SemaDecl.cpp

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//===--- SemaDecl.cpp - Semantic Analysis for Declarations ----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
//  This file implements semantic analysis for declarations.
//
//===----------------------------------------------------------------------===//
 
#include "TypeLocBuilder.h"
#include "clang/AST/ASTConsumer.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/ASTLambda.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/CharUnits.h"
#include "clang/AST/CommentDiagnostic.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/DeclTemplate.h"
#include "clang/AST/EvaluatedExprVisitor.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/NonTrivialTypeVisitor.h"
#include "clang/AST/Randstruct.h"
#include "clang/AST/StmtCXX.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/HLSLRuntime.h"
#include "clang/Basic/PartialDiagnostic.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Lex/HeaderSearch.h" // TODO: Sema shouldn't depend on Lex
#include "clang/Lex/Lexer.h" // TODO: Extract static functions to fix layering.
#include "clang/Lex/ModuleLoader.h" // TODO: Sema shouldn't depend on Lex
#include "clang/Lex/Preprocessor.h" // Included for isCodeCompletionEnabled()
#include "clang/Sema/CXXFieldCollector.h"
#include "clang/Sema/DeclSpec.h"
#include "clang/Sema/DelayedDiagnostic.h"
#include "clang/Sema/Initialization.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/ParsedTemplate.h"
#include "clang/Sema/Scope.h"
#include "clang/Sema/ScopeInfo.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/TargetParser/Triple.h"
#include <algorithm>
#include <cstring>
#include <functional>
#include <optional>
#include <unordered_map>
 
using namespace clang;
using namespace sema;
 
Sema::DeclGroupPtrTy Sema::ConvertDeclToDeclGroup(Decl *Ptr, Decl *OwnedType) {
  if (OwnedType) {
    Decl *Group[2] = { OwnedType, Ptr };
    return DeclGroupPtrTy::make(DeclGroupRef::Create(Context, Group, 2));
  }
 
  return DeclGroupPtrTy::make(DeclGroupRef(Ptr));
}
 
namespace {
 
class TypeNameValidatorCCC final : public CorrectionCandidateCallback {
 public:
   TypeNameValidatorCCC(bool AllowInvalid, bool WantClass = false,
                        bool AllowTemplates = false,
                        bool AllowNonTemplates = true)
       : AllowInvalidDecl(AllowInvalid), WantClassName(WantClass),
         AllowTemplates(AllowTemplates), AllowNonTemplates(AllowNonTemplates) {
     WantExpressionKeywords = false;
     WantCXXNamedCasts = false;
     WantRemainingKeywords = false;
  }
 
  bool ValidateCandidate(const TypoCorrection &candidate) override {
    if (NamedDecl *ND = candidate.getCorrectionDecl()) {
      if (!AllowInvalidDecl && ND->isInvalidDecl())
        return false;
 
      if (getAsTypeTemplateDecl(ND))
        return AllowTemplates;
 
      bool IsType = isa<TypeDecl>(ND) || isa<ObjCInterfaceDecl>(ND);
      if (!IsType)
        return false;
 
      if (AllowNonTemplates)
        return true;
 
      // An injected-class-name of a class template (specialization) is valid
      // as a template or as a non-template.
      if (AllowTemplates) {
        auto *RD = dyn_cast<CXXRecordDecl>(ND);
        if (!RD || !RD->isInjectedClassName())
          return false;
        RD = cast<CXXRecordDecl>(RD->getDeclContext());
        return RD->getDescribedClassTemplate() ||
               isa<ClassTemplateSpecializationDecl>(RD);
      }
 
      return false;
    }
 
    return !WantClassName && candidate.isKeyword();
  }
 
  std::unique_ptr<CorrectionCandidateCallback> clone() override {
    return std::make_unique<TypeNameValidatorCCC>(*this);
  }
 
 private:
  bool AllowInvalidDecl;
  bool WantClassName;
  bool AllowTemplates;
  bool AllowNonTemplates;
};
 
} // end anonymous namespace
 
/// Determine whether the token kind starts a simple-type-specifier.
bool Sema::isSimpleTypeSpecifier(tok::TokenKind Kind) const {
  switch (Kind) {
  // FIXME: Take into account the current language when deciding whether a
  // token kind is a valid type specifier
  case tok::kw_short:
  case tok::kw_long:
  case tok::kw___int64:
  case tok::kw___int128:
  case tok::kw_signed:
  case tok::kw_unsigned:
  case tok::kw_void:
  case tok::kw_char:
  case tok::kw_int:
  case tok::kw_half:
  case tok::kw_float:
  case tok::kw_double:
  case tok::kw___bf16:
  case tok::kw__Float16:
  case tok::kw___float128:
  case tok::kw___ibm128:
  case tok::kw_wchar_t:
  case tok::kw_bool:
#define TRANSFORM_TYPE_TRAIT_DEF(_, Trait) case tok::kw___##Trait:
#include "clang/Basic/TransformTypeTraits.def"
  case tok::kw___auto_type:
    return true;
 
  case tok::annot_typename:
  case tok::kw_char16_t:
  case tok::kw_char32_t:
  case tok::kw_typeof:
  case tok::annot_decltype:
  case tok::kw_decltype:
    return getLangOpts().CPlusPlus;
 
  case tok::kw_char8_t:
    return getLangOpts().Char8;
 
  default:
    break;
  }
 
  return false;
}
 
namespace {
enum class UnqualifiedTypeNameLookupResult {
  NotFound,
  FoundNonType,
  FoundType
};
} // end anonymous namespace
 
/// Tries to perform unqualified lookup of the type decls in bases for
/// dependent class.
/// \return \a NotFound if no any decls is found, \a FoundNotType if found not a
/// type decl, \a FoundType if only type decls are found.
static UnqualifiedTypeNameLookupResult
lookupUnqualifiedTypeNameInBase(Sema &S, const IdentifierInfo &II,
                                SourceLocation NameLoc,
                                const CXXRecordDecl *RD) {
  if (!RD->hasDefinition())
    return UnqualifiedTypeNameLookupResult::NotFound;
  // Look for type decls in base classes.
  UnqualifiedTypeNameLookupResult FoundTypeDecl =
      UnqualifiedTypeNameLookupResult::NotFound;
  for (const auto &Base : RD->bases()) {
    const CXXRecordDecl *BaseRD = nullptr;
    if (auto *BaseTT = Base.getType()->getAs<TagType>())
      BaseRD = BaseTT->getAsCXXRecordDecl();
    else if (auto *TST = Base.getType()->getAs<TemplateSpecializationType>()) {
      // Look for type decls in dependent base classes that have known primary
      // templates.
      if (!TST || !TST->isDependentType())
        continue;
      auto *TD = TST->getTemplateName().getAsTemplateDecl();
      if (!TD)
        continue;
      if (auto *BasePrimaryTemplate =
          dyn_cast_or_null<CXXRecordDecl>(TD->getTemplatedDecl())) {
        if (BasePrimaryTemplate->getCanonicalDecl() != RD->getCanonicalDecl())
          BaseRD = BasePrimaryTemplate;
        else if (auto *CTD = dyn_cast<ClassTemplateDecl>(TD)) {
          if (const ClassTemplatePartialSpecializationDecl *PS =
                  CTD->findPartialSpecialization(Base.getType()))
            if (PS->getCanonicalDecl() != RD->getCanonicalDecl())
              BaseRD = PS;
        }
      }
    }
    if (BaseRD) {
      for (NamedDecl *ND : BaseRD->lookup(&II)) {
        if (!isa<TypeDecl>(ND))
          return UnqualifiedTypeNameLookupResult::FoundNonType;
        FoundTypeDecl = UnqualifiedTypeNameLookupResult::FoundType;
      }
      if (FoundTypeDecl == UnqualifiedTypeNameLookupResult::NotFound) {
        switch (lookupUnqualifiedTypeNameInBase(S, II, NameLoc, BaseRD)) {
        case UnqualifiedTypeNameLookupResult::FoundNonType:
          return UnqualifiedTypeNameLookupResult::FoundNonType;
        case UnqualifiedTypeNameLookupResult::FoundType:
          FoundTypeDecl = UnqualifiedTypeNameLookupResult::FoundType;
          break;
        case UnqualifiedTypeNameLookupResult::NotFound:
          break;
        }
      }
    }
  }
 
  return FoundTypeDecl;
}
 
static ParsedType recoverFromTypeInKnownDependentBase(Sema &S,
                                                      const IdentifierInfo &II,
                                                      SourceLocation NameLoc) {
  // Lookup in the parent class template context, if any.
  const CXXRecordDecl *RD = nullptr;
  UnqualifiedTypeNameLookupResult FoundTypeDecl =
      UnqualifiedTypeNameLookupResult::NotFound;
  for (DeclContext *DC = S.CurContext;
       DC && FoundTypeDecl == UnqualifiedTypeNameLookupResult::NotFound;
       DC = DC->getParent()) {
    // Look for type decls in dependent base classes that have known primary
    // templates.
    RD = dyn_cast<CXXRecordDecl>(DC);
    if (RD && RD->getDescribedClassTemplate())
      FoundTypeDecl = lookupUnqualifiedTypeNameInBase(S, II, NameLoc, RD);
  }
  if (FoundTypeDecl != UnqualifiedTypeNameLookupResult::FoundType)
    return nullptr;
 
  // We found some types in dependent base classes.  Recover as if the user
  // wrote 'typename MyClass::II' instead of 'II'.  We'll fully resolve the
  // lookup during template instantiation.
  S.Diag(NameLoc, diag::ext_found_in_dependent_base) << &II;
 
  ASTContext &Context = S.Context;
  auto *NNS = NestedNameSpecifier::Create(Context, nullptr, false,
                                          cast<Type>(Context.getRecordType(RD)));
  QualType T = Context.getDependentNameType(ETK_Typename, NNS, &II);
 
  CXXScopeSpec SS;
  SS.MakeTrivial(Context, NNS, SourceRange(NameLoc));
 
  TypeLocBuilder Builder;
  DependentNameTypeLoc DepTL = Builder.push<DependentNameTypeLoc>(T);
  DepTL.setNameLoc(NameLoc);
  DepTL.setElaboratedKeywordLoc(SourceLocation());
  DepTL.setQualifierLoc(SS.getWithLocInContext(Context));
  return S.CreateParsedType(T, Builder.getTypeSourceInfo(Context, T));
}
 
/// Build a ParsedType for a simple-type-specifier with a nested-name-specifier.
static ParsedType buildNamedType(Sema &S, const CXXScopeSpec *SS, QualType T,
                                 SourceLocation NameLoc,
                                 bool WantNontrivialTypeSourceInfo = true) {
  switch (T->getTypeClass()) {
  case Type::DeducedTemplateSpecialization:
  case Type::Enum:
  case Type::InjectedClassName:
  case Type::Record:
  case Type::Typedef:
  case Type::UnresolvedUsing:
  case Type::Using:
    break;
  // These can never be qualified so an ElaboratedType node
  // would carry no additional meaning.
  case Type::ObjCInterface:
  case Type::ObjCTypeParam:
  case Type::TemplateTypeParm:
    return ParsedType::make(T);
  default:
    llvm_unreachable("Unexpected Type Class");
  }
 
  if (!SS || SS->isEmpty())
    return ParsedType::make(
        S.Context.getElaboratedType(ETK_None, nullptr, T, nullptr));
 
  QualType ElTy = S.getElaboratedType(ETK_None, *SS, T);
  if (!WantNontrivialTypeSourceInfo)
    return ParsedType::make(ElTy);
 
  TypeLocBuilder Builder;
  Builder.pushTypeSpec(T).setNameLoc(NameLoc);
  ElaboratedTypeLoc ElabTL = Builder.push<ElaboratedTypeLoc>(ElTy);
  ElabTL.setElaboratedKeywordLoc(SourceLocation());
  ElabTL.setQualifierLoc(SS->getWithLocInContext(S.Context));
  return S.CreateParsedType(ElTy, Builder.getTypeSourceInfo(S.Context, ElTy));
}
 
/// If the identifier refers to a type name within this scope,
/// return the declaration of that type.
///
/// This routine performs ordinary name lookup of the identifier II
/// within the given scope, with optional C++ scope specifier SS, to
/// determine whether the name refers to a type. If so, returns an
/// opaque pointer (actually a QualType) corresponding to that
/// type. Otherwise, returns NULL.
ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
                             Scope *S, CXXScopeSpec *SS, bool isClassName,
                             bool HasTrailingDot, ParsedType ObjectTypePtr,
                             bool IsCtorOrDtorName,
                             bool WantNontrivialTypeSourceInfo,
                             bool IsClassTemplateDeductionContext,
                             ImplicitTypenameContext AllowImplicitTypename,
                             IdentifierInfo **CorrectedII) {
  // FIXME: Consider allowing this outside C++1z mode as an extension.
  bool AllowDeducedTemplate = IsClassTemplateDeductionContext &&
                              getLangOpts().CPlusPlus17 && !IsCtorOrDtorName &&
                              !isClassName && !HasTrailingDot;
 
  // Determine where we will perform name lookup.
  DeclContext *LookupCtx = nullptr;
  if (ObjectTypePtr) {
    QualType ObjectType = ObjectTypePtr.get();
    if (ObjectType->isRecordType())
      LookupCtx = computeDeclContext(ObjectType);
  } else if (SS && SS->isNotEmpty()) {
    LookupCtx = computeDeclContext(*SS, false);
 
    if (!LookupCtx) {
      if (isDependentScopeSpecifier(*SS)) {
        // C++ [temp.res]p3:
        //   A qualified-id that refers to a type and in which the
        //   nested-name-specifier depends on a template-parameter (14.6.2)
        //   shall be prefixed by the keyword typename to indicate that the
        //   qualified-id denotes a type, forming an
        //   elaborated-type-specifier (7.1.5.3).
        //
        // We therefore do not perform any name lookup if the result would
        // refer to a member of an unknown specialization.
        // In C++2a, in several contexts a 'typename' is not required. Also
        // allow this as an extension.
        if (AllowImplicitTypename == ImplicitTypenameContext::No &&
            !isClassName && !IsCtorOrDtorName)
          return nullptr;
        bool IsImplicitTypename = !isClassName && !IsCtorOrDtorName;
        if (IsImplicitTypename) {
          SourceLocation QualifiedLoc = SS->getRange().getBegin();
          if (getLangOpts().CPlusPlus20)
            Diag(QualifiedLoc, diag::warn_cxx17_compat_implicit_typename);
          else
            Diag(QualifiedLoc, diag::ext_implicit_typename)
                << SS->getScopeRep() << II.getName()
                << FixItHint::CreateInsertion(QualifiedLoc, "typename ");
        }
 
        // We know from the grammar that this name refers to a type,
        // so build a dependent node to describe the type.
        if (WantNontrivialTypeSourceInfo)
          return ActOnTypenameType(S, SourceLocation(), *SS, II, NameLoc,
                                   (ImplicitTypenameContext)IsImplicitTypename)
              .get();
 
        NestedNameSpecifierLoc QualifierLoc = SS->getWithLocInContext(Context);
        QualType T =
            CheckTypenameType(IsImplicitTypename ? ETK_Typename : ETK_None,
                              SourceLocation(), QualifierLoc, II, NameLoc);
        return ParsedType::make(T);
      }
 
      return nullptr;
    }
 
    if (!LookupCtx->isDependentContext() &&
        RequireCompleteDeclContext(*SS, LookupCtx))
      return nullptr;
  }
 
  // FIXME: LookupNestedNameSpecifierName isn't the right kind of
  // lookup for class-names.
  LookupNameKind Kind = isClassName ? LookupNestedNameSpecifierName :
                                      LookupOrdinaryName;
  LookupResult Result(*this, &II, NameLoc, Kind);
  if (LookupCtx) {
    // Perform "qualified" name lookup into the declaration context we
    // computed, which is either the type of the base of a member access
    // expression or the declaration context associated with a prior
    // nested-name-specifier.
    LookupQualifiedName(Result, LookupCtx);
 
    if (ObjectTypePtr && Result.empty()) {
      // C++ [basic.lookup.classref]p3:
      //   If the unqualified-id is ~type-name, the type-name is looked up
      //   in the context of the entire postfix-expression. If the type T of
      //   the object expression is of a class type C, the type-name is also
      //   looked up in the scope of class C. At least one of the lookups shall
      //   find a name that refers to (possibly cv-qualified) T.
      LookupName(Result, S);
    }
  } else {
    // Perform unqualified name lookup.
    LookupName(Result, S);
 
    // For unqualified lookup in a class template in MSVC mode, look into
    // dependent base classes where the primary class template is known.
    if (Result.empty() && getLangOpts().MSVCCompat && (!SS || SS->isEmpty())) {
      if (ParsedType TypeInBase =
              recoverFromTypeInKnownDependentBase(*this, II, NameLoc))
        return TypeInBase;
    }
  }
 
  NamedDecl *IIDecl = nullptr;
  UsingShadowDecl *FoundUsingShadow = nullptr;
  switch (Result.getResultKind()) {
  case LookupResult::NotFound:
  case LookupResult::NotFoundInCurrentInstantiation:
    if (CorrectedII) {
      TypeNameValidatorCCC CCC(/*AllowInvalid=*/true, isClassName,
                               AllowDeducedTemplate);
      TypoCorrection Correction = CorrectTypo(Result.getLookupNameInfo(), Kind,
                                              S, SS, CCC, CTK_ErrorRecovery);
      IdentifierInfo *NewII = Correction.getCorrectionAsIdentifierInfo();
      TemplateTy Template;
      bool MemberOfUnknownSpecialization;
      UnqualifiedId TemplateName;
      TemplateName.setIdentifier(NewII, NameLoc);
      NestedNameSpecifier *NNS = Correction.getCorrectionSpecifier();
      CXXScopeSpec NewSS, *NewSSPtr = SS;
      if (SS && NNS) {
        NewSS.MakeTrivial(Context, NNS, SourceRange(NameLoc));
        NewSSPtr = &NewSS;
      }
      if (Correction && (NNS || NewII != &II) &&
          // Ignore a correction to a template type as the to-be-corrected
          // identifier is not a template (typo correction for template names
          // is handled elsewhere).
          !(getLangOpts().CPlusPlus && NewSSPtr &&
            isTemplateName(S, *NewSSPtr, false, TemplateName, nullptr, false,
                           Template, MemberOfUnknownSpecialization))) {
        ParsedType Ty = getTypeName(*NewII, NameLoc, S, NewSSPtr,
                                    isClassName, HasTrailingDot, ObjectTypePtr,
                                    IsCtorOrDtorName,
                                    WantNontrivialTypeSourceInfo,
                                    IsClassTemplateDeductionContext);
        if (Ty) {
          diagnoseTypo(Correction,
                       PDiag(diag::err_unknown_type_or_class_name_suggest)
                         << Result.getLookupName() << isClassName);
          if (SS && NNS)
            SS->MakeTrivial(Context, NNS, SourceRange(NameLoc));
          *CorrectedII = NewII;
          return Ty;
        }
      }
    }
    // If typo correction failed or was not performed, fall through
    [[fallthrough]];
  case LookupResult::FoundOverloaded:
  case LookupResult::FoundUnresolvedValue:
    Result.suppressDiagnostics();
    return nullptr;
 
  case LookupResult::Ambiguous:
    // Recover from type-hiding ambiguities by hiding the type.  We'll
    // do the lookup again when looking for an object, and we can
    // diagnose the error then.  If we don't do this, then the error
    // about hiding the type will be immediately followed by an error
    // that only makes sense if the identifier was treated like a type.
    if (Result.getAmbiguityKind() == LookupResult::AmbiguousTagHiding) {
      Result.suppressDiagnostics();
      return nullptr;
    }
 
    // Look to see if we have a type anywhere in the list of results.
    for (LookupResult::iterator Res = Result.begin(), ResEnd = Result.end();
         Res != ResEnd; ++Res) {
      NamedDecl *RealRes = (*Res)->getUnderlyingDecl();
      if (isa<TypeDecl, ObjCInterfaceDecl, UnresolvedUsingIfExistsDecl>(
              RealRes) ||
          (AllowDeducedTemplate && getAsTypeTemplateDecl(RealRes))) {
        if (!IIDecl ||
            // Make the selection of the recovery decl deterministic.
            RealRes->getLocation() < IIDecl->getLocation()) {
          IIDecl = RealRes;
          FoundUsingShadow = dyn_cast<UsingShadowDecl>(*Res);
        }
      }
    }
 
    if (!IIDecl) {
      // None of the entities we found is a type, so there is no way
      // to even assume that the result is a type. In this case, don't
      // complain about the ambiguity. The parser will either try to
      // perform this lookup again (e.g., as an object name), which
      // will produce the ambiguity, or will complain that it expected
      // a type name.
      Result.suppressDiagnostics();
      return nullptr;
    }
 
    // We found a type within the ambiguous lookup; diagnose the
    // ambiguity and then return that type. This might be the right
    // answer, or it might not be, but it suppresses any attempt to
    // perform the name lookup again.
    break;
 
  case LookupResult::Found:
    IIDecl = Result.getFoundDecl();
    FoundUsingShadow = dyn_cast<UsingShadowDecl>(*Result.begin());
    break;
  }
 
  assert(IIDecl && "Didn't find decl");
 
  QualType T;
  if (TypeDecl *TD = dyn_cast<TypeDecl>(IIDecl)) {
    // C++ [class.qual]p2: A lookup that would find the injected-class-name
    // instead names the constructors of the class, except when naming a class.
    // This is ill-formed when we're not actually forming a ctor or dtor name.
    auto *LookupRD = dyn_cast_or_null<CXXRecordDecl>(LookupCtx);
    auto *FoundRD = dyn_cast<CXXRecordDecl>(TD);
    if (!isClassName && !IsCtorOrDtorName && LookupRD && FoundRD &&
        FoundRD->isInjectedClassName() &&
        declaresSameEntity(LookupRD, cast<Decl>(FoundRD->getParent())))
      Diag(NameLoc, diag::err_out_of_line_qualified_id_type_names_constructor)
          << &II << /*Type*/1;
 
    DiagnoseUseOfDecl(IIDecl, NameLoc);
 
    T = Context.getTypeDeclType(TD);
    MarkAnyDeclReferenced(TD->getLocation(), TD, /*OdrUse=*/false);
  } else if (ObjCInterfaceDecl *IDecl = dyn_cast<ObjCInterfaceDecl>(IIDecl)) {
    (void)DiagnoseUseOfDecl(IDecl, NameLoc);
    if (!HasTrailingDot)
      T = Context.getObjCInterfaceType(IDecl);
    FoundUsingShadow = nullptr; // FIXME: Target must be a TypeDecl.
  } else if (auto *UD = dyn_cast<UnresolvedUsingIfExistsDecl>(IIDecl)) {
    (void)DiagnoseUseOfDecl(UD, NameLoc);
    // Recover with 'int'
    return ParsedType::make(Context.IntTy);
  } else if (AllowDeducedTemplate) {
    if (auto *TD = getAsTypeTemplateDecl(IIDecl)) {
      assert(!FoundUsingShadow || FoundUsingShadow->getTargetDecl() == TD);
      TemplateName Template =
          FoundUsingShadow ? TemplateName(FoundUsingShadow) : TemplateName(TD);
      T = Context.getDeducedTemplateSpecializationType(Template, QualType(),
                                                       false);
      // Don't wrap in a further UsingType.
      FoundUsingShadow = nullptr;
    }
  }
 
  if (T.isNull()) {
    // If it's not plausibly a type, suppress diagnostics.
    Result.suppressDiagnostics();
    return nullptr;
  }
 
  if (FoundUsingShadow)
    T = Context.getUsingType(FoundUsingShadow, T);
 
  return buildNamedType(*this, SS, T, NameLoc, WantNontrivialTypeSourceInfo);
}
 
// Builds a fake NNS for the given decl context.
static NestedNameSpecifier *
synthesizeCurrentNestedNameSpecifier(ASTContext &Context, DeclContext *DC) {
  for (;; DC = DC->getLookupParent()) {
    DC = DC->getPrimaryContext();
    auto *ND = dyn_cast<NamespaceDecl>(DC);
    if (ND && !ND->isInline() && !ND->isAnonymousNamespace())
      return NestedNameSpecifier::Create(Context, nullptr, ND);
    else if (auto *RD = dyn_cast<CXXRecordDecl>(DC))
      return NestedNameSpecifier::Create(Context, nullptr, RD->isTemplateDecl(),
                                         RD->getTypeForDecl());
    else if (isa<TranslationUnitDecl>(DC))
      return NestedNameSpecifier::GlobalSpecifier(Context);
  }
  llvm_unreachable("something isn't in TU scope?");
}
 
/// Find the parent class with dependent bases of the innermost enclosing method
/// context. Do not look for enclosing CXXRecordDecls directly, or we will end
/// up allowing unqualified dependent type names at class-level, which MSVC
/// correctly rejects.
static const CXXRecordDecl *
findRecordWithDependentBasesOfEnclosingMethod(const DeclContext *DC) {
  for (; DC && DC->isDependentContext(); DC = DC->getLookupParent()) {
    DC = DC->getPrimaryContext();
    if (const auto *MD = dyn_cast<CXXMethodDecl>(DC))
      if (MD->getParent()->hasAnyDependentBases())
        return MD->getParent();
  }
  return nullptr;
}
 
ParsedType Sema::ActOnMSVCUnknownTypeName(const IdentifierInfo &II,
                                          SourceLocation NameLoc,
                                          bool IsTemplateTypeArg) {
  assert(getLangOpts().MSVCCompat && "shouldn't be called in non-MSVC mode");
 
  NestedNameSpecifier *NNS = nullptr;
  if (IsTemplateTypeArg && getCurScope()->isTemplateParamScope()) {
    // If we weren't able to parse a default template argument, delay lookup
    // until instantiation time by making a non-dependent DependentTypeName. We
    // pretend we saw a NestedNameSpecifier referring to the current scope, and
    // lookup is retried.
    // FIXME: This hurts our diagnostic quality, since we get errors like "no
    // type named 'Foo' in 'current_namespace'" when the user didn't write any
    // name specifiers.
    NNS = synthesizeCurrentNestedNameSpecifier(Context, CurContext);
    Diag(NameLoc, diag::ext_ms_delayed_template_argument) << &II;
  } else if (const CXXRecordDecl *RD =
                 findRecordWithDependentBasesOfEnclosingMethod(CurContext)) {
    // Build a DependentNameType that will perform lookup into RD at
    // instantiation time.
    NNS = NestedNameSpecifier::Create(Context, nullptr, RD->isTemplateDecl(),
                                      RD->getTypeForDecl());
 
    // Diagnose that this identifier was undeclared, and retry the lookup during
    // template instantiation.
    Diag(NameLoc, diag::ext_undeclared_unqual_id_with_dependent_base) << &II
                                                                      << RD;
  } else {
    // This is not a situation that we should recover from.
    return ParsedType();
  }
 
  QualType T = Context.getDependentNameType(ETK_None, NNS, &II);
 
  // Build type location information.  We synthesized the qualifier, so we have
  // to build a fake NestedNameSpecifierLoc.
  NestedNameSpecifierLocBuilder NNSLocBuilder;
  NNSLocBuilder.MakeTrivial(Context, NNS, SourceRange(NameLoc));
  NestedNameSpecifierLoc QualifierLoc = NNSLocBuilder.getWithLocInContext(Context);
 
  TypeLocBuilder Builder;
  DependentNameTypeLoc DepTL = Builder.push<DependentNameTypeLoc>(T);
  DepTL.setNameLoc(NameLoc);
  DepTL.setElaboratedKeywordLoc(SourceLocation());
  DepTL.setQualifierLoc(QualifierLoc);
  return CreateParsedType(T, Builder.getTypeSourceInfo(Context, T));
}
 
/// isTagName() - This method is called *for error recovery purposes only*
/// to determine if the specified name is a valid tag name ("struct foo").  If
/// so, this returns the TST for the tag corresponding to it (TST_enum,
/// TST_union, TST_struct, TST_interface, TST_class).  This is used to diagnose
/// cases in C where the user forgot to specify the tag.
DeclSpec::TST Sema::isTagName(IdentifierInfo &II, Scope *S) {
  // Do a tag name lookup in this scope.
  LookupResult R(*this, &II, SourceLocation(), LookupTagName);
  LookupName(R, S, false);
  R.suppressDiagnostics();
  if (R.getResultKind() == LookupResult::Found)
    if (const TagDecl *TD = R.getAsSingle<TagDecl>()) {
      switch (TD->getTagKind()) {
      case TTK_Struct: return DeclSpec::TST_struct;
      case TTK_Interface: return DeclSpec::TST_interface;
      case TTK_Union:  return DeclSpec::TST_union;
      case TTK_Class:  return DeclSpec::TST_class;
      case TTK_Enum:   return DeclSpec::TST_enum;
      }
    }
 
  return DeclSpec::TST_unspecified;
}
 
/// isMicrosoftMissingTypename - In Microsoft mode, within class scope,
/// if a CXXScopeSpec's type is equal to the type of one of the base classes
/// then downgrade the missing typename error to a warning.
/// This is needed for MSVC compatibility; Example:
/// @code
/// template<class T> class A {
/// public:
///   typedef int TYPE;
/// };
/// template<class T> class B : public A<T> {
/// public:
///   A<T>::TYPE a; // no typename required because A<T> is a base class.
/// };
/// @endcode
bool Sema::isMicrosoftMissingTypename(const CXXScopeSpec *SS, Scope *S) {
  if (CurContext->isRecord()) {
    if (SS->getScopeRep()->getKind() == NestedNameSpecifier::Super)
      return true;
 
    const Type *Ty = SS->getScopeRep()->getAsType();
 
    CXXRecordDecl *RD = cast<CXXRecordDecl>(CurContext);
    for (const auto &Base : RD->bases())
      if (Ty && Context.hasSameUnqualifiedType(QualType(Ty, 1), Base.getType()))
        return true;
    return S->isFunctionPrototypeScope();
  }
  return CurContext->isFunctionOrMethod() || S->isFunctionPrototypeScope();
}
 
void Sema::DiagnoseUnknownTypeName(IdentifierInfo *&II,
                                   SourceLocation IILoc,
                                   Scope *S,
                                   CXXScopeSpec *SS,
                                   ParsedType &SuggestedType,
                                   bool IsTemplateName) {
  // Don't report typename errors for editor placeholders.
  if (II->isEditorPlaceholder())
    return;
  // We don't have anything to suggest (yet).
  SuggestedType = nullptr;
 
  // There may have been a typo in the name of the type. Look up typo
  // results, in case we have something that we can suggest.
  TypeNameValidatorCCC CCC(/*AllowInvalid=*/false, /*WantClass=*/false,
                           /*AllowTemplates=*/IsTemplateName,
                           /*AllowNonTemplates=*/!IsTemplateName);
  if (TypoCorrection Corrected =
          CorrectTypo(DeclarationNameInfo(II, IILoc), LookupOrdinaryName, S, SS,
                      CCC, CTK_ErrorRecovery)) {
    // FIXME: Support error recovery for the template-name case.
    bool CanRecover = !IsTemplateName;
    if (Corrected.isKeyword()) {
      // We corrected to a keyword.
      diagnoseTypo(Corrected,
                   PDiag(IsTemplateName ? diag::err_no_template_suggest
                                        : diag::err_unknown_typename_suggest)
                       << II);
      II = Corrected.getCorrectionAsIdentifierInfo();
    } else {
      // We found a similarly-named type or interface; suggest that.
      if (!SS || !SS->isSet()) {
        diagnoseTypo(Corrected,
                     PDiag(IsTemplateName ? diag::err_no_template_suggest
                                          : diag::err_unknown_typename_suggest)
                         << II, CanRecover);
      } else if (DeclContext *DC = computeDeclContext(*SS, false)) {
        std::string CorrectedStr(Corrected.getAsString(getLangOpts()));
        bool DroppedSpecifier = Corrected.WillReplaceSpecifier() &&
                                II->getName().equals(CorrectedStr);
        diagnoseTypo(Corrected,
                     PDiag(IsTemplateName
                               ? diag::err_no_member_template_suggest
                               : diag::err_unknown_nested_typename_suggest)
                         << II << DC << DroppedSpecifier << SS->getRange(),
                     CanRecover);
      } else {
        llvm_unreachable("could not have corrected a typo here");
      }
 
      if (!CanRecover)
        return;
 
      CXXScopeSpec tmpSS;
      if (Corrected.getCorrectionSpecifier())
        tmpSS.MakeTrivial(Context, Corrected.getCorrectionSpecifier(),
                          SourceRange(IILoc));
      // FIXME: Support class template argument deduction here.
      SuggestedType =
          getTypeName(*Corrected.getCorrectionAsIdentifierInfo(), IILoc, S,
                      tmpSS.isSet() ? &tmpSS : SS, false, false, nullptr,
                      /*IsCtorOrDtorName=*/false,
                      /*WantNontrivialTypeSourceInfo=*/true);
    }
    return;
  }
 
  if (getLangOpts().CPlusPlus && !IsTemplateName) {
    // See if II is a class template that the user forgot to pass arguments to.
    UnqualifiedId Name;
    Name.setIdentifier(II, IILoc);
    CXXScopeSpec EmptySS;
    TemplateTy TemplateResult;
    bool MemberOfUnknownSpecialization;
    if (isTemplateName(S, SS ? *SS : EmptySS, /*hasTemplateKeyword=*/false,
                       Name, nullptr, true, TemplateResult,
                       MemberOfUnknownSpecialization) == TNK_Type_template) {
      diagnoseMissingTemplateArguments(TemplateResult.get(), IILoc);
      return;
    }
  }
 
  // FIXME: Should we move the logic that tries to recover from a missing tag
  // (struct, union, enum) from Parser::ParseImplicitInt here, instead?
 
  if (!SS || (!SS->isSet() && !SS->isInvalid()))
    Diag(IILoc, IsTemplateName ? diag::err_no_template
                               : diag::err_unknown_typename)
        << II;
  else if (DeclContext *DC = computeDeclContext(*SS, false))
    Diag(IILoc, IsTemplateName ? diag::err_no_member_template
                               : diag::err_typename_nested_not_found)
        << II << DC << SS->getRange();
  else if (SS->isValid() && SS->getScopeRep()->containsErrors()) {
    SuggestedType =
        ActOnTypenameType(S, SourceLocation(), *SS, *II, IILoc).get();
  } else if (isDependentScopeSpecifier(*SS)) {
    unsigned DiagID = diag::err_typename_missing;
    if (getLangOpts().MSVCCompat && isMicrosoftMissingTypename(SS, S))
      DiagID = diag::ext_typename_missing;
 
    Diag(SS->getRange().getBegin(), DiagID)
      << SS->getScopeRep() << II->getName()
      << SourceRange(SS->getRange().getBegin(), IILoc)
      << FixItHint::CreateInsertion(SS->getRange().getBegin(), "typename ");
    SuggestedType = ActOnTypenameType(S, SourceLocation(),
                                      *SS, *II, IILoc).get();
  } else {
    assert(SS && SS->isInvalid() &&
           "Invalid scope specifier has already been diagnosed");
  }
}
 
/// Determine whether the given result set contains either a type name
/// or
static bool isResultTypeOrTemplate(LookupResult &R, const Token &NextToken) {
  bool CheckTemplate = R.getSema().getLangOpts().CPlusPlus &&
                       NextToken.is(tok::less);
 
  for (LookupResult::iterator I = R.begin(), IEnd = R.end(); I != IEnd; ++I) {
    if (isa<TypeDecl>(*I) || isa<ObjCInterfaceDecl>(*I))
      return true;
 
    if (CheckTemplate && isa<TemplateDecl>(*I))
      return true;
  }
 
  return false;
}
 
static bool isTagTypeWithMissingTag(Sema &SemaRef, LookupResult &Result,
                                    Scope *S, CXXScopeSpec &SS,
                                    IdentifierInfo *&Name,
                                    SourceLocation NameLoc) {
  LookupResult R(SemaRef, Name, NameLoc, Sema::LookupTagName);
  SemaRef.LookupParsedName(R, S, &SS);
  if (TagDecl *Tag = R.getAsSingle<TagDecl>()) {
    StringRef FixItTagName;
    switch (Tag->getTagKind()) {
      case TTK_Class:
        FixItTagName = "class ";
        break;
 
      case TTK_Enum:
        FixItTagName = "enum ";
        break;
 
      case TTK_Struct:
        FixItTagName = "struct ";
        break;
 
      case TTK_Interface:
        FixItTagName = "__interface ";
        break;
 
      case TTK_Union:
        FixItTagName = "union ";
        break;
    }
 
    StringRef TagName = FixItTagName.drop_back();
    SemaRef.Diag(NameLoc, diag::err_use_of_tag_name_without_tag)
      << Name << TagName << SemaRef.getLangOpts().CPlusPlus
      << FixItHint::CreateInsertion(NameLoc, FixItTagName);
 
    for (LookupResult::iterator I = Result.begin(), IEnd = Result.end();
         I != IEnd; ++I)
      SemaRef.Diag((*I)->getLocation(), diag::note_decl_hiding_tag_type)
        << Name << TagName;
 
    // Replace lookup results with just the tag decl.
    Result.clear(Sema::LookupTagName);
    SemaRef.LookupParsedName(Result, S, &SS);
    return true;
  }
 
  return false;
}
 
Sema::NameClassification Sema::ClassifyName(Scope *S, CXXScopeSpec &SS,
                                            IdentifierInfo *&Name,
                                            SourceLocation NameLoc,
                                            const Token &NextToken,
                                            CorrectionCandidateCallback *CCC) {
  DeclarationNameInfo NameInfo(Name, NameLoc);
  ObjCMethodDecl *CurMethod = getCurMethodDecl();
 
  assert(NextToken.isNot(tok::coloncolon) &&
         "parse nested name specifiers before calling ClassifyName");
  if (getLangOpts().CPlusPlus && SS.isSet() &&
      isCurrentClassName(*Name, S, &SS)) {
    // Per [class.qual]p2, this names the constructors of SS, not the
    // injected-class-name. We don't have a classification for that.
    // There's not much point caching this result, since the parser
    // will reject it later.
    return NameClassification::Unknown();
  }
 
  LookupResult Result(*this, Name, NameLoc, LookupOrdinaryName);
  LookupParsedName(Result, S, &SS, !CurMethod);
 
  if (SS.isInvalid())
    return NameClassification::Error();
 
  // For unqualified lookup in a class template in MSVC mode, look into
  // dependent base classes where the primary class template is known.
  if (Result.empty() && SS.isEmpty() && getLangOpts().MSVCCompat) {
    if (ParsedType TypeInBase =
            recoverFromTypeInKnownDependentBase(*this, *Name, NameLoc))
      return TypeInBase;
  }
 
  // Perform lookup for Objective-C instance variables (including automatically
  // synthesized instance variables), if we're in an Objective-C method.
  // FIXME: This lookup really, really needs to be folded in to the normal
  // unqualified lookup mechanism.
  if (SS.isEmpty() && CurMethod && !isResultTypeOrTemplate(Result, NextToken)) {
    DeclResult Ivar = LookupIvarInObjCMethod(Result, S, Name);
    if (Ivar.isInvalid())
      return NameClassification::Error();
    if (Ivar.isUsable())
      return NameClassification::NonType(cast<NamedDecl>(Ivar.get()));
 
    // We defer builtin creation until after ivar lookup inside ObjC methods.
    if (Result.empty())
      LookupBuiltin(Result);
  }
 
  bool SecondTry = false;
  bool IsFilteredTemplateName = false;
 
Corrected:
  switch (Result.getResultKind()) {
  case LookupResult::NotFound:
    // If an unqualified-id is followed by a '(', then we have a function
    // call.
    if (SS.isEmpty() && NextToken.is(tok::l_paren)) {
      // In C++, this is an ADL-only call.
      // FIXME: Reference?
      if (getLangOpts().CPlusPlus)
        return NameClassification::UndeclaredNonType();
 
      // C90 6.3.2.2:
      //   If the expression that precedes the parenthesized argument list in a
      //   function call consists solely of an identifier, and if no
      //   declaration is visible for this identifier, the identifier is
      //   implicitly declared exactly as if, in the innermost block containing
      //   the function call, the declaration
      //
      //     extern int identifier ();
      //
      //   appeared.
      //
      // We also allow this in C99 as an extension. However, this is not
      // allowed in all language modes as functions without prototypes may not
      // be supported.
      if (getLangOpts().implicitFunctionsAllowed()) {
        if (NamedDecl *D = ImplicitlyDefineFunction(NameLoc, *Name, S))
          return NameClassification::NonType(D);
      }
    }
 
    if (getLangOpts().CPlusPlus20 && SS.isEmpty() && NextToken.is(tok::less)) {
      // In C++20 onwards, this could be an ADL-only call to a function
      // template, and we're required to assume that this is a template name.
      //
      // FIXME: Find a way to still do typo correction in this case.
      TemplateName Template =
          Context.getAssumedTemplateName(NameInfo.getName());
      return NameClassification::UndeclaredTemplate(Template);
    }
 
    // In C, we first see whether there is a tag type by the same name, in
    // which case it's likely that the user just forgot to write "enum",
    // "struct", or "union".
    if (!getLangOpts().CPlusPlus && !SecondTry &&
        isTagTypeWithMissingTag(*this, Result, S, SS, Name, NameLoc)) {
      break;
    }
 
    // Perform typo correction to determine if there is another name that is
    // close to this name.
    if (!SecondTry && CCC) {
      SecondTry = true;
      if (TypoCorrection Corrected =
              CorrectTypo(Result.getLookupNameInfo(), Result.getLookupKind(), S,
                          &SS, *CCC, CTK_ErrorRecovery)) {
        unsigned UnqualifiedDiag = diag::err_undeclared_var_use_suggest;
        unsigned QualifiedDiag = diag::err_no_member_suggest;
 
        NamedDecl *FirstDecl = Corrected.getFoundDecl();
        NamedDecl *UnderlyingFirstDecl = Corrected.getCorrectionDecl();
        if (getLangOpts().CPlusPlus && NextToken.is(tok::less) &&
            UnderlyingFirstDecl && isa<TemplateDecl>(UnderlyingFirstDecl)) {
          UnqualifiedDiag = diag::err_no_template_suggest;
          QualifiedDiag = diag::err_no_member_template_suggest;
        } else if (UnderlyingFirstDecl &&
                   (isa<TypeDecl>(UnderlyingFirstDecl) ||
                    isa<ObjCInterfaceDecl>(UnderlyingFirstDecl) ||
                    isa<ObjCCompatibleAliasDecl>(UnderlyingFirstDecl))) {
          UnqualifiedDiag = diag::err_unknown_typename_suggest;
          QualifiedDiag = diag::err_unknown_nested_typename_suggest;
        }
 
        if (SS.isEmpty()) {
          diagnoseTypo(Corrected, PDiag(UnqualifiedDiag) << Name);
        } else {// FIXME: is this even reachable? Test it.
          std::string CorrectedStr(Corrected.getAsString(getLangOpts()));
          bool DroppedSpecifier = Corrected.WillReplaceSpecifier() &&
                                  Name->getName().equals(CorrectedStr);
          diagnoseTypo(Corrected, PDiag(QualifiedDiag)
                                    << Name << computeDeclContext(SS, false)
                                    << DroppedSpecifier << SS.getRange());
        }
 
        // Update the name, so that the caller has the new name.
        Name = Corrected.getCorrectionAsIdentifierInfo();
 
        // Typo correction corrected to a keyword.
        if (Corrected.isKeyword())
          return Name;
 
        // Also update the LookupResult...
        // FIXME: This should probably go away at some point
        Result.clear();
        Result.setLookupName(Corrected.getCorrection());
        if (FirstDecl)
          Result.addDecl(FirstDecl);
 
        // If we found an Objective-C instance variable, let
        // LookupInObjCMethod build the appropriate expression to
        // reference the ivar.
        // FIXME: This is a gross hack.
        if (ObjCIvarDecl *Ivar = Result.getAsSingle<ObjCIvarDecl>()) {
          DeclResult R =
              LookupIvarInObjCMethod(Result, S, Ivar->getIdentifier());
          if (R.isInvalid())
            return NameClassification::Error();
          if (R.isUsable())
            return NameClassification::NonType(Ivar);
        }
 
        goto Corrected;
      }
    }
 
    // We failed to correct; just fall through and let the parser deal with it.
    Result.suppressDiagnostics();
    return NameClassification::Unknown();
 
  case LookupResult::NotFoundInCurrentInstantiation: {
    // We performed name lookup into the current instantiation, and there were
    // dependent bases, so we treat this result the same way as any other
    // dependent nested-name-specifier.
 
    // C++ [temp.res]p2:
    //   A name used in a template declaration or definition and that is
    //   dependent on a template-parameter is assumed not to name a type
    //   unless the applicable name lookup finds a type name or the name is
    //   qualified by the keyword typename.
    //
    // FIXME: If the next token is '<', we might want to ask the parser to
    // perform some heroics to see if we actually have a
    // template-argument-list, which would indicate a missing 'template'
    // keyword here.
    return NameClassification::DependentNonType();
  }
 
  case LookupResult::Found:
  case LookupResult::FoundOverloaded:
  case LookupResult::FoundUnresolvedValue:
    break;
 
  case LookupResult::Ambiguous:
    if (getLangOpts().CPlusPlus && NextToken.is(tok::less) &&
        hasAnyAcceptableTemplateNames(Result, /*AllowFunctionTemplates=*/true,
                                      /*AllowDependent=*/false)) {
      // C++ [temp.local]p3:
      //   A lookup that finds an injected-class-name (10.2) can result in an
      //   ambiguity in certain cases (for example, if it is found in more than
      //   one base class). If all of the injected-class-names that are found
      //   refer to specializations of the same class template, and if the name
      //   is followed by a template-argument-list, the reference refers to the
      //   class template itself and not a specialization thereof, and is not
      //   ambiguous.
      //
      // This filtering can make an ambiguous result into an unambiguous one,
      // so try again after filtering out template names.
      FilterAcceptableTemplateNames(Result);
      if (!Result.isAmbiguous()) {
        IsFilteredTemplateName = true;
        break;
      }
    }
 
    // Diagnose the ambiguity and return an error.
    return NameClassification::Error();
  }
 
  if (getLangOpts().CPlusPlus && NextToken.is(tok::less) &&
      (IsFilteredTemplateName ||
       hasAnyAcceptableTemplateNames(
           Result, /*AllowFunctionTemplates=*/true,
           /*AllowDependent=*/false,
           /*AllowNonTemplateFunctions*/ SS.isEmpty() &&
               getLangOpts().CPlusPlus20))) {
    // C++ [temp.names]p3:
    //   After name lookup (3.4) finds that a name is a template-name or that
    //   an operator-function-id or a literal- operator-id refers to a set of
    //   overloaded functions any member of which is a function template if
    //   this is followed by a <, the < is always taken as the delimiter of a
    //   template-argument-list and never as the less-than operator.
    // C++2a [temp.names]p2:
    //   A name is also considered to refer to a template if it is an
    //   unqualified-id followed by a < and name lookup finds either one
    //   or more functions or finds nothing.
    if (!IsFilteredTemplateName)
      FilterAcceptableTemplateNames(Result);
 
    bool IsFunctionTemplate;
    bool IsVarTemplate;
    TemplateName Template;
    if (Result.end() - Result.begin() > 1) {
      IsFunctionTemplate = true;
      Template = Context.getOverloadedTemplateName(Result.begin(),
                                                   Result.end());
    } else if (!Result.empty()) {
      auto *TD = cast<TemplateDecl>(getAsTemplateNameDecl(
          *Result.begin(), /*AllowFunctionTemplates=*/true,
          /*AllowDependent=*/false));
      IsFunctionTemplate = isa<FunctionTemplateDecl>(TD);
      IsVarTemplate = isa<VarTemplateDecl>(TD);
 
      UsingShadowDecl *FoundUsingShadow =
          dyn_cast<UsingShadowDecl>(*Result.begin());
      assert(!FoundUsingShadow ||
             TD == cast<TemplateDecl>(FoundUsingShadow->getTargetDecl()));
      Template =
          FoundUsingShadow ? TemplateName(FoundUsingShadow) : TemplateName(TD);
      if (SS.isNotEmpty())
        Template = Context.getQualifiedTemplateName(SS.getScopeRep(),
                                                    /*TemplateKeyword=*/false,
                                                    Template);
    } else {
      // All results were non-template functions. This is a function template
      // name.
      IsFunctionTemplate = true;
      Template = Context.getAssumedTemplateName(NameInfo.getName());
    }
 
    if (IsFunctionTemplate) {
      // Function templates always go through overload resolution, at which
      // point we'll perform the various checks (e.g., accessibility) we need
      // to based on which function we selected.
      Result.suppressDiagnostics();
 
      return NameClassification::FunctionTemplate(Template);
    }
 
    return IsVarTemplate ? NameClassification::VarTemplate(Template)
                         : NameClassification::TypeTemplate(Template);
  }
 
  auto BuildTypeFor = [&](TypeDecl *Type, NamedDecl *Found) {
    QualType T = Context.getTypeDeclType(Type);
    if (const auto *USD = dyn_cast<UsingShadowDecl>(Found))
      T = Context.getUsingType(USD, T);
    return buildNamedType(*this, &SS, T, NameLoc);
  };
 
  NamedDecl *FirstDecl = (*Result.begin())->getUnderlyingDecl();
  if (TypeDecl *Type = dyn_cast<TypeDecl>(FirstDecl)) {
    DiagnoseUseOfDecl(Type, NameLoc);
    MarkAnyDeclReferenced(Type->getLocation(), Type, /*OdrUse=*/false);
    return BuildTypeFor(Type, *Result.begin());
  }
 
  ObjCInterfaceDecl *Class = dyn_cast<ObjCInterfaceDecl>(FirstDecl);
  if (!Class) {
    // FIXME: It's unfortunate that we don't have a Type node for handling this.
    if (ObjCCompatibleAliasDecl *Alias =
            dyn_cast<ObjCCompatibleAliasDecl>(FirstDecl))
      Class = Alias->getClassInterface();
  }
 
  if (Class) {
    DiagnoseUseOfDecl(Class, NameLoc);
 
    if (NextToken.is(tok::period)) {
      // Interface. <something> is parsed as a property reference expression.
      // Just return "unknown" as a fall-through for now.
      Result.suppressDiagnostics();
      return NameClassification::Unknown();
    }
 
    QualType T = Context.getObjCInterfaceType(Class);
    return ParsedType::make(T);
  }
 
  if (isa<ConceptDecl>(FirstDecl))
    return NameClassification::Concept(
        TemplateName(cast<TemplateDecl>(FirstDecl)));
 
  if (auto *EmptyD = dyn_cast<UnresolvedUsingIfExistsDecl>(FirstDecl)) {
    (void)DiagnoseUseOfDecl(EmptyD, NameLoc);
    return NameClassification::Error();
  }
 
  // We can have a type template here if we're classifying a template argument.
  if (isa<TemplateDecl>(FirstDecl) && !isa<FunctionTemplateDecl>(FirstDecl) &&
      !isa<VarTemplateDecl>(FirstDecl))
    return NameClassification::TypeTemplate(
        TemplateName(cast<TemplateDecl>(FirstDecl)));
 
  // Check for a tag type hidden by a non-type decl in a few cases where it
  // seems likely a type is wanted instead of the non-type that was found.
  bool NextIsOp = NextToken.isOneOf(tok::amp, tok::star);
  if ((NextToken.is(tok::identifier) ||
       (NextIsOp &&
        FirstDecl->getUnderlyingDecl()->isFunctionOrFunctionTemplate())) &&
      isTagTypeWithMissingTag(*this, Result, S, SS, Name, NameLoc)) {
    TypeDecl *Type = Result.getAsSingle<TypeDecl>();
    DiagnoseUseOfDecl(Type, NameLoc);
    return BuildTypeFor(Type, *Result.begin());
  }
 
  // If we already know which single declaration is referenced, just annotate
  // that declaration directly. Defer resolving even non-overloaded class
  // member accesses, as we need to defer certain access checks until we know
  // the context.
  bool ADL = UseArgumentDependentLookup(SS, Result, NextToken.is(tok::l_paren));
  if (Result.isSingleResult() && !ADL &&
      (!FirstDecl->isCXXClassMember() || isa<EnumConstantDecl>(FirstDecl)))
    return NameClassification::NonType(Result.getRepresentativeDecl());
 
  // Otherwise, this is an overload set that we will need to resolve later.
  Result.suppressDiagnostics();
  return NameClassification::OverloadSet(UnresolvedLookupExpr::Create(
      Context, Result.getNamingClass(), SS.getWithLocInContext(Context),
      Result.getLookupNameInfo(), ADL, Result.isOverloadedResult(),
      Result.begin(), Result.end()));
}
 
ExprResult
Sema::ActOnNameClassifiedAsUndeclaredNonType(IdentifierInfo *Name,
                                             SourceLocation NameLoc) {
  assert(getLangOpts().CPlusPlus && "ADL-only call in C?");
  CXXScopeSpec SS;
  LookupResult Result(*this, Name, NameLoc, LookupOrdinaryName);
  return BuildDeclarationNameExpr(SS, Result, /*ADL=*/true);
}
 
ExprResult
Sema::ActOnNameClassifiedAsDependentNonType(const CXXScopeSpec &SS,
                                            IdentifierInfo *Name,
                                            SourceLocation NameLoc,
                                            bool IsAddressOfOperand) {
  DeclarationNameInfo NameInfo(Name, NameLoc);
  return ActOnDependentIdExpression(SS, /*TemplateKWLoc=*/SourceLocation(),
                                    NameInfo, IsAddressOfOperand,
                                    /*TemplateArgs=*/nullptr);
}
 
ExprResult Sema::ActOnNameClassifiedAsNonType(Scope *S, const CXXScopeSpec &SS,
                                              NamedDecl *Found,
                                              SourceLocation NameLoc,
                                              const Token &NextToken) {
  if (getCurMethodDecl() && SS.isEmpty())
    if (auto *Ivar = dyn_cast<ObjCIvarDecl>(Found->getUnderlyingDecl()))
      return BuildIvarRefExpr(S, NameLoc, Ivar);
 
  // Reconstruct the lookup result.
  LookupResult Result(*this, Found->getDeclName(), NameLoc, LookupOrdinaryName);
  Result.addDecl(Found);
  Result.resolveKind();
 
  bool ADL = UseArgumentDependentLookup(SS, Result, NextToken.is(tok::l_paren));
  return BuildDeclarationNameExpr(SS, Result, ADL, /*AcceptInvalidDecl=*/true);
}
 
ExprResult Sema::ActOnNameClassifiedAsOverloadSet(Scope *S, Expr *E) {
  // For an implicit class member access, transform the result into a member
  // access expression if necessary.
  auto *ULE = cast<UnresolvedLookupExpr>(E);
  if ((*ULE->decls_begin())->isCXXClassMember()) {
    CXXScopeSpec SS;
    SS.Adopt(ULE->getQualifierLoc());
 
    // Reconstruct the lookup result.
    LookupResult Result(*this, ULE->getName(), ULE->getNameLoc(),
                        LookupOrdinaryName);
    Result.setNamingClass(ULE->getNamingClass());
    for (auto I = ULE->decls_begin(), E = ULE->decls_end(); I != E; ++I)
      Result.addDecl(*I, I.getAccess());
    Result.resolveKind();
    return BuildPossibleImplicitMemberExpr(SS, SourceLocation(), Result,
                                           nullptr, S);
  }
 
  // Otherwise, this is already in the form we needed, and no further checks
  // are necessary.
  return ULE;
}
 
Sema::TemplateNameKindForDiagnostics
Sema::getTemplateNameKindForDiagnostics(TemplateName Name) {
  auto *TD = Name.getAsTemplateDecl();
  if (!TD)
    return TemplateNameKindForDiagnostics::DependentTemplate;
  if (isa<ClassTemplateDecl>(TD))
    return TemplateNameKindForDiagnostics::ClassTemplate;
  if (isa<FunctionTemplateDecl>(TD))
    return TemplateNameKindForDiagnostics::FunctionTemplate;
  if (isa<VarTemplateDecl>(TD))
    return TemplateNameKindForDiagnostics::VarTemplate;
  if (isa<TypeAliasTemplateDecl>(TD))
    return TemplateNameKindForDiagnostics::AliasTemplate;
  if (isa<TemplateTemplateParmDecl>(TD))
    return TemplateNameKindForDiagnostics::TemplateTemplateParam;
  if (isa<ConceptDecl>(TD))
    return TemplateNameKindForDiagnostics::Concept;
  return TemplateNameKindForDiagnostics::DependentTemplate;
}
 
void Sema::PushDeclContext(Scope *S, DeclContext *DC) {
  assert(DC->getLexicalParent() == CurContext &&
      "The next DeclContext should be lexically contained in the current one.");
  CurContext = DC;
  S->setEntity(DC);
}
 
void Sema::PopDeclContext() {
  assert(CurContext && "DeclContext imbalance!");
 
  CurContext = CurContext->getLexicalParent();
  assert(CurContext && "Popped translation unit!");
}
 
Sema::SkippedDefinitionContext Sema::ActOnTagStartSkippedDefinition(Scope *S,
                                                                    Decl *D) {
  // Unlike PushDeclContext, the context to which we return is not necessarily
  // the containing DC of TD, because the new context will be some pre-existing
  // TagDecl definition instead of a fresh one.
  auto Result = static_cast<SkippedDefinitionContext>(CurContext);
  CurContext = cast<TagDecl>(D)->getDefinition();
  assert(CurContext && "skipping definition of undefined tag");
  // Start lookups from the parent of the current context; we don't want to look
  // into the pre-existing complete definition.
  S->setEntity(CurContext->getLookupParent());
  return Result;
}
 
void Sema::ActOnTagFinishSkippedDefinition(SkippedDefinitionContext Context) {
  CurContext = static_cast<decltype(CurContext)>(Context);
}
 
/// EnterDeclaratorContext - Used when we must lookup names in the context
/// of a declarator's nested name specifier.
///
void Sema::EnterDeclaratorContext(Scope *S, DeclContext *DC) {
  // C++0x [basic.lookup.unqual]p13:
  //   A name used in the definition of a static data member of class
  //   X (after the qualified-id of the static member) is looked up as
  //   if the name was used in a member function of X.
  // C++0x [basic.lookup.unqual]p14:
  //   If a variable member of a namespace is defined outside of the
  //   scope of its namespace then any name used in the definition of
  //   the variable member (after the declarator-id) is looked up as
  //   if the definition of the variable member occurred in its
  //   namespace.
  // Both of these imply that we should push a scope whose context
  // is the semantic context of the declaration.  We can't use
  // PushDeclContext here because that context is not necessarily
  // lexically contained in the current context.  Fortunately,
  // the containing scope should have the appropriate information.
 
  assert(!S->getEntity() && "scope already has entity");
 
#ifndef NDEBUG
  Scope *Ancestor = S->getParent();
  while (!Ancestor->getEntity()) Ancestor = Ancestor->getParent();
  assert(Ancestor->getEntity() == CurContext && "ancestor context mismatch");
#endif
 
  CurContext = DC;
  S->setEntity(DC);
 
  if (S->getParent()->isTemplateParamScope()) {
    // Also set the corresponding entities for all immediately-enclosing
    // template parameter scopes.
    EnterTemplatedContext(S->getParent(), DC);
  }
}
 
void Sema::ExitDeclaratorContext(Scope *S) {
  assert(S->getEntity() == CurContext && "Context imbalance!");
 
  // Switch back to the lexical context.  The safety of this is
  // enforced by an assert in EnterDeclaratorContext.
  Scope *Ancestor = S->getParent();
  while (!Ancestor->getEntity()) Ancestor = Ancestor->getParent();
  CurContext = Ancestor->getEntity();
 
  // We don't need to do anything with the scope, which is going to
  // disappear.
}
 
void Sema::EnterTemplatedContext(Scope *S, DeclContext *DC) {
  assert(S->isTemplateParamScope() &&
         "expected to be initializing a template parameter scope");
 
  // C++20 [temp.local]p7:
  //   In the definition of a member of a class template that appears outside
  //   of the class template definition, the name of a member of the class
  //   template hides the name of a template-parameter of any enclosing class
  //   templates (but not a template-parameter of the member if the member is a
  //   class or function template).
  // C++20 [temp.local]p9:
  //   In the definition of a class template or in the definition of a member
  //   of such a template that appears outside of the template definition, for
  //   each non-dependent base class (13.8.2.1), if the name of the base class
  //   or the name of a member of the base class is the same as the name of a
  //   template-parameter, the base class name or member name hides the
  //   template-parameter name (6.4.10).
  //
  // This means that a template parameter scope should be searched immediately
  // after searching the DeclContext for which it is a template parameter
  // scope. For example, for
  //   template<typename T> template<typename U> template<typename V>
  //     void N::A<T>::B<U>::f(...)
  // we search V then B<U> (and base classes) then U then A<T> (and base
  // classes) then T then N then ::.
  unsigned ScopeDepth = getTemplateDepth(S);
  for (; S && S->isTemplateParamScope(); S = S->getParent(), --ScopeDepth) {
    DeclContext *SearchDCAfterScope = DC;
    for (; DC; DC = DC->getLookupParent()) {
      if (const TemplateParameterList *TPL =
              cast<Decl>(DC)->getDescribedTemplateParams()) {
        unsigned DCDepth = TPL->getDepth() + 1;
        if (DCDepth > ScopeDepth)
          continue;
        if (ScopeDepth == DCDepth)
          SearchDCAfterScope = DC = DC->getLookupParent();
        break;
      }
    }
    S->setLookupEntity(SearchDCAfterScope);
  }
}
 
void Sema::ActOnReenterFunctionContext(Scope* S, Decl *D) {
  // We assume that the caller has already called
  // ActOnReenterTemplateScope so getTemplatedDecl() works.
  FunctionDecl *FD = D->getAsFunction();
  if (!FD)
    return;
 
  // Same implementation as PushDeclContext, but enters the context
  // from the lexical parent, rather than the top-level class.
  assert(CurContext == FD->getLexicalParent() &&
    "The next DeclContext should be lexically contained in the current one.");
  CurContext = FD;
  S->setEntity(CurContext);
 
  for (unsigned P = 0, NumParams = FD->getNumParams(); P < NumParams; ++P) {
    ParmVarDecl *Param = FD->getParamDecl(P);
    // If the parameter has an identifier, then add it to the scope
    if (Param->getIdentifier()) {
      S->AddDecl(Param);
      IdResolver.AddDecl(Param);
    }
  }
}
 
void Sema::ActOnExitFunctionContext() {
  // Same implementation as PopDeclContext, but returns to the lexical parent,
  // rather than the top-level class.
  assert(CurContext && "DeclContext imbalance!");
  CurContext = CurContext->getLexicalParent();
  assert(CurContext && "Popped translation unit!");
}
 
/// Determine whether overloading is allowed for a new function
/// declaration considering prior declarations of the same name.
///
/// This routine determines whether overloading is possible, not
/// whether a new declaration actually overloads a previous one.
/// It will return true in C++ (where overloads are alway permitted)
/// or, as a C extension, when either the new declaration or a
/// previous one is declared with the 'overloadable' attribute.
static bool AllowOverloadingOfFunction(const LookupResult &Previous,
                                       ASTContext &Context,
                                       const FunctionDecl *New) {
  if (Context.getLangOpts().CPlusPlus || New->hasAttr<OverloadableAttr>())
    return true;
 
  // Multiversion function declarations are not overloads in the
  // usual sense of that term, but lookup will report that an
  // overload set was found if more than one multiversion function
  // declaration is present for the same name. It is therefore
  // inadequate to assume that some prior declaration(s) had
  // the overloadable attribute; checking is required. Since one
  // declaration is permitted to omit the attribute, it is necessary
  // to check at least two; hence the 'any_of' check below. Note that
  // the overloadable attribute is implicitly added to declarations
  // that were required to have it but did not.
  if (Previous.getResultKind() == LookupResult::FoundOverloaded) {
    return llvm::any_of(Previous, [](const NamedDecl *ND) {
      return ND->hasAttr<OverloadableAttr>();
    });
  } else if (Previous.getResultKind() == LookupResult::Found)
    return Previous.getFoundDecl()->hasAttr<OverloadableAttr>();
 
  return false;
}
 
/// Add this decl to the scope shadowed decl chains.
void Sema::PushOnScopeChains(NamedDecl *D, Scope *S, bool AddToContext) {
  // Move up the scope chain until we find the nearest enclosing
  // non-transparent context. The declaration will be introduced into this
  // scope.
  while (S->getEntity() && S->getEntity()->isTransparentContext())
    S = S->getParent();
 
  // Add scoped declarations into their context, so that they can be
  // found later. Declarations without a context won't be inserted
  // into any context.
  if (AddToContext)
    CurContext->addDecl(D);
 
  // Out-of-line definitions shouldn't be pushed into scope in C++, unless they
  // are function-local declarations.
  if (getLangOpts().CPlusPlus && D->isOutOfLine() && !S->getFnParent())
    return;
 
  // Template instantiations should also not be pushed into scope.
  if (isa<FunctionDecl>(D) &&
      cast<FunctionDecl>(D)->isFunctionTemplateSpecialization())
    return;
 
  // If this replaces anything in the current scope,
  IdentifierResolver::iterator I = IdResolver.begin(D->getDeclName()),
                               IEnd = IdResolver.end();
  for (; I != IEnd; ++I) {
    if (S->isDeclScope(*I) && D->declarationReplaces(*I)) {
      S->RemoveDecl(*I);
      IdResolver.RemoveDecl(*I);
 
      // Should only need to replace one decl.
      break;
    }
  }
 
  S->AddDecl(D);
 
  if (isa<LabelDecl>(D) && !cast<LabelDecl>(D)->isGnuLocal()) {
    // Implicitly-generated labels may end up getting generated in an order that
    // isn't strictly lexical, which breaks name lookup. Be careful to insert
    // the label at the appropriate place in the identifier chain.
    for (I = IdResolver.begin(D->getDeclName()); I != IEnd; ++I) {
      DeclContext *IDC = (*I)->getLexicalDeclContext()->getRedeclContext();
      if (IDC == CurContext) {
        if (!S->isDeclScope(*I))
          continue;
      } else if (IDC->Encloses(CurContext))
        break;
    }
 
    IdResolver.InsertDeclAfter(I, D);
  } else {
    IdResolver.AddDecl(D);
  }
  warnOnReservedIdentifier(D);
}
 
bool Sema::isDeclInScope(NamedDecl *D, DeclContext *Ctx, Scope *S,
                         bool AllowInlineNamespace) const {
  return IdResolver.isDeclInScope(D, Ctx, S, AllowInlineNamespace);
}
 
Scope *Sema::getScopeForDeclContext(Scope *S, DeclContext *DC) {
  DeclContext *TargetDC = DC->getPrimaryContext();
  do {
    if (DeclContext *ScopeDC = S->getEntity())
      if (ScopeDC->getPrimaryContext() == TargetDC)
        return S;
  } while ((S = S->getParent()));
 
  return nullptr;
}
 
static bool isOutOfScopePreviousDeclaration(NamedDecl *,
                                            DeclContext*,
                                            ASTContext&);
 
/// Filters out lookup results that don't fall within the given scope
/// as determined by isDeclInScope.
void Sema::FilterLookupForScope(LookupResult &R, DeclContext *Ctx, Scope *S,
                                bool ConsiderLinkage,
                                bool AllowInlineNamespace) {
  LookupResult::Filter F = R.makeFilter();
  while (F.hasNext()) {
    NamedDecl *D = F.next();
 
    if (isDeclInScope(D, Ctx, S, AllowInlineNamespace))
      continue;
 
    if (ConsiderLinkage && isOutOfScopePreviousDeclaration(D, Ctx, Context))
      continue;
 
    F.erase();
  }
 
  F.done();
}
 
/// We've determined that \p New is a redeclaration of \p Old. Check that they
/// have compatible owning modules.
bool Sema::CheckRedeclarationModuleOwnership(NamedDecl *New, NamedDecl *Old) {
  // [module.interface]p7:
  // A declaration is attached to a module as follows:
  // - If the declaration is a non-dependent friend declaration that nominates a
  // function with a declarator-id that is a qualified-id or template-id or that
  // nominates a class other than with an elaborated-type-specifier with neither
  // a nested-name-specifier nor a simple-template-id, it is attached to the
  // module to which the friend is attached ([basic.link]).
  if (New->getFriendObjectKind() &&
      Old->getOwningModuleForLinkage() != New->getOwningModuleForLinkage()) {
    New->setLocalOwningModule(Old->getOwningModule());
    makeMergedDefinitionVisible(New);
    return false;
  }
 
  Module *NewM = New->getOwningModule();
  Module *OldM = Old->getOwningModule();
 
  if (NewM && NewM->isPrivateModule())
    NewM = NewM->Parent;
  if (OldM && OldM->isPrivateModule())
    OldM = OldM->Parent;
 
  if (NewM == OldM)
    return false;
 
  if (NewM && OldM) {
    // A module implementation unit has visibility of the decls in its
    // implicitly imported interface.
    if (NewM->isModuleImplementation() && OldM == ThePrimaryInterface)
      return false;
 
    // Partitions are part of the module, but a partition could import another
    // module, so verify that the PMIs agree.
    if ((NewM->isModulePartition() || OldM->isModulePartition()) &&
        NewM->getPrimaryModuleInterfaceName() ==
            OldM->getPrimaryModuleInterfaceName())
      return false;
  }
 
  bool NewIsModuleInterface = NewM && NewM->isModulePurview();
  bool OldIsModuleInterface = OldM && OldM->isModulePurview();
  if (NewIsModuleInterface || OldIsModuleInterface) {
    // C++ Modules TS [basic.def.odr] 6.2/6.7 [sic]:
    //   if a declaration of D [...] appears in the purview of a module, all
    //   other such declarations shall appear in the purview of the same module
    Diag(New->getLocation(), diag::err_mismatched_owning_module)
      << New
      << NewIsModuleInterface
      << (NewIsModuleInterface ? NewM->getFullModuleName() : "")
      << OldIsModuleInterface
      << (OldIsModuleInterface ? OldM->getFullModuleName() : "");
    Diag(Old->getLocation(), diag::note_previous_declaration);
    New->setInvalidDecl();
    return true;
  }
 
  return false;
}
 
// [module.interface]p6:
// A redeclaration of an entity X is implicitly exported if X was introduced by
// an exported declaration; otherwise it shall not be exported.
bool Sema::CheckRedeclarationExported(NamedDecl *New, NamedDecl *Old) {
  // [module.interface]p1:
  // An export-declaration shall inhabit a namespace scope.
  //
  // So it is meaningless to talk about redeclaration which is not at namespace
  // scope.
  if (!New->getLexicalDeclContext()
           ->getNonTransparentContext()
           ->isFileContext() ||
      !Old->getLexicalDeclContext()
           ->getNonTransparentContext()
           ->isFileContext())
    return false;
 
  bool IsNewExported = New->isInExportDeclContext();
  bool IsOldExported = Old->isInExportDeclContext();
 
  // It should be irrevelant if both of them are not exported.
  if (!IsNewExported && !IsOldExported)
    return false;
 
  if (IsOldExported)
    return false;
 
  assert(IsNewExported);
 
  auto Lk = Old->getFormalLinkage();
  int S = 0;
  if (Lk == Linkage::InternalLinkage)
    S = 1;
  else if (Lk == Linkage::ModuleLinkage)
    S = 2;
  Diag(New->getLocation(), diag::err_redeclaration_non_exported) << New << S;
  Diag(Old->getLocation(), diag::note_previous_declaration);
  return true;
}
 
// A wrapper function for checking the semantic restrictions of
// a redeclaration within a module.
bool Sema::CheckRedeclarationInModule(NamedDecl *New, NamedDecl *Old) {
  if (CheckRedeclarationModuleOwnership(New, Old))
    return true;
 
  if (CheckRedeclarationExported(New, Old))
    return true;
 
  return false;
}
 
// Check the redefinition in C++20 Modules.
//
// [basic.def.odr]p14:
// For any definable item D with definitions in multiple translation units,
// - if D is a non-inline non-templated function or variable, or
// - if the definitions in different translation units do not satisfy the
// following requirements,
//   the program is ill-formed; a diagnostic is required only if the definable
//   item is attached to a named module and a prior definition is reachable at
//   the point where a later definition occurs.
// - Each such definition shall not be attached to a named module
// ([module.unit]).
// - Each such definition shall consist of the same sequence of tokens, ...
// ...
//
// Return true if the redefinition is not allowed. Return false otherwise.
bool Sema::IsRedefinitionInModule(const NamedDecl *New,
                                     const NamedDecl *Old) const {
  assert(getASTContext().isSameEntity(New, Old) &&
         "New and Old are not the same definition, we should diagnostic it "
         "immediately instead of checking it.");
  assert(const_cast<Sema *>(this)->isReachable(New) &&
         const_cast<Sema *>(this)->isReachable(Old) &&
         "We shouldn't see unreachable definitions here.");
 
  Module *NewM = New->getOwningModule();
  Module *OldM = Old->getOwningModule();
 
  // We only checks for named modules here. The header like modules is skipped.
  // FIXME: This is not right if we import the header like modules in the module
  // purview.
  //
  // For example, assuming "header.h" provides definition for `D`.
  // ```C++
  // //--- M.cppm
  // export module M;
  // import "header.h"; // or #include "header.h" but import it by clang modules
  // actually.
  //
  // //--- Use.cpp
  // import M;
  // import "header.h"; // or uses clang modules.
  // ```
  //
  // In this case, `D` has multiple definitions in multiple TU (M.cppm and
  // Use.cpp) and `D` is attached to a named module `M`. The compiler should
  // reject it. But the current implementation couldn't detect the case since we
  // don't record the information about the importee modules.
  //
  // But this might not be painful in practice. Since the design of C++20 Named
  // Modules suggests us to use headers in global module fragment instead of
  // module purview.
  if (NewM && NewM->isHeaderLikeModule())
    NewM = nullptr;
  if (OldM && OldM->isHeaderLikeModule())
    OldM = nullptr;
 
  if (!NewM && !OldM)
    return true;
 
  // [basic.def.odr]p14.3
  // Each such definition shall not be attached to a named module
  // ([module.unit]).
  if ((NewM && NewM->isModulePurview()) || (OldM && OldM->isModulePurview()))
    return true;
 
  // Then New and Old lives in the same TU if their share one same module unit.
  if (NewM)
    NewM = NewM->getTopLevelModule();
  if (OldM)
    OldM = OldM->getTopLevelModule();
  return OldM == NewM;
}
 
static bool isUsingDecl(NamedDecl *D) {
  return isa<UsingShadowDecl>(D) ||
         isa<UnresolvedUsingTypenameDecl>(D) ||
         isa<UnresolvedUsingValueDecl>(D);
}
 
/// Removes using shadow declarations from the lookup results.
static void RemoveUsingDecls(LookupResult &R) {
  LookupResult::Filter F = R.makeFilter();
  while (F.hasNext())
    if (isUsingDecl(F.next()))
      F.erase();
 
  F.done();
}
 
/// Check for this common pattern:
/// @code
/// class S {
///   S(const S&); // DO NOT IMPLEMENT
///   void operator=(const S&); // DO NOT IMPLEMENT
/// };
/// @endcode
static bool IsDisallowedCopyOrAssign(const CXXMethodDecl *D) {
  // FIXME: Should check for private access too but access is set after we get
  // the decl here.
  if (D->doesThisDeclarationHaveABody())
    return false;
 
  if (const CXXConstructorDecl *CD = dyn_cast<CXXConstructorDecl>(D))
    return CD->isCopyConstructor();
  return D->isCopyAssignmentOperator();
}
 
// We need this to handle
//
// typedef struct {
//   void *foo() { return 0; }
// } A;
//
// When we see foo we don't know if after the typedef we will get 'A' or '*A'
// for example. If 'A', foo will have external linkage. If we have '*A',
// foo will have no linkage. Since we can't know until we get to the end
// of the typedef, this function finds out if D might have non-external linkage.
// Callers should verify at the end of the TU if it D has external linkage or
// not.
bool Sema::mightHaveNonExternalLinkage(const DeclaratorDecl *D) {
  const DeclContext *DC = D->getDeclContext();
  while (!DC->isTranslationUnit()) {
    if (const RecordDecl *RD = dyn_cast<RecordDecl>(DC)){
      if (!RD->hasNameForLinkage())
        return true;
    }
    DC = DC->getParent();
  }
 
  return !D->isExternallyVisible();
}
 
// FIXME: This needs to be refactored; some other isInMainFile users want
// these semantics.
static bool isMainFileLoc(const Sema &S, SourceLocation Loc) {
  if (S.TUKind != TU_Complete || S.getLangOpts().IsHeaderFile)
    return false;
  return S.SourceMgr.isInMainFile(Loc);
}
 
bool Sema::ShouldWarnIfUnusedFileScopedDecl(const DeclaratorDecl *D) const {
  assert(D);
 
  if (D->isInvalidDecl() || D->isUsed() || D->hasAttr<UnusedAttr>())
    return false;
 
  // Ignore all entities declared within templates, and out-of-line definitions
  // of members of class templates.
  if (D->getDeclContext()->isDependentContext() ||
      D->getLexicalDeclContext()->isDependentContext())
    return false;
 
  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
    if (FD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
      return false;
    // A non-out-of-line declaration of a member specialization was implicitly
    // instantiated; it's the out-of-line declaration that we're interested in.
    if (FD->getTemplateSpecializationKind() == TSK_ExplicitSpecialization &&
        FD->getMemberSpecializationInfo() && !FD->isOutOfLine())
      return false;
 
    if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(FD)) {
      if (MD->isVirtual() || IsDisallowedCopyOrAssign(MD))
        return false;
    } else {
      // 'static inline' functions are defined in headers; don't warn.
      if (FD->isInlined() && !isMainFileLoc(*this, FD->getLocation()))
        return false;
    }
 
    if (FD->doesThisDeclarationHaveABody() &&
        Context.DeclMustBeEmitted(FD))
      return false;
  } else if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
    // Constants and utility variables are defined in headers with internal
    // linkage; don't warn.  (Unlike functions, there isn't a convenient marker
    // like "inline".)
    if (!isMainFileLoc(*this, VD->getLocation()))
      return false;
 
    if (Context.DeclMustBeEmitted(VD))
      return false;
 
    if (VD->isStaticDataMember() &&
        VD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
      return false;
    if (VD->isStaticDataMember() &&
        VD->getTemplateSpecializationKind() == TSK_ExplicitSpecialization &&
        VD->getMemberSpecializationInfo() && !VD->isOutOfLine())
      return false;
 
    if (VD->isInline() && !isMainFileLoc(*this, VD->getLocation()))
      return false;
  } else {
    return false;
  }
 
  // Only warn for unused decls internal to the translation unit.
  // FIXME: This seems like a bogus check; it suppresses -Wunused-function
  // for inline functions defined in the main source file, for instance.
  return mightHaveNonExternalLinkage(D);
}
 
void Sema::MarkUnusedFileScopedDecl(const DeclaratorDecl *D) {
  if (!D)
    return;
 
  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
    const FunctionDecl *First = FD->getFirstDecl();
    if (FD != First && ShouldWarnIfUnusedFileScopedDecl(First))
      return; // First should already be in the vector.
  }
 
  if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
    const VarDecl *First = VD->getFirstDecl();
    if (VD != First && ShouldWarnIfUnusedFileScopedDecl(First))
      return; // First should already be in the vector.
  }
 
  if (ShouldWarnIfUnusedFileScopedDecl(D))
    UnusedFileScopedDecls.push_back(D);
}
 
static bool ShouldDiagnoseUnusedDecl(const NamedDecl *D) {
  if (D->isInvalidDecl())
    return false;
 
  if (auto *DD = dyn_cast<DecompositionDecl>(D)) {
    // For a decomposition declaration, warn if none of the bindings are
    // referenced, instead of if the variable itself is referenced (which
    // it is, by the bindings' expressions).
    for (auto *BD : DD->bindings())
      if (BD->isReferenced())
        return false;
  } else if (!D->getDeclName()) {
    return false;
  } else if (D->isReferenced() || D->isUsed()) {
    return false;
  }
 
  if (D->hasAttr<UnusedAttr>() || D->hasAttr<ObjCPreciseLifetimeAttr>())
    return false;
 
  if (isa<LabelDecl>(D))
    return true;
 
  // Except for labels, we only care about unused decls that are local to
  // functions.
  bool WithinFunction = D->getDeclContext()->isFunctionOrMethod();
  if (const auto *R = dyn_cast<CXXRecordDecl>(D->getDeclContext()))
    // For dependent types, the diagnostic is deferred.
    WithinFunction =
        WithinFunction || (R->isLocalClass() && !R->isDependentType());
  if (!WithinFunction)
    return false;
 
  if (isa<TypedefNameDecl>(D))
    return true;
 
  // White-list anything that isn't a local variable.
  if (!isa<VarDecl>(D) || isa<ParmVarDecl>(D) || isa<ImplicitParamDecl>(D))
    return false;
 
  // Types of valid local variables should be complete, so this should succeed.
  if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
 
    const Expr *Init = VD->getInit();
    if (const auto *Cleanups = dyn_cast_or_null<ExprWithCleanups>(Init))
      Init = Cleanups->getSubExpr();
 
    const auto *Ty = VD->getType().getTypePtr();
 
    // Only look at the outermost level of typedef.
    if (const TypedefType *TT = Ty->getAs<TypedefType>()) {
      // Allow anything marked with __attribute__((unused)).
      if (TT->getDecl()->hasAttr<UnusedAttr>())
        return false;
    }
 
    // Warn for reference variables whose initializtion performs lifetime
    // extension.
    if (const auto *MTE = dyn_cast_or_null<MaterializeTemporaryExpr>(Init)) {
      if (MTE->getExtendingDecl()) {
        Ty = VD->getType().getNonReferenceType().getTypePtr();
        Init = MTE->getSubExpr()->IgnoreImplicitAsWritten();
      }
    }
 
    // If we failed to complete the type for some reason, or if the type is
    // dependent, don't diagnose the variable.
    if (Ty->isIncompleteType() || Ty->isDependentType())
      return false;
 
    // Look at the element type to ensure that the warning behaviour is
    // consistent for both scalars and arrays.
    Ty = Ty->getBaseElementTypeUnsafe();
 
    if (const TagType *TT = Ty->getAs<TagType>()) {
      const TagDecl *Tag = TT->getDecl();
      if (Tag->hasAttr<UnusedAttr>())
        return false;
 
      if (const CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Tag)) {
        if (!RD->hasTrivialDestructor() && !RD->hasAttr<WarnUnusedAttr>())
          return false;
 
        if (Init) {
          const CXXConstructExpr *Construct =
            dyn_cast<CXXConstructExpr>(Init);
          if (Construct && !Construct->isElidable()) {
            CXXConstructorDecl *CD = Construct->getConstructor();
            if (!CD->isTrivial() && !RD->hasAttr<WarnUnusedAttr>() &&
                (VD->getInit()->isValueDependent() || !VD->evaluateValue()))
              return false;
          }
 
          // Suppress the warning if we don't know how this is constructed, and
          // it could possibly be non-trivial constructor.
          if (Init->isTypeDependent()) {
            for (const CXXConstructorDecl *Ctor : RD->ctors())
              if (!Ctor->isTrivial())
                return false;
          }
 
          // Suppress the warning if the constructor is unresolved because
          // its arguments are dependent.
          if (isa<CXXUnresolvedConstructExpr>(Init))
            return false;
        }
      }
    }
 
    // TODO: __attribute__((unused)) templates?
  }
 
  return true;
}
 
static void GenerateFixForUnusedDecl(const NamedDecl *D, ASTContext &Ctx,
                                     FixItHint &Hint) {
  if (isa<LabelDecl>(D)) {
    SourceLocation AfterColon = Lexer::findLocationAfterToken(
        D->getEndLoc(), tok::colon, Ctx.getSourceManager(), Ctx.getLangOpts(),
        true);
    if (AfterColon.isInvalid())
      return;
    Hint = FixItHint::CreateRemoval(
        CharSourceRange::getCharRange(D->getBeginLoc(), AfterColon));
  }
}
 
void Sema::DiagnoseUnusedNestedTypedefs(const RecordDecl *D) {
  DiagnoseUnusedNestedTypedefs(
      D, [this](SourceLocation Loc, PartialDiagnostic PD) { Diag(Loc, PD); });
}
 
void Sema::DiagnoseUnusedNestedTypedefs(const RecordDecl *D,
                                        DiagReceiverTy DiagReceiver) {
  if (D->getTypeForDecl()->isDependentType())
    return;
 
  for (auto *TmpD : D->decls()) {
    if (const auto *T = dyn_cast<TypedefNameDecl>(TmpD))
      DiagnoseUnusedDecl(T, DiagReceiver);
    else if(const auto *R = dyn_cast<RecordDecl>(TmpD))
      DiagnoseUnusedNestedTypedefs(R, DiagReceiver);
  }
}
 
void Sema::DiagnoseUnusedDecl(const NamedDecl *D) {
  DiagnoseUnusedDecl(
      D, [this](SourceLocation Loc, PartialDiagnostic PD) { Diag(Loc, PD); });
}
 
/// DiagnoseUnusedDecl - Emit warnings about declarations that are not used
/// unless they are marked attr(unused).
void Sema::DiagnoseUnusedDecl(const NamedDecl *D, DiagReceiverTy DiagReceiver) {
  if (!ShouldDiagnoseUnusedDecl(D))
    return;
 
  if (auto *TD = dyn_cast<TypedefNameDecl>(D)) {
    // typedefs can be referenced later on, so the diagnostics are emitted
    // at end-of-translation-unit.
    UnusedLocalTypedefNameCandidates.insert(TD);
    return;
  }
 
  FixItHint Hint;
  GenerateFixForUnusedDecl(D, Context, Hint);
 
  unsigned DiagID;
  if (isa<VarDecl>(D) && cast<VarDecl>(D)->isExceptionVariable())
    DiagID = diag::warn_unused_exception_param;
  else if (isa<LabelDecl>(D))
    DiagID = diag::warn_unused_label;
  else
    DiagID = diag::warn_unused_variable;
 
  DiagReceiver(D->getLocation(), PDiag(DiagID) << D << Hint);
}
 
void Sema::DiagnoseUnusedButSetDecl(const VarDecl *VD,
                                    DiagReceiverTy DiagReceiver) {
  // If it's not referenced, it can't be set. If it has the Cleanup attribute,
  // it's not really unused.
  if (!VD->isReferenced() || !VD->getDeclName() || VD->hasAttr<UnusedAttr>() ||
      VD->hasAttr<CleanupAttr>())
    return;
 
  const auto *Ty = VD->getType().getTypePtr()->getBaseElementTypeUnsafe();
 
  if (Ty->isReferenceType() || Ty->isDependentType())
    return;
 
  if (const TagType *TT = Ty->getAs<TagType>()) {
    const TagDecl *Tag = TT->getDecl();
    if (Tag->hasAttr<UnusedAttr>())
      return;
    // In C++, don't warn for record types that don't have WarnUnusedAttr, to
    // mimic gcc's behavior.
    if (const CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Tag)) {
      if (!RD->hasAttr<WarnUnusedAttr>())
        return;
    }
  }
 
  // Don't warn about __block Objective-C pointer variables, as they might
  // be assigned in the block but not used elsewhere for the purpose of lifetime
  // extension.
  if (VD->hasAttr<BlocksAttr>() && Ty->isObjCObjectPointerType())
    return;
 
  // Don't warn about Objective-C pointer variables with precise lifetime
  // semantics; they can be used to ensure ARC releases the object at a known
  // time, which may mean assignment but no other references.
  if (VD->hasAttr<ObjCPreciseLifetimeAttr>() && Ty->isObjCObjectPointerType())
    return;
 
  auto iter = RefsMinusAssignments.find(VD);
  if (iter == RefsMinusAssignments.end())
    return;
 
  assert(iter->getSecond() >= 0 &&
         "Found a negative number of references to a VarDecl");
  if (iter->getSecond() != 0)
    return;
  unsigned DiagID = isa<ParmVarDecl>(VD) ? diag::warn_unused_but_set_parameter
                                         : diag::warn_unused_but_set_variable;
  DiagReceiver(VD->getLocation(), PDiag(DiagID) << VD);
}
 
static void CheckPoppedLabel(LabelDecl *L, Sema &S,
                             Sema::DiagReceiverTy DiagReceiver) {
  // Verify that we have no forward references left.  If so, there was a goto
  // or address of a label taken, but no definition of it.  Label fwd
  // definitions are indicated with a null substmt which is also not a resolved
  // MS inline assembly label name.
  bool Diagnose = false;
  if (L->isMSAsmLabel())
    Diagnose = !L->isResolvedMSAsmLabel();
  else
    Diagnose = L->getStmt() == nullptr;
  if (Diagnose)
    DiagReceiver(L->getLocation(), S.PDiag(diag::err_undeclared_label_use)
                                       << L);
}
 
void Sema::ActOnPopScope(SourceLocation Loc, Scope *S) {
  S->applyNRVO();
 
  if (S->decl_empty()) return;
  assert((S->getFlags() & (Scope::DeclScope | Scope::TemplateParamScope)) &&
         "Scope shouldn't contain decls!");
 
  /// We visit the decls in non-deterministic order, but we want diagnostics
  /// emitted in deterministic order. Collect any diagnostic that may be emitted
  /// and sort the diagnostics before emitting them, after we visited all decls.
  struct LocAndDiag {
    SourceLocation Loc;
    std::optional<SourceLocation> PreviousDeclLoc;
    PartialDiagnostic PD;
  };
  SmallVector<LocAndDiag, 16> DeclDiags;
  auto addDiag = [&DeclDiags](SourceLocation Loc, PartialDiagnostic PD) {
    DeclDiags.push_back(LocAndDiag{Loc, std::nullopt, std::move(PD)});
  };
  auto addDiagWithPrev = [&DeclDiags](SourceLocation Loc,
                                      SourceLocation PreviousDeclLoc,
                                      PartialDiagnostic PD) {
    DeclDiags.push_back(LocAndDiag{Loc, PreviousDeclLoc, std::move(PD)});
  };
 
  for (auto *TmpD : S->decls()) {
    assert(TmpD && "This decl didn't get pushed??");
 
    assert(isa<NamedDecl>(TmpD) && "Decl isn't NamedDecl?");
    NamedDecl *D = cast<NamedDecl>(TmpD);
 
    // Diagnose unused variables in this scope.
    if (!S->hasUnrecoverableErrorOccurred()) {
      DiagnoseUnusedDecl(D, addDiag);
      if (const auto *RD = dyn_cast<RecordDecl>(D))
        DiagnoseUnusedNestedTypedefs(RD, addDiag);
      if (VarDecl *VD = dyn_cast<VarDecl>(D)) {
        DiagnoseUnusedButSetDecl(VD, addDiag);
        RefsMinusAssignments.erase(VD);
      }
    }
 
    if (!D->getDeclName()) continue;
 
    // If this was a forward reference to a label, verify it was defined.
    if (LabelDecl *LD = dyn_cast<LabelDecl>(D))
      CheckPoppedLabel(LD, *this, addDiag);
 
    // Remove this name from our lexical scope, and warn on it if we haven't
    // already.
    IdResolver.RemoveDecl(D);
    auto ShadowI = ShadowingDecls.find(D);
    if (ShadowI != ShadowingDecls.end()) {
      if (const auto *FD = dyn_cast<FieldDecl>(ShadowI->second)) {
        addDiagWithPrev(D->getLocation(), FD->getLocation(),
                        PDiag(diag::warn_ctor_parm_shadows_field)
                            << D << FD << FD->getParent());
      }
      ShadowingDecls.erase(ShadowI);
    }
  }
 
  llvm::sort(DeclDiags,
             [](const LocAndDiag &LHS, const LocAndDiag &RHS) -> bool {
               // The particular order for diagnostics is not important, as long
               // as the order is deterministic. Using the raw location is going
               // to generally be in source order unless there are macro
               // expansions involved.
               return LHS.Loc.getRawEncoding() < RHS.Loc.getRawEncoding();
             });
  for (const LocAndDiag &D : DeclDiags) {
    Diag(D.Loc, D.PD);
    if (D.PreviousDeclLoc)
      Diag(*D.PreviousDeclLoc, diag::note_previous_declaration);
  }
}
 
/// Look for an Objective-C class in the translation unit.
///
/// \param Id The name of the Objective-C class we're looking for. If
/// typo-correction fixes this name, the Id will be updated
/// to the fixed name.
///
/// \param IdLoc The location of the name in the translation unit.
///
/// \param DoTypoCorrection If true, this routine will attempt typo correction
/// if there is no class with the given name.
///
/// \returns The declaration of the named Objective-C class, or NULL if the
/// class could not be found.
ObjCInterfaceDecl *Sema::getObjCInterfaceDecl(IdentifierInfo *&Id,
                                              SourceLocation IdLoc,
                                              bool DoTypoCorrection) {
  // The third "scope" argument is 0 since we aren't enabling lazy built-in
  // creation from this context.
  NamedDecl *IDecl = LookupSingleName(TUScope, Id, IdLoc, LookupOrdinaryName);
 
  if (!IDecl && DoTypoCorrection) {
    // Perform typo correction at the given location, but only if we
    // find an Objective-C class name.
    DeclFilterCCC<ObjCInterfaceDecl> CCC{};
    if (TypoCorrection C =
            CorrectTypo(DeclarationNameInfo(Id, IdLoc), LookupOrdinaryName,
                        TUScope, nullptr, CCC, CTK_ErrorRecovery)) {
      diagnoseTypo(C, PDiag(diag::err_undef_interface_suggest) << Id);
      IDecl = C.getCorrectionDeclAs<ObjCInterfaceDecl>();
      Id = IDecl->getIdentifier();
    }
  }
  ObjCInterfaceDecl *Def = dyn_cast_or_null<ObjCInterfaceDecl>(IDecl);
  // This routine must always return a class definition, if any.
  if (Def && Def->getDefinition())
      Def = Def->getDefinition();
  return Def;
}
 
/// getNonFieldDeclScope - Retrieves the innermost scope, starting
/// from S, where a non-field would be declared. This routine copes
/// with the difference between C and C++ scoping rules in structs and
/// unions. For example, the following code is well-formed in C but
/// ill-formed in C++:
/// @code
/// struct S6 {
///   enum { BAR } e;
/// };
///
/// void test_S6() {
///   struct S6 a;
///   a.e = BAR;
/// }
/// @endcode
/// For the declaration of BAR, this routine will return a different
/// scope. The scope S will be the scope of the unnamed enumeration
/// within S6. In C++, this routine will return the scope associated
/// with S6, because the enumeration's scope is a transparent
/// context but structures can contain non-field names. In C, this
/// routine will return the translation unit scope, since the
/// enumeration's scope is a transparent context and structures cannot
/// contain non-field names.
Scope *Sema::getNonFieldDeclScope(Scope *S) {
  while (((S->getFlags() & Scope::DeclScope) == 0) ||
         (S->getEntity() && S->getEntity()->isTransparentContext()) ||
         (S->isClassScope() && !getLangOpts().CPlusPlus))
    S = S->getParent();
  return S;
}
 
static StringRef getHeaderName(Builtin::Context &BuiltinInfo, unsigned ID,
                               ASTContext::GetBuiltinTypeError Error) {
  switch (Error) {
  case ASTContext::GE_None:
    return "";
  case ASTContext::GE_Missing_type:
    return BuiltinInfo.getHeaderName(ID);
  case ASTContext::GE_Missing_stdio:
    return "stdio.h";
  case ASTContext::GE_Missing_setjmp:
    return "setjmp.h";
  case ASTContext::GE_Missing_ucontext:
    return "ucontext.h";
  }
  llvm_unreachable("unhandled error kind");
}
 
FunctionDecl *Sema::CreateBuiltin(IdentifierInfo *II, QualType Type,
                                  unsigned ID, SourceLocation Loc) {
  DeclContext *Parent = Context.getTranslationUnitDecl();
 
  if (getLangOpts().CPlusPlus) {
    LinkageSpecDecl *CLinkageDecl = LinkageSpecDecl::Create(
        Context, Parent, Loc, Loc, LinkageSpecDecl::lang_c, false);
    CLinkageDecl->setImplicit();
    Parent->addDecl(CLinkageDecl);
    Parent = CLinkageDecl;
  }
 
  FunctionDecl *New = FunctionDecl::Create(Context, Parent, Loc, Loc, II, Type,
                                           /*TInfo=*/nullptr, SC_Extern,
                                           getCurFPFeatures().isFPConstrained(),
                                           false, Type->isFunctionProtoType());
  New->setImplicit();
  New->addAttr(BuiltinAttr::CreateImplicit(Context, ID));
 
  // Create Decl objects for each parameter, adding them to the
  // FunctionDecl.
  if (const FunctionProtoType *FT = dyn_cast<FunctionProtoType>(Type)) {
    SmallVector<ParmVarDecl *, 16> Params;
    for (unsigned i = 0, e = FT->getNumParams(); i != e; ++i) {
      ParmVarDecl *parm = ParmVarDecl::Create(
          Context, New, SourceLocation(), SourceLocation(), nullptr,
          FT->getParamType(i), /*TInfo=*/nullptr, SC_None, nullptr);
      parm->setScopeInfo(0, i);
      Params.push_back(parm);
    }
    New->setParams(Params);
  }
 
  AddKnownFunctionAttributes(New);
  return New;
}
 
/// LazilyCreateBuiltin - The specified Builtin-ID was first used at
/// file scope.  lazily create a decl for it. ForRedeclaration is true
/// if we're creating this built-in in anticipation of redeclaring the
/// built-in.
NamedDecl *Sema::LazilyCreateBuiltin(IdentifierInfo *II, unsigned ID,
                                     Scope *S, bool ForRedeclaration,
                                     SourceLocation Loc) {
  LookupNecessaryTypesForBuiltin(S, ID);
 
  ASTContext::GetBuiltinTypeError Error;
  QualType R = Context.GetBuiltinType(ID, Error);
  if (Error) {
    if (!ForRedeclaration)
      return nullptr;
 
    // If we have a builtin without an associated type we should not emit a
    // warning when we were not able to find a type for it.
    if (Error == ASTContext::GE_Missing_type ||
        Context.BuiltinInfo.allowTypeMismatch(ID))
      return nullptr;
 
    // If we could not find a type for setjmp it is because the jmp_buf type was
    // not defined prior to the setjmp declaration.
    if (Error == ASTContext::GE_Missing_setjmp) {
      Diag(Loc, diag::warn_implicit_decl_no_jmp_buf)
          << Context.BuiltinInfo.getName(ID);
      return nullptr;
    }
 
    // Generally, we emit a warning that the declaration requires the
    // appropriate header.
    Diag(Loc, diag::warn_implicit_decl_requires_sysheader)
        << getHeaderName(Context.BuiltinInfo, ID, Error)
        << Context.BuiltinInfo.getName(ID);
    return nullptr;
  }
 
  if (!ForRedeclaration &&
      (Context.BuiltinInfo.isPredefinedLibFunction(ID) ||
       Context.BuiltinInfo.isHeaderDependentFunction(ID))) {
    Diag(Loc, LangOpts.C99 ? diag::ext_implicit_lib_function_decl_c99
                           : diag::ext_implicit_lib_function_decl)
        << Context.BuiltinInfo.getName(ID) << R;
    if (const char *Header = Context.BuiltinInfo.getHeaderName(ID))
      Diag(Loc, diag::note_include_header_or_declare)
          << Header << Context.BuiltinInfo.getName(ID);
  }
 
  if (R.isNull())
    return nullptr;
 
  FunctionDecl *New = CreateBuiltin(II, R, ID, Loc);
  RegisterLocallyScopedExternCDecl(New, S);
 
  // TUScope is the translation-unit scope to insert this function into.
  // FIXME: This is hideous. We need to teach PushOnScopeChains to
  // relate Scopes to DeclContexts, and probably eliminate CurContext
  // entirely, but we're not there yet.
  DeclContext *SavedContext = CurContext;
  CurContext = New->getDeclContext();
  PushOnScopeChains(New, TUScope);
  CurContext = SavedContext;
  return New;
}
 
/// Typedef declarations don't have linkage, but they still denote the same
/// entity if their types are the same.
/// FIXME: This is notionally doing the same thing as ASTReaderDecl's
/// isSameEntity.
static void filterNonConflictingPreviousTypedefDecls(Sema &S,
                                                     TypedefNameDecl *Decl,
                                                     LookupResult &Previous) {
  // This is only interesting when modules are enabled.
  if (!S.getLangOpts().Modules && !S.getLangOpts().ModulesLocalVisibility)
    return;
 
  // Empty sets are uninteresting.
  if (Previous.empty())
    return;
 
  LookupResult::Filter Filter = Previous.makeFilter();
  while (Filter.hasNext()) {
    NamedDecl *Old = Filter.next();
 
    // Non-hidden declarations are never ignored.
    if (S.isVisible(Old))
      continue;
 
    // Declarations of the same entity are not ignored, even if they have
    // different linkages.
    if (auto *OldTD = dyn_cast<TypedefNameDecl>(Old)) {
      if (S.Context.hasSameType(OldTD->getUnderlyingType(),
                                Decl->getUnderlyingType()))
        continue;
 
      // If both declarations give a tag declaration a typedef name for linkage
      // purposes, then they declare the same entity.
      if (OldTD->getAnonDeclWithTypedefName(/*AnyRedecl*/true) &&
          Decl->getAnonDeclWithTypedefName())
        continue;
    }
 
    Filter.erase();
  }
 
  Filter.done();
}
 
bool Sema::isIncompatibleTypedef(TypeDecl *Old, TypedefNameDecl *New) {
  QualType OldType;
  if (TypedefNameDecl *OldTypedef = dyn_cast<TypedefNameDecl>(Old))
    OldType = OldTypedef->getUnderlyingType();
  else
    OldType = Context.getTypeDeclType(Old);
  QualType NewType = New->getUnderlyingType();
 
  if (NewType->isVariablyModifiedType()) {
    // Must not redefine a typedef with a variably-modified type.
    int Kind = isa<TypeAliasDecl>(Old) ? 1 : 0;
    Diag(New->getLocation(), diag::err_redefinition_variably_modified_typedef)
      << Kind << NewType;
    if (Old->getLocation().isValid())
      notePreviousDefinition(Old, New->getLocation());
    New->setInvalidDecl();
    return true;
  }
 
  if (OldType != NewType &&
      !OldType->isDependentType() &&
      !NewType->isDependentType() &&
      !Context.hasSameType(OldType, NewType)) {
    int Kind = isa<TypeAliasDecl>(Old) ? 1 : 0;
    Diag(New->getLocation(), diag::err_redefinition_different_typedef)
      << Kind << NewType << OldType;
    if (Old->getLocation().isValid())
      notePreviousDefinition(Old, New->getLocation());
    New->setInvalidDecl();
    return true;
  }
  return false;
}
 
/// MergeTypedefNameDecl - We just parsed a typedef 'New' which has the
/// same name and scope as a previous declaration 'Old'.  Figure out
/// how to resolve this situation, merging decls or emitting
/// diagnostics as appropriate. If there was an error, set New to be invalid.
///
void Sema::MergeTypedefNameDecl(Scope *S, TypedefNameDecl *New,
                                LookupResult &OldDecls) {
  // If the new decl is known invalid already, don't bother doing any
  // merging checks.
  if (New->isInvalidDecl()) return;
 
  // Allow multiple definitions for ObjC built-in typedefs.
  // FIXME: Verify the underlying types are equivalent!
  if (getLangOpts().ObjC) {
    const IdentifierInfo *TypeID = New->getIdentifier();
    switch (TypeID->getLength()) {
    default: break;
    case 2:
      {
        if (!TypeID->isStr("id"))
          break;
        QualType T = New->getUnderlyingType();
        if (!T->isPointerType())
          break;
        if (!T->isVoidPointerType()) {
          QualType PT = T->castAs<PointerType>()->getPointeeType();
          if (!PT->isStructureType())
            break;
        }
        Context.setObjCIdRedefinitionType(T);
        // Install the built-in type for 'id', ignoring the current definition.
        New->setTypeForDecl(Context.getObjCIdType().getTypePtr());
        return;
      }
    case 5:
      if (!TypeID->isStr("Class"))
        break;
      Context.setObjCClassRedefinitionType(New->getUnderlyingType());
      // Install the built-in type for 'Class', ignoring the current definition.
      New->setTypeForDecl(Context.getObjCClassType().getTypePtr());
      return;
    case 3:
      if (!TypeID->isStr("SEL"))
        break;
      Context.setObjCSelRedefinitionType(New->getUnderlyingType());
      // Install the built-in type for 'SEL', ignoring the current definition.
      New->setTypeForDecl(Context.getObjCSelType().getTypePtr());
      return;
    }
    // Fall through - the typedef name was not a builtin type.
  }
 
  // Verify the old decl was also a type.
  TypeDecl *Old = OldDecls.getAsSingle<TypeDecl>();
  if (!Old) {
    Diag(New->getLocation(), diag::err_redefinition_different_kind)
      << New->getDeclName();
 
    NamedDecl *OldD = OldDecls.getRepresentativeDecl();
    if (OldD->getLocation().isValid())
      notePreviousDefinition(OldD, New->getLocation());
 
    return New->setInvalidDecl();
  }
 
  // If the old declaration is invalid, just give up here.
  if (Old->isInvalidDecl())
    return New->setInvalidDecl();
 
  if (auto *OldTD = dyn_cast<TypedefNameDecl>(Old)) {
    auto *OldTag = OldTD->getAnonDeclWithTypedefName(/*AnyRedecl*/true);
    auto *NewTag = New->getAnonDeclWithTypedefName();
    NamedDecl *Hidden = nullptr;
    if (OldTag && NewTag &&
        OldTag->getCanonicalDecl() != NewTag->getCanonicalDecl() &&
        !hasVisibleDefinition(OldTag, &Hidden)) {
      // There is a definition of this tag, but it is not visible. Use it
      // instead of our tag.
      New->setTypeForDecl(OldTD->getTypeForDecl());
      if (OldTD->isModed())
        New->setModedTypeSourceInfo(OldTD->getTypeSourceInfo(),
                                    OldTD->getUnderlyingType());
      else
        New->setTypeSourceInfo(OldTD->getTypeSourceInfo());
 
      // Make the old tag definition visible.
      makeMergedDefinitionVisible(Hidden);
 
      // If this was an unscoped enumeration, yank all of its enumerators
      // out of the scope.
      if (isa<EnumDecl>(NewTag)) {
        Scope *EnumScope = getNonFieldDeclScope(S);
        for (auto *D : NewTag->decls()) {
          auto *ED = cast<EnumConstantDecl>(D);
          assert(EnumScope->isDeclScope(ED));
          EnumScope->RemoveDecl(ED);
          IdResolver.RemoveDecl(ED);
          ED->getLexicalDeclContext()->removeDecl(ED);
        }
      }
    }
  }
 
  // If the typedef types are not identical, reject them in all languages and
  // with any extensions enabled.
  if (isIncompatibleTypedef(Old, New))
    return;
 
  // The types match.  Link up the redeclaration chain and merge attributes if
  // the old declaration was a typedef.
  if (TypedefNameDecl *Typedef = dyn_cast<TypedefNameDecl>(Old)) {
    New->setPreviousDecl(Typedef);
    mergeDeclAttributes(New, Old);
  }
 
  if (getLangOpts().MicrosoftExt)
    return;
 
  if (getLangOpts().CPlusPlus) {
    // C++ [dcl.typedef]p2:
    //   In a given non-class scope, a typedef specifier can be used to
    //   redefine the name of any type declared in that scope to refer
    //   to the type to which it already refers.
    if (!isa<CXXRecordDecl>(CurContext))
      return;
 
    // C++0x [dcl.typedef]p4:
    //   In a given class scope, a typedef specifier can be used to redefine
    //   any class-name declared in that scope that is not also a typedef-name
    //   to refer to the type to which it already refers.
    //
    // This wording came in via DR424, which was a correction to the
    // wording in DR56, which accidentally banned code like:
    //
    //   struct S {
    //     typedef struct A { } A;
    //   };
    //
    // in the C++03 standard. We implement the C++0x semantics, which
    // allow the above but disallow
    //
    //   struct S {
    //     typedef int I;
    //     typedef int I;
    //   };
    //
    // since that was the intent of DR56.
    if (!isa<TypedefNameDecl>(Old))
      return;
 
    Diag(New->getLocation(), diag::err_redefinition)
      << New->getDeclName();
    notePreviousDefinition(Old, New->getLocation());
    return New->setInvalidDecl();
  }
 
  // Modules always permit redefinition of typedefs, as does C11.
  if (getLangOpts().Modules || getLangOpts().C11)
    return;
 
  // If we have a redefinition of a typedef in C, emit a warning.  This warning
  // is normally mapped to an error, but can be controlled with
  // -Wtypedef-redefinition.  If either the original or the redefinition is
  // in a system header, don't emit this for compatibility with GCC.
  if (getDiagnostics().getSuppressSystemWarnings() &&
      // Some standard types are defined implicitly in Clang (e.g. OpenCL).
      (Old->isImplicit() ||
       Context.getSourceManager().isInSystemHeader(Old->getLocation()) ||
       Context.getSourceManager().isInSystemHeader(New->getLocation())))
    return;
 
  Diag(New->getLocation(), diag::ext_redefinition_of_typedef)
    << New->getDeclName();
  notePreviousDefinition(Old, New->getLocation());
}
 
/// DeclhasAttr - returns true if decl Declaration already has the target
/// attribute.
static bool DeclHasAttr(const Decl *D, const Attr *A) {
  const OwnershipAttr *OA = dyn_cast<OwnershipAttr>(A);
  const AnnotateAttr *Ann = dyn_cast<AnnotateAttr>(A);
  for (const auto *i : D->attrs())
    if (i->getKind() == A->getKind()) {
      if (Ann) {
        if (Ann->getAnnotation() == cast<AnnotateAttr>(i)->getAnnotation())
          return true;
        continue;
      }
      // FIXME: Don't hardcode this check
      if (OA && isa<OwnershipAttr>(i))
        return OA->getOwnKind() == cast<OwnershipAttr>(i)->getOwnKind();
      return true;
    }
 
  return false;
}
 
static bool isAttributeTargetADefinition(Decl *D) {
  if (VarDecl *VD = dyn_cast<VarDecl>(D))
    return VD->isThisDeclarationADefinition();
  if (TagDecl *TD = dyn_cast<TagDecl>(D))
    return TD->isCompleteDefinition() || TD->isBeingDefined();
  return true;
}
 
/// Merge alignment attributes from \p Old to \p New, taking into account the
/// special semantics of C11's _Alignas specifier and C++11's alignas attribute.
///
/// \return \c true if any attributes were added to \p New.
static bool mergeAlignedAttrs(Sema &S, NamedDecl *New, Decl *Old) {
  // Look for alignas attributes on Old, and pick out whichever attribute
  // specifies the strictest alignment requirement.
  AlignedAttr *OldAlignasAttr = nullptr;
  AlignedAttr *OldStrictestAlignAttr = nullptr;
  unsigned OldAlign = 0;
  for (auto *I : Old->specific_attrs<AlignedAttr>()) {
    // FIXME: We have no way of representing inherited dependent alignments
    // in a case like:
    //   template<int A, int B> struct alignas(A) X;
    //   template<int A, int B> struct alignas(B) X {};
    // For now, we just ignore any alignas attributes which are not on the
    // definition in such a case.
    if (I->isAlignmentDependent())
      return false;
 
    if (I->isAlignas())
      OldAlignasAttr = I;
 
    unsigned Align = I->getAlignment(S.Context);
    if (Align > OldAlign) {
      OldAlign = Align;
      OldStrictestAlignAttr = I;
    }
  }
 
  // Look for alignas attributes on New.
  AlignedAttr *NewAlignasAttr = nullptr;
  unsigned NewAlign = 0;
  for (auto *I : New->specific_attrs<AlignedAttr>()) {
    if (I->isAlignmentDependent())
      return false;
 
    if (I->isAlignas())
      NewAlignasAttr = I;
 
    unsigned Align = I->getAlignment(S.Context);
    if (Align > NewAlign)
      NewAlign = Align;
  }
 
  if (OldAlignasAttr && NewAlignasAttr && OldAlign != NewAlign) {
    // Both declarations have 'alignas' attributes. We require them to match.
    // C++11 [dcl.align]p6 and C11 6.7.5/7 both come close to saying this, but
    // fall short. (If two declarations both have alignas, they must both match
    // every definition, and so must match each other if there is a definition.)
 
    // If either declaration only contains 'alignas(0)' specifiers, then it
    // specifies the natural alignment for the type.
    if (OldAlign == 0 || NewAlign == 0) {
      QualType Ty;
      if (ValueDecl *VD = dyn_cast<ValueDecl>(New))
        Ty = VD->getType();
      else
        Ty = S.Context.getTagDeclType(cast<TagDecl>(New));
 
      if (OldAlign == 0)
        OldAlign = S.Context.getTypeAlign(Ty);
      if (NewAlign == 0)
        NewAlign = S.Context.getTypeAlign(Ty);
    }
 
    if (OldAlign != NewAlign) {
      S.Diag(NewAlignasAttr->getLocation(), diag::err_alignas_mismatch)
        << (unsigned)S.Context.toCharUnitsFromBits(OldAlign).getQuantity()
        << (unsigned)S.Context.toCharUnitsFromBits(NewAlign).getQuantity();
      S.Diag(OldAlignasAttr->getLocation(), diag::note_previous_declaration);
    }
  }
 
  if (OldAlignasAttr && !NewAlignasAttr && isAttributeTargetADefinition(New)) {
    // C++11 [dcl.align]p6:
    //   if any declaration of an entity has an alignment-specifier,
    //   every defining declaration of that entity shall specify an
    //   equivalent alignment.
    // C11 6.7.5/7:
    //   If the definition of an object does not have an alignment
    //   specifier, any other declaration of that object shall also
    //   have no alignment specifier.
    S.Diag(New->getLocation(), diag::err_alignas_missing_on_definition)
      << OldAlignasAttr;
    S.Diag(OldAlignasAttr->getLocation(), diag::note_alignas_on_declaration)
      << OldAlignasAttr;
  }
 
  bool AnyAdded = false;
 
  // Ensure we have an attribute representing the strictest alignment.
  if (OldAlign > NewAlign) {
    AlignedAttr *Clone = OldStrictestAlignAttr->clone(S.Context);
    Clone->setInherited(true);
    New->addAttr(Clone);
    AnyAdded = true;
  }
 
  // Ensure we have an alignas attribute if the old declaration had one.
  if (OldAlignasAttr && !NewAlignasAttr &&
      !(AnyAdded && OldStrictestAlignAttr->isAlignas())) {
    AlignedAttr *Clone = OldAlignasAttr->clone(S.Context);
    Clone->setInherited(true);
    New->addAttr(Clone);
    AnyAdded = true;
  }
 
  return AnyAdded;
}
 
#define WANT_DECL_MERGE_LOGIC
#include "clang/Sema/AttrParsedAttrImpl.inc"
#undef WANT_DECL_MERGE_LOGIC
 
static bool mergeDeclAttribute(Sema &S, NamedDecl *D,
                               const InheritableAttr *Attr,
                               Sema::AvailabilityMergeKind AMK) {
  // Diagnose any mutual exclusions between the attribute that we want to add
  // and attributes that already exist on the declaration.
  if (!DiagnoseMutualExclusions(S, D, Attr))
    return false;
 
  // This function copies an attribute Attr from a previous declaration to the
  // new declaration D if the new declaration doesn't itself have that attribute
  // yet or if that attribute allows duplicates.
  // If you're adding a new attribute that requires logic different from
  // "use explicit attribute on decl if present, else use attribute from
  // previous decl", for example if the attribute needs to be consistent
  // between redeclarations, you need to call a custom merge function here.
  InheritableAttr *NewAttr = nullptr;
  if (const auto *AA = dyn_cast<AvailabilityAttr>(Attr))
    NewAttr = S.mergeAvailabilityAttr(
        D, *AA, AA->getPlatform(), AA->isImplicit(), AA->getIntroduced(),
        AA->getDeprecated(), AA->getObsoleted(), AA->getUnavailable(),
        AA->getMessage(), AA->getStrict(), AA->getReplacement(), AMK,
        AA->getPriority());
  else if (const auto *VA = dyn_cast<VisibilityAttr>(Attr))
    NewAttr = S.mergeVisibilityAttr(D, *VA, VA->getVisibility());
  else if (const auto *VA = dyn_cast<TypeVisibilityAttr>(Attr))
    NewAttr = S.mergeTypeVisibilityAttr(D, *VA, VA->getVisibility());
  else if (const auto *ImportA = dyn_cast<DLLImportAttr>(Attr))
    NewAttr = S.mergeDLLImportAttr(D, *ImportA);
  else if (const auto *ExportA = dyn_cast<DLLExportAttr>(Attr))
    NewAttr = S.mergeDLLExportAttr(D, *ExportA);
  else if (const auto *EA = dyn_cast<ErrorAttr>(Attr))
    NewAttr = S.mergeErrorAttr(D, *EA, EA->getUserDiagnostic());
  else if (const auto *FA = dyn_cast<FormatAttr>(Attr))
    NewAttr = S.mergeFormatAttr(D, *FA, FA->getType(), FA->getFormatIdx(),
                                FA->getFirstArg());
  else if (const auto *SA = dyn_cast<SectionAttr>(Attr))
    NewAttr = S.mergeSectionAttr(D, *SA, SA->getName());
  else if (const auto *CSA = dyn_cast<CodeSegAttr>(Attr))
    NewAttr = S.mergeCodeSegAttr(D, *CSA, CSA->getName());
  else if (const auto *IA = dyn_cast<MSInheritanceAttr>(Attr))
    NewAttr = S.mergeMSInheritanceAttr(D, *IA, IA->getBestCase(),
                                       IA->getInheritanceModel());
  else if (const auto *AA = dyn_cast<AlwaysInlineAttr>(Attr))
    NewAttr = S.mergeAlwaysInlineAttr(D, *AA,
                                      &S.Context.Idents.get(AA->getSpelling()));
  else if (S.getLangOpts().CUDA && isa<FunctionDecl>(D) &&
           (isa<CUDAHostAttr>(Attr) || isa<CUDADeviceAttr>(Attr) ||
            isa<CUDAGlobalAttr>(Attr))) {
    // CUDA target attributes are part of function signature for
    // overloading purposes and must not be merged.
    return false;
  } else if (const auto *MA = dyn_cast<MinSizeAttr>(Attr))
    NewAttr = S.mergeMinSizeAttr(D, *MA);
  else if (const auto *SNA = dyn_cast<SwiftNameAttr>(Attr))
    NewAttr = S.mergeSwiftNameAttr(D, *SNA, SNA->getName());
  else if (const auto *OA = dyn_cast<OptimizeNoneAttr>(Attr))
    NewAttr = S.mergeOptimizeNoneAttr(D, *OA);
  else if (const auto *InternalLinkageA = dyn_cast<InternalLinkageAttr>(Attr))
    NewAttr = S.mergeInternalLinkageAttr(D, *InternalLinkageA);
  else if (isa<AlignedAttr>(Attr))
    // AlignedAttrs are handled separately, because we need to handle all
    // such attributes on a declaration at the same time.
    NewAttr = nullptr;
  else if ((isa<DeprecatedAttr>(Attr) || isa<UnavailableAttr>(Attr)) &&
           (AMK == Sema::AMK_Override ||
            AMK == Sema::AMK_ProtocolImplementation ||
            AMK == Sema::AMK_OptionalProtocolImplementation))
    NewAttr = nullptr;
  else if (const auto *UA = dyn_cast<UuidAttr>(Attr))
    NewAttr = S.mergeUuidAttr(D, *UA, UA->getGuid(), UA->getGuidDecl());
  else if (const auto *IMA = dyn_cast<WebAssemblyImportModuleAttr>(Attr))
    NewAttr = S.mergeImportModuleAttr(D, *IMA);
  else if (const auto *INA = dyn_cast<WebAssemblyImportNameAttr>(Attr))
    NewAttr = S.mergeImportNameAttr(D, *INA);
  else if (const auto *TCBA = dyn_cast<EnforceTCBAttr>(Attr))
    NewAttr = S.mergeEnforceTCBAttr(D, *TCBA);
  else if (const auto *TCBLA = dyn_cast<EnforceTCBLeafAttr>(Attr))
    NewAttr = S.mergeEnforceTCBLeafAttr(D, *TCBLA);
  else if (const auto *BTFA = dyn_cast<BTFDeclTagAttr>(Attr))
    NewAttr = S.mergeBTFDeclTagAttr(D, *BTFA);
  else if (const auto *NT = dyn_cast<HLSLNumThreadsAttr>(Attr))
    NewAttr =
        S.mergeHLSLNumThreadsAttr(D, *NT, NT->getX(), NT->getY(), NT->getZ());
  else if (const auto *SA = dyn_cast<HLSLShaderAttr>(Attr))
    NewAttr = S.mergeHLSLShaderAttr(D, *SA, SA->getType());
  else if (Attr->shouldInheritEvenIfAlreadyPresent() || !DeclHasAttr(D, Attr))
    NewAttr = cast<InheritableAttr>(Attr->clone(S.Context));
 
  if (NewAttr) {
    NewAttr->setInherited(true);
    D->addAttr(NewAttr);
    if (isa<MSInheritanceAttr>(NewAttr))
      S.Consumer.AssignInheritanceModel(cast<CXXRecordDecl>(D));
    return true;
  }
 
  return false;
}
 
static const NamedDecl *getDefinition(const Decl *D) {
  if (const TagDecl *TD = dyn_cast<TagDecl>(D))
    return TD->getDefinition();
  if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
    const VarDecl *Def = VD->getDefinition();
    if (Def)
      return Def;
    return VD->getActingDefinition();
  }
  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {
    const FunctionDecl *Def = nullptr;
    if (FD->isDefined(Def, true))
      return Def;
  }
  return nullptr;
}
 
static bool hasAttribute(const Decl *D, attr::Kind Kind) {
  for (const auto *Attribute : D->attrs())
    if (Attribute->getKind() == Kind)
      return true;
  return false;
}
 
/// checkNewAttributesAfterDef - If we already have a definition, check that
/// there are no new attributes in this declaration.
static void checkNewAttributesAfterDef(Sema &S, Decl *New, const Decl *Old) {
  if (!New->hasAttrs())
    return;
 
  const NamedDecl *Def = getDefinition(Old);
  if (!Def || Def == New)
    return;
 
  AttrVec &NewAttributes = New->getAttrs();
  for (unsigned I = 0, E = NewAttributes.size(); I != E;) {
    const Attr *NewAttribute = NewAttributes[I];
 
    if (isa<AliasAttr>(NewAttribute) || isa<IFuncAttr>(NewAttribute)) {
      if (FunctionDecl *FD = dyn_cast<FunctionDecl>(New)) {
        Sema::SkipBodyInfo SkipBody;
        S.CheckForFunctionRedefinition(FD, cast<FunctionDecl>(Def), &SkipBody);
 
        // If we're skipping this definition, drop the "alias" attribute.
        if (SkipBody.ShouldSkip) {
          NewAttributes.erase(NewAttributes.begin() + I);
          --E;
          continue;
        }
      } else {
        VarDecl *VD = cast<VarDecl>(New);
        unsigned Diag = cast<VarDecl>(Def)->isThisDeclarationADefinition() ==
                                VarDecl::TentativeDefinition
                            ? diag::err_alias_after_tentative
                            : diag::err_redefinition;
        S.Diag(VD->getLocation(), Diag) << VD->getDeclName();
        if (Diag == diag::err_redefinition)
          S.notePreviousDefinition(Def, VD->getLocation());
        else
          S.Diag(Def->getLocation(), diag::note_previous_definition);
        VD->setInvalidDecl();
      }
      ++I;
      continue;
    }
 
    if (const VarDecl *VD = dyn_cast<VarDecl>(Def)) {
      // Tentative definitions are only interesting for the alias check above.
      if (VD->isThisDeclarationADefinition() != VarDecl::Definition) {
        ++I;
        continue;
      }
    }
 
    if (hasAttribute(Def, NewAttribute->getKind())) {
      ++I;
      continue; // regular attr merging will take care of validating this.
    }
 
    if (isa<C11NoReturnAttr>(NewAttribute)) {
      // C's _Noreturn is allowed to be added to a function after it is defined.
      ++I;
      continue;
    } else if (isa<UuidAttr>(NewAttribute)) {
      // msvc will allow a subsequent definition to add an uuid to a class
      ++I;
      continue;
    } else if (const AlignedAttr *AA = dyn_cast<AlignedAttr>(NewAttribute)) {
      if (AA->isAlignas()) {
        // C++11 [dcl.align]p6:
        //   if any declaration of an entity has an alignment-specifier,
        //   every defining declaration of that entity shall specify an
        //   equivalent alignment.
        // C11 6.7.5/7:
        //   If the definition of an object does not have an alignment
        //   specifier, any other declaration of that object shall also
        //   have no alignment specifier.
        S.Diag(Def->getLocation(), diag::err_alignas_missing_on_definition)
          << AA;
        S.Diag(NewAttribute->getLocation(), diag::note_alignas_on_declaration)
          << AA;
        NewAttributes.erase(NewAttributes.begin() + I);
        --E;
        continue;
      }
    } else if (isa<LoaderUninitializedAttr>(NewAttribute)) {
      // If there is a C definition followed by a redeclaration with this
      // attribute then there are two different definitions. In C++, prefer the
      // standard diagnostics.
      if (!S.getLangOpts().CPlusPlus) {
        S.Diag(NewAttribute->getLocation(),
               diag::err_loader_uninitialized_redeclaration);
        S.Diag(Def->getLocation(), diag::note_previous_definition);
        NewAttributes.erase(NewAttributes.begin() + I);
        --E;
        continue;
      }
    } else if (isa<SelectAnyAttr>(NewAttribute) &&
               cast<VarDecl>(New)->isInline() &&
               !cast<VarDecl>(New)->isInlineSpecified()) {
      // Don't warn about applying selectany to implicitly inline variables.
      // Older compilers and language modes would require the use of selectany
      // to make such variables inline, and it would have no effect if we
      // honored it.
      ++I;
      continue;
    } else if (isa<OMPDeclareVariantAttr>(NewAttribute)) {
      // We allow to add OMP[Begin]DeclareVariantAttr to be added to
      // declarations after definitions.
      ++I;
      continue;
    }
 
    S.Diag(NewAttribute->getLocation(),
           diag::warn_attribute_precede_definition);
    S.Diag(Def->getLocation(), diag::note_previous_definition);
    NewAttributes.erase(NewAttributes.begin() + I);
    --E;
  }
}
 
static void diagnoseMissingConstinit(Sema &S, const VarDecl *InitDecl,
                                     const ConstInitAttr *CIAttr,
                                     bool AttrBeforeInit) {
  SourceLocation InsertLoc = InitDecl->getInnerLocStart();
 
  // Figure out a good way to write this specifier on the old declaration.
  // FIXME: We should just use the spelling of CIAttr, but we don't preserve
  // enough of the attribute list spelling information to extract that without
  // heroics.
  std::string SuitableSpelling;
  if (S.getLangOpts().CPlusPlus20)
    SuitableSpelling = std::string(
        S.PP.getLastMacroWithSpelling(InsertLoc, {tok::kw_constinit}));
  if (SuitableSpelling.empty() && S.getLangOpts().CPlusPlus11)
    SuitableSpelling = std::string(S.PP.getLastMacroWithSpelling(
        InsertLoc, {tok::l_square, tok::l_square,
                    S.PP.getIdentifierInfo("clang"), tok::coloncolon,
                    S.PP.getIdentifierInfo("require_constant_initialization"),
                    tok::r_square, tok::r_square}));
  if (SuitableSpelling.empty())
    SuitableSpelling = std::string(S.PP.getLastMacroWithSpelling(
        InsertLoc, {tok::kw___attribute, tok::l_paren, tok::r_paren,
                    S.PP.getIdentifierInfo("require_constant_initialization"),
                    tok::r_paren, tok::r_paren}));
  if (SuitableSpelling.empty() && S.getLangOpts().CPlusPlus20)
    SuitableSpelling = "constinit";
  if (SuitableSpelling.empty() && S.getLangOpts().CPlusPlus11)
    SuitableSpelling = "[[clang::require_constant_initialization]]";
  if (SuitableSpelling.empty())
    SuitableSpelling = "__attribute__((require_constant_initialization))";
  SuitableSpelling += " ";
 
  if (AttrBeforeInit) {
    // extern constinit int a;
    // int a = 0; // error (missing 'constinit'), accepted as extension
    assert(CIAttr->isConstinit() && "should not diagnose this for attribute");
    S.Diag(InitDecl->getLocation(), diag::ext_constinit_missing)
        << InitDecl << FixItHint::CreateInsertion(InsertLoc, SuitableSpelling);
    S.Diag(CIAttr->getLocation(), diag::note_constinit_specified_here);
  } else {
    // int a = 0;
    // constinit extern int a; // error (missing 'constinit')
    S.Diag(CIAttr->getLocation(),
           CIAttr->isConstinit() ? diag::err_constinit_added_too_late
                                 : diag::warn_require_const_init_added_too_late)
        << FixItHint::CreateRemoval(SourceRange(CIAttr->getLocation()));
    S.Diag(InitDecl->getLocation(), diag::note_constinit_missing_here)
        << CIAttr->isConstinit()
        << FixItHint::CreateInsertion(InsertLoc, SuitableSpelling);
  }
}
 
/// mergeDeclAttributes - Copy attributes from the Old decl to the New one.
void Sema::mergeDeclAttributes(NamedDecl *New, Decl *Old,
                               AvailabilityMergeKind AMK) {
  if (UsedAttr *OldAttr = Old->getMostRecentDecl()->getAttr<UsedAttr>()) {
    UsedAttr *NewAttr = OldAttr->clone(Context);
    NewAttr->setInherited(true);
    New->addAttr(NewAttr);
  }
  if (RetainAttr *OldAttr = Old->getMostRecentDecl()->getAttr<RetainAttr>()) {
    RetainAttr *NewAttr = OldAttr->clone(Context);
    NewAttr->setInherited(true);
    New->addAttr(NewAttr);
  }
 
  if (!Old->hasAttrs() && !New->hasAttrs())
    return;
 
  // [dcl.constinit]p1:
  //   If the [constinit] specifier is applied to any declaration of a
  //   variable, it shall be applied to the initializing declaration.
  const auto *OldConstInit = Old->getAttr<ConstInitAttr>();
  const auto *NewConstInit = New->getAttr<ConstInitAttr>();
  if (bool(OldConstInit) != bool(NewConstInit)) {
    const auto *OldVD = cast<VarDecl>(Old);
    auto *NewVD = cast<VarDecl>(New);
 
    // Find the initializing declaration. Note that we might not have linked
    // the new declaration into the redeclaration chain yet.
    const VarDecl *InitDecl = OldVD->getInitializingDeclaration();
    if (!InitDecl &&
        (NewVD->hasInit() || NewVD->isThisDeclarationADefinition()))
      InitDecl = NewVD;
 
    if (InitDecl == NewVD) {
      // This is the initializing declaration. If it would inherit 'constinit',
      // that's ill-formed. (Note that we do not apply this to the attribute
      // form).
      if (OldConstInit && OldConstInit->isConstinit())
        diagnoseMissingConstinit(*this, NewVD, OldConstInit,
                                 /*AttrBeforeInit=*/true);
    } else if (NewConstInit) {
      // This is the first time we've been told that this declaration should
      // have a constant initializer. If we already saw the initializing
      // declaration, this is too late.
      if (InitDecl && InitDecl != NewVD) {
        diagnoseMissingConstinit(*this, InitDecl, NewConstInit,
                                 /*AttrBeforeInit=*/false);
        NewVD->dropAttr<ConstInitAttr>();
      }
    }
  }
 
  // Attributes declared post-definition are currently ignored.
  checkNewAttributesAfterDef(*this, New, Old);
 
  if (AsmLabelAttr *NewA = New->getAttr<AsmLabelAttr>()) {
    if (AsmLabelAttr *OldA = Old->getAttr<AsmLabelAttr>()) {
      if (!OldA->isEquivalent(NewA)) {
        // This redeclaration changes __asm__ label.
        Diag(New->getLocation(), diag::err_different_asm_label);
        Diag(OldA->getLocation(), diag::note_previous_declaration);
      }
    } else if (Old->isUsed()) {
      // This redeclaration adds an __asm__ label to a declaration that has
      // already been ODR-used.
      Diag(New->getLocation(), diag::err_late_asm_label_name)
        << isa<FunctionDecl>(Old) << New->getAttr<AsmLabelAttr>()->getRange();
    }
  }
 
  // Re-declaration cannot add abi_tag's.
  if (const auto *NewAbiTagAttr = New->getAttr<AbiTagAttr>()) {
    if (const auto *OldAbiTagAttr = Old->getAttr<AbiTagAttr>()) {
      for (const auto &NewTag : NewAbiTagAttr->tags()) {
        if (!llvm::is_contained(OldAbiTagAttr->tags(), NewTag)) {
          Diag(NewAbiTagAttr->getLocation(),
               diag::err_new_abi_tag_on_redeclaration)
              << NewTag;
          Diag(OldAbiTagAttr->getLocation(), diag::note_previous_declaration);
        }
      }
    } else {
      Diag(NewAbiTagAttr->getLocation(), diag::err_abi_tag_on_redeclaration);
      Diag(Old->getLocation(), diag::note_previous_declaration);
    }
  }
 
  // This redeclaration adds a section attribute.
  if (New->hasAttr<SectionAttr>() && !Old->hasAttr<SectionAttr>()) {
    if (auto *VD = dyn_cast<VarDecl>(New)) {
      if (VD->isThisDeclarationADefinition() == VarDecl::DeclarationOnly) {
        Diag(New->getLocation(), diag::warn_attribute_section_on_redeclaration);
        Diag(Old->getLocation(), diag::note_previous_declaration);
      }
    }
  }
 
  // Redeclaration adds code-seg attribute.
  const auto *NewCSA = New->getAttr<CodeSegAttr>();
  if (NewCSA && !Old->hasAttr<CodeSegAttr>() &&
      !NewCSA->isImplicit() && isa<CXXMethodDecl>(New)) {
    Diag(New->getLocation(), diag::warn_mismatched_section)
         << 0 /*codeseg*/;
    Diag(Old->getLocation(), diag::note_previous_declaration);
  }
 
  if (!Old->hasAttrs())
    return;
 
  bool foundAny = New->hasAttrs();
 
  // Ensure that any moving of objects within the allocated map is done before
  // we process them.
  if (!foundAny) New->setAttrs(AttrVec());
 
  for (auto *I : Old->specific_attrs<InheritableAttr>()) {
    // Ignore deprecated/unavailable/availability attributes if requested.
    AvailabilityMergeKind LocalAMK = AMK_None;
    if (isa<DeprecatedAttr>(I) ||
        isa<UnavailableAttr>(I) ||
        isa<AvailabilityAttr>(I)) {
      switch (AMK) {
      case AMK_None:
        continue;
 
      case AMK_Redeclaration:
      case AMK_Override:
      case AMK_ProtocolImplementation:
      case AMK_OptionalProtocolImplementation:
        LocalAMK = AMK;
        break;
      }
    }
 
    // Already handled.
    if (isa<UsedAttr>(I) || isa<RetainAttr>(I))
      continue;
 
    if (mergeDeclAttribute(*this, New, I, LocalAMK))
      foundAny = true;
  }
 
  if (mergeAlignedAttrs(*this, New, Old))
    foundAny = true;
 
  if (!foundAny) New->dropAttrs();
}
 
/// mergeParamDeclAttributes - Copy attributes from the old parameter
/// to the new one.
static void mergeParamDeclAttributes(ParmVarDecl *newDecl,
                                     const ParmVarDecl *oldDecl,
                                     Sema &S) {
  // C++11 [dcl.attr.depend]p2:
  //   The first declaration of a function shall specify the
  //   carries_dependency attribute for its declarator-id if any declaration
  //   of the function specifies the carries_dependency attribute.
  const CarriesDependencyAttr *CDA = newDecl->getAttr<CarriesDependencyAttr>();
  if (CDA && !oldDecl->hasAttr<CarriesDependencyAttr>()) {
    S.Diag(CDA->getLocation(),
           diag::err_carries_dependency_missing_on_first_decl) << 1/*Param*/;
    // Find the first declaration of the parameter.
    // FIXME: Should we build redeclaration chains for function parameters?
    const FunctionDecl *FirstFD =
      cast<FunctionDecl>(oldDecl->getDeclContext())->getFirstDecl();
    const ParmVarDecl *FirstVD =
      FirstFD->getParamDecl(oldDecl->getFunctionScopeIndex());
    S.Diag(FirstVD->getLocation(),
           diag::note_carries_dependency_missing_first_decl) << 1/*Param*/;
  }
 
  if (!oldDecl->hasAttrs())
    return;
 
  bool foundAny = newDecl->hasAttrs();
 
  // Ensure that any moving of objects within the allocated map is
  // done before we process them.
  if (!foundAny) newDecl->setAttrs(AttrVec());
 
  for (const auto *I : oldDecl->specific_attrs<InheritableParamAttr>()) {
    if (!DeclHasAttr(newDecl, I)) {
      InheritableAttr *newAttr =
        cast<InheritableParamAttr>(I->clone(S.Context));
      newAttr->setInherited(true);
      newDecl->addAttr(newAttr);
      foundAny = true;
    }
  }
 
  if (!foundAny) newDecl->dropAttrs();
}
 
static bool EquivalentArrayTypes(QualType Old, QualType New,
                                 const ASTContext &Ctx) {
 
  auto NoSizeInfo = [&Ctx](QualType Ty) {
    if (Ty->isIncompleteArrayType() || Ty->isPointerType())
      return true;
    if (const auto *VAT = Ctx.getAsVariableArrayType(Ty))
      return VAT->getSizeModifier() == ArrayType::ArraySizeModifier::Star;
    return false;
  };
 
  // `type[]` is equivalent to `type *` and `type[*]`.
  if (NoSizeInfo(Old) && NoSizeInfo(New))
    return true;
 
  // Don't try to compare VLA sizes, unless one of them has the star modifier.
  if (Old->isVariableArrayType() && New->isVariableArrayType()) {
    const auto *OldVAT = Ctx.getAsVariableArrayType(Old);
    const auto *NewVAT = Ctx.getAsVariableArrayType(New);
    if ((OldVAT->getSizeModifier() == ArrayType::ArraySizeModifier::Star) ^
        (NewVAT->getSizeModifier() == ArrayType::ArraySizeModifier::Star))
      return false;
    return true;
  }
 
  // Only compare size, ignore Size modifiers and CVR.
  if (Old->isConstantArrayType() && New->isConstantArrayType()) {
    return Ctx.getAsConstantArrayType(Old)->getSize() ==
           Ctx.getAsConstantArrayType(New)->getSize();
  }
 
  // Don't try to compare dependent sized array
  if (Old->isDependentSizedArrayType() && New->isDependentSizedArrayType()) {
    return true;
  }
 
  return Old == New;
}
 
static void mergeParamDeclTypes(ParmVarDecl *NewParam,
                                const ParmVarDecl *OldParam,
                                Sema &S) {
  if (auto Oldnullability = OldParam->getType()->getNullability()) {
    if (auto Newnullability = NewParam->getType()->getNullability()) {
      if (*Oldnullability != *Newnullability) {
        S.Diag(NewParam->getLocation(), diag::warn_mismatched_nullability_attr)
          << DiagNullabilityKind(
               *Newnullability,
               ((NewParam->getObjCDeclQualifier() & Decl::OBJC_TQ_CSNullability)
                != 0))
          << DiagNullabilityKind(
               *Oldnullability,
               ((OldParam->getObjCDeclQualifier() & Decl::OBJC_TQ_CSNullability)
                != 0));
        S.Diag(OldParam->getLocation(), diag::note_previous_declaration);
      }
    } else {
      QualType NewT = NewParam->getType();
      NewT = S.Context.getAttributedType(
                         AttributedType::getNullabilityAttrKind(*Oldnullability),
                         NewT, NewT);
      NewParam->setType(NewT);
    }
  }
  const auto *OldParamDT = dyn_cast<DecayedType>(OldParam->getType());
  const auto *NewParamDT = dyn_cast<DecayedType>(NewParam->getType());
  if (OldParamDT && NewParamDT &&
      OldParamDT->getPointeeType() == NewParamDT->getPointeeType()) {
    QualType OldParamOT = OldParamDT->getOriginalType();
    QualType NewParamOT = NewParamDT->getOriginalType();
    if (!EquivalentArrayTypes(OldParamOT, NewParamOT, S.getASTContext())) {
      S.Diag(NewParam->getLocation(), diag::warn_inconsistent_array_form)
          << NewParam << NewParamOT;
      S.Diag(OldParam->getLocation(), diag::note_previous_declaration_as)
          << OldParamOT;
    }
  }
}
 
namespace {
 
/// Used in MergeFunctionDecl to keep track of function parameters in
/// C.
struct GNUCompatibleParamWarning {
  ParmVarDecl *OldParm;
  ParmVarDecl *NewParm;
  QualType PromotedType;
};
 
} // end anonymous namespace
 
// Determine whether the previous declaration was a definition, implicit
// declaration, or a declaration.
template <typename T>
static std::pair<diag::kind, SourceLocation>
getNoteDiagForInvalidRedeclaration(const T *Old, const T *New) {
  diag::kind PrevDiag;
  SourceLocation OldLocation = Old->getLocation();
  if (Old->isThisDeclarationADefinition())
    PrevDiag = diag::note_previous_definition;
  else if (Old->isImplicit()) {
    PrevDiag = diag::note_previous_implicit_declaration;
    if (const auto *FD = dyn_cast<FunctionDecl>(Old)) {
      if (FD->getBuiltinID())
        PrevDiag = diag::note_previous_builtin_declaration;
    }
    if (OldLocation.isInvalid())
      OldLocation = New->getLocation();
  } else
    PrevDiag = diag::note_previous_declaration;
  return std::make_pair(PrevDiag, OldLocation);
}
 
/// canRedefineFunction - checks if a function can be redefined. Currently,
/// only extern inline functions can be redefined, and even then only in
/// GNU89 mode.
static bool canRedefineFunction(const FunctionDecl *FD,
                                const LangOptions& LangOpts) {
  return ((FD->hasAttr<GNUInlineAttr>() || LangOpts.GNUInline) &&
          !LangOpts.CPlusPlus &&
          FD->isInlineSpecified() &&
          FD->getStorageClass() == SC_Extern);
}
 
const AttributedType *Sema::getCallingConvAttributedType(QualType T) const {
  const AttributedType *AT = T->getAs<AttributedType>();
  while (AT && !AT->isCallingConv())
    AT = AT->getModifiedType()->getAs<AttributedType>();
  return AT;
}
 
template <typename T>
static bool haveIncompatibleLanguageLinkages(const T *Old, const T *New) {
  const DeclContext *DC = Old->getDeclContext();
  if (DC->isRecord())
    return false;
 
  LanguageLinkage OldLinkage = Old->getLanguageLinkage();
  if (OldLinkage == CXXLanguageLinkage && New->isInExternCContext())
    return true;
  if (OldLinkage == CLanguageLinkage && New->isInExternCXXContext())
    return true;
  return false;
}
 
template<typename T> static bool isExternC(T *D) { return D->isExternC(); }
static bool isExternC(VarTemplateDecl *) { return false; }
static bool isExternC(FunctionTemplateDecl *) { return false; }
 
/// Check whether a redeclaration of an entity introduced by a
/// using-declaration is valid, given that we know it's not an overload
/// (nor a hidden tag declaration).
template<typename ExpectedDecl>
static bool checkUsingShadowRedecl(Sema &S, UsingShadowDecl *OldS,
                                   ExpectedDecl *New) {
  // C++11 [basic.scope.declarative]p4:
  //   Given a set of declarations in a single declarative region, each of
  //   which specifies the same unqualified name,
  //   -- they shall all refer to the same entity, or all refer to functions
  //      and function templates; or
  //   -- exactly one declaration shall declare a class name or enumeration
  //      name that is not a typedef name and the other declarations shall all
  //      refer to the same variable or enumerator, or all refer to functions
  //      and function templates; in this case the class name or enumeration
  //      name is hidden (3.3.10).
 
  // C++11 [namespace.udecl]p14:
  //   If a function declaration in namespace scope or block scope has the
  //   same name and the same parameter-type-list as a function introduced
  //   by a using-declaration, and the declarations do not declare the same
  //   function, the program is ill-formed.
 
  auto *Old = dyn_cast<ExpectedDecl>(OldS->getTargetDecl());
  if (Old &&
      !Old->getDeclContext()->getRedeclContext()->Equals(
          New->getDeclContext()->getRedeclContext()) &&
      !(isExternC(Old) && isExternC(New)))
    Old = nullptr;
 
  if (!Old) {
    S.Diag(New->getLocation(), diag::err_using_decl_conflict_reverse);
    S.Diag(OldS->getTargetDecl()->getLocation(), diag::note_using_decl_target);
    S.Diag(OldS->getIntroducer()->getLocation(), diag::note_using_decl) << 0;
    return true;
  }
  return false;
}
 
static bool hasIdenticalPassObjectSizeAttrs(const FunctionDecl *A,
                                            const FunctionDecl *B) {
  assert(A->getNumParams() == B->getNumParams());
 
  auto AttrEq = [](const ParmVarDecl *A, const ParmVarDecl *B) {
    const auto *AttrA = A->getAttr<PassObjectSizeAttr>();
    const auto *AttrB = B->getAttr<PassObjectSizeAttr>();
    if (AttrA == AttrB)
      return true;
    return AttrA && AttrB && AttrA->getType() == AttrB->getType() &&
           AttrA->isDynamic() == AttrB->isDynamic();
  };
 
  return std::equal(A->param_begin(), A->param_end(), B->param_begin(), AttrEq);
}
 
/// If necessary, adjust the semantic declaration context for a qualified
/// declaration to name the correct inline namespace within the qualifier.
static void adjustDeclContextForDeclaratorDecl(DeclaratorDecl *NewD,
                                               DeclaratorDecl *OldD) {
  // The only case where we need to update the DeclContext is when
  // redeclaration lookup for a qualified name finds a declaration
  // in an inline namespace within the context named by the qualifier:
  //
  //   inline namespace N { int f(); }
  //   int ::f(); // Sema DC needs adjusting from :: to N::.
  //
  // For unqualified declarations, the semantic context *can* change
  // along the redeclaration chain (for local extern declarations,
  // extern "C" declarations, and friend declarations in particular).
  if (!NewD->getQualifier())
    return;
 
  // NewD is probably already in the right context.
  auto *NamedDC = NewD->getDeclContext()->getRedeclContext();
  auto *SemaDC = OldD->getDeclContext()->getRedeclContext();
  if (NamedDC->Equals(SemaDC))
    return;
 
  assert((NamedDC->InEnclosingNamespaceSetOf(SemaDC) ||
          NewD->isInvalidDecl() || OldD->isInvalidDecl()) &&
         "unexpected context for redeclaration");
 
  auto *LexDC = NewD->getLexicalDeclContext();
  auto FixSemaDC = [=](NamedDecl *D) {
    if (!D)
      return;
    D->setDeclContext(SemaDC);
    D->setLexicalDeclContext(LexDC);
  };
 
  FixSemaDC(NewD);
  if (auto *FD = dyn_cast<FunctionDecl>(NewD))
    FixSemaDC(FD->getDescribedFunctionTemplate());
  else if (auto *VD = dyn_cast<VarDecl>(NewD))
    FixSemaDC(VD->getDescribedVarTemplate());
}
 
/// MergeFunctionDecl - We just parsed a function 'New' from
/// declarator D which has the same name and scope as a previous
/// declaration 'Old'.  Figure out how to resolve this situation,
/// merging decls or emitting diagnostics as appropriate.
///
/// In C++, New and Old must be declarations that are not
/// overloaded. Use IsOverload to determine whether New and Old are
/// overloaded, and to select the Old declaration that New should be
/// merged with.
///
/// Returns true if there was an error, false otherwise.
bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
                             bool MergeTypeWithOld, bool NewDeclIsDefn) {
  // Verify the old decl was also a function.
  FunctionDecl *Old = OldD->getAsFunction();
  if (!Old) {
    if (UsingShadowDecl *Shadow = dyn_cast<UsingShadowDecl>(OldD)) {
      if (New->getFriendObjectKind()) {
        Diag(New->getLocation(), diag::err_using_decl_friend);
        Diag(Shadow->getTargetDecl()->getLocation(),
             diag::note_using_decl_target);
        Diag(Shadow->getIntroducer()->getLocation(), diag::note_using_decl)
            << 0;
        return true;
      }
 
      // Check whether the two declarations might declare the same function or
      // function template.
      if (FunctionTemplateDecl *NewTemplate =
              New->getDescribedFunctionTemplate()) {
        if (checkUsingShadowRedecl<FunctionTemplateDecl>(*this, Shadow,
                                                         NewTemplate))
          return true;
        OldD = Old = cast<FunctionTemplateDecl>(Shadow->getTargetDecl())
                         ->getAsFunction();
      } else {
        if (checkUsingShadowRedecl<FunctionDecl>(*this, Shadow, New))
          return true;
        OldD = Old = cast<FunctionDecl>(Shadow->getTargetDecl());
      }
    } else {
      Diag(New->getLocation(), diag::err_redefinition_different_kind)
        << New->getDeclName();
      notePreviousDefinition(OldD, New->getLocation());
      return true;
    }
  }
 
  // If the old declaration was found in an inline namespace and the new
  // declaration was qualified, update the DeclContext to match.
  adjustDeclContextForDeclaratorDecl(New, Old);
 
  // If the old declaration is invalid, just give up here.
  if (Old->isInvalidDecl())
    return true;
 
  // Disallow redeclaration of some builtins.
  if (!getASTContext().canBuiltinBeRedeclared(Old)) {
    Diag(New->getLocation(), diag::err_builtin_redeclare) << Old->getDeclName();
    Diag(Old->getLocation(), diag::note_previous_builtin_declaration)
        << Old << Old->getType();
    return true;
  }
 
  diag::kind PrevDiag;
  SourceLocation OldLocation;
  std::tie(PrevDiag, OldLocation) =
      getNoteDiagForInvalidRedeclaration(Old, New);
 
  // Don't complain about this if we're in GNU89 mode and the old function
  // is an extern inline function.
  // Don't complain about specializations. They are not supposed to have
  // storage classes.
  if (!isa<CXXMethodDecl>(New) && !isa<CXXMethodDecl>(Old) &&
      New->getStorageClass() == SC_Static &&
      Old->hasExternalFormalLinkage() &&
      !New->getTemplateSpecializationInfo() &&
      !canRedefineFunction(Old, getLangOpts())) {
    if (getLangOpts().MicrosoftExt) {
      Diag(New->getLocation(), diag::ext_static_non_static) << New;
      Diag(OldLocation, PrevDiag);
    } else {
      Diag(New->getLocation(), diag::err_static_non_static) << New;
      Diag(OldLocation, PrevDiag);
      return true;
    }
  }
 
  if (const auto *ILA = New->getAttr<InternalLinkageAttr>())
    if (!Old->hasAttr<InternalLinkageAttr>()) {
      Diag(New->getLocation(), diag::err_attribute_missing_on_first_decl)
          << ILA;
      Diag(Old->getLocation(), diag::note_previous_declaration);
      New->dropAttr<InternalLinkageAttr>();
    }
 
  if (auto *EA = New->getAttr<ErrorAttr>()) {
    if (!Old->hasAttr<ErrorAttr>()) {
      Diag(EA->getLocation(), diag::err_attribute_missing_on_first_decl) << EA;
      Diag(Old->getLocation(), diag::note_previous_declaration);
      New->dropAttr<ErrorAttr>();
    }
  }
 
  if (CheckRedeclarationInModule(New, Old))
    return true;
 
  if (!getLangOpts().CPlusPlus) {
    bool OldOvl = Old->hasAttr<OverloadableAttr>();
    if (OldOvl != New->hasAttr<OverloadableAttr>() && !Old->isImplicit()) {
      Diag(New->getLocation(), diag::err_attribute_overloadable_mismatch)
        << New << OldOvl;
 
      // Try our best to find a decl that actually has the overloadable
      // attribute for the note. In most cases (e.g. programs with only one
      // broken declaration/definition), this won't matter.
      //
      // FIXME: We could do this if we juggled some extra state in
      // OverloadableAttr, rather than just removing it.
      const Decl *DiagOld = Old;
      if (OldOvl) {
        auto OldIter = llvm::find_if(Old->redecls(), [](const Decl *D) {
          const auto *A = D->getAttr<OverloadableAttr>();
          return A && !A->isImplicit();
        });
        // If we've implicitly added *all* of the overloadable attrs to this
        // chain, emitting a "previous redecl" note is pointless.
        DiagOld = OldIter == Old->redecls_end() ? nullptr : *OldIter;
      }
 
      if (DiagOld)
        Diag(DiagOld->getLocation(),
             diag::note_attribute_overloadable_prev_overload)
          << OldOvl;
 
      if (OldOvl)
        New->addAttr(OverloadableAttr::CreateImplicit(Context));
      else
        New->dropAttr<OverloadableAttr>();
    }
  }
 
  // If a function is first declared with a calling convention, but is later
  // declared or defined without one, all following decls assume the calling
  // convention of the first.
  //
  // It's OK if a function is first declared without a calling convention,
  // but is later declared or defined with the default calling convention.
  //
  // To test if either decl has an explicit calling convention, we look for
  // AttributedType sugar nodes on the type as written.  If they are missing or
  // were canonicalized away, we assume the calling convention was implicit.
  //
  // Note also that we DO NOT return at this point, because we still have
  // other tests to run.
  QualType OldQType = Context.getCanonicalType(Old->getType());
  QualType NewQType = Context.getCanonicalType(New->getType());
  const FunctionType *OldType = cast<FunctionType>(OldQType);
  const FunctionType *NewType = cast<FunctionType>(NewQType);
  FunctionType::ExtInfo OldTypeInfo = OldType->getExtInfo();
  FunctionType::ExtInfo NewTypeInfo = NewType->getExtInfo();
  bool RequiresAdjustment = false;
 
  if (OldTypeInfo.getCC() != NewTypeInfo.getCC()) {
    FunctionDecl *First = Old->getFirstDecl();
    const FunctionType *FT =
        First->getType().getCanonicalType()->castAs<FunctionType>();
    FunctionType::ExtInfo FI = FT->getExtInfo();
    bool NewCCExplicit = getCallingConvAttributedType(New->getType());
    if (!NewCCExplicit) {
      // Inherit the CC from the previous declaration if it was specified
      // there but not here.
      NewTypeInfo = NewTypeInfo.withCallingConv(OldTypeInfo.getCC());
      RequiresAdjustment = true;
    } else if (Old->getBuiltinID()) {
      // Builtin attribute isn't propagated to the new one yet at this point,
      // so we check if the old one is a builtin.
 
      // Calling Conventions on a Builtin aren't really useful and setting a
      // default calling convention and cdecl'ing some builtin redeclarations is
      // common, so warn and ignore the calling convention on the redeclaration.
      Diag(New->getLocation(), diag::warn_cconv_unsupported)
          << FunctionType::getNameForCallConv(NewTypeInfo.getCC())
          << (int)CallingConventionIgnoredReason::BuiltinFunction;
      NewTypeInfo = NewTypeInfo.withCallingConv(OldTypeInfo.getCC());
      RequiresAdjustment = true;
    } else {
      // Calling conventions aren't compatible, so complain.
      bool FirstCCExplicit = getCallingConvAttributedType(First->getType());
      Diag(New->getLocation(), diag::err_cconv_change)
        << FunctionType::getNameForCallConv(NewTypeInfo.getCC())
        << !FirstCCExplicit
        << (!FirstCCExplicit ? "" :
            FunctionType::getNameForCallConv(FI.getCC()));
 
      // Put the note on the first decl, since it is the one that matters.
      Diag(First->getLocation(), diag::note_previous_declaration);
      return true;
    }
  }
 
  // FIXME: diagnose the other way around?
  if (OldTypeInfo.getNoReturn() && !NewTypeInfo.getNoReturn()) {
    NewTypeInfo = NewTypeInfo.withNoReturn(true);
    RequiresAdjustment = true;
  }
 
  // Merge regparm attribute.
  if (OldTypeInfo.getHasRegParm() != NewTypeInfo.getHasRegParm() ||
      OldTypeInfo.getRegParm() != NewTypeInfo.getRegParm()) {
    if (NewTypeInfo.getHasRegParm()) {
      Diag(New->getLocation(), diag::err_regparm_mismatch)
        << NewType->getRegParmType()
        << OldType->getRegParmType();
      Diag(OldLocation, diag::note_previous_declaration);
      return true;
    }
 
    NewTypeInfo = NewTypeInfo.withRegParm(OldTypeInfo.getRegParm());
    RequiresAdjustment = true;
  }
 
  // Merge ns_returns_retained attribute.
  if (OldTypeInfo.getProducesResult() != NewTypeInfo.getProducesResult()) {
    if (NewTypeInfo.getProducesResult()) {
      Diag(New->getLocation(), diag::err_function_attribute_mismatch)
          << "'ns_returns_retained'";
      Diag(OldLocation, diag::note_previous_declaration);
      return true;
    }
 
    NewTypeInfo = NewTypeInfo.withProducesResult(true);
    RequiresAdjustment = true;
  }
 
  if (OldTypeInfo.getNoCallerSavedRegs() !=
      NewTypeInfo.getNoCallerSavedRegs()) {
    if (NewTypeInfo.getNoCallerSavedRegs()) {
      AnyX86NoCallerSavedRegistersAttr *Attr =
        New->getAttr<AnyX86NoCallerSavedRegistersAttr>();
      Diag(New->getLocation(), diag::err_function_attribute_mismatch) << Attr;
      Diag(OldLocation, diag::note_previous_declaration);
      return true;
    }
 
    NewTypeInfo = NewTypeInfo.withNoCallerSavedRegs(true);
    RequiresAdjustment = true;
  }
 
  if (RequiresAdjustment) {
    const FunctionType *AdjustedType = New->getType()->getAs<FunctionType>();
    AdjustedType = Context.adjustFunctionType(AdjustedType, NewTypeInfo);
    New->setType(QualType(AdjustedType, 0));
    NewQType = Context.getCanonicalType(New->getType());
  }
 
  // If this redeclaration makes the function inline, we may need to add it to
  // UndefinedButUsed.
  if (!Old->isInlined() && New->isInlined() &&
      !New->hasAttr<GNUInlineAttr>() &&
      !getLangOpts().GNUInline &&
      Old->isUsed(false) &&
      !Old->isDefined() && !New->isThisDeclarationADefinition())
    UndefinedButUsed.insert(std::make_pair(Old->getCanonicalDecl(),
                                           SourceLocation()));
 
  // If this redeclaration makes it newly gnu_inline, we don't want to warn
  // about it.
  if (New->hasAttr<GNUInlineAttr>() &&
      Old->isInlined() && !Old->hasAttr<GNUInlineAttr>()) {
    UndefinedButUsed.erase(Old->getCanonicalDecl());
  }
 
  // If pass_object_size params don't match up perfectly, this isn't a valid
  // redeclaration.
  if (Old->getNumParams() > 0 && Old->getNumParams() == New->getNumParams() &&
      !hasIdenticalPassObjectSizeAttrs(Old, New)) {
    Diag(New->getLocation(), diag::err_different_pass_object_size_params)
        << New->getDeclName();
    Diag(OldLocation, PrevDiag) << Old << Old->getType();
    return true;
  }
 
  if (getLangOpts().CPlusPlus) {
    // C++1z [over.load]p2
    //   Certain function declarations cannot be overloaded:
    //     -- Function declarations that differ only in the return type,
    //        the exception specification, or both cannot be overloaded.
 
    // Check the exception specifications match. This may recompute the type of
    // both Old and New if it resolved exception specifications, so grab the
    // types again after this. Because this updates the type, we do this before
    // any of the other checks below, which may update the "de facto" NewQType
    // but do not necessarily update the type of New.
    if (CheckEquivalentExceptionSpec(Old, New))
      return true;
    OldQType = Context.getCanonicalType(Old->getType());
    NewQType = Context.getCanonicalType(New->getType());
 
    // Go back to the type source info to compare the declared return types,
    // per C++1y [dcl.type.auto]p13:
    //   Redeclarations or specializations of a function or function template
    //   with a declared return type that uses a placeholder type shall also
    //   use that placeholder, not a deduced type.
    QualType OldDeclaredReturnType = Old->getDeclaredReturnType();
    QualType NewDeclaredReturnType = New->getDeclaredReturnType();
    if (!Context.hasSameType(OldDeclaredReturnType, NewDeclaredReturnType) &&
        canFullyTypeCheckRedeclaration(New, Old, NewDeclaredReturnType,
                                       OldDeclaredReturnType)) {
      QualType ResQT;
      if (NewDeclaredReturnType->isObjCObjectPointerType() &&
          OldDeclaredReturnType->isObjCObjectPointerType())
        // FIXME: This does the wrong thing for a deduced return type.
        ResQT = Context.mergeObjCGCQualifiers(NewQType, OldQType);
      if (ResQT.isNull()) {
        if (New->isCXXClassMember() && New->isOutOfLine())
          Diag(New->getLocation(), diag::err_member_def_does_not_match_ret_type)
              << New << New->getReturnTypeSourceRange();
        else
          Diag(New->getLocation(), diag::err_ovl_diff_return_type)
              << New->getReturnTypeSourceRange();
        Diag(OldLocation, PrevDiag) << Old << Old->getType()
                                    << Old->getReturnTypeSourceRange();
        return true;
      }
      else
        NewQType = ResQT;
    }
 
    QualType OldReturnType = OldType->getReturnType();
    QualType NewReturnType = cast<FunctionType>(NewQType)->getReturnType();
    if (OldReturnType != NewReturnType) {
      // If this function has a deduced return type and has already been
      // defined, copy the deduced value from the old declaration.
      AutoType *OldAT = Old->getReturnType()->getContainedAutoType();
      if (OldAT && OldAT->isDeduced()) {
        QualType DT = OldAT->getDeducedType();
        if (DT.isNull()) {
          New->setType(SubstAutoTypeDependent(New->getType()));
          NewQType = Context.getCanonicalType(SubstAutoTypeDependent(NewQType));
        } else {
          New->setType(SubstAutoType(New->getType(), DT));
          NewQType = Context.getCanonicalType(SubstAutoType(NewQType, DT));
        }
      }
    }
 
    const CXXMethodDecl *OldMethod = dyn_cast<CXXMethodDecl>(Old);
    CXXMethodDecl *NewMethod = dyn_cast<CXXMethodDecl>(New);
    if (OldMethod && NewMethod) {
      // Preserve triviality.
      NewMethod->setTrivial(OldMethod->isTrivial());
 
      // MSVC allows explicit template specialization at class scope:
      // 2 CXXMethodDecls referring to the same function will be injected.
      // We don't want a redeclaration error.
      bool IsClassScopeExplicitSpecialization =
                              OldMethod->isFunctionTemplateSpecialization() &&
                              NewMethod->isFunctionTemplateSpecialization();
      bool isFriend = NewMethod->getFriendObjectKind();
 
      if (!isFriend && NewMethod->getLexicalDeclContext()->isRecord() &&
          !IsClassScopeExplicitSpecialization) {
        //    -- Member function declarations with the same name and the
        //       same parameter types cannot be overloaded if any of them
        //       is a static member function declaration.
        if (OldMethod->isStatic() != NewMethod->isStatic()) {
          Diag(New->getLocation(), diag::err_ovl_static_nonstatic_member);
          Diag(OldLocation, PrevDiag) << Old << Old->getType();
          return true;
        }
 
        // C++ [class.mem]p1:
        //   [...] A member shall not be declared twice in the
        //   member-specification, except that a nested class or member
        //   class template can be declared and then later defined.
        if (!inTemplateInstantiation()) {
          unsigned NewDiag;
          if (isa<CXXConstructorDecl>(OldMethod))
            NewDiag = diag::err_constructor_redeclared;
          else if (isa<CXXDestructorDecl>(NewMethod))
            NewDiag = diag::err_destructor_redeclared;
          else if (isa<CXXConversionDecl>(NewMethod))
            NewDiag = diag::err_conv_function_redeclared;
          else
            NewDiag = diag::err_member_redeclared;
 
          Diag(New->getLocation(), NewDiag);
        } else {
          Diag(New->getLocation(), diag::err_member_redeclared_in_instantiation)
            << New << New->getType();
        }
        Diag(OldLocation, PrevDiag) << Old << Old->getType();
        return true;
 
      // Complain if this is an explicit declaration of a special
      // member that was initially declared implicitly.
      //
      // As an exception, it's okay to befriend such methods in order
      // to permit the implicit constructor/destructor/operator calls.
      } else if (OldMethod->isImplicit()) {
        if (isFriend) {
          NewMethod->setImplicit();
        } else {
          Diag(NewMethod->getLocation(),
               diag::err_definition_of_implicitly_declared_member)
            << New << getSpecialMember(OldMethod);
          return true;
        }
      } else if (OldMethod->getFirstDecl()->isExplicitlyDefaulted() && !isFriend) {
        Diag(NewMethod->getLocation(),
             diag::err_definition_of_explicitly_defaulted_member)
          << getSpecialMember(OldMethod);
        return true;
      }
    }
 
    // C++11 [dcl.attr.noreturn]p1:
    //   The first declaration of a function shall specify the noreturn
    //   attribute if any declaration of that function specifies the noreturn
    //   attribute.
    if (const auto *NRA = New->getAttr<CXX11NoReturnAttr>())
      if (!Old->hasAttr<CXX11NoReturnAttr>()) {
        Diag(NRA->getLocation(), diag::err_attribute_missing_on_first_decl)
            << NRA;
        Diag(Old->getLocation(), diag::note_previous_declaration);
      }
 
    // C++11 [dcl.attr.depend]p2:
    //   The first declaration of a function shall specify the
    //   carries_dependency attribute for its declarator-id if any declaration
    //   of the function specifies the carries_dependency attribute.
    const CarriesDependencyAttr *CDA = New->getAttr<CarriesDependencyAttr>();
    if (CDA && !Old->hasAttr<CarriesDependencyAttr>()) {
      Diag(CDA->getLocation(),
           diag::err_carries_dependency_missing_on_first_decl) << 0/*Function*/;
      Diag(Old->getFirstDecl()->getLocation(),
           diag::note_carries_dependency_missing_first_decl) << 0/*Function*/;
    }
 
    // (C++98 8.3.5p3):
    //   All declarations for a function shall agree exactly in both the
    //   return type and the parameter-type-list.
    // We also want to respect all the extended bits except noreturn.
 
    // noreturn should now match unless the old type info didn't have it.
    QualType OldQTypeForComparison = OldQType;
    if (!OldTypeInfo.getNoReturn() && NewTypeInfo.getNoReturn()) {
      auto *OldType = OldQType->castAs<FunctionProtoType>();
      const FunctionType *OldTypeForComparison
        = Context.adjustFunctionType(OldType, OldTypeInfo.withNoReturn(true));
      OldQTypeForComparison = QualType(OldTypeForComparison, 0);
      assert(OldQTypeForComparison.isCanonical());
    }
 
    if (haveIncompatibleLanguageLinkages(Old, New)) {
      // As a special case, retain the language linkage from previous
      // declarations of a friend function as an extension.
      //
      // This liberal interpretation of C++ [class.friend]p3 matches GCC/MSVC
      // and is useful because there's otherwise no way to specify language
      // linkage within class scope.
      //
      // Check cautiously as the friend object kind isn't yet complete.
      if (New->getFriendObjectKind() != Decl::FOK_None) {
        Diag(New->getLocation(), diag::ext_retained_language_linkage) << New;
        Diag(OldLocation, PrevDiag);
      } else {
        Diag(New->getLocation(), diag::err_different_language_linkage) << New;
        Diag(OldLocation, PrevDiag);
        return true;
      }
    }
 
    // If the function types are compatible, merge the declarations. Ignore the
    // exception specifier because it was already checked above in
    // CheckEquivalentExceptionSpec, and we don't want follow-on diagnostics
    // about incompatible types under -fms-compatibility.
    if (Context.hasSameFunctionTypeIgnoringExceptionSpec(OldQTypeForComparison,
                                                         NewQType))
      return MergeCompatibleFunctionDecls(New, Old, S, MergeTypeWithOld);
 
    // If the types are imprecise (due to dependent constructs in friends or
    // local extern declarations), it's OK if they differ. We'll check again
    // during instantiation.
    if (!canFullyTypeCheckRedeclaration(New, Old, NewQType, OldQType))
      return false;
 
    // Fall through for conflicting redeclarations and redefinitions.
  }
 
  // C: Function types need to be compatible, not identical. This handles
  // duplicate function decls like "void f(int); void f(enum X);" properly.
  if (!getLangOpts().CPlusPlus) {
    // C99 6.7.5.3p15: ...If one type has a parameter type list and the other
    // type is specified by a function definition that contains a (possibly
    // empty) identifier list, both shall agree in the number of parameters
    // and the type of each parameter shall be compatible with the type that
    // results from the application of default argument promotions to the
    // type of the corresponding identifier. ...
    // This cannot be handled by ASTContext::typesAreCompatible() because that
    // doesn't know whether the function type is for a definition or not when
    // eventually calling ASTContext::mergeFunctionTypes(). The only situation
    // we need to cover here is that the number of arguments agree as the
    // default argument promotion rules were already checked by
    // ASTContext::typesAreCompatible().
    if (Old->hasPrototype() && !New->hasWrittenPrototype() && NewDeclIsDefn &&
        Old->getNumParams() != New->getNumParams() && !Old->isImplicit()) {
      if (Old->hasInheritedPrototype())
        Old = Old->getCanonicalDecl();
      Diag(New->getLocation(), diag::err_conflicting_types) << New;
      Diag(Old->getLocation(), PrevDiag) << Old << Old->getType();
      return true;
    }
 
    // If we are merging two functions where only one of them has a prototype,
    // we may have enough information to decide to issue a diagnostic that the
    // function without a protoype will change behavior in C2x. This handles
    // cases like:
    //   void i(); void i(int j);
    //   void i(int j); void i();
    //   void i(); void i(int j) {}
    // See ActOnFinishFunctionBody() for other cases of the behavior change
    // diagnostic. See GetFullTypeForDeclarator() for handling of a function
    // type without a prototype.
    if (New->hasWrittenPrototype() != Old->hasWrittenPrototype() &&
        !New->isImplicit() && !Old->isImplicit()) {
      const FunctionDecl *WithProto, *WithoutProto;
      if (New->hasWrittenPrototype()) {
        WithProto = New;
        WithoutProto = Old;
      } else {
        WithProto = Old;
        WithoutProto = New;
      }
 
      if (WithProto->getNumParams() != 0) {
        if (WithoutProto->getBuiltinID() == 0 && !WithoutProto->isImplicit()) {
          // The one without the prototype will be changing behavior in C2x, so
          // warn about that one so long as it's a user-visible declaration.
          bool IsWithoutProtoADef = false, IsWithProtoADef = false;
          if (WithoutProto == New)
            IsWithoutProtoADef = NewDeclIsDefn;
          else
            IsWithProtoADef = NewDeclIsDefn;
          Diag(WithoutProto->getLocation(),
               diag::warn_non_prototype_changes_behavior)
              << IsWithoutProtoADef << (WithoutProto->getNumParams() ? 0 : 1)
              << (WithoutProto == Old) << IsWithProtoADef;
 
          // The reason the one without the prototype will be changing behavior
          // is because of the one with the prototype, so note that so long as
          // it's a user-visible declaration. There is one exception to this:
          // when the new declaration is a definition without a prototype, the
          // old declaration with a prototype is not the cause of the issue,
          // and that does not need to be noted because the one with a
          // prototype will not change behavior in C2x.
          if (WithProto->getBuiltinID() == 0 && !WithProto->isImplicit() &&
              !IsWithoutProtoADef)
            Diag(WithProto->getLocation(), diag::note_conflicting_prototype);
        }
      }
    }
 
    if (Context.typesAreCompatible(OldQType, NewQType)) {
      const FunctionType *OldFuncType = OldQType->getAs<FunctionType>();
      const FunctionType *NewFuncType = NewQType->getAs<FunctionType>();
      const FunctionProtoType *OldProto = nullptr;
      if (MergeTypeWithOld && isa<FunctionNoProtoType>(NewFuncType) &&
          (OldProto = dyn_cast<FunctionProtoType>(OldFuncType))) {
        // The old declaration provided a function prototype, but the
        // new declaration does not. Merge in the prototype.
        assert(!OldProto->hasExceptionSpec() && "Exception spec in C");
        NewQType = Context.getFunctionType(NewFuncType->getReturnType(),
                                           OldProto->getParamTypes(),
                                           OldProto->getExtProtoInfo());
        New->setType(NewQType);
        New->setHasInheritedPrototype();
 
        // Synthesize parameters with the same types.
        SmallVector<ParmVarDecl *, 16> Params;
        for (const auto &ParamType : OldProto->param_types()) {
          ParmVarDecl *Param = ParmVarDecl::Create(
              Context, New, SourceLocation(), SourceLocation(), nullptr,
              ParamType, /*TInfo=*/nullptr, SC_None, nullptr);
          Param->setScopeInfo(0, Params.size());
          Param->setImplicit();
          Params.push_back(Param);
        }
 
        New->setParams(Params);
      }
 
      return MergeCompatibleFunctionDecls(New, Old, S, MergeTypeWithOld);
    }
  }
 
  // Check if the function types are compatible when pointer size address
  // spaces are ignored.
  if (Context.hasSameFunctionTypeIgnoringPtrSizes(OldQType, NewQType))
    return false;
 
  // GNU C permits a K&R definition to follow a prototype declaration
  // if the declared types of the parameters in the K&R definition
  // match the types in the prototype declaration, even when the
  // promoted types of the parameters from the K&R definition differ
  // from the types in the prototype. GCC then keeps the types from
  // the prototype.
  //
  // If a variadic prototype is followed by a non-variadic K&R definition,
  // the K&R definition becomes variadic.  This is sort of an edge case, but
  // it's legal per the standard depending on how you read C99 6.7.5.3p15 and
  // C99 6.9.1p8.
  if (!getLangOpts().CPlusPlus &&
      Old->hasPrototype() && !New->hasPrototype() &&
      New->getType()->getAs<FunctionProtoType>() &&
      Old->getNumParams() == New->getNumParams()) {
    SmallVector<QualType, 16> ArgTypes;
    SmallVector<GNUCompatibleParamWarning, 16> Warnings;
    const FunctionProtoType *OldProto
      = Old->getType()->getAs<FunctionProtoType>();
    const FunctionProtoType *NewProto
      = New->getType()->getAs<FunctionProtoType>();
 
    // Determine whether this is the GNU C extension.
    QualType MergedReturn = Context.mergeTypes(OldProto->getReturnType(),
                                               NewProto->getReturnType());
    bool LooseCompatible = !MergedReturn.isNull();
    for (unsigned Idx = 0, End = Old->getNumParams();
         LooseCompatible && Idx != End; ++Idx) {
      ParmVarDecl *OldParm = Old->getParamDecl(Idx);
      ParmVarDecl *NewParm = New->getParamDecl(Idx);
      if (Context.typesAreCompatible(OldParm->getType(),
                                     NewProto->getParamType(Idx))) {
        ArgTypes.push_back(NewParm->getType());
      } else if (Context.typesAreCompatible(OldParm->getType(),
                                            NewParm->getType(),
                                            /*CompareUnqualified=*/true)) {
        GNUCompatibleParamWarning Warn = { OldParm, NewParm,
                                           NewProto->getParamType(Idx) };
        Warnings.push_back(Warn);
        ArgTypes.push_back(NewParm->getType());
      } else
        LooseCompatible = false;
    }
 
    if (LooseCompatible) {
      for (unsigned Warn = 0; Warn < Warnings.size(); ++Warn) {
        Diag(Warnings[Warn].NewParm->getLocation(),
             diag::ext_param_promoted_not_compatible_with_prototype)
          << Warnings[Warn].PromotedType
          << Warnings[Warn].OldParm->getType();
        if (Warnings[Warn].OldParm->getLocation().isValid())
          Diag(Warnings[Warn].OldParm->getLocation(),
               diag::note_previous_declaration);
      }
 
      if (MergeTypeWithOld)
        New->setType(Context.getFunctionType(MergedReturn, ArgTypes,
                                             OldProto->getExtProtoInfo()));
      return MergeCompatibleFunctionDecls(New, Old, S, MergeTypeWithOld);
    }
 
    // Fall through to diagnose conflicting types.
  }
 
  // A function that has already been declared has been redeclared or
  // defined with a different type; show an appropriate diagnostic.
 
  // If the previous declaration was an implicitly-generated builtin
  // declaration, then at the very least we should use a specialized note.
  unsigned BuiltinID;
  if (Old->isImplicit() && (BuiltinID = Old->getBuiltinID())) {
    // If it's actually a library-defined builtin function like 'malloc'
    // or 'printf', just warn about the incompatible redeclaration.
    if (Context.BuiltinInfo.isPredefinedLibFunction(BuiltinID)) {
      Diag(New->getLocation(), diag::warn_redecl_library_builtin) << New;
      Diag(OldLocation, diag::note_previous_builtin_declaration)
        << Old << Old->getType();
      return false;
    }
 
    PrevDiag = diag::note_previous_builtin_declaration;
  }
 
  Diag(New->getLocation(), diag::err_conflicting_types) << New->getDeclName();
  Diag(OldLocation, PrevDiag) << Old << Old->getType();
  return true;
}
 
/// Completes the merge of two function declarations that are
/// known to be compatible.
///
/// This routine handles the merging of attributes and other
/// properties of function declarations from the old declaration to
/// the new declaration, once we know that New is in fact a
/// redeclaration of Old.
///
/// \returns false
bool Sema::MergeCompatibleFunctionDecls(FunctionDecl *New, FunctionDecl *Old,
                                        Scope *S, bool MergeTypeWithOld) {
  // Merge the attributes
  mergeDeclAttributes(New, Old);
 
  // Merge "pure" flag.
  if (Old->isPure())
    New->setPure();
 
  // Merge "used" flag.
  if (Old->getMostRecentDecl()->isUsed(false))
    New->setIsUsed();
 
  // Merge attributes from the parameters.  These can mismatch with K&R
  // declarations.
  if (New->getNumParams() == Old->getNumParams())
      for (unsigned i = 0, e = New->getNumParams(); i != e; ++i) {
        ParmVarDecl *NewParam = New->getParamDecl(i);
        ParmVarDecl *OldParam = Old->getParamDecl(i);
        mergeParamDeclAttributes(NewParam, OldParam, *this);
        mergeParamDeclTypes(NewParam, OldParam, *this);
      }
 
  if (getLangOpts().CPlusPlus)
    return MergeCXXFunctionDecl(New, Old, S);
 
  // Merge the function types so the we get the composite types for the return
  // and argument types. Per C11 6.2.7/4, only update the type if the old decl
  // was visible.
  QualType Merged = Context.mergeTypes(Old->getType(), New->getType());
  if (!Merged.isNull() && MergeTypeWithOld)
    New->setType(Merged);
 
  return false;
}
 
void Sema::mergeObjCMethodDecls(ObjCMethodDecl *newMethod,
                                ObjCMethodDecl *oldMethod) {
  // Merge the attributes, including deprecated/unavailable
  AvailabilityMergeKind MergeKind =
      isa<ObjCProtocolDecl>(oldMethod->getDeclContext())
          ? (oldMethod->isOptional() ? AMK_OptionalProtocolImplementation
                                     : AMK_ProtocolImplementation)
          : isa<ObjCImplDecl>(newMethod->getDeclContext()) ? AMK_Redeclaration
                                                           : AMK_Override;
 
  mergeDeclAttributes(newMethod, oldMethod, MergeKind);
 
  // Merge attributes from the parameters.
  ObjCMethodDecl::param_const_iterator oi = oldMethod->param_begin(),
                                       oe = oldMethod->param_end();
  for (ObjCMethodDecl::param_iterator
         ni = newMethod->param_begin(), ne = newMethod->param_end();
       ni != ne && oi != oe; ++ni, ++oi)
    mergeParamDeclAttributes(*ni, *oi, *this);
 
  CheckObjCMethodOverride(newMethod, oldMethod);
}
 
static void diagnoseVarDeclTypeMismatch(Sema &S, VarDecl *New, VarDecl* Old) {
  assert(!S.Context.hasSameType(New->getType(), Old->getType()));
 
  S.Diag(New->getLocation(), New->isThisDeclarationADefinition()
         ? diag::err_redefinition_different_type
         : diag::err_redeclaration_different_type)
    << New->getDeclName() << New->getType() << Old->getType();
 
  diag::kind PrevDiag;
  SourceLocation OldLocation;
  std::tie(PrevDiag, OldLocation)
    = getNoteDiagForInvalidRedeclaration(Old, New);
  S.Diag(OldLocation, PrevDiag);
  New->setInvalidDecl();
}
 
/// MergeVarDeclTypes - We parsed a variable 'New' which has the same name and
/// scope as a previous declaration 'Old'.  Figure out how to merge their types,
/// emitting diagnostics as appropriate.
///
/// Declarations using the auto type specifier (C++ [decl.spec.auto]) call back
/// to here in AddInitializerToDecl. We can't check them before the initializer
/// is attached.
void Sema::MergeVarDeclTypes(VarDecl *New, VarDecl *Old,
                             bool MergeTypeWithOld) {
  if (New->isInvalidDecl() || Old->isInvalidDecl())
    return;
 
  QualType MergedT;
  if (getLangOpts().CPlusPlus) {
    if (New->getType()->isUndeducedType()) {
      // We don't know what the new type is until the initializer is attached.
      return;
    } else if (Context.hasSameType(New->getType(), Old->getType())) {
      // These could still be something that needs exception specs checked.
      return MergeVarDeclExceptionSpecs(New, Old);
    }
    // C++ [basic.link]p10:
    //   [...] the types specified by all declarations referring to a given
    //   object or function shall be identical, except that declarations for an
    //   array object can specify array types that differ by the presence or
    //   absence of a major array bound (8.3.4).
    else if (Old->getType()->isArrayType() && New->getType()->isArrayType()) {
      const ArrayType *OldArray = Context.getAsArrayType(Old->getType());
      const ArrayType *NewArray = Context.getAsArrayType(New->getType());
 
      // We are merging a variable declaration New into Old. If it has an array
      // bound, and that bound differs from Old's bound, we should diagnose the
      // mismatch.
      if (!NewArray->isIncompleteArrayType() && !NewArray->isDependentType()) {
        for (VarDecl *PrevVD = Old->getMostRecentDecl(); PrevVD;
             PrevVD = PrevVD->getPreviousDecl()) {
          QualType PrevVDTy = PrevVD->getType();
          if (PrevVDTy->isIncompleteArrayType() || PrevVDTy->isDependentType())
            continue;
 
          if (!Context.hasSameType(New->getType(), PrevVDTy))
            return diagnoseVarDeclTypeMismatch(*this, New, PrevVD);
        }
      }
 
      if (OldArray->isIncompleteArrayType() && NewArray->isArrayType()) {
        if (Context.hasSameType(OldArray->getElementType(),
                                NewArray->getElementType()))
          MergedT = New->getType();
      }
      // FIXME: Check visibility. New is hidden but has a complete type. If New
      // has no array bound, it should not inherit one from Old, if Old is not
      // visible.
      else if (OldArray->isArrayType() && NewArray->isIncompleteArrayType()) {
        if (Context.hasSameType(OldArray->getElementType(),
                                NewArray->getElementType()))
          MergedT = Old->getType();
      }
    }
    else if (New->getType()->isObjCObjectPointerType() &&
               Old->getType()->isObjCObjectPointerType()) {
      MergedT = Context.mergeObjCGCQualifiers(New->getType(),
                                              Old->getType());
    }
  } else {
    // C 6.2.7p2:
    //   All declarations that refer to the same object or function shall have
    //   compatible type.
    MergedT = Context.mergeTypes(New->getType(), Old->getType());
  }
  if (MergedT.isNull()) {
    // It's OK if we couldn't merge types if either type is dependent, for a
    // block-scope variable. In other cases (static data members of class
    // templates, variable templates, ...), we require the types to be
    // equivalent.
    // FIXME: The C++ standard doesn't say anything about this.
    if ((New->getType()->isDependentType() ||
         Old->getType()->isDependentType()) && New->isLocalVarDecl()) {
      // If the old type was dependent, we can't merge with it, so the new type
      // becomes dependent for now. We'll reproduce the original type when we
      // instantiate the TypeSourceInfo for the variable.
      if (!New->getType()->isDependentType() && MergeTypeWithOld)
        New->setType(Context.DependentTy);
      return;
    }
    return diagnoseVarDeclTypeMismatch(*this, New, Old);
  }
 
  // Don't actually update the type on the new declaration if the old
  // declaration was an extern declaration in a different scope.
  if (MergeTypeWithOld)
    New->setType(MergedT);
}
 
static bool mergeTypeWithPrevious(Sema &S, VarDecl *NewVD, VarDecl *OldVD,
                                  LookupResult &Previous) {
  // C11 6.2.7p4:
  //   For an identifier with internal or external linkage declared
  //   in a scope in which a prior declaration of that identifier is
  //   visible, if the prior declaration specifies internal or
  //   external linkage, the type of the identifier at the later
  //   declaration becomes the composite type.
  //
  // If the variable isn't visible, we do not merge with its type.
  if (Previous.isShadowed())
    return false;
 
  if (S.getLangOpts().CPlusPlus) {
    // C++11 [dcl.array]p3:
    //   If there is a preceding declaration of the entity in the same
    //   scope in which the bound was specified, an omitted array bound
    //   is taken to be the same as in that earlier declaration.
    return NewVD->isPreviousDeclInSameBlockScope() ||
           (!OldVD->getLexicalDeclContext()->isFunctionOrMethod() &&
            !NewVD->getLexicalDeclContext()->isFunctionOrMethod());
  } else {
    // If the old declaration was function-local, don't merge with its
    // type unless we're in the same function.
    return !OldVD->getLexicalDeclContext()->isFunctionOrMethod() ||
           OldVD->getLexicalDeclContext() == NewVD->getLexicalDeclContext();
  }
}
 
/// MergeVarDecl - We just parsed a variable 'New' which has the same name
/// and scope as a previous declaration 'Old'.  Figure out how to resolve this
/// situation, merging decls or emitting diagnostics as appropriate.
///
/// Tentative definition rules (C99 6.9.2p2) are checked by
/// FinalizeDeclaratorGroup. Unfortunately, we can't analyze tentative
/// definitions here, since the initializer hasn't been attached.
///
void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
  // If the new decl is already invalid, don't do any other checking.
  if (New->isInvalidDecl())
    return;
 
  if (!shouldLinkPossiblyHiddenDecl(Previous, New))
    return;
 
  VarTemplateDecl *NewTemplate = New->getDescribedVarTemplate();
 
  // Verify the old decl was also a variable or variable template.
  VarDecl *Old = nullptr;
  VarTemplateDecl *OldTemplate = nullptr;
  if (Previous.isSingleResult()) {
    if (NewTemplate) {
      OldTemplate = dyn_cast<VarTemplateDecl>(Previous.getFoundDecl());
      Old = OldTemplate ? OldTemplate->getTemplatedDecl() : nullptr;
 
      if (auto *Shadow =
              dyn_cast<UsingShadowDecl>(Previous.getRepresentativeDecl()))
        if (checkUsingShadowRedecl<VarTemplateDecl>(*this, Shadow, NewTemplate))
          return New->setInvalidDecl();
    } else {
      Old = dyn_cast<VarDecl>(Previous.getFoundDecl());
 
      if (auto *Shadow =
              dyn_cast<UsingShadowDecl>(Previous.getRepresentativeDecl()))
        if (checkUsingShadowRedecl<VarDecl>(*this, Shadow, New))
          return New->setInvalidDecl();
    }
  }
  if (!Old) {
    Diag(New->getLocation(), diag::err_redefinition_different_kind)
        << New->getDeclName();
    notePreviousDefinition(Previous.getRepresentativeDecl(),
                           New->getLocation());
    return New->setInvalidDecl();
  }
 
  // If the old declaration was found in an inline namespace and the new
  // declaration was qualified, update the DeclContext to match.
  adjustDeclContextForDeclaratorDecl(New, Old);
 
  // Ensure the template parameters are compatible.
  if (NewTemplate &&
      !TemplateParameterListsAreEqual(NewTemplate->getTemplateParameters(),
                                      OldTemplate->getTemplateParameters(),
                                      /*Complain=*/true, TPL_TemplateMatch))
    return New->setInvalidDecl();
 
  // C++ [class.mem]p1:
  //   A member shall not be declared twice in the member-specification [...]
  //
  // Here, we need only consider static data members.
  if (Old->isStaticDataMember() && !New->isOutOfLine()) {
    Diag(New->getLocation(), diag::err_duplicate_member)
      << New->getIdentifier();
    Diag(Old->getLocation(), diag::note_previous_declaration);
    New->setInvalidDecl();
  }
 
  mergeDeclAttributes(New, Old);
  // Warn if an already-declared variable is made a weak_import in a subsequent
  // declaration
  if (New->hasAttr<WeakImportAttr>() &&
      Old->getStorageClass() == SC_None &&
      !Old->hasAttr<WeakImportAttr>()) {
    Diag(New->getLocation(), diag::warn_weak_import) << New->getDeclName();
    Diag(Old->getLocation(), diag::note_previous_declaration);
    // Remove weak_import attribute on new declaration.
    New->dropAttr<WeakImportAttr>();
  }
 
  if (const auto *ILA = New->getAttr<InternalLinkageAttr>())
    if (!Old->hasAttr<InternalLinkageAttr>()) {
      Diag(New->getLocation(), diag::err_attribute_missing_on_first_decl)
          << ILA;
      Diag(Old->getLocation(), diag::note_previous_declaration);
      New->dropAttr<InternalLinkageAttr>();
    }
 
  // Merge the types.
  VarDecl *MostRecent = Old->getMostRecentDecl();
  if (MostRecent != Old) {
    MergeVarDeclTypes(New, MostRecent,
                      mergeTypeWithPrevious(*this, New, MostRecent, Previous));
    if (New->isInvalidDecl())
      return;
  }
 
  MergeVarDeclTypes(New, Old, mergeTypeWithPrevious(*this, New, Old, Previous));
  if (New->isInvalidDecl())
    return;
 
  diag::kind PrevDiag;
  SourceLocation OldLocation;
  std::tie(PrevDiag, OldLocation) =
      getNoteDiagForInvalidRedeclaration(Old, New);
 
  // [dcl.stc]p8: Check if we have a non-static decl followed by a static.
  if (New->getStorageClass() == SC_Static &&
      !New->isStaticDataMember() &&
      Old->hasExternalFormalLinkage()) {
    if (getLangOpts().MicrosoftExt) {
      Diag(New->getLocation(), diag::ext_static_non_static)
          << New->getDeclName();
      Diag(OldLocation, PrevDiag);
    } else {
      Diag(New->getLocation(), diag::err_static_non_static)
          << New->getDeclName();
      Diag(OldLocation, PrevDiag);
      return New->setInvalidDecl();
    }
  }
  // C99 6.2.2p4:
  //   For an identifier declared with the storage-class specifier
  //   extern in a scope in which a prior declaration of that
  //   identifier is visible,23) if the prior declaration specifies
  //   internal or external linkage, the linkage of the identifier at
  //   the later declaration is the same as the linkage specified at
  //   the prior declaration. If no prior declaration is visible, or
  //   if the prior declaration specifies no linkage, then the
  //   identifier has external linkage.
  if (New->hasExternalStorage() && Old->hasLinkage())
    /* Okay */;
  else if (New->getCanonicalDecl()->getStorageClass() != SC_Static &&
           !New->isStaticDataMember() &&
           Old->getCanonicalDecl()->getStorageClass() == SC_Static) {
    Diag(New->getLocation(), diag::err_non_static_static) << New->getDeclName();
    Diag(OldLocation, PrevDiag);
    return New->setInvalidDecl();
  }
 
  // Check if extern is followed by non-extern and vice-versa.
  if (New->hasExternalStorage() &&
      !Old->hasLinkage() && Old->isLocalVarDeclOrParm()) {
    Diag(New->getLocation(), diag::err_extern_non_extern) << New->getDeclName();
    Diag(OldLocation, PrevDiag);
    return New->setInvalidDecl();
  }
  if (Old->hasLinkage() && New->isLocalVarDeclOrParm() &&
      !New->hasExternalStorage()) {
    Diag(New->getLocation(), diag::err_non_extern_extern) << New->getDeclName();
    Diag(OldLocation, PrevDiag);
    return New->setInvalidDecl();
  }
 
  if (CheckRedeclarationInModule(New, Old))
    return;
 
  // Variables with external linkage are analyzed in FinalizeDeclaratorGroup.
 
  // FIXME: The test for external storage here seems wrong? We still
  // need to check for mismatches.
  if (!New->hasExternalStorage() && !New->isFileVarDecl() &&
      // Don't complain about out-of-line definitions of static members.
      !(Old->getLexicalDeclContext()->isRecord() &&
        !New->getLexicalDeclContext()->isRecord())) {
    Diag(New->getLocation(), diag::err_redefinition) << New->getDeclName();
    Diag(OldLocation, PrevDiag);
    return New->setInvalidDecl();
  }
 
  if (New->isInline() && !Old->getMostRecentDecl()->isInline()) {
    if (VarDecl *Def = Old->getDefinition()) {
      // C++1z [dcl.fcn.spec]p4:
      //   If the definition of a variable appears in a translation unit before
      //   its first declaration as inline, the program is ill-formed.
      Diag(New->getLocation(), diag::err_inline_decl_follows_def) << New;
      Diag(Def->getLocation(), diag::note_previous_definition);
    }
  }
 
  // If this redeclaration makes the variable inline, we may need to add it to
  // UndefinedButUsed.
  if (!Old->isInline() && New->isInline() && Old->isUsed(false) &&
      !Old->getDefinition() && !New->isThisDeclarationADefinition())
    UndefinedButUsed.insert(std::make_pair(Old->getCanonicalDecl(),
                                           SourceLocation()));
 
  if (New->getTLSKind() != Old->getTLSKind()) {
    if (!Old->getTLSKind()) {
      Diag(New->getLocation(), diag::err_thread_non_thread) << New->getDeclName();
      Diag(OldLocation, PrevDiag);
    } else if (!New->getTLSKind()) {
      Diag(New->getLocation(), diag::err_non_thread_thread) << New->getDeclName();
      Diag(OldLocation, PrevDiag);
    } else {
      // Do not allow redeclaration to change the variable between requiring
      // static and dynamic initialization.
      // FIXME: GCC allows this, but uses the TLS keyword on the first
      // declaration to determine the kind. Do we need to be compatible here?
      Diag(New->getLocation(), diag::err_thread_thread_different_kind)
        << New->getDeclName() << (New->getTLSKind() == VarDecl::TLS_Dynamic);
      Diag(OldLocation, PrevDiag);
    }
  }
 
  // C++ doesn't have tentative definitions, so go right ahead and check here.
  if (getLangOpts().CPlusPlus) {
    if (Old->isStaticDataMember() && Old->getCanonicalDecl()->isInline() &&
        Old->getCanonicalDecl()->isConstexpr()) {
      // This definition won't be a definition any more once it's been merged.
      Diag(New->getLocation(),
           diag::warn_deprecated_redundant_constexpr_static_def);
    } else if (New->isThisDeclarationADefinition() == VarDecl::Definition) {
      VarDecl *Def = Old->getDefinition();
      if (Def && checkVarDeclRedefinition(Def, New))
        return;
    }
  }
 
  if (haveIncompatibleLanguageLinkages(Old, New)) {
    Diag(New->getLocation(), diag::err_different_language_linkage) << New;
    Diag(OldLocation, PrevDiag);
    New->setInvalidDecl();
    return;
  }
 
  // Merge "used" flag.
  if (Old->getMostRecentDecl()->isUsed(false))
    New->setIsUsed();
 
  // Keep a chain of previous declarations.
  New->setPreviousDecl(Old);
  if (NewTemplate)
    NewTemplate->setPreviousDecl(OldTemplate);
 
  // Inherit access appropriately.
  New->setAccess(Old->getAccess());
  if (NewTemplate)
    NewTemplate->setAccess(New->getAccess());
 
  if (Old->isInline())
    New->setImplicitlyInline();
}
 
void Sema::notePreviousDefinition(const NamedDecl *Old, SourceLocation New) {
  SourceManager &SrcMgr = getSourceManager();
  auto FNewDecLoc = SrcMgr.getDecomposedLoc(New);
  auto FOldDecLoc = SrcMgr.getDecomposedLoc(Old->getLocation());
  auto *FNew = SrcMgr.getFileEntryForID(FNewDecLoc.first);
  auto *FOld = SrcMgr.getFileEntryForID(FOldDecLoc.first);
  auto &HSI = PP.getHeaderSearchInfo();
  StringRef HdrFilename =
      SrcMgr.getFilename(SrcMgr.getSpellingLoc(Old->getLocation()));
 
  auto noteFromModuleOrInclude = [&](Module *Mod,
                                     SourceLocation IncLoc) -> bool {
    // Redefinition errors with modules are common with non modular mapped
    // headers, example: a non-modular header H in module A that also gets
    // included directly in a TU. Pointing twice to the same header/definition
    // is confusing, try to get better diagnostics when modules is on.
    if (IncLoc.isValid()) {
      if (Mod) {
        Diag(IncLoc, diag::note_redefinition_modules_same_file)
            << HdrFilename.str() << Mod->getFullModuleName();
        if (!Mod->DefinitionLoc.isInvalid())
          Diag(Mod->DefinitionLoc, diag::note_defined_here)
              << Mod->getFullModuleName();
      } else {
        Diag(IncLoc, diag::note_redefinition_include_same_file)
            << HdrFilename.str();
      }
      return true;
    }
 
    return false;
  };
 
  // Is it the same file and same offset? Provide more information on why
  // this leads to a redefinition error.
  if (FNew == FOld && FNewDecLoc.second == FOldDecLoc.second) {
    SourceLocation OldIncLoc = SrcMgr.getIncludeLoc(FOldDecLoc.first);
    SourceLocation NewIncLoc = SrcMgr.getIncludeLoc(FNewDecLoc.first);
    bool EmittedDiag =
        noteFromModuleOrInclude(Old->getOwningModule(), OldIncLoc);
    EmittedDiag |= noteFromModuleOrInclude(getCurrentModule(), NewIncLoc);
 
    // If the header has no guards, emit a note suggesting one.
    if (FOld && !HSI.isFileMultipleIncludeGuarded(FOld))
      Diag(Old->getLocation(), diag::note_use_ifdef_guards);
 
    if (EmittedDiag)
      return;
  }
 
  // Redefinition coming from different files or couldn't do better above.
  if (Old->getLocation().isValid())
    Diag(Old->getLocation(), diag::note_previous_definition);
}
 
/// We've just determined that \p Old and \p New both appear to be definitions
/// of the same variable. Either diagnose or fix the problem.
bool Sema::checkVarDeclRedefinition(VarDecl *Old, VarDecl *New) {
  if (!hasVisibleDefinition(Old) &&
      (New->getFormalLinkage() == InternalLinkage ||
       New->isInline() ||
       isa<VarTemplateSpecializationDecl>(New) ||
       New->getDescribedVarTemplate() ||
       New->getNumTemplateParameterLists() ||
       New->getDeclContext()->isDependentContext())) {
    // The previous definition is hidden, and multiple definitions are
    // permitted (in separate TUs). Demote this to a declaration.
    New->demoteThisDefinitionToDeclaration();
 
    // Make the canonical definition visible.
    if (auto *OldTD = Old->getDescribedVarTemplate())
      makeMergedDefinitionVisible(OldTD);
    makeMergedDefinitionVisible(Old);
    return false;
  } else {
    Diag(New->getLocation(), diag::err_redefinition) << New;
    notePreviousDefinition(Old, New->getLocation());
    New->setInvalidDecl();
    return true;
  }
}
 
/// ParsedFreeStandingDeclSpec - This method is invoked when a declspec with
/// no declarator (e.g. "struct foo;") is parsed.
Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
                                       DeclSpec &DS,
                                       const ParsedAttributesView &DeclAttrs,
                                       RecordDecl *&AnonRecord) {
  return ParsedFreeStandingDeclSpec(
      S, AS, DS, DeclAttrs, MultiTemplateParamsArg(), false, AnonRecord);
}
 
// The MS ABI changed between VS2013 and VS2015 with regard to numbers used to
// disambiguate entities defined in different scopes.
// While the VS2015 ABI fixes potential miscompiles, it is also breaks
// compatibility.
// We will pick our mangling number depending on which version of MSVC is being
// targeted.
static unsigned getMSManglingNumber(const LangOptions &LO, Scope *S) {
  return LO.isCompatibleWithMSVC(LangOptions::MSVC2015)
             ? S->getMSCurManglingNumber()
             : S->getMSLastManglingNumber();
}
 
void Sema::handleTagNumbering(const TagDecl *Tag, Scope *TagScope) {
  if (!Context.getLangOpts().CPlusPlus)
    return;
 
  if (isa<CXXRecordDecl>(Tag->getParent())) {
    // If this tag is the direct child of a class, number it if
    // it is anonymous.
    if (!Tag->getName().empty() || Tag->getTypedefNameForAnonDecl())
      return;
    MangleNumberingContext &MCtx =
        Context.getManglingNumberContext(Tag->getParent());
    Context.setManglingNumber(
        Tag, MCtx.getManglingNumber(
                 Tag, getMSManglingNumber(getLangOpts(), TagScope)));
    return;
  }
 
  // If this tag isn't a direct child of a class, number it if it is local.
  MangleNumberingContext *MCtx;
  Decl *ManglingContextDecl;
  std::tie(MCtx, ManglingContextDecl) =
      getCurrentMangleNumberContext(Tag->getDeclContext());
  if (MCtx) {
    Context.setManglingNumber(
        Tag, MCtx->getManglingNumber(
                 Tag, getMSManglingNumber(getLangOpts(), TagScope)));
  }
}
 
namespace {
struct NonCLikeKind {
  enum {
    None,
    BaseClass,
    DefaultMemberInit,
    Lambda,
    Friend,
    OtherMember,
    Invalid,
  } Kind = None;
  SourceRange Range;
 
  explicit operator bool() { return Kind != None; }
};
}
 
/// Determine whether a class is C-like, according to the rules of C++
/// [dcl.typedef] for anonymous classes with typedef names for linkage.
static NonCLikeKind getNonCLikeKindForAnonymousStruct(const CXXRecordDecl *RD) {
  if (RD->isInvalidDecl())
    return {NonCLikeKind::Invalid, {}};
 
  // C++ [dcl.typedef]p9: [P1766R1]
  //   An unnamed class with a typedef name for linkage purposes shall not
  //
  //    -- have any base classes
  if (RD->getNumBases())
    return {NonCLikeKind::BaseClass,
            SourceRange(RD->bases_begin()->getBeginLoc(),
                        RD->bases_end()[-1].getEndLoc())};
  bool Invalid = false;
  for (Decl *D : RD->decls()) {
    // Don't complain about things we already diagnosed.
    if (D->isInvalidDecl()) {
      Invalid = true;
      continue;
    }
 
    //  -- have any [...] default member initializers
    if (auto *FD = dyn_cast<FieldDecl>(D)) {
      if (FD->hasInClassInitializer()) {
        auto *Init = FD->getInClassInitializer();
        return {NonCLikeKind::DefaultMemberInit,
                Init ? Init->getSourceRange() : D->getSourceRange()};
      }
      continue;
    }
 
    // FIXME: We don't allow friend declarations. This violates the wording of
    // P1766, but not the intent.
    if (isa<FriendDecl>(D))
      return {NonCLikeKind::Friend, D->getSourceRange()};
 
    //  -- declare any members other than non-static data members, member
    //     enumerations, or member classes,
    if (isa<StaticAssertDecl>(D) || isa<IndirectFieldDecl>(D) ||
        isa<EnumDecl>(D))
      continue;
    auto *MemberRD = dyn_cast<CXXRecordDecl>(D);
    if (!MemberRD) {
      if (D->isImplicit())
        continue;
      return {NonCLikeKind::OtherMember, D->getSourceRange()};
    }
 
    //  -- contain a lambda-expression,
    if (MemberRD->isLambda())
      return {NonCLikeKind::Lambda, MemberRD->getSourceRange()};
 
    //  and all member classes shall also satisfy these requirements
    //  (recursively).
    if (MemberRD->isThisDeclarationADefinition()) {
      if (auto Kind = getNonCLikeKindForAnonymousStruct(MemberRD))
        return Kind;
    }
  }
 
  return {Invalid ? NonCLikeKind::Invalid : NonCLikeKind::None, {}};
}
 
void Sema::setTagNameForLinkagePurposes(TagDecl *TagFromDeclSpec,
                                        TypedefNameDecl *NewTD) {
  if (TagFromDeclSpec->isInvalidDecl())
    return;
 
  // Do nothing if the tag already has a name for linkage purposes.
  if (TagFromDeclSpec->hasNameForLinkage())
    return;
 
  // A well-formed anonymous tag must always be a TUK_Definition.
  assert(TagFromDeclSpec->isThisDeclarationADefinition());
 
  // The type must match the tag exactly;  no qualifiers allowed.
  if (!Context.hasSameType(NewTD->getUnderlyingType(),
                           Context.getTagDeclType(TagFromDeclSpec))) {
    if (getLangOpts().CPlusPlus)
      Context.addTypedefNameForUnnamedTagDecl(TagFromDeclSpec, NewTD);
    return;
  }
 
  // C++ [dcl.typedef]p9: [P1766R1, applied as DR]
  //   An unnamed class with a typedef name for linkage purposes shall [be
  //   C-like].
  //
  // FIXME: Also diagnose if we