInterfaceDecl() == ToPtr2->getInterfaceDecl();
          if (IsFirstSame) {
            if (!IsSecondSame)
              return ImplicitConversionSequence::Better;
          } else if (IsSecondSame)
            return ImplicitConversionSequence::Worse;
        }
        return ToAssignLeft? ImplicitConversionSequence::Worse
                           : ImplicitConversionSequence::Better;
      }
 
      //   -- "conversion of B* to A* is better than conversion of C* to A*,"
      if (S.Context.hasSameUnqualifiedType(ToType1, ToType2) &&
          (FromAssignLeft != FromAssignRight))
        return FromAssignLeft? ImplicitConversionSequence::Better
        : ImplicitConversionSequence::Worse;
    }
  }
 
  // Ranking of member-pointer types.
  if (SCS1.Second == ICK_Pointer_Member && SCS2.Second == ICK_Pointer_Member &&
      FromType1->isMemberPointerType() && FromType2->isMemberPointerType() &&
      ToType1->isMemberPointerType() && ToType2->isMemberPointerType()) {
    const auto *FromMemPointer1 = FromType1->castAs<MemberPointerType>();
    const auto *ToMemPointer1 = ToType1->castAs<MemberPointerType>();
    const auto *FromMemPointer2 = FromType2->castAs<MemberPointerType>();