SemaOverload.cpp

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//===--- SemaOverload.cpp - C++ Overloading -------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file provides Sema routines for C++ overloading.
//
//===----------------------------------------------------------------------===//

#include "clang/Sema/Overload.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/ExprObjC.h"
#include "clang/AST/TypeOrdering.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/Basic/DiagnosticOptions.h"
#include "clang/Basic/PartialDiagnostic.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/Initialization.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "clang/Sema/TemplateDeduction.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallString.h"
#include <algorithm>
#include <cstdlib>

using namespace clang;
using namespace sema;

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();
  if (auto *FPT = Fn->getType()->getAs<FunctionProtoType>())
    S.ResolveExceptionSpec(Loc, FPT);
  DeclRefExpr *DRE = new (S.Context) DeclRefExpr(Fn, false, Fn->getType(),
                                                 VK_LValue, Loc, LocInfo);
  if (HadMultipleCandidates)
    DRE->setHadMultipleCandidates(true);

  S.MarkDeclRefReferenced(DRE, Base);
  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,
                        bool 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[(int)ICK_Num_Conversion_Kinds] = {
    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_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_C_Conversion,
    ICR_C_Conversion_Extension
  };
  return Rank[(int)Kind];
}

/// GetImplicitConversionName - Return the name of this kind of
/// implicit conversion.
static const char* GetImplicitConversionName(ImplicitConversionKind Kind) {
  static const char* const Name[(int)ICK_Num_Conversion_Kinds] = {
    "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",
    "Vector splat",
    "Complex-real conversion",
    "Block Pointer conversion",
    "Transparent Union Conversion",
    "Writeback conversion",
    "OpenCL Zero Event Conversion",
    "C specific type conversion",
    "Incompatible pointer conversion"
  };
  return Name[Kind];
}

/// StandardConversionSequence - Set the standard conversion
/// sequence to the identity conversion.
void StandardConversionSequence::setAsIdentityConversion() {
  First = ICK_Identity;
  Second = 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  (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() ||
       getFromType()->isNullPtrType() ||
       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(const Expr *Converted) {
  while (const ImplicitCastExpr *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 && "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;
    // Boolean conversions can be from pointers and pointers to members
    // [conv.bool], and those aren't considered narrowing conversions.
    return NK_Not_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;
      llvm::APSInt IntConstantValue;
      const Expr *Initializer = IgnoreNarrowingConversion(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 (Initializer->isIntegerConstantExpr(IntConstantValue, 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(Converted);

      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;

      if (Initializer->isCXX11ConstantExpr(Ctx, &ConstantValue)) {
        // Constant!
        assert(ConstantValue.isFloat());
        llvm::APFloat FloatVal = ConstantValue.getFloat();
        // Convert the source value into the target type.
        bool ignored;
        llvm::APFloat::opStatus ConvertStatus = FloatVal.convert(
          Ctx.getFloatTypeSemantics(ToType),
          llvm::APFloat::rmNearestTiesToEven, &ignored);
        // 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
  //    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();
    const unsigned FromWidth = Ctx.getIntWidth(FromType);
    const bool ToSigned = ToType->isSignedIntegerOrEnumerationType();
    const unsigned ToWidth = Ctx.getIntWidth(ToType);

    if (FromWidth > ToWidth ||
        (FromWidth == ToWidth && FromSigned != ToSigned) ||
        (FromSigned && !ToSigned)) {
      // Not all values of FromType can be represented in ToType.
      llvm::APSInt InitializerValue;
      const Expr *Initializer = IgnoreNarrowingConversion(Converted);

      // If it's value-dependent, we can't tell whether it's narrowing.
      if (Initializer->isValueDependent())
        return NK_Dependent_Narrowing;

      if (!Initializer->isIntegerConstantExpr(InitializerValue, Ctx)) {
        // Such conversions on variables are always narrowing.
        return NK_Variable_Narrowing;
      }
      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;
  }

  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 (isStdInitializerListElement())
    OS << "Worst std::initializer_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;
  };
}

/// Convert from Sema's representation of template deduction information
/// to the form used in overload-candidate information.
DeductionFailureInfo
clang::MakeDeductionFailureInfo(ASTContext &Context,
                                Sema::TemplateDeductionResult TDK,
                                TemplateDeductionInfo &Info) {
  DeductionFailureInfo Result;
  Result.Result = static_cast<unsigned>(TDK);
  Result.HasDiagnostic = false;
  switch (TDK) {
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_MiscellaneousDeductionFailure:
  case Sema::TDK_CUDATargetMismatch:
    Result.Data = nullptr;
    break;

  case Sema::TDK_Incomplete:
  case Sema::TDK_InvalidExplicitArguments:
    Result.Data = Info.Param.getOpaqueValue();
    break;

  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_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.take();
    Saved->CallArgIndex = Info.CallArgIndex;
    Result.Data = Saved;
    break;
  }

  case Sema::TDK_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 Sema::TDK_IncompletePack:
    // FIXME: It's slightly wasteful to allocate two TemplateArguments for this.
  case Sema::TDK_Inconsistent:
  case Sema::TDK_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 Sema::TDK_SubstitutionFailure:
    Result.Data = Info.take();
    if (Info.hasSFINAEDiagnostic()) {
      PartialDiagnosticAt *Diag = new (Result.Diagnostic) PartialDiagnosticAt(
          SourceLocation(), PartialDiagnostic::NullDiagnostic());
      Info.takeSFINAEDiagnostic(*Diag);
      Result.HasDiagnostic = true;
    }
    break;

  case Sema::TDK_Success:
  case Sema::TDK_NonDependentConversionFailure:
    llvm_unreachable("not a deduction failure");
  }

  return Result;
}

void DeductionFailureInfo::Destroy() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_Incomplete:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    break;

  case Sema::TDK_IncompletePack:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
    // FIXME: Destroy the data?
    Data = nullptr;
    break;

  case Sema::TDK_SubstitutionFailure:
    // FIXME: Destroy the template argument list?
    Data = nullptr;
    if (PartialDiagnosticAt *Diag = getSFINAEDiagnostic()) {
      Diag->~PartialDiagnosticAt();
      HasDiagnostic = false;
    }
    break;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
    break;
  }
}

PartialDiagnosticAt *DeductionFailureInfo::getSFINAEDiagnostic() {
  if (HasDiagnostic)
    return static_cast<PartialDiagnosticAt*>(static_cast<void*>(Diagnostic));
  return nullptr;
}

TemplateParameter DeductionFailureInfo::getTemplateParameter() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    return TemplateParameter();

  case Sema::TDK_Incomplete:
  case Sema::TDK_InvalidExplicitArguments:
    return TemplateParameter::getFromOpaqueValue(Data);

  case Sema::TDK_IncompletePack:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
    return static_cast<DFIParamWithArguments*>(Data)->Param;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
    break;
  }

  return TemplateParameter();
}

TemplateArgumentList *DeductionFailureInfo::getTemplateArgumentList() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_Incomplete:
  case Sema::TDK_IncompletePack:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_NonDeducedMismatch:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    return nullptr;

  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
    return static_cast<DFIDeducedMismatchArgs*>(Data)->TemplateArgs;

  case Sema::TDK_SubstitutionFailure:
    return static_cast<TemplateArgumentList*>(Data);

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
    break;
  }

  return nullptr;
}

const TemplateArgument *DeductionFailureInfo::getFirstArg() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_Incomplete:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    return nullptr;

  case Sema::TDK_IncompletePack:
  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
    return &static_cast<DFIArguments*>(Data)->FirstArg;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
    break;
  }

  return nullptr;
}

const TemplateArgument *DeductionFailureInfo::getSecondArg() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_Success:
  case Sema::TDK_Invalid:
  case Sema::TDK_InstantiationDepth:
  case Sema::TDK_Incomplete:
  case Sema::TDK_IncompletePack:
  case Sema::TDK_TooManyArguments:
  case Sema::TDK_TooFewArguments:
  case Sema::TDK_InvalidExplicitArguments:
  case Sema::TDK_SubstitutionFailure:
  case Sema::TDK_CUDATargetMismatch:
  case Sema::TDK_NonDependentConversionFailure:
    return nullptr;

  case Sema::TDK_Inconsistent:
  case Sema::TDK_Underqualified:
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
  case Sema::TDK_NonDeducedMismatch:
    return &static_cast<DFIArguments*>(Data)->SecondArg;

  // Unhandled
  case Sema::TDK_MiscellaneousDeductionFailure:
    break;
  }

  return nullptr;
}

llvm::Optional<unsigned> DeductionFailureInfo::getCallArgIndex() {
  switch (static_cast<Sema::TemplateDeductionResult>(Result)) {
  case Sema::TDK_DeducedMismatch:
  case Sema::TDK_DeducedMismatchNested:
    return static_cast<DFIDeducedMismatchArgs*>(Data)->CallArgIndex;

  default:
    return llvm::None;
  }
}

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.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;
}

/// Determine whether the given New declaration is an overload of the
/// declarations in Old. This routine returns Ovl_Match or Ovl_NonFunction if
/// New and Old cannot be overloaded, e.g., if New has the same signature as
/// some function in Old (C++ 1.3.10) or if the Old declarations aren't
/// functions (or function templates) at all. When it does return Ovl_Match or
/// Ovl_NonFunction, MatchedDecl will point to the decl that New cannot be
/// overloaded with. This decl may be a UsingShadowDecl on top of the underlying
/// declaration.
///
/// Example: Given the following input:
///
///   void f(int, float); // #1
///   void f(int, int); // #2
///   int f(int, int); // #3
///
/// When we process #1, there is no previous declaration of "f", so IsOverload
/// will not be used.
///
/// When we process #2, Old contains only the FunctionDecl for #1. By comparing
/// the parameter types, we see that #1 and #2 are overloaded (since they have
/// different signatures), so this routine returns Ovl_Overload; MatchedDecl is
/// unchanged.
///
/// When we process #3, Old is an overload set containing #1 and #2. We compare
/// the signatures of #3 to #1 (they're overloaded, so we do nothing) and then
/// #3 to #2. Since the signatures of #3 and #2 are identical (return types of
/// functions are not part of the signature), IsOverload returns Ovl_Match and
/// MatchedDecl will be set to point to the FunctionDecl for #2.
///
/// 'NewIsUsingShadowDecl' indicates that 'New' is being introduced into a class
/// by a using declaration. The rules for whether to hide shadow declarations
/// ignore some properties which otherwise figure into a function template's
/// signature.
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;
    }
  }

  return Ovl_Overload;
}

bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
                      bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs) {
  // 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;

  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 = Context.getCanonicalType(Old->getType());
  QualType NewQType = 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 FunctionProtoType *OldType = cast<FunctionProtoType>(OldQType);
  const FunctionProtoType *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->getNumParams() != NewType->getNumParams() ||
       OldType->isVariadic() != NewType->isVariadic() ||
       !FunctionParamTypesAreEqual(OldType, NewType)))
    return true;

  // 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.
  //
  // However, we don't consider either of these when deciding whether
  // a member introduced by a shadow declaration is hidden.
  if (!UseMemberUsingDeclRules && NewTemplate &&
      (!TemplateParameterListsAreEqual(NewTemplate->getTemplateParameters(),
                                       OldTemplate->getTemplateParameters(),
                                       false, TPL_TemplateMatch) ||
       !Context.hasSameType(Old->getDeclaredReturnType(),
                            New->getDeclaredReturnType())))
    return true;

  // If the function is a class member, its signature includes the
  // cv-qualifiers (if any) and ref-qualifier (if any) on the function itself.
  //
  // As part of this, also check whether one of the member functions
  // is static, in which case they are not overloads (C++
  // 13.1p2). While not part of the definition of the signature,
  // this check is important to determine whether these functions
  // can be overloaded.
  CXXMethodDecl *OldMethod = dyn_cast<CXXMethodDecl>(Old);
  CXXMethodDecl *NewMethod = dyn_cast<CXXMethodDecl>(New);
  if (OldMethod && NewMethod &&
      !OldMethod->isStatic() && !NewMethod->isStatic()) {
    if (OldMethod->getRefQualifier() != NewMethod->getRefQualifier()) {
      if (!UseMemberUsingDeclRules &&
          (OldMethod->getRefQualifier() == RQ_None ||
           NewMethod->getRefQualifier() == RQ_None)) {
        // C++0x [over.load]p2:
        //   - Member function declarations with the same name and the same
        //     parameter-type-list as well as member function template
        //     declarations with the same name, the same parameter-type-list, and
        //     the same template parameter lists cannot be overloaded if any of
        //     them, but not all, have a ref-qualifier (8.3.5).
        Diag(NewMethod->getLocation(), diag::err_ref_qualifier_overload)
          << NewMethod->getRefQualifier() << OldMethod->getRefQualifier();
        Diag(OldMethod->getLocation(), diag::note_previous_declaration);
      }
      return true;
    }

    // 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.
    // FIXME: OpenCL: Need to consider address spaces
    unsigned OldQuals = OldMethod->getTypeQualifiers().getCVRUQualifiers();
    unsigned NewQuals = NewMethod->getTypeQualifiers().getCVRUQualifiers();
    if (!getLangOpts().CPlusPlus14 && NewMethod->isConstexpr() &&
        !isa<CXXConstructorDecl>(NewMethod))
      NewQuals |= Qualifiers::Const;

    // We do not allow overloading based off of '__restrict'.
    OldQuals &= ~Qualifiers::Restrict;
    NewQuals &= ~Qualifiers::Restrict;
    if (OldQuals != NewQuals)
      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, Context, true);
    OldI->getCond()->Profile(OldID, Context, true);
    if (NewID != OldID)
      return true;
  }

  if (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))
      return false;

    CUDAFunctionTarget NewTarget = IdentifyCUDATarget(New),
                       OldTarget = IdentifyCUDATarget(Old);
    if (NewTarget == CFT_InvalidTarget)
      return false;

    assert((OldTarget != CFT_InvalidTarget) && "Unexpected invalid target.");

    // Allow overloading of functions with same signature and different CUDA
    // target attributes.
    return NewTarget != OldTarget;
  }

  // The signatures match; this is not an overload.
  return false;
}

/// Checks availability of the function depending on the current
/// function context. Inside an unavailable function, unavailability is ignored.
///
/// \returns true if \arg FD is unavailable and current context is inside
/// an available function, false otherwise.
bool Sema::isFunctionConsideredUnavailable(FunctionDecl *FD) {
  if (!FD->isUnavailable())
    return false;

  // Walk up the context of the caller.
  Decl *C = cast<Decl>(CurContext);
  do {
    if (C->isUnavailable())
      return false;
  } while ((C = cast_or_null<Decl>(C->getDeclContext())));
  return 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,
                         bool 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 FromCanon
        = S.Context.getCanonicalType(From->getType().getUnqualifiedType());
      QualType ToCanon
        = S.Context.getCanonicalType(ToType).getUnqualifiedType();
      if (Constructor->isCopyConstructor() &&
          (FromCanon == ToCanon ||
           S.IsDerivedFrom(From->getBeginLoc(), 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(From->getType());
        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->Viable)
        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,
                      bool 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;
  }

  return TryUserDefinedConversion(S, From, ToType, SuppressUserConversions,
                                  AllowExplicit, InOverloadResolution, CStyle,
                                  AllowObjCWritebackConversion,
                                  AllowObjCConversionOnExplicit);
}

ImplicitConversionSequence
Sema::TryImplicitConversion(Expr *From, QualType ToType,
                            bool SuppressUserConversions,
                            bool AllowExplicit,
                            bool InOverloadResolution,
                            bool CStyle,
                            bool AllowObjCWritebackConversion) {
  return ::TryImplicitConversion(*this, From, ToType,
                                 SuppressUserConversions, AllowExplicit,
                                 InOverloadResolution, CStyle,
                                 AllowObjCWritebackConversion,
                                 /*AllowObjCConversionOnExplicit=*/false);
}

/// PerformImplicitConversion - Perform an implicit conversion of the
/// expression From to the type ToType. Returns the
/// converted expression. Flavor is the kind of conversion we're
/// performing, used in the error message. If @p AllowExplicit,
/// explicit user-defined conversions are permitted.
ExprResult
Sema::PerformImplicitConversion(Expr *From, QualType ToType,
                                AssignmentAction Action, bool AllowExplicit) {
  ImplicitConversionSequence ICS;
  return PerformImplicitConversion(From, ToType, Action, AllowExplicit, ICS);
}

ExprResult
Sema::PerformImplicitConversion(Expr *From, QualType ToType,
                                AssignmentAction Action, bool AllowExplicit,
                                ImplicitConversionSequence& ICS) {
  if (checkPlaceholderForOverload(*this, From))
    return ExprError();

  // Objective-C ARC: Determine whether we will allow the writeback conversion.
  bool AllowObjCWritebackConversion
    = getLangOpts().ObjCAutoRefCount &&
      (Action == AA_Passing || Action == AA_Sending);
  if (getLangOpts().ObjC)
    CheckObjCBridgeRelatedConversions(From->getBeginLoc(), ToType,
                                      From->getType(), From);
  ICS = ::TryImplicitConversion(*this, From, ToType,
                                /*SuppressUserConversions=*/false,
                                AllowExplicit,
                                /*InOverloadResolution=*/false,
                                /*CStyle=*/false,
                                AllowObjCWritebackConversion,
                                /*AllowObjCConversionOnExplicit=*/false);
  return PerformImplicitConversion(From, ToType, ICS, Action);
}

/// Determine whether the conversion from FromType to ToType is a valid
/// conversion that strips "noexcept" or "noreturn" off the nested function
/// type.
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.getAs<PointerType>()->getPointeeType();
      CanFrom = CanFrom.getAs<PointerType>()->getPointeeType();
    } else if (TyClass == Type::BlockPointer) {
      CanTo = CanTo.getAs<BlockPointerType>()->getPointeeType();
      CanFrom = CanFrom.getAs<BlockPointerType>()->getPointeeType();
    } else if (TyClass == Type::MemberPointer) {
      auto ToMPT = CanTo.getAs<MemberPointerType>();
      auto FromMPT = CanFrom.getAs<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;
    }
  }

  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
/// 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) {
  // 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;

  // There are no conversions between extended vector types, only identity.
  if (ToType->isExtVectorType()) {
    // There are no conversions between extended vector types other than the
    // identity conversion.
    if (FromType->isExtVectorType())
      return false;

    // Vector splat from any arithmetic type to a vector.
    if (FromType->isArithmeticType()) {
      ICK = ICK_Vector_Splat;
      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
  if (ToType->isVectorType() && FromType->isVectorType()) {
    if (S.Context.areCompatibleVectorTypes(FromType, ToType) ||
        S.isLaxVectorConversion(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()) {
        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);
      }

      // Check that we've computed the proper type after overload resolution.
      // FIXME: FixOverloadedFunctionReference has side-effects; we shouldn't
      // be calling it from within an NDEBUG block.
      assert(S.Context.hasSameType(
        FromType,
        S.FixOverloadedFunctionReference(From, AccessPair, Fn)->getType()));
    } else {
      return false;
    }
  }
  // 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.
  bool argIsLValue = From->isGLValue();
  if (argIsLValue &&
      !FromType->isFunctionType() && !FromType->isArrayType() &&
      S.Context.getCanonicalType(FromType) != S.Context.OverloadTy) {
    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;
  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() ||
              FromType->isNullPtrType())) {
    // 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 (FromType->isRealFloatingType() && ToType->isRealFloatingType()) {
    // FIXME: disable conversions between long double 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.
    if (&S.Context.getFloatTypeSemantics(FromType) !=
        &S.Context.getFloatTypeSemantics(ToType)) {
      bool Float128AndLongDouble = ((FromType == S.Context.Float128Ty &&
                                    ToType == S.Context.LongDoubleTy) ||
                                   (FromType == S.Context.LongDoubleTy &&
                                    ToType == S.Context.Float128Ty));
      if (Float128AndLongDouble &&
          (&S.Context.getFloatTypeSemantics(S.Context.LongDoubleTy) ==
           &llvm::APFloat::PPCDoubleDouble()))
        return false;
    }
    // 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.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)) {
    SCS.Second = SecondICK;
    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 {
    // 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, /*ObjCWritebackConversion=*/false)) {
      ToType = it->getType();
      return true;
    }
  }
  return false;
}

/// IsIntegralPromotion - Determines whether the conversion from the
/// expression From (whose potentially-adjusted type is FromType) to
/// ToType is an integral promotion (C++ 4.5). If so, returns true and
/// sets PromotedType to the promoted type.
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 (FromType->isPromotableIntegerType() && !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()) {
      llvm::APSInt BitWidth;
      if (FromType->isIntegralType(Context) &&
          MemberDecl->getBitWidth()->isIntegerConstantExpr(BitWidth, 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;
  }

  return false;
}

/// IsFloatingPointPromotion - Determines whether the conversion from
/// FromType to ToType is a floating point promotion (C++ 4.6). If so,
/// returns true and sets PromotedType to the promoted type.
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))
        return true;

      // Half can be promoted to float.
      if (!getLangOpts().NativeHalfType &&
           FromBuiltin->getKind() == BuiltinType::Half &&
          ToBuiltin->getKind() == BuiltinType::Float)
        return true;
    }

  return false;
}

/// Determine if a conversion is a complex promotion.
///
/// A complex promotion is defined as a complex -> complex conversion
/// where the conversion between the underlying real types is a
/// floating-point or integral promotion.
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);
}

/// IsPointerConversion - Determines whether the conversion of the
/// expression From, which has the (possibly adjusted) type FromType,
/// can be converted to the type ToType via a pointer conversion (C++
/// 4.10). If so, returns true and places the converted type (that
/// might differ from ToType in its cv-qualifiers at some level) into
/// ConvertedType.
///
/// This routine also supports conversions to and from block pointers
/// and conversions with Objective-C's 'id', 'id<protocols...>', and
/// pointers to interfaces. FIXME: Once we've determined the
/// appropriate overloading rules for Objective-C, we may want to
/// split the Objective-C checks into a different routine; however,
/// GCC seems to consider all of these conversions to be pointer
/// conversions, so for now they live here. IncompatibleObjC will be
/// set if the conversion is an allowed Objective-C conversion that
/// should result in a warning.
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->getAs<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->getAs<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))
    return Context.getQualifiedType(T, Qs);

  return Context.getQualifiedType(T.getUnqualifiedType(), Qs);
}

/// isObjCPointerConversion - Determines whether this is an
/// Objective-C pointer conversion. Subroutine of IsPointerConversion,
/// with the same arguments and return values.
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()))
        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->getTypeQuals() != ToFunctionType->getTypeQuals())
      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;
}

/// Determine whether this is an Objective-C writeback conversion,
/// used for parameter passing when performing automatic reference counting.
///
/// \param FromType The type we're converting form.
///
/// \param ToType The type we're converting to.
///
/// \param ConvertedType The type that will be produced after applying
/// this conversion.
bool Sema::isObjCWritebackConversion(QualType FromType, QualType ToType,
                                     QualType &ConvertedType) {
  if (!getLangOpts().ObjCAutoRefCount ||
      Context.hasSameUnqualifiedType(FromType, ToType))
    return false;

  // Parameter must be a pointer to __autoreleasing (with no other qualifiers).
  QualType ToPointee;
  if (const PointerType *ToPointer = ToType->getAs<PointerType>())
    ToPointee = ToPointer->getPointeeType();
  else
    return false;

  Qualifiers ToQuals = ToPointee.getQualifiers();
  if (!ToPointee->isObjCLifetimeType() ||
      ToQuals.getObjCLifetime() != Qualifiers::OCL_Autoreleasing ||
      !ToQuals.withoutObjCLifetime().empty())
    return false;

  // Argument must be a pointer to __strong to __weak.
  QualType FromPointee;
  if (const PointerType *FromPointer = FromType->getAs<PointerType>())
    FromPointee = FromPointer->getPointeeType();
  else
    return false;

  Qualifiers FromQuals = FromPointee.getQualifiers();
  if (!FromPointee->isObjCLifetimeType() ||
      (FromQuals.getObjCLifetime() != Qualifiers::OCL_Strong &&
       FromQuals.getObjCLifetime() != Qualifiers::OCL_Weak))
    return false;

  // Make sure that we have compatible qualifiers.
  FromQuals.setObjCLifetime(Qualifiers::OCL_Autoreleasing);
  if (!ToQuals.compatiblyIncludes(FromQuals))
    return false;

  // Remove qualifiers from the pointee type we're converting from; they
  // aren't used in the compatibility check belong, and we'll be adding back
  // qualifiers (with __autoreleasing) if the compatibility check succeeds.
  FromPointee = FromPointee.getUnqualifiedType();

  // The unqualified form of the pointee types must be compatible.
  ToPointee = ToPointee.getUnqualifiedType();
  bool IncompatibleObjC;
  if (Context.typesAreCompatible(FromPointee, ToPointee))
    FromPointee = ToPointee;
  else if (!isObjCPointerConversion(FromPointee, ToPointee, FromPointee,
                                    IncompatibleObjC))
    return false;

  /// Construct the type we're converting to, which is a pointer to
  /// __autoreleasing pointee.
  FromPointee = Context.getQualifiedType(FromPointee, FromQuals);
  ConvertedType = Context.getPointerType(FromPointee);
  return true;
}

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;
}

/// HandleFunctionTypeMismatch - Gives diagnostic information for differeing
/// function types.  Catches different number of parameter, mismatch in
/// parameter types, and different return types.
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 MemberPointerType *FromMember = FromType->getAs<MemberPointerType>(),
                            *ToMember = ToType->getAs<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;
  }

  // FIXME: OpenCL: Need to consider address spaces
  unsigned FromQuals = FromFunction->getTypeQuals().getCVRUQualifiers();
  unsigned ToQuals = ToFunction->getTypeQuals().getCVRUQualifiers();
  if (FromQuals != ToQuals) {
    PDiag << ft_qualifer_mismatch << ToQuals << FromQuals;
    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;
}

/// FunctionParamTypesAreEqual - This routine checks two function proto types
/// for equality of their argument types. Caller has already checked that
/// they have same number of arguments.  If the parameters are different,
/// ArgPos will have the parameter index of the first different parameter.
bool Sema::FunctionParamTypesAreEqual(const FunctionProtoType *OldType,
                                      const FunctionProtoType *NewType,
                                      unsigned *ArgPos) {
  for (FunctionProtoType::param_type_iterator O = OldType->param_type_begin(),
                                              N = NewType->param_type_begin(),
                                              E = OldType->param_type_end();
       O && (O != E); ++O, ++N) {
    if (!Context.hasSameType(O->getUnqualifiedType(),
                             N->getUnqualifiedType())) {
      if (ArgPos)
        *ArgPos = O - OldType->param_type_begin();
      return false;
    }
  }
  return true;
}

/// CheckPointerConversion - Check the pointer conversion from the
/// expression From to the type ToType. This routine checks for
/// ambiguous or inaccessible derived-to-base pointer
/// conversions for which IsPointerConversion has already returned
/// true. It returns true and produces a diagnostic if there was an
/// error, or returns false otherwise.
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 AmbigiousID = 0;
        if (Diagnose) {
          InaccessibleID = diag::err_upcast_to_inaccessible_base;
          AmbigiousID = diag::err_ambiguous_derived_to_base_conv;
        }
        if (CheckDerivedToBaseConversion(
                FromPointeeType, ToPointeeType, InaccessibleID, AmbigiousID,
                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;
}

/// IsMemberPointerConversion - Determines whether the conversion of the
/// expression From, which has the (possibly adjusted) type FromType, can be
/// converted to the type ToType via a member pointer conversion (C++ 4.11).
/// If so, returns true and places the converted type (that might differ from
/// ToType in its cv-qualifiers at some level) into ConvertedType.
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;
}

/// CheckMemberPointerConversion - Check the member pointer conversion from the
/// expression From to the type ToType. This routine checks for ambiguous or
/// virtual or inaccessible base-to-derived member pointer conversions
/// for which IsMemberPointerConversion has already returned true. It returns
/// true and produces a diagnostic if there was an error, or returns false
/// otherwise.
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;
}

/// IsQualificationConversion - Determines whether the conversion from
/// an rvalue of type FromType to ToType is a qualification conversion
/// (C++ 4.4).
///
/// \param ObjCLifetimeConversion Output parameter that will be set to indicate
/// when the qualification conversion involves a change in the Objective-C
/// object lifetime.
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)) {
    // Within each iteration of the loop, we check the qualifiers to
    // determine if this still looks like a qualification
    // conversion. Then, if all is well, we unwrap one more level of
    // pointers or pointers-to-members and do it all again
    // until there are no more pointers or pointers-to-members left to
    // unwrap.
    UnwrappedAnyPointer = true;

    Qualifiers FromQuals = FromType.getQualifiers();
    Qualifiers ToQuals = ToType.getQualifiers();

    // Ignore __unaligned qualifier if this type is void.
    if (ToType.getUnqualifiedType()->isVoidType())
      FromQuals.removeUnaligned();

    // Objective-C ARC:
    //   Check Objective-C lifetime conversions.
    if (FromQuals.getObjCLifetime() != ToQuals.getObjCLifetime() &&
        UnwrappedAnyPointer) {
      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))
      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;

    // Keep track of whether all prior cv-qualifiers in the "to" type
    // include const.
    PreviousToQualsIncludeConst
      = PreviousToQualsIncludeConst && ToQuals.hasConst();
  }

  // Allows address space promotion by language rules implemented in
  // Type::Qualifiers::isAddressSpaceSupersetOf.
  Qualifiers FromQuals = FromType.getQualifiers();
  Qualifiers ToQuals = ToType.getQualifiers();
  if (!ToQuals.isAddressSpaceSupersetOf(FromQuals) &&
      !FromQuals.isAddressSpaceSupersetOf(ToQuals)) {
    return false;
  }

  // We are left with FromType and ToType being the pointee types
  // after unwrapping the original FromType and ToType the same number
  // of types. 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 FunctionProtoType *CtorType =
      Constructor->getType()->getAs<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) &&
                  (AllowExplicit || !Info.Constructor->isExplicit());
    if (Usable) {
      // If the first argument is (a reference to) the target type,
      // suppress conversions.
      bool SuppressUserConversions = isFirstArgumentCompatibleWithType(
          S.Context, Info.Constructor, ToType);
      if (Info.ConstructorTmpl)
        S.AddTemplateOverloadCandidate(Info.ConstructorTmpl, Info.FoundDecl,
                                       /*ExplicitArgs*/ nullptr, From,
                                       CandidateSet, SuppressUserConversions);
      else
        S.AddOverloadCandidate(Info.Constructor, Info.FoundDecl, From,
                               CandidateSet, SuppressUserConversions);
    }
  }

  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->getThisType(S.Context);
    // 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->getAs<PointerType>()->getPointeeType());
    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,
                        bool AllowExplicit,
                        bool AllowObjCConversionOnExplicit) {
  assert(AllowExplicit || !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);
        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 && (AllowExplicit || !Info.Constructor->isExplicit());
        else
          Usable = Usable &&
                   Info.Constructor->isConvertingConstructor(AllowExplicit);
        if (Usable) {
          bool SuppressUserConversions = !ConstructorsOnly;
          if (SuppressUserConversions && ListInitializing) {
            SuppressUserConversions = false;
            if (NumArgs == 1) {
              // If the first argument is (a reference to) the target type,
              // suppress conversions.
              SuppressUserConversions = isFirstArgumentCompatibleWithType(
                  S.Context, Info.Constructor, ToType);
            }
          }
          if (Info.ConstructorTmpl)
            S.AddTemplateOverloadCandidate(
                Info.ConstructorTmpl, Info.FoundDecl,
                /*ExplicitArgs*/ nullptr, llvm::makeArrayRef(Args, NumArgs),
                CandidateSet, SuppressUserConversions);
          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::makeArrayRef(Args, NumArgs),
                                   CandidateSet, SuppressUserConversions);
        }
      }
    }
  }

  // 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 (AllowExplicit || !Conv->isExplicit()) {
          if (ConvTemplate)
            S.AddTemplateConversionCandidate(ConvTemplate, FoundDecl,
                                             ActingContext, From, ToType,
                                             CandidateSet,
                                             AllowObjCConversionOnExplicit);
          else
            S.AddConversionCandidate(Conv, FoundDecl, ActingContext,
                                     From, ToType, CandidateSet,
                                     AllowObjCConversionOnExplicit);
        }
      }
    }
  }

  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.
      //
      QualType ThisType = Constructor->getThisType(S.Context);
      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(ThisType->getAs<PointerType>()->getPointeeType());
      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, false, false);
  if (OvResult == OR_Ambiguous)
    Diag(From->getBeginLoc(), diag::err_typecheck_ambiguous_condition)
        << From->getType() << ToType << From->getSourceRange();
  else if (OvResult == 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;
  } else
    return false;
  CandidateSet.NoteCandidates(*this, OCD_AllCandidates, From);
  return true;
}

/// 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) {
  if (!S.getLangOpts().ObjC || !S.getLangOpts().CPlusPlus11)
    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.
  CXXConversionDecl *Conv1 = dyn_cast_or_null<CXXConversionDecl>(Function1);
  if (!Conv1)
    return ImplicitConversionSequence::Indistinguishable;

  CXXConversionDecl *Conv2 = dyn_cast<CXXConversionDecl>(Function2);
  if (!Conv2)
    return ImplicitConversionSequence::Indistinguishable;

  if (Conv1->getParent()->isLambda() && Conv2->getParent()->isLambda()) {
    bool Block1 = Conv1->getConversionType()->isBlockPointerType();
    bool Block2 = Conv2->getConversionType()->isBlockPointerType();
    if (Block1 != Block2)
      return Block1 ? ImplicitConversionSequence::Worse
                    : ImplicitConversionSequence::Better;
  }

  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))
    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 converts to type "array of N1 T", L2 converts to type "array of N2 T",
  //   and N1 is smaller than N2.,
  // even if one of the other rules in this paragraph would otherwise apply.
  if (!ICS1.isBad()) {
    if (ICS1.isStdInitializerListElement() &&
        !ICS2.isStdInitializerListElement())
      return ImplicitConversionSequence::Better;
    if (!ICS1.isStdInitializerListElement() &&
        ICS2.isStdInitializerListElement())
      return 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);
}

/// 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++ [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;
      }
    }
  }

  // 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) {
    // 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;

    // 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))
        return ImplicitConversionSequence::Better;
      else if (T1.isMoreQualifiedThan(T2))
        return ImplicitConversionSequence::Worse;
    }
  }

  // In Microsoft mode, 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 && 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;
  }

  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++ 13.3.3.2p3:
  //  -- S1 and S2 differ only in their qualification conversion and
  //     yield similar types T1 and T2 (C++ 4.4), respectively, 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).
  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);
  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 unwrapped
  // them.
  if (UnqualT1 == UnqualT2)
    return ImplicitConversionSequence::Indistinguishable;

  // 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);

  ImplicitConversionSequence::CompareKind Result
    = ImplicitConversionSequence::Indistinguishable;

  // Objective-C++ ARC:
  //   Prefer qualification conversions not involving a change in lifetime
  //   to qualification conversions that do not change lifetime.
  if (SCS1.QualificationIncludesObjCLifetime !=
                                      SCS2.QualificationIncludesObjCLifetime) {
    Result = SCS1.QualificationIncludesObjCLifetime
               ? ImplicitConversionSequence::Worse
               : ImplicitConversionSequence::Better;
  }

  while (S.Context.UnwrapSimilarTypes(T1, T2)) {
    // Within each iteration of the loop, we check the qualifiers to
    // determine if this still looks like a qualification
    // conversion. Then, if all is well, we unwrap one more level of
    // pointers or pointers-to-members and do it all again
    // until there are no more pointers or pointers-to-members left
    // to unwrap. This essentially mimics what
    // IsQualificationConversion does, but here we're checking for a
    // strict subset of qualifiers.
    if (T1.getCVRQualifiers() == T2.getCVRQualifiers())
      // The qualifiers are the same, so this doesn't tell us anything
      // about how the sequences rank.
      ;
    else if (T2.isMoreQualifiedThan(T1)) {
      // T1 has fewer qualifiers, so it could be the better sequence.
      if (Result == ImplicitConversionSequence::Worse)
        // Neither has qualifiers that are a subset of the other's
        // qualifiers.
        return ImplicitConversionSequence::Indistinguishable;

      Result = ImplicitConversionSequence::Better;
    } else if (T1.isMoreQualifiedThan(T2)) {
      // T2 has fewer qualifiers, so it could be the better sequence.
      if (Result == ImplicitConversionSequence::Better)
        // Neither has qualifiers that are a subset of the other's
        // qualifiers.
        return ImplicitConversionSequence::Indistinguishable;

      Result = ImplicitConversionSequence::Worse;
    } else {
      // Qualifiers are disjoint.
      return ImplicitConversionSequence::Indistinguishable;
    }

    // If the types after this point are equivalent, we're done.
    if (S.Context.hasSameUnqualifiedType(T1, T2))
      break;
  }

  // Check that the winning standard conversion sequence isn't using
  // the deprecated string literal array to pointer conversion.
  switch (Result) {
  case ImplicitConversionSequence::Better:
    if (SCS1.DeprecatedStringLiteralToCharPtr)
      Result = ImplicitConversionSequence::Indistinguishable;
    break;

  case ImplicitConversionSequence::Indistinguishable:
    break;

  case ImplicitConversionSequence::Worse:
    if (SCS2.DeprecatedStringLiteralToCharPtr)
      Result = ImplicitConversionSequence::Indistinguishable;
    break;
  }

  return Result;
}

/// 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->getAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType ToPointee1
      = ToType1->getAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType FromPointee2
      = FromType2->getAs<PointerType>()->getPointeeType().getUnqualifiedType();
    QualType ToPointee2
      = ToType2->getAs<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 MemberPointerType * FromMemPointer1 =
                                        FromType1->getAs<MemberPointerType>();
    const MemberPointerType * ToMemPointer1 =
                                          ToType1->getAs<MemberPointerType>();
    const MemberPointerType * FromMemPointer2 =
                                          FromType2->getAs<MemberPointerType>();
    const MemberPointerType * ToMemPointer2 =
                                          ToType2->getAs<MemberPointerType>();
    const Type *FromPointeeType1 = FromMemPointer1->getClass();
    const Type *ToPointeeType1 = ToMemPointer1->getClass();
    const Type *FromPointeeType2 = FromMemPointer2->getClass();
    const Type *ToPointeeType2 = ToMemPointer2->getClass();
    QualType FromPointee1 = QualType(FromPointeeType1, 0).getUnqualifiedType();
    QualType ToPointee1 = QualType(ToPointeeType1, 0).getUnqualifiedType();
    QualType FromPointee2 = QualType(FromPointeeType2, 0).getUnqualifiedType();
    QualType ToPointee2 = QualType(ToPointeeType2, 0).getUnqualifiedType();
    // conversion of A::* to B::* is better than conversion of A::* to C::*,
    if (FromPointee1 == FromPointee2 && ToPointee1 != ToPointee2) {
      if (S.IsDerivedFrom(Loc, ToPointee1, ToPointee2))
        return ImplicitConversionSequence::Worse;
      else if (S.IsDerivedFrom(Loc, ToPointee2, ToPointee1))
        return ImplicitConversionSequence::Better;
    }
    // conversion of B::* to C::* is better than conversion of A::* to C::*
    if (ToPointee1 == ToPointee2 && FromPointee1 != FromPointee2) {
      if (S.IsDerivedFrom(Loc, FromPointee1, FromPointee2))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, FromPointee2, FromPointee1))
        return ImplicitConversionSequence::Worse;
    }
  }

  if (SCS1.Second == ICK_Derived_To_Base) {
    //   -- conversion of C to B is better than conversion of C to A,
    //   -- binding of an expression of type C to a reference of type
    //      B& is better than binding an expression of type C to a
    //      reference of type A&,
    if (S.Context.hasSameUnqualifiedType(FromType1, FromType2) &&
        !S.Context.hasSameUnqualifiedType(ToType1, ToType2)) {
      if (S.IsDerivedFrom(Loc, ToType1, ToType2))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, ToType2, ToType1))
        return ImplicitConversionSequence::Worse;
    }

    //   -- conversion of B to A is better than conversion of C to A.
    //   -- binding of an expression of type B to a reference of type
    //      A& is better than binding an expression of type C to a
    //      reference of type A&,
    if (!S.Context.hasSameUnqualifiedType(FromType1, FromType2) &&
        S.Context.hasSameUnqualifiedType(ToType1, ToType2)) {
      if (S.IsDerivedFrom(Loc, FromType2, FromType1))
        return ImplicitConversionSequence::Better;
      else if (S.IsDerivedFrom(Loc, FromType1, FromType2))
        return ImplicitConversionSequence::Worse;
    }
  }

  return ImplicitConversionSequence::Indistinguishable;
}

/// Determine whether the given type is valid, e.g., it is not an invalid
/// C++ class.
static bool isTypeValid(QualType T) {
  if (CXXRecordDecl *Record = T->getAsCXXRecordDecl())
    return !Record->isInvalidDecl();

  return true;
}

/// CompareReferenceRelationship - Compare the two types T1 and T2 to
/// determine whether they are reference-related,
/// reference-compatible, reference-compatible with added
/// qualification, or incompatible, for use in C++ initialization by
/// reference (C++ [dcl.ref.init]p4). Neither type can be a reference
/// type, and the first type (T1) is the pointee type of the reference
/// type being initialized.
Sema::ReferenceCompareResult
Sema::CompareReferenceRelationship(SourceLocation Loc,
                                   QualType OrigT1, QualType OrigT2,
                                   bool &DerivedToBase,
                                   bool &ObjCConversion,
                                   bool &ObjCLifetimeConversion) {
  assert(!OrigT1->isReferenceType() &&
    "T1 must be the pointee type of the reference type");
  assert(!OrigT2->isReferenceType() && "T2 cannot be a reference type");

  QualType T1 = Context.getCanonicalType(OrigT1);
  QualType T2 = Context.getCanonicalType(OrigT2);
  Qualifiers T1Quals, T2Quals;
  QualType UnqualT1 = Context.getUnqualifiedArrayType(T1, T1Quals);
  QualType UnqualT2 = Context.getUnqualifiedArrayType(T2, T2Quals);

  // C++ [dcl.init.ref]p4:
  //   Given types "cv1 T1" and "cv2 T2," "cv1 T1" is
  //   reference-related to "cv2 T2" if T1 is the same type as T2, or
  //   T1 is a base class of T2.
  DerivedToBase = false;
  ObjCConversion = false;
  ObjCLifetimeConversion = false;
  QualType ConvertedT2;
  if (UnqualT1 == UnqualT2) {
    // Nothing to do.
  } else if (isCompleteType(Loc, OrigT2) &&
             isTypeValid(UnqualT1) && isTypeValid(UnqualT2) &&
             IsDerivedFrom(Loc, UnqualT2, UnqualT1))
    DerivedToBase = true;
  else if (UnqualT1->isObjCObjectOrInterfaceType() &&
           UnqualT2->isObjCObjectOrInterfaceType() &&
           Context.canBindObjCObjectType(UnqualT1, UnqualT2))
    ObjCConversion = true;
  else if (UnqualT2->isFunctionType() &&
           IsFunctionConversion(UnqualT2, UnqualT1, ConvertedT2))
    // C++1z [dcl.init.ref]p4:
    //   cv1 T1" is reference-compatible with "cv2 T2" if [...] T2 is "noexcept
    //   function" and T1 is "function"
    //
    // We extend this to also apply to 'noreturn', so allow any function
    // conversion between function types.
    return Ref_Compatible;
  else
    return Ref_Incompatible;

  // At this point, we know that T1 and T2 are reference-related (at
  // least).

  // If the type is an array type, promote the element qualifiers to the type
  // for comparison.
  if (isa<ArrayType>(T1) && T1Quals)
    T1 = Context.getQualifiedType(UnqualT1, T1Quals);
  if (isa<ArrayType>(T2) && T2Quals)
    T2 = Context.getQualifiedType(UnqualT2, T2Quals);

  // C++ [dcl.init.ref]p4:
  //   "cv1 T1" is reference-compatible with "cv2 T2" if T1 is
  //   reference-related to T2 and cv1 is the same cv-qualification
  //   as, or greater cv-qualification than, cv2. For purposes of
  //   overload resolution, cases for which cv1 is greater
  //   cv-qualification than cv2 are identified as
  //   reference-compatible with added qualification (see 13.3.3.2).
  //
  // Note that we also require equivalence of Objective-C GC and address-space
  // qualifiers when performing these computations, so that e.g., an int in
  // address space 1 is not reference-compatible with an int in address
  // space 2.
  if (T1Quals.getObjCLifetime() != T2Quals.getObjCLifetime() &&
      T1Quals.compatiblyIncludesObjCLifetime(T2Quals)) {
    if (isNonTrivialObjCLifetimeConversion(T2Quals, T1Quals))
      ObjCLifetimeConversion = true;

    T1Quals.removeObjCLifetime();
    T2Quals.removeObjCLifetime();
  }

  // MS compiler ignores __unaligned qualifier for references; do the same.
  T1Quals.removeUnaligned();
  T2Quals.removeUnaligned();

  if (T1Quals.compatiblyIncludes(T2Quals))
    return Ref_Compatible;
  else
    return Ref_Related;
}

/// Look for a user-defined conversion to a value reference-compatible
///        with DeclType. Return true if something definite is found.
static bool
FindConversionForRefInit(Sema &S, ImplicitConversionSequence &ICS,
                         QualType DeclType, SourceLocation DeclLoc,
                         Expr *Init, QualType T2, bool AllowRvalues,
                         bool AllowExplicit) {
  assert(T2->isRecordType() && "Can only find conversions of record types.");
  CXXRecordDecl *T2RecordDecl
    = dyn_cast<CXXRecordDecl>(T2->getAs<RecordType>()->getDecl());

  OverloadCandidateSet CandidateSet(
      DeclLoc, OverloadCandidateSet::CSK_InitByUserDefinedConversion);
  const auto &Conversions = T2RecordDecl->getVisibleConversionFunctions();
  for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
    NamedDecl *D = *I;
    CXXRecordDecl *ActingDC = cast<CXXRecordDecl>(D->getDeclContext());
    if (isa<UsingShadowDecl>(D))
      D = cast<UsingShadowDecl>(D)->getTargetDecl();

    FunctionTemplateDecl *ConvTemplate
      = dyn_cast<FunctionTemplateDecl>(D);
    CXXConversionDecl *Conv;
    if (ConvTemplate)
      Conv = cast<CXXConversionDecl>(ConvTemplate->getTemplatedDecl());
    else
      Conv = cast<CXXConversionDecl>(D);

    // If this is an explicit conversion, and we're not allowed to consider
    // explicit conversions, skip it.
    if (!AllowExplicit && Conv->isExplicit())
      continue;

    if (AllowRvalues) {
      bool DerivedToBase = false;
      bool ObjCConversion = false;
      bool ObjCLifetimeConversion = false;

      // If we are initializing an rvalue reference, don't permit conversion
      // functions that return lvalues.
      if (!ConvTemplate && DeclType->isRValueReferenceType()) {
        const ReferenceType *RefType
          = Conv->getConversionType()->getAs<LValueReferenceType>();
        if (RefType && !RefType->getPointeeType()->isFunctionType())
          continue;
      }

      if (!ConvTemplate &&
          S.CompareReferenceRelationship(
            DeclLoc,
            Conv->getConversionType().getNonReferenceType()
              .getUnqualifiedType(),
            DeclType.getNonReferenceType().getUnqualifiedType(),
            DerivedToBase, ObjCConversion, ObjCLifetimeConversion) ==
          Sema::Ref_Incompatible)
        continue;
    } else {
      // If the conversion function doesn't return a reference type,
      // it can't be considered for this conversion. An rvalue reference
      // is only acceptable if its referencee is a function type.

      const ReferenceType *RefType =
        Conv->getConversionType()->getAs<ReferenceType>();
      if (!RefType ||
          (!RefType->isLValueReferenceType() &&
           !RefType->getPointeeType()->isFunctionType()))
        continue;
    }

    if (ConvTemplate)
      S.AddTemplateConversionCandidate(ConvTemplate, I.getPair(), ActingDC,
                                       Init, DeclType, CandidateSet,
                                       /*AllowObjCConversionOnExplicit=*/false);
    else
      S.AddConversionCandidate(Conv, I.getPair(), ActingDC, Init,
                               DeclType, CandidateSet,
                               /*AllowObjCConversionOnExplicit=*/false);
  }

  bool HadMultipleCandidates = (CandidateSet.size() > 1);

  OverloadCandidateSet::iterator Best;
  switch (CandidateSet.BestViableFunction(S, DeclLoc, Best)) {
  case OR_Success:
    // C++ [over.ics.ref]p1:
    //
    //   [...] If the parameter binds directly to the result of
    //   applying a conversion function to the argument
    //   expression, the implicit conversion sequence is a
    //   user-defined conversion sequence (13.3.3.1.2), with the
    //   second standard conversion sequence either an identity
    //   conversion or, if the conversion function returns an
    //   entity of a type that is a derived class of the parameter
    //   type, a derived-to-base Conversion.
    if (!Best->FinalConversion.DirectBinding)
      return false;

    ICS.setUserDefined();
    ICS.UserDefined.Before = Best->Conversions[0].Standard;
    ICS.UserDefined.After = Best->FinalConversion;
    ICS.UserDefined.HadMultipleCandidates = HadMultipleCandidates;
    ICS.UserDefined.ConversionFunction = Best->Function;
    ICS.UserDefined.FoundConversionFunction = Best->FoundDecl;
    ICS.UserDefined.EllipsisConversion = false;
    assert(ICS.UserDefined.After.ReferenceBinding &&
           ICS.UserDefined.After.DirectBinding &&
           "Expected a direct reference binding!");
    return true;

  case OR_Ambiguous:
    ICS.setAmbiguous();
    for (OverloadCandidateSet::iterator Cand = CandidateSet.begin();
         Cand != CandidateSet.end(); ++Cand)
      if (Cand->Viable)
        ICS.Ambiguous.addConversion(Cand->FoundDecl, Cand->Function);
    return true;

  case OR_No_Viable_Function:
  case OR_Deleted:
    // There was no suitable conversion, or we found a deleted
    // conversion; continue with other checks.
    return false;
  }

  llvm_unreachable("Invalid OverloadResult!");
}

/// Compute an implicit conversion sequence for reference
/// initialization.
static ImplicitConversionSequence
TryReferenceInit(Sema &S, Expr *Init, QualType DeclType,
                 SourceLocation DeclLoc,
                 bool SuppressUserConversions,
                 bool AllowExplicit) {
  assert(DeclType->isReferenceType() && "Reference init needs a reference");

  // Most paths end in a failed conversion.
  ImplicitConversionSequence ICS;
  ICS.setBad(BadConversionSequence::no_conversion, Init, DeclType);

  QualType T1 = DeclType->getAs<ReferenceType>()->getPointeeType();
  QualType T2 = Init->getType();

  // If the initializer is the address of an overloaded function, try
  // to resolve the overloaded function. If all goes well, T2 is the
  // type of the resulting function.
  if (S.Context.getCanonicalType(T2) == S.Context.OverloadTy) {
    DeclAccessPair Found;
    if (FunctionDecl *Fn = S.ResolveAddressOfOverloadedFunction(Init, DeclType,
                                                                false, Found))
      T2 = Fn->getType();
  }

  // Compute some basic properties of the types and the initializer.
  bool isRValRef = DeclType->isRValueReferenceType();
  bool DerivedToBase = false;
  bool ObjCConversion = false;
  bool ObjCLifetimeConversion = false;
  Expr::Classification InitCategory = Init->Classify(S.Context);
  Sema::ReferenceCompareResult RefRelationship
    = S.CompareReferenceRelationship(DeclLoc, T1, T2, DerivedToBase,
                                     ObjCConversion, ObjCLifetimeConversion);


  // C++0x [dcl.init.ref]p5:
  //   A reference to type "cv1 T1" is initialized by an expression
  //   of type "cv2 T2" as follows:

  //     -- If reference is an lvalue reference and the initializer expression
  if (!isRValRef) {
    //     -- is an lvalue (but is not a bit-field), and "cv1 T1" is
    //        reference-compatible with "cv2 T2," or
    //
    // Per C++ [over.ics.ref]p4, we don't check the bit-field property here.
    if (InitCategory.isLValue() && RefRelationship == Sema::Ref_Compatible) {
      // C++ [over.ics.ref]p1:
      //   When a parameter of reference type binds directly (8.5.3)
      //   to an argument expression, the implicit conversion sequence
      //   is the identity conversion, unless the argument expression
      //   has a type that is a derived class of the parameter type,
      //   in which case the implicit conversion sequence is a
      //   derived-to-base Conversion (13.3.3.1).
      ICS.setStandard();
      ICS.Standard.First = ICK_Identity;
      ICS.Standard.Second = DerivedToBase? ICK_Derived_To_Base
                         : ObjCConversion? ICK_Compatible_Conversion
                         : ICK_Identity;
      ICS.Standard.Third = ICK_Identity;
      ICS.Standard.FromTypePtr = T2.getAsOpaquePtr();
      ICS.Standard.setToType(0, T2);
      ICS.Standard.setToType(1, T1);
      ICS.Standard.setToType(2, T1);
      ICS.Standard.ReferenceBinding = true;
      ICS.Standard.DirectBinding = true;
      ICS.Standard.IsLvalueReference = !isRValRef;
      ICS.Standard.BindsToFunctionLvalue = T2->isFunctionType();
      ICS.Standard.BindsToRvalue = false;
      ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
      ICS.Standard.ObjCLifetimeConversionBinding = ObjCLifetimeConversion;
      ICS.Standard.CopyConstructor = nullptr;
      ICS.Standard.DeprecatedStringLiteralToCharPtr = false;

      // Nothing more to do: the inaccessibility/ambiguity check for
      // derived-to-base conversions is suppressed when we're
      // computing the implicit conversion sequence (C++
      // [over.best.ics]p2).
      return ICS;
    }

    //       -- has a class type (i.e., T2 is a class type), where T1 is
    //          not reference-related to T2, and can be implicitly
    //          converted to an lvalue of type "cv3 T3," where "cv1 T1"
    //          is reference-compatible with "cv3 T3" 92) (this
    //          conversion is selected by enumerating the applicable
    //          conversion functions (13.3.1.6) and choosing the best
    //          one through overload resolution (13.3)),
    if (!SuppressUserConversions && T2->isRecordType() &&
        S.isCompleteType(DeclLoc, T2) &&
        RefRelationship == Sema::Ref_Incompatible) {
      if (FindConversionForRefInit(S, ICS, DeclType, DeclLoc,
                                   Init, T2, /*AllowRvalues=*/false,
                                   AllowExplicit))
        return ICS;
    }
  }

  //     -- Otherwise, the reference shall be an lvalue reference to a
  //        non-volatile const type (i.e., cv1 shall be const), or the reference
  //        shall be an rvalue reference.
  if (!isRValRef && (!T1.isConstQualified() || T1.isVolatileQualified()))
    return ICS;

  //       -- If the initializer expression
  //
  //            -- is an xvalue, class prvalue, array prvalue or function
  //               lvalue and "cv1 T1" is reference-compatible with "cv2 T2", or
  if (RefRelationship == Sema::Ref_Compatible &&
      (InitCategory.isXValue() ||
       (InitCategory.isPRValue() && (T2->isRecordType() || T2->isArrayType())) ||
       (InitCategory.isLValue() && T2->isFunctionType()))) {
    ICS.setStandard();
    ICS.Standard.First = ICK_Identity;
    ICS.Standard.Second = DerivedToBase? ICK_Derived_To_Base
                      : ObjCConversion? ICK_Compatible_Conversion
                      : ICK_Identity;
    ICS.Standard.Third = ICK_Identity;
    ICS.Standard.FromTypePtr = T2.getAsOpaquePtr();
    ICS.Standard.setToType(0, T2);
    ICS.Standard.setToType(1, T1);
    ICS.Standard.setToType(2, T1);
    ICS.Standard.ReferenceBinding = true;
    // In C++0x, this is always a direct binding. In C++98/03, it's a direct
    // binding unless we're binding to a class prvalue.
    // Note: Although xvalues wouldn't normally show up in C++98/03 code, we
    // allow the use of rvalue references in C++98/03 for the benefit of
    // standard library implementors; therefore, we need the xvalue check here.
    ICS.Standard.DirectBinding =
      S.getLangOpts().CPlusPlus11 ||
      !(InitCategory.isPRValue() || T2->isRecordType());
    ICS.Standard.IsLvalueReference = !isRValRef;
    ICS.Standard.BindsToFunctionLvalue = T2->isFunctionType();
    ICS.Standard.BindsToRvalue = InitCategory.isRValue();
    ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
    ICS.Standard.ObjCLifetimeConversionBinding = ObjCLifetimeConversion;
    ICS.Standard.CopyConstructor = nullptr;
    ICS.Standard.DeprecatedStringLiteralToCharPtr = false;
    return ICS;
  }

  //            -- has a class type (i.e., T2 is a class type), where T1 is not
  //               reference-related to T2, and can be implicitly converted to
  //               an xvalue, class prvalue, or function lvalue of type
  //               "cv3 T3", where "cv1 T1" is reference-compatible with
  //               "cv3 T3",
  //
  //          then the reference is bound to the value of the initializer
  //          expression in the first case and to the result of the conversion
  //          in the second case (or, in either case, to an appropriate base
  //          class subobject).
  if (!SuppressUserConversions && RefRelationship == Sema::Ref_Incompatible &&
      T2->isRecordType() && S.isCompleteType(DeclLoc, T2) &&
      FindConversionForRefInit(S, ICS, DeclType, DeclLoc,
                               Init, T2, /*AllowRvalues=*/true,
                               AllowExplicit)) {
    // In the second case, if the reference is an rvalue reference
    // and the second standard conversion sequence of the
    // user-defined conversion sequence includes an lvalue-to-rvalue
    // conversion, the program is ill-formed.
    if (ICS.isUserDefined() && isRValRef &&
        ICS.UserDefined.After.First == ICK_Lvalue_To_Rvalue)
      ICS.setBad(BadConversionSequence::no_conversion, Init, DeclType);

    return ICS;
  }

  // A temporary of function type cannot be created; don't even try.
  if (T1->isFunctionType())
    return ICS;

  //       -- Otherwise, a temporary of type "cv1 T1" is created and
  //          initialized from the initializer expression using the
  //          rules for a non-reference copy initialization (8.5). The
  //          reference is then bound to the temporary. If T1 is
  //          reference-related to T2, cv1 must be the same
  //          cv-qualification as, or greater cv-qualification than,
  //          cv2; otherwise, the program is ill-formed.
  if (RefRelationship == Sema::Ref_Related) {
    // If cv1 == cv2 or cv1 is a greater cv-qualified than cv2, then
    // we would be reference-compatible or reference-compatible with
    // added qualification. But that wasn't the case, so the reference
    // initialization fails.
    //
    // Note that we only want to check address spaces and cvr-qualifiers here.
    // ObjC GC, lifetime and unaligned qualifiers aren't important.
    Qualifiers T1Quals = T1.getQualifiers();
    Qualifiers T2Quals = T2.getQualifiers();
    T1Quals.removeObjCGCAttr();
    T1Quals.removeObjCLifetime();
    T2Quals.removeObjCGCAttr();
    T2Quals.removeObjCLifetime();
    // MS compiler ignores __unaligned qualifier for references; do the same.
    T1Quals.removeUnaligned();
    T2Quals.removeUnaligned();
    if (!T1Quals.compatiblyIncludes(T2Quals))
      return ICS;
  }

  // If at least one of the types is a class type, the types are not
  // related, and we aren't allowed any user conversions, the
  // reference binding fails. This case is important for breaking
  // recursion, since TryImplicitConversion below will attempt to
  // create a temporary through the use of a copy constructor.
  if (SuppressUserConversions && RefRelationship == Sema::Ref_Incompatible &&
      (T1->isRecordType() || T2->isRecordType()))
    return ICS;

  // If T1 is reference-related to T2 and the reference is an rvalue
  // reference, the initializer expression shall not be an lvalue.
  if (RefRelationship >= Sema::Ref_Related &&
      isRValRef && Init->Classify(S.Context).isLValue())
    return ICS;

  // C++ [over.ics.ref]p2:
  //   When a parameter of reference type is not bound directly to
  //   an argument expression, the conversion sequence is the one
  //   required to convert the argument expression to the
  //   underlying type of the reference according to
  //   13.3.3.1. Conceptually, this conversion sequence corresponds
  //   to copy-initializing a temporary of the underlying type with
  //   the argument expression. Any difference in top-level
  //   cv-qualification is subsumed by the initialization itself
  //   and does not constitute a conversion.
  ICS = TryImplicitConversion(S, Init, T1, SuppressUserConversions,
                              /*AllowExplicit=*/false,
                              /*InOverloadResolution=*/false,
                              /*CStyle=*/false,
                              /*AllowObjCWritebackConversion=*/false,
                              /*AllowObjCConversionOnExplicit=*/false);

  // Of course, that's still a reference binding.
  if (ICS.isStandard()) {
    ICS.Standard.ReferenceBinding = true;
    ICS.Standard.IsLvalueReference = !isRValRef;
    ICS.Standard.BindsToFunctionLvalue = false;
    ICS.Standard.BindsToRvalue = true;
    ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier = false;
    ICS.Standard.ObjCLifetimeConversionBinding = false;
  } else if (ICS.isUserDefined()) {
    const ReferenceType *LValRefType =
        ICS.UserDefined.ConversionFunction->getReturnType()
            ->getAs<LValueReferenceType>();

    // C++ [over.ics.ref]p3:
    //   Except for an implicit object parameter, for which see 13.3.1, a
    //   standard conversion sequence cannot be formed if it requires [...]
    //   binding an rvalue reference to an lvalue other than a function
    //   lvalue.
    // Note that the function case is not possible here.
    if (DeclType->isRValueReferenceType() && LValRefType) {
      // FIXME: This is the wrong BadConversionSequence. The problem is binding
      // an rvalue reference to a (non-function) lvalue, not binding an lvalue
      // reference to an rvalue!
      ICS.setBad(BadConversionSequence::lvalue_ref_to_rvalue, Init, DeclType);
      return ICS;
    }

    ICS.UserDefined.After.ReferenceBinding = true;
    ICS.UserDefined.After.IsLvalueReference = !isRValRef;
    ICS.UserDefined.After.BindsToFunctionLvalue = false;
    ICS.UserDefined.After.BindsToRvalue = !LValRefType;
    ICS.UserDefined.After.BindsImplicitObjectArgumentWithoutRefQualifier = false;
    ICS.UserDefined.After.ObjCLifetimeConversionBinding = false;
  }

  return ICS;
}

static ImplicitConversionSequence
TryCopyInitialization(Sema &S, Expr *From, QualType ToType,
                      bool SuppressUserConversions,
                      bool InOverloadResolution,
                      bool AllowObjCWritebackConversion,
                      bool AllowExplicit = false);

/// TryListConversion - Try to copy-initialize a value of type ToType from the
/// initializer list From.
static ImplicitConversionSequence
TryListConversion(Sema &S, InitListExpr *From, QualType ToType,
                  bool SuppressUserConversions,
                  bool InOverloadResolution,
                  bool AllowObjCWritebackConversion) {
  // C++11 [over.ics.list]p1:
  //   When an argument is an initializer list, it is not an expression and
  //   special rules apply for converting it to a parameter type.

  ImplicitConversionSequence Result;
  Result.setBad(BadConversionSequence::no_conversion, From, ToType);

  // We need a complete type for what follows. Incomplete types can never be
  // initialized from init lists.
  if (!S.isCompleteType(From->getBeginLoc(), ToType))
    return Result;

  // Per DR1467:
  //   If the parameter type is a class X 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 is the one required to convert the element
  //   to the parameter type.
  //
  //   Otherwise, if the parameter type is a character array [... ]
  //   and the initializer list has a single element that is an
  //   appropriately-typed string literal (8.5.2 [dcl.init.string]), the
  //   implicit conversion sequence is the identity conversion.
  if (From->getNumInits() == 1) {
    if (ToType->isRecordType()) {
      QualType InitType = From->getInit(0)->getType();
      if (S.Context.hasSameUnqualifiedType(InitType, ToType) ||
          S.IsDerivedFrom(From->getBeginLoc(), InitType, ToType))
        return TryCopyInitialization(S, From->getInit(0), ToType,
                                     SuppressUserConversions,
                                     InOverloadResolution,
                                     AllowObjCWritebackConversion);
    }
    // FIXME: Check the other conditions here: array of character type,
    // initializer is a string literal.
    if (ToType->isArrayType()) {
      InitializedEntity Entity =
        InitializedEntity::InitializeParameter(S.Context, ToType,
                                               /*Consumed=*/false);
      if (S.CanPerformCopyInitialization(Entity, From)) {
        Result.setStandard();
        Result.Standard.setAsIdentityConversion();
        Result.Standard.setFromType(ToType);
        Result.Standard.setAllToTypes(ToType);
        return Result;
      }
    }
  }

  // C++14 [over.ics.list]p2: Otherwise, if the parameter type [...] (below).
  // C++11 [over.ics.list]p2:
  //   If the parameter type is std::initializer_list<X> or "array of X" and
  //   all the elements can be implicitly converted to X, the implicit
  //   conversion sequence is the worst conversion necessary to convert an
  //   element of the list to X.
  //
  // C++14 [over.ics.list]p3:
  //   Otherwise, if the parameter type is "array of N X", if the initializer
  //   list has exactly N elements or if it has fewer than N elements and X is
  //   default-constructible, and if all the elements of the initializer list
  //   can be implicitly converted to X, the implicit conversion sequence is
  //   the worst conversion necessary to convert an element of the list to X.
  //
  // FIXME: We're missing a lot of these checks.
  bool toStdInitializerList = false;
  QualType X;
  if (ToType->isArrayType())
    X = S.Context.getAsArrayType(ToType)->getElementType();
  else
    toStdInitializerList = S.isStdInitializerList(ToType, &X);
  if (!X.isNull()) {
    for (unsigned i = 0, e = From->getNumInits(); i < e; ++i) {
      Expr *Init = From->getInit(i);
      ImplicitConversionSequence ICS =
          TryCopyInitialization(S, Init, X, SuppressUserConversions,
                                InOverloadResolution,
                                AllowObjCWritebackConversion);
      // If a single element isn't convertible, fail.
      if (ICS.isBad()) {
        Result = ICS;
        break;
      }
      // Otherwise, look for the worst conversion.
      if (Result.isBad() || CompareImplicitConversionSequences(
                                S, From->getBeginLoc(), ICS, Result) ==
                                ImplicitConversionSequence::Worse)
        Result = ICS;
    }

    // For an empty list, we won't have computed any conversion sequence.
    // Introduce the identity conversion sequence.
    if (From->getNumInits() == 0) {
      Result.setStandard();
      Result.Standard.setAsIdentityConversion();
      Result.Standard.setFromType(ToType);
      Result.Standard.setAllToTypes(ToType);
    }

    Result.setStdInitializerListElement(toStdInitializerList);
    return Result;
  }

  // C++14 [over.ics.list]p4:
  // C++11 [over.ics.list]p3:
  //   Otherwise, if the parameter is a non-aggregate class X and overload
  //   resolution chooses a single best constructor [...] the implicit
  //   conversion sequence is a user-defined conversion sequence. If multiple
  //   constructors are viable but none is better than the others, the
  //   implicit conversion sequence is a user-defined conversion sequence.
  if (ToType->isRecordType() && !ToType->isAggregateType()) {
    // This function can deal with initializer lists.
    return TryUserDefinedConversion(S, From, ToType, SuppressUserConversions,
                                    /*AllowExplicit=*/false,
                                    InOverloadResolution, /*CStyle=*/false,
                                    AllowObjCWritebackConversion,
                                    /*AllowObjCConversionOnExplicit=*/false);
  }

  // C++14 [over.ics.list]p5:
  // C++11 [over.ics.list]p4:
  //   Otherwise, if the parameter has an aggregate type which can be
  //   initialized from the initializer list [...] the implicit conversion
  //   sequence is a user-defined conversion sequence.
  if (ToType->isAggregateType()) {
    // Type is an aggregate, argument is an init list. At this point it comes
    // down to checking whether the initialization works.
    // FIXME: Find out whether this parameter is consumed or not.
    // FIXME: Expose SemaInit's aggregate initialization code so that we don't
    // need to call into the initialization code here; overload resolution
    // should not be doing that.
    InitializedEntity Entity =
        InitializedEntity::InitializeParameter(S.Context, ToType,
                                               /*Consumed=*/false);
    if (S.CanPerformCopyInitialization(Entity, From)) {
      Result.setUserDefined();
      Result.UserDefined.Before.setAsIdentityConversion();
      // Initializer lists don't have a type.
      Result.UserDefined.Before.setFromType(QualType());
      Result.UserDefined.Before.setAllToTypes(QualType());

      Result.UserDefined.After.setAsIdentityConversion();
      Result.UserDefined.After.setFromType(ToType);
      Result.UserDefined.After.setAllToTypes(ToType);
      Result.UserDefined.ConversionFunction = nullptr;
    }
    return Result;
  }

  // C++14 [over.ics.list]p6:
  // C++11 [over.ics.list]p5:
  //   Otherwise, if the parameter is a reference, see 13.3.3.1.4.
  if (ToType->isReferenceType()) {
    // The standard is notoriously unclear here, since 13.3.3.1.4 doesn't
    // mention initializer lists in any way. So we go by what list-
    // initialization would do and try to extrapolate from that.

    QualType T1 = ToType->getAs<ReferenceType>()->getPointeeType();

    // If the initializer list has a single element that is reference-related
    // to the parameter type, we initialize the reference from that.
    if (From->getNumInits() == 1) {
      Expr *Init = From->getInit(0);

      QualType T2 = Init->getType();

      // If the initializer is the address of an overloaded function, try
      // to resolve the overloaded function. If all goes well, T2 is the
      // type of the resulting function.
      if (S.Context.getCanonicalType(T2) == S.Context.OverloadTy) {
        DeclAccessPair Found;
        if (FunctionDecl *Fn = S.ResolveAddressOfOverloadedFunction(
                                   Init, ToType, false, Found))
          T2 = Fn->getType();
      }

      // Compute some basic properties of the types and the initializer.
      bool dummy1 = false;
      bool dummy2 = false;
      bool dummy3 = false;
      Sema::ReferenceCompareResult RefRelationship =
          S.CompareReferenceRelationship(From->getBeginLoc(), T1, T2, dummy1,
                                         dummy2, dummy3);

      if (RefRelationship >= Sema::Ref_Related) {
        return TryReferenceInit(S, Init, ToType, /*FIXME*/ From->getBeginLoc(),
                                SuppressUserConversions,
                                /*AllowExplicit=*/false);
      }
    }

    // Otherwise, we bind the reference to a temporary created from the
    // initializer list.
    Result = TryListConversion(S, From, T1, SuppressUserConversions,
                               InOverloadResolution,
                               AllowObjCWritebackConversion);
    if (Result.isFailure())
      return Result;
    assert(!Result.isEllipsis() &&
           "Sub-initialization cannot result in ellipsis conversion.");

    // Can we even bind to a temporary?
    if (ToType->isRValueReferenceType() ||
        (T1.isConstQualified() && !T1.isVolatileQualified())) {
      StandardConversionSequence &SCS = Result.isStandard() ? Result.Standard :
                                            Result.UserDefined.After;
      SCS.ReferenceBinding = true;
      SCS.IsLvalueReference = ToType->isLValueReferenceType();
      SCS.BindsToRvalue = true;
      SCS.BindsToFunctionLvalue = false;
      SCS.BindsImplicitObjectArgumentWithoutRefQualifier = false;
      SCS.ObjCLifetimeConversionBinding = false;
    } else
      Result.setBad(BadConversionSequence::lvalue_ref_to_rvalue,
                    From, ToType);
    return Result;
  }

  // C++14 [over.ics.list]p7:
  // C++11 [over.ics.list]p6:
  //   Otherwise, if the parameter type is not a class:
  if (!ToType->isRecordType()) {
    //    - if the initializer list has one element that is not itself an
    //      initializer list, the implicit conversion sequence is the one
    //      required to convert the element to the parameter type.
    unsigned NumInits = From->getNumInits();
    if (NumInits == 1 && !isa<InitListExpr>(From->getInit(0)))
      Result = TryCopyInitialization(S, From->getInit(0), ToType,
                                     SuppressUserConversions,
                                     InOverloadResolution,
                                     AllowObjCWritebackConversion);
    //    - if the initializer list has no elements, the implicit conversion
    //      sequence is the identity conversion.
    else if (NumInits == 0) {
      Result.setStandard();
      Result.Standard.setAsIdentityConversion();
      Result.Standard.setFromType(ToType);
      Result.Standard.setAllToTypes(ToType);
    }
    return Result;
  }

  // C++14 [over.ics.list]p8:
  // C++11 [over.ics.list]p7:
  //   In all cases other than those enumerated above, no conversion is possible
  return Result;
}

/// TryCopyInitialization - Try to copy-initialize a value of type
/// ToType from the expression From. Return the implicit conversion
/// sequence required to pass this argument, which may be a bad
/// conversion sequence (meaning that the argument cannot be passed to
/// a parameter of this type). If @p SuppressUserConversions, then we
/// do not permit any user-defined conversion sequences.
static ImplicitConversionSequence
TryCopyInitialization(Sema &S, Expr *From, QualType ToType,
                      bool SuppressUserConversions,
                      bool InOverloadResolution,
                      bool AllowObjCWritebackConversion,
                      bool AllowExplicit) {
  if (InitListExpr *FromInitList = dyn_cast<InitListExpr>(From))
    return TryListConversion(S, FromInitList, ToType, SuppressUserConversions,
                             InOverloadResolution,AllowObjCWritebackConversion);

  if (ToType->isReferenceType())
    return TryReferenceInit(S, From, ToType,
                            /*FIXME:*/ From->getBeginLoc(),
                            SuppressUserConversions, AllowExplicit);

  return TryImplicitConversion(S, From, ToType,
                               SuppressUserConversions,
                               /*AllowExplicit=*/false,
                               InOverloadResolution,
                               /*CStyle=*/false,
                               AllowObjCWritebackConversion,
                               /*AllowObjCConversionOnExplicit=*/false);
}

static bool TryCopyInitialization(const CanQualType FromQTy,
                                  const CanQualType ToQTy,
                                  Sema &S,
                                  SourceLocation Loc,
                                  ExprValueKind FromVK) {
  OpaqueValueExpr TmpExpr(Loc, FromQTy, FromVK);
  ImplicitConversionSequence ICS =
    TryCopyInitialization(S, &TmpExpr, ToQTy, true, true, false);

  return !ICS.isBad();
}

/// TryObjectArgumentInitialization - Try to initialize the object
/// parameter of the given member function (@c Method) from the
/// expression @p From.
static ImplicitConversionSequence
TryObjectArgumentInitialization(Sema &S, SourceLocation Loc, QualType FromType,
                                Expr::Classification FromClassification,
                                CXXMethodDecl *Method,
                                CXXRecordDecl *ActingContext) {
  QualType ClassType = S.Context.getTypeDeclType(ActingContext);
  // [class.dtor]p2: A destructor can be invoked for a const, volatile or
  //                 const volatile object.
  Qualifiers Quals;
  if (isa<CXXDestructorDecl>(Method)) {
    Quals.addConst();
    Quals.addVolatile();
  } else {
    Quals = Method->getTypeQualifiers();
  }

  QualType ImplicitParamType = S.Context.getQualifiedType(ClassType, Quals);

  // Set up the conversion sequence as a "bad" conversion, to allow us
  // to exit early.
  ImplicitConversionSequence ICS;

  // We need to have an object of class type.
  if (const PointerType *PT = FromType->getAs<PointerType>()) {
    FromType = PT->getPointeeType();

    // When we had a pointer, it's implicitly dereferenced, so we
    // better have an lvalue.
    assert(FromClassification.isLValue());
  }

  assert(FromType->isRecordType());

  // C++0x [over.match.funcs]p4:
  //   For non-static member functions, the type of the implicit object
  //   parameter is
  //
  //     - "lvalue reference to cv X" for functions declared without a
  //        ref-qualifier or with the & ref-qualifier
  //     - "rvalue reference to cv X" for functions declared with the &&
  //        ref-qualifier
  //
  // where X is the class of which the function is a member and cv is the
  // cv-qualification on the member function declaration.
  //
  // However, when finding an implicit conversion sequence for the argument, we
  // are not allowed to perform user-defined conversions
  // (C++ [over.match.funcs]p5). We perform a simplified version of
  // reference binding here, that allows class rvalues to bind to
  // non-constant references.

  // First check the qualifiers.
  QualType FromTypeCanon = S.Context.getCanonicalType(FromType);
  if (ImplicitParamType.getCVRQualifiers()
                                    != FromTypeCanon.getLocalCVRQualifiers() &&
      !ImplicitParamType.isAtLeastAsQualifiedAs(FromTypeCanon)) {
    ICS.setBad(BadConversionSequence::bad_qualifiers,
               FromType, ImplicitParamType);
    return ICS;
  }

  // Check that we have either the same type or a derived type. It
  // affects the conversion rank.
  QualType ClassTypeCanon = S.Context.getCanonicalType(ClassType);
  ImplicitConversionKind SecondKind;
  if (ClassTypeCanon == FromTypeCanon.getLocalUnqualifiedType()) {
    SecondKind = ICK_Identity;
  } else if (S.IsDerivedFrom(Loc, FromType, ClassType))
    SecondKind = ICK_Derived_To_Base;
  else {
    ICS.setBad(BadConversionSequence::unrelated_class,
               FromType, ImplicitParamType);
    return ICS;
  }

  // Check the ref-qualifier.
  switch (Method->getRefQualifier()) {
  case RQ_None:
    // Do nothing; we don't care about lvalueness or rvalueness.
    break;

  case RQ_LValue:
    if (!FromClassification.isLValue() && !Quals.hasOnlyConst()) {
      // non-const lvalue reference cannot bind to an rvalue
      ICS.setBad(BadConversionSequence::lvalue_ref_to_rvalue, FromType,
                 ImplicitParamType);
      return ICS;
    }
    break;

  case RQ_RValue:
    if (!FromClassification.isRValue()) {
      // rvalue reference cannot bind to an lvalue
      ICS.setBad(BadConversionSequence::rvalue_ref_to_lvalue, FromType,
                 ImplicitParamType);
      return ICS;
    }
    break;
  }

  // Success. Mark this as a reference binding.
  ICS.setStandard();
  ICS.Standard.setAsIdentityConversion();
  ICS.Standard.Second = SecondKind;
  ICS.Standard.setFromType(FromType);
  ICS.Standard.setAllToTypes(ImplicitParamType);
  ICS.Standard.ReferenceBinding = true;
  ICS.Standard.DirectBinding = true;
  ICS.Standard.IsLvalueReference = Method->getRefQualifier() != RQ_RValue;
  ICS.Standard.BindsToFunctionLvalue = false;
  ICS.Standard.BindsToRvalue = FromClassification.isRValue();
  ICS.Standard.BindsImplicitObjectArgumentWithoutRefQualifier
    = (Method->getRefQualifier() == RQ_None);
  return ICS;
}

/// PerformObjectArgumentInitialization - Perform initialization of
/// the implicit object parameter for the given Method with the given
/// expression.
ExprResult
Sema::PerformObjectArgumentInitialization(Expr *From,
                                          NestedNameSpecifier *Qualifier,
                                          NamedDecl *FoundDecl,
                                          CXXMethodDecl *Method) {
  QualType FromRecordType, DestType;
  QualType ImplicitParamRecordType  =
    Method->getThisType(Context)->getAs<PointerType>()->getPointeeType();

  Expr::Classification FromClassification;
  if (const PointerType *PT = From->getType()->getAs<PointerType>()) {
    FromRecordType = PT->getPointeeType();
    DestType = Method->getThisType(Context);
    FromClassification = Expr::Classification::makeSimpleLValue();
  } else {
    FromRecordType = From->getType();
    DestType = ImplicitParamRecordType;
    FromClassification = From->Classify(Context);

    // When performing member access on an rvalue, materialize a temporary.
    if (From->isRValue()) {
      From = CreateMaterializeTemporaryExpr(FromRecordType, From,
                                            Method->getRefQualifier() !=
                                                RefQualifierKind::RQ_RValue);
    }
  }

  // Note that we always use the true parent context when performing
  // the actual argument initialization.
  ImplicitConversionSequence ICS = TryObjectArgumentInitialization(
      *this, From->getBeginLoc(), From->getType(), FromClassification, Method,
      Method->getParent());
  if (ICS.isBad()) {
    switch (ICS.Bad.Kind) {
    case BadConversionSequence::bad_qualifiers: {
      Qualifiers FromQs = FromRecordType.getQualifiers();
      Qualifiers ToQs = DestType.getQualifiers();
      unsigned CVR = FromQs.getCVRQualifiers() & ~ToQs.getCVRQualifiers();
      if (CVR) {
        Diag(From->getBeginLoc(), diag::err_member_function_call_bad_cvr)
            << Method->getDeclName() << FromRecordType << (CVR - 1)
            << From->getSourceRange();
        Diag(Method->getLocation(), diag::note_previous_decl)
          << Method->getDeclName();
        return ExprError();
      }
      break;
    }

    case BadConversionSequence::lvalue_ref_to_rvalue:
    case BadConversionSequence::rvalue_ref_to_lvalue: {
      bool IsRValueQualified =
        Method->getRefQualifier() == RefQualifierKind::RQ_RValue;
      Diag(From->getBeginLoc(), diag::err_member_function_call_bad_ref)
          << Method->getDeclName() << FromClassification.isRValue()
          << IsRValueQualified;
      Diag(Method->getLocation(), diag::note_previous_decl)
        << Method->getDeclName();
      return ExprError();
    }

    case BadConversionSequence::no_conversion:
    case BadConversionSequence::unrelated_class:
      break;
    }

    return Diag(From->getBeginLoc(), diag::err_member_function_call_bad_type)
           << ImplicitParamRecordType << FromRecordType
           << From->getSourceRange();
  }

  if (ICS.Standard.Second == ICK_Derived_To_Base) {
    ExprResult FromRes =
      PerformObjectMemberConversion(From, Qualifier, FoundDecl, Method);
    if (FromRes.isInvalid())
      return ExprError();
    From = FromRes.get();
  }

  if (!Context.hasSameType(From->getType(), DestType)) {
    if (From->getType().getAddressSpace() != DestType.getAddressSpace())
      From = ImpCastExprToType(From, DestType, CK_AddressSpaceConversion,
                             From->getValueKind()).get();
    else
      From = ImpCastExprToType(From, DestType, CK_NoOp,
                             From->getValueKind()).get();
  }
  return From;
}

/// TryContextuallyConvertToBool - Attempt to contextually convert the
/// expression From to bool (C++0x [conv]p3).
static ImplicitConversionSequence
TryContextuallyConvertToBool(Sema &S, Expr *From) {
  return TryImplicitConversion(S, From, S.Context.BoolTy,
                               /*SuppressUserConversions=*/false,
                               /*AllowExplicit=*/true,
                               /*InOverloadResolution=*/false,
                               /*CStyle=*/false,
                               /*AllowObjCWritebackConversion=*/false,
                               /*AllowObjCConversionOnExplicit=*/false);
}

/// PerformContextuallyConvertToBool - Perform a contextual conversion
/// of the expression From to bool (C++0x [conv]p3).
ExprResult Sema::PerformContextuallyConvertToBool(Expr *From) {
  if (checkPlaceholderForOverload(*this, From))
    return ExprError();

  ImplicitConversionSequence ICS = TryContextuallyConvertToBool(*this, From);
  if (!ICS.isBad())
    return PerformImplicitConversion(From, Context.BoolTy, ICS, AA_Converting);

  if (!DiagnoseMultipleUserDefinedConversion(From, Context.BoolTy))
    return Diag(From->getBeginLoc(), diag::err_typecheck_bool_condition)
           << From->getType() << From->getSourceRange();
  return ExprError();
}

/// Check that the specified conversion is permitted in a converted constant
/// expression, according to C++11 [expr.const]p3. Return true if the conversion
/// is acceptable.
static bool CheckConvertedConstantConversions(Sema &S,
                                              StandardConversionSequence &SCS) {
  // Since we know that the target type is an integral or unscoped enumeration
  // type, most conversion kinds are impossible. All possible First and Third
  // conversions are fine.
  switch (SCS.Second) {
  case ICK_Identity:
  case ICK_Function_Conversion:
  case ICK_Integral_Promotion:
  case ICK_Integral_Conversion: // Narrowing conversions are checked elsewhere.
  case ICK_Zero_Queue_Conversion:
    return true;

  case ICK_Boolean_Conversion:
    // Conversion from an integral or unscoped enumeration type to bool is
    // classified as ICK_Boolean_Conversion, but it's also arguably an integral
    // conversion, so we allow it in a converted constant expression.
    //
    // FIXME: Per core issue 1407, we should not allow this, but that breaks
    // a lot of popular code. We should at least add a warning for this
    // (non-conforming) extension.
    return SCS.getFromType()->isIntegralOrUnscopedEnumerationType() &&
           SCS.getToType(2)->isBooleanType();

  case ICK_Pointer_Conversion:
  case ICK_Pointer_Member:
    // C++1z: null pointer conversions and null member pointer conversions are
    // only permitted if the source type is std::nullptr_t.
    return SCS.getFromType()->isNullPtrType();

  case ICK_Floating_Promotion:
  case ICK_Complex_Promotion:
  case ICK_Floating_Conversion:
  case ICK_Complex_Conversion:
  case ICK_Floating_Integral:
  case ICK_Compatible_Conversion:
  case ICK_Derived_To_Base:
  case ICK_Vector_Conversion:
  case ICK_Vector_Splat:
  case ICK_Complex_Real:
  case ICK_Block_Pointer_Conversion:
  case ICK_TransparentUnionConversion:
  case ICK_Writeback_Conversion:
  case ICK_Zero_Event_Conversion:
  case ICK_C_Only_Conversion:
  case ICK_Incompatible_Pointer_Conversion: