SemaTemplate.cpp

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//===------- SemaTemplate.cpp - Semantic Analysis for C++ Templates -------===//
//
// 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 C++ templates.
//===----------------------------------------------------------------------===//
 
#include "TreeTransform.h"
#include "clang/AST/ASTConsumer.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Decl.h"
#include "clang/AST/DeclFriend.h"
#include "clang/AST/DeclTemplate.h"
#include "clang/AST/DynamicRecursiveASTVisitor.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/TemplateName.h"
#include "clang/AST/TypeVisitor.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/DiagnosticSema.h"
#include "clang/Basic/LangOptions.h"
#include "clang/Basic/PartialDiagnostic.h"
#include "clang/Basic/SourceLocation.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/DeclSpec.h"
#include "clang/Sema/EnterExpressionEvaluationContext.h"
#include "clang/Sema/Initialization.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/Overload.h"
#include "clang/Sema/ParsedTemplate.h"
#include "clang/Sema/Scope.h"
#include "clang/Sema/SemaCUDA.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "clang/Sema/TemplateDeduction.h"
#include "llvm/ADT/SmallBitVector.h"
#include "llvm/ADT/StringExtras.h"
 
#include <optional>
using namespace clang;
using namespace sema;
 
// Exported for use by Parser.
SourceRange
clang::getTemplateParamsRange(TemplateParameterList const * const *Ps,
                              unsigned N) {
  if (!N) return SourceRange();
  return SourceRange(Ps[0]->getTemplateLoc(), Ps[N-1]->getRAngleLoc());
}
 
unsigned Sema::getTemplateDepth(Scope *S) const {
  unsigned Depth = 0;
 
  // Each template parameter scope represents one level of template parameter
  // depth.
  for (Scope *TempParamScope = S->getTemplateParamParent(); TempParamScope;
       TempParamScope = TempParamScope->getParent()->getTemplateParamParent()) {
    ++Depth;
  }
 
  // Note that there are template parameters with the given depth.
  auto ParamsAtDepth = [&](unsigned D) { Depth = std::max(Depth, D + 1); };
 
  // Look for parameters of an enclosing generic lambda. We don't create a
  // template parameter scope for these.
  for (FunctionScopeInfo *FSI : getFunctionScopes()) {
    if (auto *LSI = dyn_cast<LambdaScopeInfo>(FSI)) {
      if (!LSI->TemplateParams.empty()) {
        ParamsAtDepth(LSI->AutoTemplateParameterDepth);
        break;
      }
      if (LSI->GLTemplateParameterList) {
        ParamsAtDepth(LSI->GLTemplateParameterList->getDepth());
        break;
      }
    }
  }
 
  // Look for parameters of an enclosing terse function template. We don't
  // create a template parameter scope for these either.
  for (const InventedTemplateParameterInfo &Info :
       getInventedParameterInfos()) {
    if (!Info.TemplateParams.empty()) {
      ParamsAtDepth(Info.AutoTemplateParameterDepth);
      break;
    }
  }
 
  return Depth;
}
 
/// \brief Determine whether the declaration found is acceptable as the name
/// of a template and, if so, return that template declaration. Otherwise,
/// returns null.
///
/// Note that this may return an UnresolvedUsingValueDecl if AllowDependent
/// is true. In all other cases it will return a TemplateDecl (or null).
NamedDecl *Sema::getAsTemplateNameDecl(NamedDecl *D,
                                       bool AllowFunctionTemplates,
                                       bool AllowDependent) {
  D = D->getUnderlyingDecl();
 
  if (isa<TemplateDecl>(D)) {
    if (!AllowFunctionTemplates && isa<FunctionTemplateDecl>(D))
      return nullptr;
 
    return D;
  }
 
  if (const auto *Record = dyn_cast<CXXRecordDecl>(D)) {
    // C++ [temp.local]p1:
    //   Like normal (non-template) classes, class templates have an
    //   injected-class-name (Clause 9). The injected-class-name
    //   can be used with or without a template-argument-list. When
    //   it is used without a template-argument-list, it is
    //   equivalent to the injected-class-name followed by the
    //   template-parameters of the class template enclosed in
    //   <>. When it is used with a template-argument-list, it
    //   refers to the specified class template specialization,
    //   which could be the current specialization or another
    //   specialization.
    if (Record->isInjectedClassName()) {
      Record = cast<CXXRecordDecl>(Record->getDeclContext());
      if (Record->getDescribedClassTemplate())
        return Record->getDescribedClassTemplate();
 
      if (const auto *Spec = dyn_cast<ClassTemplateSpecializationDecl>(Record))
        return Spec->getSpecializedTemplate();
    }
 
    return nullptr;
  }
 
  // 'using Dependent::foo;' can resolve to a template name.
  // 'using typename Dependent::foo;' cannot (not even if 'foo' is an
  // injected-class-name).
  if (AllowDependent && isa<UnresolvedUsingValueDecl>(D))
    return D;
 
  return nullptr;
}
 
void Sema::FilterAcceptableTemplateNames(LookupResult &R,
                                         bool AllowFunctionTemplates,
                                         bool AllowDependent) {
  LookupResult::Filter filter = R.makeFilter();
  while (filter.hasNext()) {
    NamedDecl *Orig = filter.next();
    if (!getAsTemplateNameDecl(Orig, AllowFunctionTemplates, AllowDependent))
      filter.erase();
  }
  filter.done();
}
 
bool Sema::hasAnyAcceptableTemplateNames(LookupResult &R,
                                         bool AllowFunctionTemplates,
                                         bool AllowDependent,
                                         bool AllowNonTemplateFunctions) {
  for (LookupResult::iterator I = R.begin(), IEnd = R.end(); I != IEnd; ++I) {
    if (getAsTemplateNameDecl(*I, AllowFunctionTemplates, AllowDependent))
      return true;
    if (AllowNonTemplateFunctions &&
        isa<FunctionDecl>((*I)->getUnderlyingDecl()))
      return true;
  }
 
  return false;
}
 
TemplateNameKind Sema::isTemplateName(Scope *S,
                                      CXXScopeSpec &SS,
                                      bool hasTemplateKeyword,
                                      const UnqualifiedId &Name,
                                      ParsedType ObjectTypePtr,
                                      bool EnteringContext,
                                      TemplateTy &TemplateResult,
                                      bool &MemberOfUnknownSpecialization,
                                      bool Disambiguation) {
  assert(getLangOpts().CPlusPlus && "No template names in C!");
 
  DeclarationName TName;
  MemberOfUnknownSpecialization = false;
 
  switch (Name.getKind()) {
  case UnqualifiedIdKind::IK_Identifier:
    TName = DeclarationName(Name.Identifier);
    break;
 
  case UnqualifiedIdKind::IK_OperatorFunctionId:
    TName = Context.DeclarationNames.getCXXOperatorName(
                                              Name.OperatorFunctionId.Operator);
    break;
 
  case UnqualifiedIdKind::IK_LiteralOperatorId:
    TName = Context.DeclarationNames.getCXXLiteralOperatorName(Name.Identifier);
    break;
 
  default:
    return TNK_Non_template;
  }
 
  QualType ObjectType = ObjectTypePtr.get();
 
  AssumedTemplateKind AssumedTemplate;
  LookupResult R(*this, TName, Name.getBeginLoc(), LookupOrdinaryName);
  if (LookupTemplateName(R, S, SS, ObjectType, EnteringContext,
                         /*RequiredTemplate=*/SourceLocation(),
                         &AssumedTemplate,
                         /*AllowTypoCorrection=*/!Disambiguation))
    return TNK_Non_template;
  MemberOfUnknownSpecialization = R.wasNotFoundInCurrentInstantiation();
 
  if (AssumedTemplate != AssumedTemplateKind::None) {
    TemplateResult = TemplateTy::make(Context.getAssumedTemplateName(TName));
    // Let the parser know whether we found nothing or found functions; if we
    // found nothing, we want to more carefully check whether this is actually
    // a function template name versus some other kind of undeclared identifier.
    return AssumedTemplate == AssumedTemplateKind::FoundNothing
               ? TNK_Undeclared_template
               : TNK_Function_template;
  }
 
  if (R.empty())
    return TNK_Non_template;
 
  NamedDecl *D = nullptr;
  UsingShadowDecl *FoundUsingShadow = dyn_cast<UsingShadowDecl>(*R.begin());
  if (R.isAmbiguous()) {
    // If we got an ambiguity involving a non-function template, treat this
    // as a template name, and pick an arbitrary template for error recovery.
    bool AnyFunctionTemplates = false;
    for (NamedDecl *FoundD : R) {
      if (NamedDecl *FoundTemplate = getAsTemplateNameDecl(FoundD)) {
        if (isa<FunctionTemplateDecl>(FoundTemplate))
          AnyFunctionTemplates = true;
        else {
          D = FoundTemplate;
          FoundUsingShadow = dyn_cast<UsingShadowDecl>(FoundD);
          break;
        }
      }
    }
 
    // If we didn't find any templates at all, this isn't a template name.
    // Leave the ambiguity for a later lookup to diagnose.
    if (!D && !AnyFunctionTemplates) {
      R.suppressDiagnostics();
      return TNK_Non_template;
    }
 
    // If the only templates were function templates, filter out the rest.
    // We'll diagnose the ambiguity later.
    if (!D)
      FilterAcceptableTemplateNames(R);
  }
 
  // At this point, we have either picked a single template name declaration D
  // or we have a non-empty set of results R containing either one template name
  // declaration or a set of function templates.
 
  TemplateName Template;
  TemplateNameKind TemplateKind;
 
  unsigned ResultCount = R.end() - R.begin();
  if (!D && ResultCount > 1) {
    // We assume that we'll preserve the qualifier from a function
    // template name in other ways.
    Template = Context.getOverloadedTemplateName(R.begin(), R.end());
    TemplateKind = TNK_Function_template;
 
    // We'll do this lookup again later.
    R.suppressDiagnostics();
  } else {
    if (!D) {
      D = getAsTemplateNameDecl(*R.begin());
      assert(D && "unambiguous result is not a template name");
    }
 
    if (isa<UnresolvedUsingValueDecl>(D)) {
      // We don't yet know whether this is a template-name or not.
      MemberOfUnknownSpecialization = true;
      return TNK_Non_template;
    }
 
    TemplateDecl *TD = cast<TemplateDecl>(D);
    Template =
        FoundUsingShadow ? TemplateName(FoundUsingShadow) : TemplateName(TD);
    assert(!FoundUsingShadow || FoundUsingShadow->getTargetDecl() == TD);
    if (!SS.isInvalid()) {
      NestedNameSpecifier *Qualifier = SS.getScopeRep();
      Template = Context.getQualifiedTemplateName(Qualifier, hasTemplateKeyword,
                                                  Template);
    }
 
    if (isa<FunctionTemplateDecl>(TD)) {
      TemplateKind = TNK_Function_template;
 
      // We'll do this lookup again later.
      R.suppressDiagnostics();
    } else {
      assert(isa<ClassTemplateDecl>(TD) || isa<TemplateTemplateParmDecl>(TD) ||
             isa<TypeAliasTemplateDecl>(TD) || isa<VarTemplateDecl>(TD) ||
             isa<BuiltinTemplateDecl>(TD) || isa<ConceptDecl>(TD));
      TemplateKind =
          isa<VarTemplateDecl>(TD) ? TNK_Var_template :
          isa<ConceptDecl>(TD) ? TNK_Concept_template :
          TNK_Type_template;
    }
  }
 
  TemplateResult = TemplateTy::make(Template);
  return TemplateKind;
}
 
bool Sema::isDeductionGuideName(Scope *S, const IdentifierInfo &Name,
                                SourceLocation NameLoc, CXXScopeSpec &SS,
                                ParsedTemplateTy *Template /*=nullptr*/) {
  // We could use redeclaration lookup here, but we don't need to: the
  // syntactic form of a deduction guide is enough to identify it even
  // if we can't look up the template name at all.
  LookupResult R(*this, DeclarationName(&Name), NameLoc, LookupOrdinaryName);
  if (LookupTemplateName(R, S, SS, /*ObjectType*/ QualType(),
                         /*EnteringContext*/ false))
    return false;
 
  if (R.empty()) return false;
  if (R.isAmbiguous()) {
    // FIXME: Diagnose an ambiguity if we find at least one template.
    R.suppressDiagnostics();
    return false;
  }
 
  // We only treat template-names that name type templates as valid deduction
  // guide names.
  TemplateDecl *TD = R.getAsSingle<TemplateDecl>();
  if (!TD || !getAsTypeTemplateDecl(TD))
    return false;
 
  if (Template) {
    TemplateName Name = Context.getQualifiedTemplateName(
        SS.getScopeRep(), /*TemplateKeyword=*/false, TemplateName(TD));
    *Template = TemplateTy::make(Name);
  }
  return true;
}
 
bool Sema::DiagnoseUnknownTemplateName(const IdentifierInfo &II,
                                       SourceLocation IILoc,
                                       Scope *S,
                                       const CXXScopeSpec *SS,
                                       TemplateTy &SuggestedTemplate,
                                       TemplateNameKind &SuggestedKind) {
  // We can't recover unless there's a dependent scope specifier preceding the
  // template name.
  // FIXME: Typo correction?
  if (!SS || !SS->isSet() || !isDependentScopeSpecifier(*SS) ||
      computeDeclContext(*SS))
    return false;
 
  // The code is missing a 'template' keyword prior to the dependent template
  // name.
  NestedNameSpecifier *Qualifier = (NestedNameSpecifier*)SS->getScopeRep();
  Diag(IILoc, diag::err_template_kw_missing)
    << Qualifier << II.getName()
    << FixItHint::CreateInsertion(IILoc, "template ");
  SuggestedTemplate
    = TemplateTy::make(Context.getDependentTemplateName(Qualifier, &II));
  SuggestedKind = TNK_Dependent_template_name;
  return true;
}
 
bool Sema::LookupTemplateName(LookupResult &Found, Scope *S, CXXScopeSpec &SS,
                              QualType ObjectType, bool EnteringContext,
                              RequiredTemplateKind RequiredTemplate,
                              AssumedTemplateKind *ATK,
                              bool AllowTypoCorrection) {
  if (ATK)
    *ATK = AssumedTemplateKind::None;
 
  if (SS.isInvalid())
    return true;
 
  Found.setTemplateNameLookup(true);
 
  // Determine where to perform name lookup
  DeclContext *LookupCtx = nullptr;
  bool IsDependent = false;
  if (!ObjectType.isNull()) {
    // This nested-name-specifier occurs in a member access expression, e.g.,
    // x->B::f, and we are looking into the type of the object.
    assert(SS.isEmpty() && "ObjectType and scope specifier cannot coexist");
    LookupCtx = computeDeclContext(ObjectType);
    IsDependent = !LookupCtx && ObjectType->isDependentType();
    assert((IsDependent || !ObjectType->isIncompleteType() ||
            !ObjectType->getAs<TagType>() ||
            ObjectType->castAs<TagType>()->isBeingDefined()) &&
           "Caller should have completed object type");
 
    // Template names cannot appear inside an Objective-C class or object type
    // or a vector type.
    //
    // FIXME: This is wrong. For example:
    //
    //   template<typename T> using Vec = T __attribute__((ext_vector_type(4)));
    //   Vec<int> vi;
    //   vi.Vec<int>::~Vec<int>();
    //
    // ... should be accepted but we will not treat 'Vec' as a template name
    // here. The right thing to do would be to check if the name is a valid
    // vector component name, and look up a template name if not. And similarly
    // for lookups into Objective-C class and object types, where the same
    // problem can arise.
    if (ObjectType->isObjCObjectOrInterfaceType() ||
        ObjectType->isVectorType()) {
      Found.clear();
      return false;
    }
  } else if (SS.isNotEmpty()) {
    // This nested-name-specifier occurs after another nested-name-specifier,
    // so long into the context associated with the prior nested-name-specifier.
    LookupCtx = computeDeclContext(SS, EnteringContext);
    IsDependent = !LookupCtx && isDependentScopeSpecifier(SS);
 
    // The declaration context must be complete.
    if (LookupCtx && RequireCompleteDeclContext(SS, LookupCtx))
      return true;
  }
 
  bool ObjectTypeSearchedInScope = false;
  bool AllowFunctionTemplatesInLookup = true;
  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(Found, LookupCtx);
 
    // FIXME: The C++ standard does not clearly specify what happens in the
    // case where the object type is dependent, and implementations vary. In
    // Clang, we treat a name after a . or -> as a template-name if lookup
    // finds a non-dependent member or member of the current instantiation that
    // is a type template, or finds no such members and lookup in the context
    // of the postfix-expression finds a type template. In the latter case, the
    // name is nonetheless dependent, and we may resolve it to a member of an
    // unknown specialization when we come to instantiate the template.
    IsDependent |= Found.wasNotFoundInCurrentInstantiation();
  }
 
  if (SS.isEmpty() && (ObjectType.isNull() || Found.empty())) {
    // C++ [basic.lookup.classref]p1:
    //   In a class member access expression (5.2.5), if the . or -> token is
    //   immediately followed by an identifier followed by a <, the
    //   identifier must be looked up to determine whether the < is the
    //   beginning of a template argument list (14.2) or a less-than operator.
    //   The identifier is first looked up in the class of the object
    //   expression. If the identifier is not found, it is then looked up in
    //   the context of the entire postfix-expression and shall name a class
    //   template.
    if (S)
      LookupName(Found, S);
 
    if (!ObjectType.isNull()) {
      //  FIXME: We should filter out all non-type templates here, particularly
      //  variable templates and concepts. But the exclusion of alias templates
      //  and template template parameters is a wording defect.
      AllowFunctionTemplatesInLookup = false;
      ObjectTypeSearchedInScope = true;
    }
 
    IsDependent |= Found.wasNotFoundInCurrentInstantiation();
  }
 
  if (Found.isAmbiguous())
    return false;
 
  if (ATK && SS.isEmpty() && ObjectType.isNull() &&
      !RequiredTemplate.hasTemplateKeyword()) {
    // 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.
    //
    // To keep our behavior consistent, we apply the "finds nothing" part in
    // all language modes, and diagnose the empty lookup in ActOnCallExpr if we
    // successfully form a call to an undeclared template-id.
    bool AllFunctions =
        getLangOpts().CPlusPlus20 && llvm::all_of(Found, [](NamedDecl *ND) {
          return isa<FunctionDecl>(ND->getUnderlyingDecl());
        });
    if (AllFunctions || (Found.empty() && !IsDependent)) {
      // If lookup found any functions, or if this is a name that can only be
      // used for a function, then strongly assume this is a function
      // template-id.
      *ATK = (Found.empty() && Found.getLookupName().isIdentifier())
                 ? AssumedTemplateKind::FoundNothing
                 : AssumedTemplateKind::FoundFunctions;
      Found.clear();
      return false;
    }
  }
 
  if (Found.empty() && !IsDependent && AllowTypoCorrection) {
    // If we did not find any names, and this is not a disambiguation, attempt
    // to correct any typos.
    DeclarationName Name = Found.getLookupName();
    Found.clear();
    // Simple filter callback that, for keywords, only accepts the C++ *_cast
    DefaultFilterCCC FilterCCC{};
    FilterCCC.WantTypeSpecifiers = false;
    FilterCCC.WantExpressionKeywords = false;
    FilterCCC.WantRemainingKeywords = false;
    FilterCCC.WantCXXNamedCasts = true;
    if (TypoCorrection Corrected =
            CorrectTypo(Found.getLookupNameInfo(), Found.getLookupKind(), S,
                        &SS, FilterCCC, CTK_ErrorRecovery, LookupCtx)) {
      if (auto *ND = Corrected.getFoundDecl())
        Found.addDecl(ND);
      FilterAcceptableTemplateNames(Found);
      if (Found.isAmbiguous()) {
        Found.clear();
      } else if (!Found.empty()) {
        Found.setLookupName(Corrected.getCorrection());
        if (LookupCtx) {
          std::string CorrectedStr(Corrected.getAsString(getLangOpts()));
          bool DroppedSpecifier = Corrected.WillReplaceSpecifier() &&
                                  Name.getAsString() == CorrectedStr;
          diagnoseTypo(Corrected, PDiag(diag::err_no_member_template_suggest)
                                    << Name << LookupCtx << DroppedSpecifier
                                    << SS.getRange());
        } else {
          diagnoseTypo(Corrected, PDiag(diag::err_no_template_suggest) << Name);
        }
      }
    }
  }
 
  NamedDecl *ExampleLookupResult =
      Found.empty() ? nullptr : Found.getRepresentativeDecl();
  FilterAcceptableTemplateNames(Found, AllowFunctionTemplatesInLookup);
  if (Found.empty()) {
    if (IsDependent) {
      Found.setNotFoundInCurrentInstantiation();
      return false;
    }
 
    // If a 'template' keyword was used, a lookup that finds only non-template
    // names is an error.
    if (ExampleLookupResult && RequiredTemplate) {
      Diag(Found.getNameLoc(), diag::err_template_kw_refers_to_non_template)
          << Found.getLookupName() << SS.getRange()
          << RequiredTemplate.hasTemplateKeyword()
          << RequiredTemplate.getTemplateKeywordLoc();
      Diag(ExampleLookupResult->getUnderlyingDecl()->getLocation(),
           diag::note_template_kw_refers_to_non_template)
          << Found.getLookupName();
      return true;
    }
 
    return false;
  }
 
  if (S && !ObjectType.isNull() && !ObjectTypeSearchedInScope &&
      !getLangOpts().CPlusPlus11) {
    // C++03 [basic.lookup.classref]p1:
    //   [...] If the lookup in the class of the object expression finds a
    //   template, the name is also looked up in the context of the entire
    //   postfix-expression and [...]
    //
    // Note: C++11 does not perform this second lookup.
    LookupResult FoundOuter(*this, Found.getLookupName(), Found.getNameLoc(),
                            LookupOrdinaryName);
    FoundOuter.setTemplateNameLookup(true);
    LookupName(FoundOuter, S);
    // FIXME: We silently accept an ambiguous lookup here, in violation of
    // [basic.lookup]/1.
    FilterAcceptableTemplateNames(FoundOuter, /*AllowFunctionTemplates=*/false);
 
    NamedDecl *OuterTemplate;
    if (FoundOuter.empty()) {
      //   - if the name is not found, the name found in the class of the
      //     object expression is used, otherwise
    } else if (FoundOuter.isAmbiguous() || !FoundOuter.isSingleResult() ||
               !(OuterTemplate =
                     getAsTemplateNameDecl(FoundOuter.getFoundDecl()))) {
      //   - if the name is found in the context of the entire
      //     postfix-expression and does not name a class template, the name
      //     found in the class of the object expression is used, otherwise
      FoundOuter.clear();
    } else if (!Found.isSuppressingAmbiguousDiagnostics()) {
      //   - if the name found is a class template, it must refer to the same
      //     entity as the one found in the class of the object expression,
      //     otherwise the program is ill-formed.
      if (!Found.isSingleResult() ||
          getAsTemplateNameDecl(Found.getFoundDecl())->getCanonicalDecl() !=
              OuterTemplate->getCanonicalDecl()) {
        Diag(Found.getNameLoc(),
             diag::ext_nested_name_member_ref_lookup_ambiguous)
          << Found.getLookupName()
          << ObjectType;
        Diag(Found.getRepresentativeDecl()->getLocation(),
             diag::note_ambig_member_ref_object_type)
          << ObjectType;
        Diag(FoundOuter.getFoundDecl()->getLocation(),
             diag::note_ambig_member_ref_scope);
 
        // Recover by taking the template that we found in the object
        // expression's type.
      }
    }
  }
 
  return false;
}
 
void Sema::diagnoseExprIntendedAsTemplateName(Scope *S, ExprResult TemplateName,
                                              SourceLocation Less,
                                              SourceLocation Greater) {
  if (TemplateName.isInvalid())
    return;
 
  DeclarationNameInfo NameInfo;
  CXXScopeSpec SS;
  LookupNameKind LookupKind;
 
  DeclContext *LookupCtx = nullptr;
  NamedDecl *Found = nullptr;
  bool MissingTemplateKeyword = false;
 
  // Figure out what name we looked up.
  if (auto *DRE = dyn_cast<DeclRefExpr>(TemplateName.get())) {
    NameInfo = DRE->getNameInfo();
    SS.Adopt(DRE->getQualifierLoc());
    LookupKind = LookupOrdinaryName;
    Found = DRE->getFoundDecl();
  } else if (auto *ME = dyn_cast<MemberExpr>(TemplateName.get())) {
    NameInfo = ME->getMemberNameInfo();
    SS.Adopt(ME->getQualifierLoc());
    LookupKind = LookupMemberName;
    LookupCtx = ME->getBase()->getType()->getAsCXXRecordDecl();
    Found = ME->getMemberDecl();
  } else if (auto *DSDRE =
                 dyn_cast<DependentScopeDeclRefExpr>(TemplateName.get())) {
    NameInfo = DSDRE->getNameInfo();
    SS.Adopt(DSDRE->getQualifierLoc());
    MissingTemplateKeyword = true;
  } else if (auto *DSME =
                 dyn_cast<CXXDependentScopeMemberExpr>(TemplateName.get())) {
    NameInfo = DSME->getMemberNameInfo();
    SS.Adopt(DSME->getQualifierLoc());
    MissingTemplateKeyword = true;
  } else {
    llvm_unreachable("unexpected kind of potential template name");
  }
 
  // If this is a dependent-scope lookup, diagnose that the 'template' keyword
  // was missing.
  if (MissingTemplateKeyword) {
    Diag(NameInfo.getBeginLoc(), diag::err_template_kw_missing)
        << "" << NameInfo.getName().getAsString() << SourceRange(Less, Greater);
    return;
  }
 
  // Try to correct the name by looking for templates and C++ named casts.
  struct TemplateCandidateFilter : CorrectionCandidateCallback {
    Sema &S;
    TemplateCandidateFilter(Sema &S) : S(S) {
      WantTypeSpecifiers = false;
      WantExpressionKeywords = false;
      WantRemainingKeywords = false;
      WantCXXNamedCasts = true;
    };
    bool ValidateCandidate(const TypoCorrection &Candidate) override {
      if (auto *ND = Candidate.getCorrectionDecl())
        return S.getAsTemplateNameDecl(ND);
      return Candidate.isKeyword();
    }
 
    std::unique_ptr<CorrectionCandidateCallback> clone() override {
      return std::make_unique<TemplateCandidateFilter>(*this);
    }
  };
 
  DeclarationName Name = NameInfo.getName();
  TemplateCandidateFilter CCC(*this);
  if (TypoCorrection Corrected = CorrectTypo(NameInfo, LookupKind, S, &SS, CCC,
                                             CTK_ErrorRecovery, LookupCtx)) {
    auto *ND = Corrected.getFoundDecl();
    if (ND)
      ND = getAsTemplateNameDecl(ND);
    if (ND || Corrected.isKeyword()) {
      if (LookupCtx) {
        std::string CorrectedStr(Corrected.getAsString(getLangOpts()));
        bool DroppedSpecifier = Corrected.WillReplaceSpecifier() &&
                                Name.getAsString() == CorrectedStr;
        diagnoseTypo(Corrected,
                     PDiag(diag::err_non_template_in_member_template_id_suggest)
                         << Name << LookupCtx << DroppedSpecifier
                         << SS.getRange(), false);
      } else {
        diagnoseTypo(Corrected,
                     PDiag(diag::err_non_template_in_template_id_suggest)
                         << Name, false);
      }
      if (Found)
        Diag(Found->getLocation(),
             diag::note_non_template_in_template_id_found);
      return;
    }
  }
 
  Diag(NameInfo.getLoc(), diag::err_non_template_in_template_id)
    << Name << SourceRange(Less, Greater);
  if (Found)
    Diag(Found->getLocation(), diag::note_non_template_in_template_id_found);
}
 
ExprResult
Sema::ActOnDependentIdExpression(const CXXScopeSpec &SS,
                                 SourceLocation TemplateKWLoc,
                                 const DeclarationNameInfo &NameInfo,
                                 bool isAddressOfOperand,
                           const TemplateArgumentListInfo *TemplateArgs) {
  if (SS.isEmpty()) {
    // FIXME: This codepath is only used by dependent unqualified names
    // (e.g. a dependent conversion-function-id, or operator= once we support
    // it). It doesn't quite do the right thing, and it will silently fail if
    // getCurrentThisType() returns null.
    QualType ThisType = getCurrentThisType();
    if (ThisType.isNull())
      return ExprError();
 
    return CXXDependentScopeMemberExpr::Create(
        Context, /*Base=*/nullptr, ThisType,
        /*IsArrow=*/!Context.getLangOpts().HLSL,
        /*OperatorLoc=*/SourceLocation(),
        /*QualifierLoc=*/NestedNameSpecifierLoc(), TemplateKWLoc,
        /*FirstQualifierFoundInScope=*/nullptr, NameInfo, TemplateArgs);
  }
  return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
}
 
ExprResult
Sema::BuildDependentDeclRefExpr(const CXXScopeSpec &SS,
                                SourceLocation TemplateKWLoc,
                                const DeclarationNameInfo &NameInfo,
                                const TemplateArgumentListInfo *TemplateArgs) {
  // DependentScopeDeclRefExpr::Create requires a valid NestedNameSpecifierLoc
  if (!SS.isValid())
    return CreateRecoveryExpr(
        SS.getBeginLoc(),
        TemplateArgs ? TemplateArgs->getRAngleLoc() : NameInfo.getEndLoc(), {});
 
  return DependentScopeDeclRefExpr::Create(
      Context, SS.getWithLocInContext(Context), TemplateKWLoc, NameInfo,
      TemplateArgs);
}
 
bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
                                          NamedDecl *Instantiation,
                                          bool InstantiatedFromMember,
                                          const NamedDecl *Pattern,
                                          const NamedDecl *PatternDef,
                                          TemplateSpecializationKind TSK,
                                          bool Complain /*= true*/) {
  assert(isa<TagDecl>(Instantiation) || isa<FunctionDecl>(Instantiation) ||
         isa<VarDecl>(Instantiation));
 
  bool IsEntityBeingDefined = false;
  if (const TagDecl *TD = dyn_cast_or_null<TagDecl>(PatternDef))
    IsEntityBeingDefined = TD->isBeingDefined();
 
  if (PatternDef && !IsEntityBeingDefined) {
    NamedDecl *SuggestedDef = nullptr;
    if (!hasReachableDefinition(const_cast<NamedDecl *>(PatternDef),
                                &SuggestedDef,
                                /*OnlyNeedComplete*/ false)) {
      // If we're allowed to diagnose this and recover, do so.
      bool Recover = Complain && !isSFINAEContext();
      if (Complain)
        diagnoseMissingImport(PointOfInstantiation, SuggestedDef,
                              Sema::MissingImportKind::Definition, Recover);
      return !Recover;
    }
    return false;
  }
 
  if (!Complain || (PatternDef && PatternDef->isInvalidDecl()))
    return true;
 
  QualType InstantiationTy;
  if (TagDecl *TD = dyn_cast<TagDecl>(Instantiation))
    InstantiationTy = Context.getTypeDeclType(TD);
  if (PatternDef) {
    Diag(PointOfInstantiation,
         diag::err_template_instantiate_within_definition)
      << /*implicit|explicit*/(TSK != TSK_ImplicitInstantiation)
      << InstantiationTy;
    // Not much point in noting the template declaration here, since
    // we're lexically inside it.
    Instantiation->setInvalidDecl();
  } else if (InstantiatedFromMember) {
    if (isa<FunctionDecl>(Instantiation)) {
      Diag(PointOfInstantiation,
           diag::err_explicit_instantiation_undefined_member)
        << /*member function*/ 1 << Instantiation->getDeclName()
        << Instantiation->getDeclContext();
      Diag(Pattern->getLocation(), diag::note_explicit_instantiation_here);
    } else {
      assert(isa<TagDecl>(Instantiation) && "Must be a TagDecl!");
      Diag(PointOfInstantiation,
           diag::err_implicit_instantiate_member_undefined)
        << InstantiationTy;
      Diag(Pattern->getLocation(), diag::note_member_declared_at);
    }
  } else {
    if (isa<FunctionDecl>(Instantiation)) {
      Diag(PointOfInstantiation,
           diag::err_explicit_instantiation_undefined_func_template)
        << Pattern;
      Diag(Pattern->getLocation(), diag::note_explicit_instantiation_here);
    } else if (isa<TagDecl>(Instantiation)) {
      Diag(PointOfInstantiation, diag::err_template_instantiate_undefined)
        << (TSK != TSK_ImplicitInstantiation)
        << InstantiationTy;
      NoteTemplateLocation(*Pattern);
    } else {
      assert(isa<VarDecl>(Instantiation) && "Must be a VarDecl!");
      if (isa<VarTemplateSpecializationDecl>(Instantiation)) {
        Diag(PointOfInstantiation,
             diag::err_explicit_instantiation_undefined_var_template)
          << Instantiation;
        Instantiation->setInvalidDecl();
      } else
        Diag(PointOfInstantiation,
             diag::err_explicit_instantiation_undefined_member)
          << /*static data member*/ 2 << Instantiation->getDeclName()
          << Instantiation->getDeclContext();
      Diag(Pattern->getLocation(), diag::note_explicit_instantiation_here);
    }
  }
 
  // In general, Instantiation isn't marked invalid to get more than one
  // error for multiple undefined instantiations. But the code that does
  // explicit declaration -> explicit definition conversion can't handle
  // invalid declarations, so mark as invalid in that case.
  if (TSK == TSK_ExplicitInstantiationDeclaration)
    Instantiation->setInvalidDecl();
  return true;
}
 
void Sema::DiagnoseTemplateParameterShadow(SourceLocation Loc, Decl *PrevDecl,
                                           bool SupportedForCompatibility) {
  assert(PrevDecl->isTemplateParameter() && "Not a template parameter");
 
  // C++23 [temp.local]p6:
  //   The name of a template-parameter shall not be bound to any following.
  //   declaration whose locus is contained by the scope to which the
  //   template-parameter belongs.
  //
  // When MSVC compatibility is enabled, the diagnostic is always a warning
  // by default. Otherwise, it an error unless SupportedForCompatibility is
  // true, in which case it is a default-to-error warning.
  unsigned DiagId =
      getLangOpts().MSVCCompat
          ? diag::ext_template_param_shadow
          : (SupportedForCompatibility ? diag::ext_compat_template_param_shadow
                                       : diag::err_template_param_shadow);
  const auto *ND = cast<NamedDecl>(PrevDecl);
  Diag(Loc, DiagId) << ND->getDeclName();
  NoteTemplateParameterLocation(*ND);
}
 
TemplateDecl *Sema::AdjustDeclIfTemplate(Decl *&D) {
  if (TemplateDecl *Temp = dyn_cast_or_null<TemplateDecl>(D)) {
    D = Temp->getTemplatedDecl();
    return Temp;
  }
  return nullptr;
}
 
ParsedTemplateArgument ParsedTemplateArgument::getTemplatePackExpansion(
                                             SourceLocation EllipsisLoc) const {
  assert(Kind == Template &&
         "Only template template arguments can be pack expansions here");
  assert(getAsTemplate().get().containsUnexpandedParameterPack() &&
         "Template template argument pack expansion without packs");
  ParsedTemplateArgument Result(*this);
  Result.EllipsisLoc = EllipsisLoc;
  return Result;
}
 
static TemplateArgumentLoc translateTemplateArgument(Sema &SemaRef,
                                            const ParsedTemplateArgument &Arg) {
 
  switch (Arg.getKind()) {
  case ParsedTemplateArgument::Type: {
    TypeSourceInfo *DI;
    QualType T = SemaRef.GetTypeFromParser(Arg.getAsType(), &DI);
    if (!DI)
      DI = SemaRef.Context.getTrivialTypeSourceInfo(T, Arg.getLocation());
    return TemplateArgumentLoc(TemplateArgument(T), DI);
  }
 
  case ParsedTemplateArgument::NonType: {
    Expr *E = static_cast<Expr *>(Arg.getAsExpr());
    return TemplateArgumentLoc(TemplateArgument(E), E);
  }
 
  case ParsedTemplateArgument::Template: {
    TemplateName Template = Arg.getAsTemplate().get();
    TemplateArgument TArg;
    if (Arg.getEllipsisLoc().isValid())
      TArg = TemplateArgument(Template, std::optional<unsigned int>());
    else
      TArg = Template;
    return TemplateArgumentLoc(
        SemaRef.Context, TArg,
        Arg.getScopeSpec().getWithLocInContext(SemaRef.Context),
        Arg.getLocation(), Arg.getEllipsisLoc());
  }
  }
 
  llvm_unreachable("Unhandled parsed template argument");
}
 
void Sema::translateTemplateArguments(const ASTTemplateArgsPtr &TemplateArgsIn,
                                      TemplateArgumentListInfo &TemplateArgs) {
 for (unsigned I = 0, Last = TemplateArgsIn.size(); I != Last; ++I)
   TemplateArgs.addArgument(translateTemplateArgument(*this,
                                                      TemplateArgsIn[I]));
}
 
static void maybeDiagnoseTemplateParameterShadow(Sema &SemaRef, Scope *S,
                                                 SourceLocation Loc,
                                                 const IdentifierInfo *Name) {
  NamedDecl *PrevDecl =
      SemaRef.LookupSingleName(S, Name, Loc, Sema::LookupOrdinaryName,
                               RedeclarationKind::ForVisibleRedeclaration);
  if (PrevDecl && PrevDecl->isTemplateParameter())
    SemaRef.DiagnoseTemplateParameterShadow(Loc, PrevDecl);
}
 
ParsedTemplateArgument Sema::ActOnTemplateTypeArgument(TypeResult ParsedType) {
  TypeSourceInfo *TInfo;
  QualType T = GetTypeFromParser(ParsedType.get(), &TInfo);
  if (T.isNull())
    return ParsedTemplateArgument();
  assert(TInfo && "template argument with no location");
 
  // If we might have formed a deduced template specialization type, convert
  // it to a template template argument.
  if (getLangOpts().CPlusPlus17) {
    TypeLoc TL = TInfo->getTypeLoc();
    SourceLocation EllipsisLoc;
    if (auto PET = TL.getAs<PackExpansionTypeLoc>()) {
      EllipsisLoc = PET.getEllipsisLoc();
      TL = PET.getPatternLoc();
    }
 
    CXXScopeSpec SS;
    if (auto ET = TL.getAs<ElaboratedTypeLoc>()) {
      SS.Adopt(ET.getQualifierLoc());
      TL = ET.getNamedTypeLoc();
    }
 
    if (auto DTST = TL.getAs<DeducedTemplateSpecializationTypeLoc>()) {
      TemplateName Name = DTST.getTypePtr()->getTemplateName();
      ParsedTemplateArgument Result(SS, TemplateTy::make(Name),
                                    DTST.getTemplateNameLoc());
      if (EllipsisLoc.isValid())
        Result = Result.getTemplatePackExpansion(EllipsisLoc);
      return Result;
    }
  }
 
  // This is a normal type template argument. Note, if the type template
  // argument is an injected-class-name for a template, it has a dual nature
  // and can be used as either a type or a template. We handle that in
  // convertTypeTemplateArgumentToTemplate.
  return ParsedTemplateArgument(ParsedTemplateArgument::Type,
                                ParsedType.get().getAsOpaquePtr(),
                                TInfo->getTypeLoc().getBeginLoc());
}
 
NamedDecl *Sema::ActOnTypeParameter(Scope *S, bool Typename,
                                    SourceLocation EllipsisLoc,
                                    SourceLocation KeyLoc,
                                    IdentifierInfo *ParamName,
                                    SourceLocation ParamNameLoc,
                                    unsigned Depth, unsigned Position,
                                    SourceLocation EqualLoc,
                                    ParsedType DefaultArg,
                                    bool HasTypeConstraint) {
  assert(S->isTemplateParamScope() &&
         "Template type parameter not in template parameter scope!");
 
  bool IsParameterPack = EllipsisLoc.isValid();
  TemplateTypeParmDecl *Param
    = TemplateTypeParmDecl::Create(Context, Context.getTranslationUnitDecl(),
                                   KeyLoc, ParamNameLoc, Depth, Position,
                                   ParamName, Typename, IsParameterPack,
                                   HasTypeConstraint);
  Param->setAccess(AS_public);
 
  if (Param->isParameterPack())
    if (auto *CSI = getEnclosingLambdaOrBlock())
      CSI->LocalPacks.push_back(Param);
 
  if (ParamName) {
    maybeDiagnoseTemplateParameterShadow(*this, S, ParamNameLoc, ParamName);
 
    // Add the template parameter into the current scope.
    S->AddDecl(Param);
    IdResolver.AddDecl(Param);
  }
 
  // C++0x [temp.param]p9:
  //   A default template-argument may be specified for any kind of
  //   template-parameter that is not a template parameter pack.
  if (DefaultArg && IsParameterPack) {
    Diag(EqualLoc, diag::err_template_param_pack_default_arg);
    DefaultArg = nullptr;
  }
 
  // Handle the default argument, if provided.
  if (DefaultArg) {
    TypeSourceInfo *DefaultTInfo;
    GetTypeFromParser(DefaultArg, &DefaultTInfo);
 
    assert(DefaultTInfo && "expected source information for type");
 
    // Check for unexpanded parameter packs.
    if (DiagnoseUnexpandedParameterPack(ParamNameLoc, DefaultTInfo,
                                        UPPC_DefaultArgument))
      return Param;
 
    // Check the template argument itself.
    if (CheckTemplateArgument(DefaultTInfo)) {
      Param->setInvalidDecl();
      return Param;
    }
 
    Param->setDefaultArgument(
        Context, TemplateArgumentLoc(DefaultTInfo->getType(), DefaultTInfo));
  }
 
  return Param;
}
 
/// Convert the parser's template argument list representation into our form.
static TemplateArgumentListInfo
makeTemplateArgumentListInfo(Sema &S, TemplateIdAnnotation &TemplateId) {
  TemplateArgumentListInfo TemplateArgs(TemplateId.LAngleLoc,
                                        TemplateId.RAngleLoc);
  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId.getTemplateArgs(),
                                     TemplateId.NumArgs);
  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgs);
  return TemplateArgs;
}
 
bool Sema::CheckTypeConstraint(TemplateIdAnnotation *TypeConstr) {
 
  TemplateName TN = TypeConstr->Template.get();
  ConceptDecl *CD = cast<ConceptDecl>(TN.getAsTemplateDecl());
 
  // C++2a [temp.param]p4:
  //     [...] The concept designated by a type-constraint shall be a type
  //     concept ([temp.concept]).
  if (!CD->isTypeConcept()) {
    Diag(TypeConstr->TemplateNameLoc,
         diag::err_type_constraint_non_type_concept);
    return true;
  }
 
  if (CheckConceptUseInDefinition(CD, TypeConstr->TemplateNameLoc))
    return true;
 
  bool WereArgsSpecified = TypeConstr->LAngleLoc.isValid();
 
  if (!WereArgsSpecified &&
      CD->getTemplateParameters()->getMinRequiredArguments() > 1) {
    Diag(TypeConstr->TemplateNameLoc,
         diag::err_type_constraint_missing_arguments)
        << CD;
    return true;
  }
  return false;
}
 
bool Sema::ActOnTypeConstraint(const CXXScopeSpec &SS,
                               TemplateIdAnnotation *TypeConstr,
                               TemplateTypeParmDecl *ConstrainedParameter,
                               SourceLocation EllipsisLoc) {
  return BuildTypeConstraint(SS, TypeConstr, ConstrainedParameter, EllipsisLoc,
                             false);
}
 
bool Sema::BuildTypeConstraint(const CXXScopeSpec &SS,
                               TemplateIdAnnotation *TypeConstr,
                               TemplateTypeParmDecl *ConstrainedParameter,
                               SourceLocation EllipsisLoc,
                               bool AllowUnexpandedPack) {
 
  if (CheckTypeConstraint(TypeConstr))
    return true;
 
  TemplateName TN = TypeConstr->Template.get();
  ConceptDecl *CD = cast<ConceptDecl>(TN.getAsTemplateDecl());
  UsingShadowDecl *USD = TN.getAsUsingShadowDecl();
 
  DeclarationNameInfo ConceptName(DeclarationName(TypeConstr->Name),
                                  TypeConstr->TemplateNameLoc);
 
  TemplateArgumentListInfo TemplateArgs;
  if (TypeConstr->LAngleLoc.isValid()) {
    TemplateArgs =
        makeTemplateArgumentListInfo(*this, *TypeConstr);
 
    if (EllipsisLoc.isInvalid() && !AllowUnexpandedPack) {
      for (TemplateArgumentLoc Arg : TemplateArgs.arguments()) {
        if (DiagnoseUnexpandedParameterPack(Arg, UPPC_TypeConstraint))
          return true;
      }
    }
  }
  return AttachTypeConstraint(
      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
      ConceptName, CD, /*FoundDecl=*/USD ? cast<NamedDecl>(USD) : CD,
      TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
      ConstrainedParameter, Context.getTypeDeclType(ConstrainedParameter),
      EllipsisLoc);
}
 
template <typename ArgumentLocAppender>
static ExprResult formImmediatelyDeclaredConstraint(
    Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
    ConceptDecl *NamedConcept, NamedDecl *FoundDecl, SourceLocation LAngleLoc,
    SourceLocation RAngleLoc, QualType ConstrainedType,
    SourceLocation ParamNameLoc, ArgumentLocAppender Appender,
    SourceLocation EllipsisLoc) {
 
  TemplateArgumentListInfo ConstraintArgs;
  ConstraintArgs.addArgument(
    S.getTrivialTemplateArgumentLoc(TemplateArgument(ConstrainedType),
                                    /*NTTPType=*/QualType(), ParamNameLoc));
 
  ConstraintArgs.setRAngleLoc(RAngleLoc);
  ConstraintArgs.setLAngleLoc(LAngleLoc);
  Appender(ConstraintArgs);
 
  // C++2a [temp.param]p4:
  //     [...] This constraint-expression E is called the immediately-declared
  //     constraint of T. [...]
  CXXScopeSpec SS;
  SS.Adopt(NS);
  ExprResult ImmediatelyDeclaredConstraint = S.CheckConceptTemplateId(
      SS, /*TemplateKWLoc=*/SourceLocation(), NameInfo,
      /*FoundDecl=*/FoundDecl ? FoundDecl : NamedConcept, NamedConcept,
      &ConstraintArgs);
  if (ImmediatelyDeclaredConstraint.isInvalid() || !EllipsisLoc.isValid())
    return ImmediatelyDeclaredConstraint;
 
  // C++2a [temp.param]p4:
  //     [...] If T is not a pack, then E is E', otherwise E is (E' && ...).
  //
  // We have the following case:
  //
  // template<typename T> concept C1 = true;
  // template<C1... T> struct s1;
  //
  // The constraint: (C1<T> && ...)
  //
  // Note that the type of C1<T> is known to be 'bool', so we don't need to do
  // any unqualified lookups for 'operator&&' here.
  return S.BuildCXXFoldExpr(/*UnqualifiedLookup=*/nullptr,
                            /*LParenLoc=*/SourceLocation(),
                            ImmediatelyDeclaredConstraint.get(), BO_LAnd,
                            EllipsisLoc, /*RHS=*/nullptr,
                            /*RParenLoc=*/SourceLocation(),
                            /*NumExpansions=*/std::nullopt);
}
 
bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
                                DeclarationNameInfo NameInfo,
                                ConceptDecl *NamedConcept, NamedDecl *FoundDecl,
                                const TemplateArgumentListInfo *TemplateArgs,
                                TemplateTypeParmDecl *ConstrainedParameter,
                                QualType ConstrainedType,
                                SourceLocation EllipsisLoc) {
  // C++2a [temp.param]p4:
  //     [...] If Q is of the form C<A1, ..., An>, then let E' be
  //     C<T, A1, ..., An>. Otherwise, let E' be C<T>. [...]
  const ASTTemplateArgumentListInfo *ArgsAsWritten =
    TemplateArgs ? ASTTemplateArgumentListInfo::Create(Context,
                                                       *TemplateArgs) : nullptr;
 
  QualType ParamAsArgument = ConstrainedType;
 
  ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
      *this, NS, NameInfo, NamedConcept, FoundDecl,
      TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
      TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
      ParamAsArgument, ConstrainedParameter->getLocation(),
      [&](TemplateArgumentListInfo &ConstraintArgs) {
        if (TemplateArgs)
          for (const auto &ArgLoc : TemplateArgs->arguments())
            ConstraintArgs.addArgument(ArgLoc);
      },
      EllipsisLoc);
  if (ImmediatelyDeclaredConstraint.isInvalid())
    return true;
 
  auto *CL = ConceptReference::Create(Context, /*NNS=*/NS,
                                      /*TemplateKWLoc=*/SourceLocation{},
                                      /*ConceptNameInfo=*/NameInfo,
                                      /*FoundDecl=*/FoundDecl,
                                      /*NamedConcept=*/NamedConcept,
                                      /*ArgsWritten=*/ArgsAsWritten);
  ConstrainedParameter->setTypeConstraint(CL,
                                          ImmediatelyDeclaredConstraint.get());
  return false;
}
 
bool Sema::AttachTypeConstraint(AutoTypeLoc TL,
                                NonTypeTemplateParmDecl *NewConstrainedParm,
                                NonTypeTemplateParmDecl *OrigConstrainedParm,
                                SourceLocation EllipsisLoc) {
  if (NewConstrainedParm->getType() != TL.getType() ||
      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
    Diag(NewConstrainedParm->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
         diag::err_unsupported_placeholder_constraint)
        << NewConstrainedParm->getTypeSourceInfo()
               ->getTypeLoc()
               .getSourceRange();
    return true;
  }
  // FIXME: Concepts: This should be the type of the placeholder, but this is
  // unclear in the wording right now.
  DeclRefExpr *Ref =
      BuildDeclRefExpr(OrigConstrainedParm, OrigConstrainedParm->getType(),
                       VK_PRValue, OrigConstrainedParm->getLocation());
  if (!Ref)
    return true;
  ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
      *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
      TL.getNamedConcept(), /*FoundDecl=*/TL.getFoundDecl(), TL.getLAngleLoc(),
      TL.getRAngleLoc(), BuildDecltypeType(Ref),
      OrigConstrainedParm->getLocation(),
      [&](TemplateArgumentListInfo &ConstraintArgs) {
        for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
          ConstraintArgs.addArgument(TL.getArgLoc(I));
      },
      EllipsisLoc);
  if (ImmediatelyDeclaredConstraint.isInvalid() ||
      !ImmediatelyDeclaredConstraint.isUsable())
    return true;
 
  NewConstrainedParm->setPlaceholderTypeConstraint(
      ImmediatelyDeclaredConstraint.get());
  return false;
}
 
QualType Sema::CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
                                                 SourceLocation Loc) {
  if (TSI->getType()->isUndeducedType()) {
    // C++17 [temp.dep.expr]p3:
    //   An id-expression is type-dependent if it contains
    //    - an identifier associated by name lookup with a non-type
    //      template-parameter declared with a type that contains a
    //      placeholder type (7.1.7.4),
    TSI = SubstAutoTypeSourceInfoDependent(TSI);
  }
 
  return CheckNonTypeTemplateParameterType(TSI->getType(), Loc);
}
 
bool Sema::RequireStructuralType(QualType T, SourceLocation Loc) {
  if (T->isDependentType())
    return false;
 
  if (RequireCompleteType(Loc, T, diag::err_template_nontype_parm_incomplete))
    return true;
 
  if (T->isStructuralType())
    return false;
 
  // Structural types are required to be object types or lvalue references.
  if (T->isRValueReferenceType()) {
    Diag(Loc, diag::err_template_nontype_parm_rvalue_ref) << T;
    return true;
  }
 
  // Don't mention structural types in our diagnostic prior to C++20. Also,
  // there's not much more we can say about non-scalar non-class types --
  // because we can't see functions or arrays here, those can only be language
  // extensions.
  if (!getLangOpts().CPlusPlus20 ||
      (!T->isScalarType() && !T->isRecordType())) {
    Diag(Loc, diag::err_template_nontype_parm_bad_type) << T;
    return true;
  }
 
  // Structural types are required to be literal types.
  if (RequireLiteralType(Loc, T, diag::err_template_nontype_parm_not_literal))
    return true;
 
  Diag(Loc, diag::err_template_nontype_parm_not_structural) << T;
 
  // Drill down into the reason why the class is non-structural.
  while (const CXXRecordDecl *RD = T->getAsCXXRecordDecl()) {
    // All members are required to be public and non-mutable, and can't be of
    // rvalue reference type. Check these conditions first to prefer a "local"
    // reason over a more distant one.
    for (const FieldDecl *FD : RD->fields()) {
      if (FD->getAccess() != AS_public) {
        Diag(FD->getLocation(), diag::note_not_structural_non_public) << T << 0;
        return true;
      }
      if (FD->isMutable()) {
        Diag(FD->getLocation(), diag::note_not_structural_mutable_field) << T;
        return true;
      }
      if (FD->getType()->isRValueReferenceType()) {
        Diag(FD->getLocation(), diag::note_not_structural_rvalue_ref_field)
            << T;
        return true;
      }
    }
 
    // All bases are required to be public.
    for (const auto &BaseSpec : RD->bases()) {
      if (BaseSpec.getAccessSpecifier() != AS_public) {
        Diag(BaseSpec.getBaseTypeLoc(), diag::note_not_structural_non_public)
            << T << 1;
        return true;
      }
    }
 
    // All subobjects are required to be of structural types.
    SourceLocation SubLoc;
    QualType SubType;
    int Kind = -1;
 
    for (const FieldDecl *FD : RD->fields()) {
      QualType T = Context.getBaseElementType(FD->getType());
      if (!T->isStructuralType()) {
        SubLoc = FD->getLocation();
        SubType = T;
        Kind = 0;
        break;
      }
    }
 
    if (Kind == -1) {
      for (const auto &BaseSpec : RD->bases()) {
        QualType T = BaseSpec.getType();
        if (!T->isStructuralType()) {
          SubLoc = BaseSpec.getBaseTypeLoc();
          SubType = T;
          Kind = 1;
          break;
        }
      }
    }
 
    assert(Kind != -1 && "couldn't find reason why type is not structural");
    Diag(SubLoc, diag::note_not_structural_subobject)
        << T << Kind << SubType;
    T = SubType;
    RD = T->getAsCXXRecordDecl();
  }
 
  return true;
}
 
QualType Sema::CheckNonTypeTemplateParameterType(QualType T,
                                                 SourceLocation Loc) {
  // We don't allow variably-modified types as the type of non-type template
  // parameters.
  if (T->isVariablyModifiedType()) {
    Diag(Loc, diag::err_variably_modified_nontype_template_param)
      << T;
    return QualType();
  }
 
  // C++ [temp.param]p4:
  //
  // A non-type template-parameter shall have one of the following
  // (optionally cv-qualified) types:
  //
  //       -- integral or enumeration type,
  if (T->isIntegralOrEnumerationType() ||
      //   -- pointer to object or pointer to function,
      T->isPointerType() ||
      //   -- lvalue reference to object or lvalue reference to function,
      T->isLValueReferenceType() ||
      //   -- pointer to member,
      T->isMemberPointerType() ||
      //   -- std::nullptr_t, or
      T->isNullPtrType() ||
      //   -- a type that contains a placeholder type.
      T->isUndeducedType()) {
    // C++ [temp.param]p5: The top-level cv-qualifiers on the template-parameter
    // are ignored when determining its type.
    return T.getUnqualifiedType();
  }
 
  // C++ [temp.param]p8:
  //
  //   A non-type template-parameter of type "array of T" or
  //   "function returning T" is adjusted to be of type "pointer to
  //   T" or "pointer to function returning T", respectively.
  if (T->isArrayType() || T->isFunctionType())
    return Context.getDecayedType(T);
 
  // If T is a dependent type, we can't do the check now, so we
  // assume that it is well-formed. Note that stripping off the
  // qualifiers here is not really correct if T turns out to be
  // an array type, but we'll recompute the type everywhere it's
  // used during instantiation, so that should be OK. (Using the
  // qualified type is equally wrong.)
  if (T->isDependentType())
    return T.getUnqualifiedType();
 
  // C++20 [temp.param]p6:
  //   -- a structural type
  if (RequireStructuralType(T, Loc))
    return QualType();
 
  if (!getLangOpts().CPlusPlus20) {
    // FIXME: Consider allowing structural types as an extension in C++17. (In
    // earlier language modes, the template argument evaluation rules are too
    // inflexible.)
    Diag(Loc, diag::err_template_nontype_parm_bad_structural_type) << T;
    return QualType();
  }
 
  Diag(Loc, diag::warn_cxx17_compat_template_nontype_parm_type) << T;
  return T.getUnqualifiedType();
}
 
NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
                                          unsigned Depth,
                                          unsigned Position,
                                          SourceLocation EqualLoc,
                                          Expr *Default) {
  TypeSourceInfo *TInfo = GetTypeForDeclarator(D);
 
  // Check that we have valid decl-specifiers specified.
  auto CheckValidDeclSpecifiers = [this, &D] {
    // C++ [temp.param]
    // p1
    //   template-parameter:
    //     ...
    //     parameter-declaration
    // p2
    //   ... A storage class shall not be specified in a template-parameter
    //   declaration.
    // [dcl.typedef]p1:
    //   The typedef specifier [...] shall not be used in the decl-specifier-seq
    //   of a parameter-declaration
    const DeclSpec &DS = D.getDeclSpec();
    auto EmitDiag = [this](SourceLocation Loc) {
      Diag(Loc, diag::err_invalid_decl_specifier_in_nontype_parm)
          << FixItHint::CreateRemoval(Loc);
    };
    if (DS.getStorageClassSpec() != DeclSpec::SCS_unspecified)
      EmitDiag(DS.getStorageClassSpecLoc());
 
    if (DS.getThreadStorageClassSpec() != TSCS_unspecified)
      EmitDiag(DS.getThreadStorageClassSpecLoc());
 
    // [dcl.inline]p1:
    //   The inline specifier can be applied only to the declaration or
    //   definition of a variable or function.
 
    if (DS.isInlineSpecified())
      EmitDiag(DS.getInlineSpecLoc());
 
    // [dcl.constexpr]p1:
    //   The constexpr specifier shall be applied only to the definition of a
    //   variable or variable template or the declaration of a function or
    //   function template.
 
    if (DS.hasConstexprSpecifier())
      EmitDiag(DS.getConstexprSpecLoc());
 
    // [dcl.fct.spec]p1:
    //   Function-specifiers can be used only in function declarations.
 
    if (DS.isVirtualSpecified())
      EmitDiag(DS.getVirtualSpecLoc());
 
    if (DS.hasExplicitSpecifier())
      EmitDiag(DS.getExplicitSpecLoc());
 
    if (DS.isNoreturnSpecified())
      EmitDiag(DS.getNoreturnSpecLoc());
  };
 
  CheckValidDeclSpecifiers();
 
  if (const auto *T = TInfo->getType()->getContainedDeducedType())
    if (isa<AutoType>(T))
      Diag(D.getIdentifierLoc(),
           diag::warn_cxx14_compat_template_nontype_parm_auto_type)
          << QualType(TInfo->getType()->getContainedAutoType(), 0);
 
  assert(S->isTemplateParamScope() &&
         "Non-type template parameter not in template parameter scope!");
  bool Invalid = false;
 
  QualType T = CheckNonTypeTemplateParameterType(TInfo, D.getIdentifierLoc());
  if (T.isNull()) {
    T = Context.IntTy; // Recover with an 'int' type.
    Invalid = true;
  }
 
  CheckFunctionOrTemplateParamDeclarator(S, D);
 
  const IdentifierInfo *ParamName = D.getIdentifier();
  bool IsParameterPack = D.hasEllipsis();
  NonTypeTemplateParmDecl *Param = NonTypeTemplateParmDecl::Create(
      Context, Context.getTranslationUnitDecl(), D.getBeginLoc(),
      D.getIdentifierLoc(), Depth, Position, ParamName, T, IsParameterPack,
      TInfo);
  Param->setAccess(AS_public);
 
  if (AutoTypeLoc TL = TInfo->getTypeLoc().getContainedAutoTypeLoc())
    if (TL.isConstrained())
      if (AttachTypeConstraint(TL, Param, Param, D.getEllipsisLoc()))
        Invalid = true;
 
  if (Invalid)
    Param->setInvalidDecl();
 
  if (Param->isParameterPack())
    if (auto *CSI = getEnclosingLambdaOrBlock())
      CSI->LocalPacks.push_back(Param);
 
  if (ParamName) {
    maybeDiagnoseTemplateParameterShadow(*this, S, D.getIdentifierLoc(),
                                         ParamName);
 
    // Add the template parameter into the current scope.
    S->AddDecl(Param);
    IdResolver.AddDecl(Param);
  }
 
  // C++0x [temp.param]p9:
  //   A default template-argument may be specified for any kind of
  //   template-parameter that is not a template parameter pack.
  if (Default && IsParameterPack) {
    Diag(EqualLoc, diag::err_template_param_pack_default_arg);
    Default = nullptr;
  }
 
  // Check the well-formedness of the default template argument, if provided.
  if (Default) {
    // Check for unexpanded parameter packs.
    if (DiagnoseUnexpandedParameterPack(Default, UPPC_DefaultArgument))
      return Param;
 
    Param->setDefaultArgument(
        Context, getTrivialTemplateArgumentLoc(TemplateArgument(Default),
                                               QualType(), SourceLocation()));
  }
 
  return Param;
}
 
NamedDecl *Sema::ActOnTemplateTemplateParameter(
    Scope *S, SourceLocation TmpLoc, TemplateParameterList *Params,
    bool Typename, SourceLocation EllipsisLoc, IdentifierInfo *Name,
    SourceLocation NameLoc, unsigned Depth, unsigned Position,
    SourceLocation EqualLoc, ParsedTemplateArgument Default) {
  assert(S->isTemplateParamScope() &&
         "Template template parameter not in template parameter scope!");
 
  // Construct the parameter object.
  bool IsParameterPack = EllipsisLoc.isValid();
  TemplateTemplateParmDecl *Param = TemplateTemplateParmDecl::Create(
      Context, Context.getTranslationUnitDecl(),
      NameLoc.isInvalid() ? TmpLoc : NameLoc, Depth, Position, IsParameterPack,
      Name, Typename, Params);
  Param->setAccess(AS_public);
 
  if (Param->isParameterPack())
    if (auto *LSI = getEnclosingLambdaOrBlock())
      LSI->LocalPacks.push_back(Param);
 
  // If the template template parameter has a name, then link the identifier
  // into the scope and lookup mechanisms.
  if (Name) {
    maybeDiagnoseTemplateParameterShadow(*this, S, NameLoc, Name);
 
    S->AddDecl(Param);
    IdResolver.AddDecl(Param);
  }
 
  if (Params->size() == 0) {
    Diag(Param->getLocation(), diag::err_template_template_parm_no_parms)
    << SourceRange(Params->getLAngleLoc(), Params->getRAngleLoc());
    Param->setInvalidDecl();
  }
 
  // C++0x [temp.param]p9:
  //   A default template-argument may be specified for any kind of
  //   template-parameter that is not a template parameter pack.
  if (IsParameterPack && !Default.isInvalid()) {
    Diag(EqualLoc, diag::err_template_param_pack_default_arg);
    Default = ParsedTemplateArgument();
  }
 
  if (!Default.isInvalid()) {
    // Check only that we have a template template argument. We don't want to
    // try to check well-formedness now, because our template template parameter
    // might have dependent types in its template parameters, which we wouldn't
    // be able to match now.
    //
    // If none of the template template parameter's template arguments mention
    // other template parameters, we could actually perform more checking here.
    // However, it isn't worth doing.
    TemplateArgumentLoc DefaultArg = translateTemplateArgument(*this, Default);
    if (DefaultArg.getArgument().getAsTemplate().isNull()) {
      Diag(DefaultArg.getLocation(), diag::err_template_arg_not_valid_template)
        << DefaultArg.getSourceRange();
      return Param;
    }
 
    // Check for unexpanded parameter packs.
    if (DiagnoseUnexpandedParameterPack(DefaultArg.getLocation(),
                                        DefaultArg.getArgument().getAsTemplate(),
                                        UPPC_DefaultArgument))
      return Param;
 
    Param->setDefaultArgument(Context, DefaultArg);
  }
 
  return Param;
}
 
namespace {
class ConstraintRefersToContainingTemplateChecker
    : public TreeTransform<ConstraintRefersToContainingTemplateChecker> {
  bool Result = false;
  const FunctionDecl *Friend = nullptr;
  unsigned TemplateDepth = 0;
 
  // Check a record-decl that we've seen to see if it is a lexical parent of the
  // Friend, likely because it was referred to without its template arguments.
  void CheckIfContainingRecord(const CXXRecordDecl *CheckingRD) {
    CheckingRD = CheckingRD->getMostRecentDecl();
    if (!CheckingRD->isTemplated())
      return;
 
    for (const DeclContext *DC = Friend->getLexicalDeclContext();
         DC && !DC->isFileContext(); DC = DC->getParent())
      if (const auto *RD = dyn_cast<CXXRecordDecl>(DC))
        if (CheckingRD == RD->getMostRecentDecl())
          Result = true;
  }
 
  void CheckNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
    if (D->getDepth() < TemplateDepth)
      Result = true;
 
    // Necessary because the type of the NTTP might be what refers to the parent
    // constriant.
    TransformType(D->getType());
  }
 
public:
  using inherited = TreeTransform<ConstraintRefersToContainingTemplateChecker>;
 
  ConstraintRefersToContainingTemplateChecker(Sema &SemaRef,
                                              const FunctionDecl *Friend,
                                              unsigned TemplateDepth)
      : inherited(SemaRef), Friend(Friend), TemplateDepth(TemplateDepth) {}
  bool getResult() const { return Result; }
 
  // This should be the only template parm type that we have to deal with.
  // SubstTempalteTypeParmPack, SubstNonTypeTemplateParmPack, and
  // FunctionParmPackExpr are all partially substituted, which cannot happen
  // with concepts at this point in translation.
  using inherited::TransformTemplateTypeParmType;
  QualType TransformTemplateTypeParmType(TypeLocBuilder &TLB,
                                         TemplateTypeParmTypeLoc TL, bool) {
    if (TL.getDecl()->getDepth() < TemplateDepth)
      Result = true;
    return inherited::TransformTemplateTypeParmType(
        TLB, TL,
        /*SuppressObjCLifetime=*/false);
  }
 
  Decl *TransformDecl(SourceLocation Loc, Decl *D) {
    if (!D)
      return D;
    // FIXME : This is possibly an incomplete list, but it is unclear what other
    // Decl kinds could be used to refer to the template parameters.  This is a
    // best guess so far based on examples currently available, but the
    // unreachable should catch future instances/cases.
    if (auto *TD = dyn_cast<TypedefNameDecl>(D))
      TransformType(TD->getUnderlyingType());
    else if (auto *NTTPD = dyn_cast<NonTypeTemplateParmDecl>(D))
      CheckNonTypeTemplateParmDecl(NTTPD);
    else if (auto *VD = dyn_cast<ValueDecl>(D))
      TransformType(VD->getType());
    else if (auto *TD = dyn_cast<TemplateDecl>(D))
      TransformTemplateParameterList(TD->getTemplateParameters());
    else if (auto *RD = dyn_cast<CXXRecordDecl>(D))
      CheckIfContainingRecord(RD);
    else if (isa<NamedDecl>(D)) {
      // No direct types to visit here I believe.
    } else
      llvm_unreachable("Don't know how to handle this declaration type yet");
    return D;
  }
};
} // namespace
 
bool Sema::ConstraintExpressionDependsOnEnclosingTemplate(
    const FunctionDecl *Friend, unsigned TemplateDepth,
    const Expr *Constraint) {
  assert(Friend->getFriendObjectKind() && "Only works on a friend");
  ConstraintRefersToContainingTemplateChecker Checker(*this, Friend,
                                                      TemplateDepth);
  Checker.TransformExpr(const_cast<Expr *>(Constraint));
  return Checker.getResult();
}
 
TemplateParameterList *
Sema::ActOnTemplateParameterList(unsigned Depth,
                                 SourceLocation ExportLoc,
                                 SourceLocation TemplateLoc,
                                 SourceLocation LAngleLoc,
                                 ArrayRef<NamedDecl *> Params,
                                 SourceLocation RAngleLoc,
                                 Expr *RequiresClause) {
  if (ExportLoc.isValid())
    Diag(ExportLoc, diag::warn_template_export_unsupported);
 
  for (NamedDecl *P : Params)
    warnOnReservedIdentifier(P);
 
  return TemplateParameterList::Create(
      Context, TemplateLoc, LAngleLoc,
      llvm::ArrayRef(Params.data(), Params.size()), RAngleLoc, RequiresClause);
}
 
static void SetNestedNameSpecifier(Sema &S, TagDecl *T,
                                   const CXXScopeSpec &SS) {
  if (SS.isSet())
    T->setQualifierInfo(SS.getWithLocInContext(S.Context));
}
 
// Returns the template parameter list with all default template argument
// information.
TemplateParameterList *Sema::GetTemplateParameterList(TemplateDecl *TD) {
  // Make sure we get the template parameter list from the most
  // recent declaration, since that is the only one that is guaranteed to
  // have all the default template argument information.
  Decl *D = TD->getMostRecentDecl();
  // C++11 N3337 [temp.param]p12:
  // A default template argument shall not be specified in a friend class
  // template declaration.
  //
  // Skip past friend *declarations* because they are not supposed to contain
  // default template arguments. Moreover, these declarations may introduce
  // template parameters living in different template depths than the
  // corresponding template parameters in TD, causing unmatched constraint
  // substitution.
  //
  // FIXME: Diagnose such cases within a class template:
  //  template <class T>
  //  struct S {
  //    template <class = void> friend struct C;
  //  };
  //  template struct S<int>;
  while (D->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None &&
         D->getPreviousDecl())
    D = D->getPreviousDecl();
  return cast<TemplateDecl>(D)->getTemplateParameters();
}
 
DeclResult Sema::CheckClassTemplate(
    Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
    CXXScopeSpec &SS, IdentifierInfo *Name, SourceLocation NameLoc,
    const ParsedAttributesView &Attr, TemplateParameterList *TemplateParams,
    AccessSpecifier AS, SourceLocation ModulePrivateLoc,
    SourceLocation FriendLoc, unsigned NumOuterTemplateParamLists,
    TemplateParameterList **OuterTemplateParamLists, SkipBodyInfo *SkipBody) {
  assert(TemplateParams && TemplateParams->size() > 0 &&
         "No template parameters");
  assert(TUK != TagUseKind::Reference &&
         "Can only declare or define class templates");
  bool Invalid = false;
 
  // Check that we can declare a template here.
  if (CheckTemplateDeclScope(S, TemplateParams))
    return true;
 
  TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
  assert(Kind != TagTypeKind::Enum &&
         "can't build template of enumerated type");
 
  // There is no such thing as an unnamed class template.
  if (!Name) {
    Diag(KWLoc, diag::err_template_unnamed_class);
    return true;
  }
 
  // Find any previous declaration with this name. For a friend with no
  // scope explicitly specified, we only look for tag declarations (per
  // C++11 [basic.lookup.elab]p2).
  DeclContext *SemanticContext;
  LookupResult Previous(*this, Name, NameLoc,
                        (SS.isEmpty() && TUK == TagUseKind::Friend)
                            ? LookupTagName
                            : LookupOrdinaryName,
                        forRedeclarationInCurContext());
  if (SS.isNotEmpty() && !SS.isInvalid()) {
    SemanticContext = computeDeclContext(SS, true);
    if (!SemanticContext) {
      // FIXME: Horrible, horrible hack! We can't currently represent this
      // in the AST, and historically we have just ignored such friend
      // class templates, so don't complain here.
      Diag(NameLoc, TUK == TagUseKind::Friend
                        ? diag::warn_template_qualified_friend_ignored
                        : diag::err_template_qualified_declarator_no_match)
          << SS.getScopeRep() << SS.getRange();
      return TUK != TagUseKind::Friend;
    }
 
    if (RequireCompleteDeclContext(SS, SemanticContext))
      return true;
 
    // If we're adding a template to a dependent context, we may need to
    // rebuilding some of the types used within the template parameter list,
    // now that we know what the current instantiation is.
    if (SemanticContext->isDependentContext()) {
      ContextRAII SavedContext(*this, SemanticContext);
      if (RebuildTemplateParamsInCurrentInstantiation(TemplateParams))
        Invalid = true;
    }
 
    if (TUK != TagUseKind::Friend && TUK != TagUseKind::Reference)
      diagnoseQualifiedDeclaration(SS, SemanticContext, Name, NameLoc,
                                   /*TemplateId-*/ nullptr,
                                   /*IsMemberSpecialization*/ false);
 
    LookupQualifiedName(Previous, SemanticContext);
  } else {
    SemanticContext = CurContext;
 
    // C++14 [class.mem]p14:
    //   If T is the name of a class, then each of the following shall have a
    //   name different from T:
    //    -- every member template of class T
    if (TUK != TagUseKind::Friend &&
        DiagnoseClassNameShadow(SemanticContext,
                                DeclarationNameInfo(Name, NameLoc)))
      return true;
 
    LookupName(Previous, S);
  }
 
  if (Previous.isAmbiguous())
    return true;
 
  // Let the template parameter scope enter the lookup chain of the current
  // class template. For example, given
  //
  //  namespace ns {
  //    template <class> bool Param = false;
  //    template <class T> struct N;
  //  }
  //
  //  template <class Param> struct ns::N { void foo(Param); };
  //
  // When we reference Param inside the function parameter list, our name lookup
  // chain for it should be like:
  //  FunctionScope foo
  //  -> RecordScope N
  //  -> TemplateParamScope (where we will find Param)
  //  -> NamespaceScope ns
  //
  // See also CppLookupName().
  if (S->isTemplateParamScope())
    EnterTemplatedContext(S, SemanticContext);
 
  NamedDecl *PrevDecl = nullptr;
  if (Previous.begin() != Previous.end())
    PrevDecl = (*Previous.begin())->getUnderlyingDecl();
 
  if (PrevDecl && PrevDecl->isTemplateParameter()) {
    // Maybe we will complain about the shadowed template parameter.
    DiagnoseTemplateParameterShadow(NameLoc, PrevDecl);
    // Just pretend that we didn't see the previous declaration.
    PrevDecl = nullptr;
  }
 
  // If there is a previous declaration with the same name, check
  // whether this is a valid redeclaration.
  ClassTemplateDecl *PrevClassTemplate =
      dyn_cast_or_null<ClassTemplateDecl>(PrevDecl);
 
  // We may have found the injected-class-name of a class template,
  // class template partial specialization, or class template specialization.
  // In these cases, grab the template that is being defined or specialized.
  if (!PrevClassTemplate && isa_and_nonnull<CXXRecordDecl>(PrevDecl) &&
      cast<CXXRecordDecl>(PrevDecl)->isInjectedClassName()) {
    PrevDecl = cast<CXXRecordDecl>(PrevDecl->getDeclContext());
    PrevClassTemplate
      = cast<CXXRecordDecl>(PrevDecl)->getDescribedClassTemplate();
    if (!PrevClassTemplate && isa<ClassTemplateSpecializationDecl>(PrevDecl)) {
      PrevClassTemplate
        = cast<ClassTemplateSpecializationDecl>(PrevDecl)
            ->getSpecializedTemplate();
    }
  }
 
  if (TUK == TagUseKind::Friend) {
    // C++ [namespace.memdef]p3:
    //   [...] When looking for a prior declaration of a class or a function
    //   declared as a friend, and when the name of the friend class or
    //   function is neither a qualified name nor a template-id, scopes outside
    //   the innermost enclosing namespace scope are not considered.
    if (!SS.isSet()) {
      DeclContext *OutermostContext = CurContext;
      while (!OutermostContext->isFileContext())
        OutermostContext = OutermostContext->getLookupParent();
 
      if (PrevDecl &&
          (OutermostContext->Equals(PrevDecl->getDeclContext()) ||
           OutermostContext->Encloses(PrevDecl->getDeclContext()))) {
        SemanticContext = PrevDecl->getDeclContext();
      } else {
        // Declarations in outer scopes don't matter. However, the outermost
        // context we computed is the semantic context for our new
        // declaration.
        PrevDecl = PrevClassTemplate = nullptr;
        SemanticContext = OutermostContext;
 
        // Check that the chosen semantic context doesn't already contain a
        // declaration of this name as a non-tag type.
        Previous.clear(LookupOrdinaryName);
        DeclContext *LookupContext = SemanticContext;
        while (LookupContext->isTransparentContext())
          LookupContext = LookupContext->getLookupParent();
        LookupQualifiedName(Previous, LookupContext);
 
        if (Previous.isAmbiguous())
          return true;
 
        if (Previous.begin() != Previous.end())
          PrevDecl = (*Previous.begin())->getUnderlyingDecl();
      }
    }
  } else if (PrevDecl && !isDeclInScope(Previous.getRepresentativeDecl(),
                                        SemanticContext, S, SS.isValid()))
    PrevDecl = PrevClassTemplate = nullptr;
 
  if (auto *Shadow = dyn_cast_or_null<UsingShadowDecl>(
          PrevDecl ? Previous.getRepresentativeDecl() : nullptr)) {
    if (SS.isEmpty() &&
        !(PrevClassTemplate &&
          PrevClassTemplate->getDeclContext()->getRedeclContext()->Equals(
              SemanticContext->getRedeclContext()))) {
      Diag(KWLoc, diag::err_using_decl_conflict_reverse);
      Diag(Shadow->getTargetDecl()->getLocation(),
           diag::note_using_decl_target);
      Diag(Shadow->getIntroducer()->getLocation(), diag::note_using_decl) << 0;
      // Recover by ignoring the old declaration.
      PrevDecl = PrevClassTemplate = nullptr;
    }
  }
 
  if (PrevClassTemplate) {
    // Ensure that the template parameter lists are compatible. Skip this check
    // for a friend in a dependent context: the template parameter list itself
    // could be dependent.
    if (!(TUK == TagUseKind::Friend && CurContext->isDependentContext()) &&
        !TemplateParameterListsAreEqual(
            TemplateCompareNewDeclInfo(SemanticContext ? SemanticContext
                                                       : CurContext,
                                       CurContext, KWLoc),
            TemplateParams, PrevClassTemplate,
            PrevClassTemplate->getTemplateParameters(), /*Complain=*/true,
            TPL_TemplateMatch))
      return true;
 
    // C++ [temp.class]p4:
    //   In a redeclaration, partial specialization, explicit
    //   specialization or explicit instantiation of a class template,
    //   the class-key shall agree in kind with the original class
    //   template declaration (7.1.5.3).
    RecordDecl *PrevRecordDecl = PrevClassTemplate->getTemplatedDecl();
    if (!isAcceptableTagRedeclaration(
            PrevRecordDecl, Kind, TUK == TagUseKind::Definition, KWLoc, Name)) {
      Diag(KWLoc, diag::err_use_with_wrong_tag)
        << Name
        << FixItHint::CreateReplacement(KWLoc, PrevRecordDecl->getKindName());
      Diag(PrevRecordDecl->getLocation(), diag::note_previous_use);
      Kind = PrevRecordDecl->getTagKind();
    }
 
    // Check for redefinition of this class template.
    if (TUK == TagUseKind::Definition) {
      if (TagDecl *Def = PrevRecordDecl->getDefinition()) {
        // If we have a prior definition that is not visible, treat this as
        // simply making that previous definition visible.
        NamedDecl *Hidden = nullptr;
        if (SkipBody && !hasVisibleDefinition(Def, &Hidden)) {
          SkipBody->ShouldSkip = true;
          SkipBody->Previous = Def;
          auto *Tmpl = cast<CXXRecordDecl>(Hidden)->getDescribedClassTemplate();
          assert(Tmpl && "original definition of a class template is not a "
                         "class template?");
          makeMergedDefinitionVisible(Hidden);
          makeMergedDefinitionVisible(Tmpl);
        } else {
          Diag(NameLoc, diag::err_redefinition) << Name;
          Diag(Def->getLocation(), diag::note_previous_definition);
          // FIXME: Would it make sense to try to "forget" the previous
          // definition, as part of error recovery?
          return true;
        }
      }
    }
  } else if (PrevDecl) {
    // C++ [temp]p5:
    //   A class template shall not have the same name as any other
    //   template, class, function, object, enumeration, enumerator,
    //   namespace, or type in the same scope (3.3), except as specified
    //   in (14.5.4).
    Diag(NameLoc, diag::err_redefinition_different_kind) << Name;
    Diag(PrevDecl->getLocation(), diag::note_previous_definition);
    return true;
  }
 
  // Check the template parameter list of this declaration, possibly
  // merging in the template parameter list from the previous class
  // template declaration. Skip this check for a friend in a dependent
  // context, because the template parameter list might be dependent.
  if (!(TUK == TagUseKind::Friend && CurContext->isDependentContext()) &&
      CheckTemplateParameterList(
          TemplateParams,
          PrevClassTemplate ? GetTemplateParameterList(PrevClassTemplate)
                            : nullptr,
          (SS.isSet() && SemanticContext && SemanticContext->isRecord() &&
           SemanticContext->isDependentContext())
              ? TPC_ClassTemplateMember
          : TUK == TagUseKind::Friend ? TPC_FriendClassTemplate
                                      : TPC_ClassTemplate,
          SkipBody))
    Invalid = true;
 
  if (SS.isSet()) {
    // If the name of the template was qualified, we must be defining the
    // template out-of-line.
    if (!SS.isInvalid() && !Invalid && !PrevClassTemplate) {
      Diag(NameLoc, TUK == TagUseKind::Friend
                        ? diag::err_friend_decl_does_not_match
                        : diag::err_member_decl_does_not_match)
          << Name << SemanticContext << /*IsDefinition*/ true << SS.getRange();
      Invalid = true;
    }
  }
 
  // If this is a templated friend in a dependent context we should not put it
  // on the redecl chain. In some cases, the templated friend can be the most
  // recent declaration tricking the template instantiator to make substitutions
  // there.
  // FIXME: Figure out how to combine with shouldLinkDependentDeclWithPrevious
  bool ShouldAddRedecl =
      !(TUK == TagUseKind::Friend && CurContext->isDependentContext());
 
  CXXRecordDecl *NewClass =
    CXXRecordDecl::Create(Context, Kind, SemanticContext, KWLoc, NameLoc, Name,
                          PrevClassTemplate && ShouldAddRedecl ?
                            PrevClassTemplate->getTemplatedDecl() : nullptr,
                          /*DelayTypeCreation=*/true);
  SetNestedNameSpecifier(*this, NewClass, SS);
  if (NumOuterTemplateParamLists > 0)
    NewClass->setTemplateParameterListsInfo(
        Context,
        llvm::ArrayRef(OuterTemplateParamLists, NumOuterTemplateParamLists));
 
  // Add alignment attributes if necessary; these attributes are checked when
  // the ASTContext lays out the structure.
  if (TUK == TagUseKind::Definition && (!SkipBody || !SkipBody->ShouldSkip)) {
    AddAlignmentAttributesForRecord(NewClass);
    AddMsStructLayoutForRecord(NewClass);
  }
 
  ClassTemplateDecl *NewTemplate
    = ClassTemplateDecl::Create(Context, SemanticContext, NameLoc,
                                DeclarationName(Name), TemplateParams,
                                NewClass);
 
  if (ShouldAddRedecl)
    NewTemplate->setPreviousDecl(PrevClassTemplate);
 
  NewClass->setDescribedClassTemplate(NewTemplate);
 
  if (ModulePrivateLoc.isValid())
    NewTemplate->setModulePrivate();
 
  // Build the type for the class template declaration now.
  QualType T = NewTemplate->getInjectedClassNameSpecialization();
  T = Context.getInjectedClassNameType(NewClass, T);
  assert(T->isDependentType() && "Class template type is not dependent?");
  (void)T;
 
  // If we are providing an explicit specialization of a member that is a
  // class template, make a note of that.
  if (PrevClassTemplate &&
      PrevClassTemplate->getInstantiatedFromMemberTemplate())
    PrevClassTemplate->setMemberSpecialization();
 
  // Set the access specifier.
  if (!Invalid && TUK != TagUseKind::Friend &&
      NewTemplate->getDeclContext()->isRecord())
    SetMemberAccessSpecifier(NewTemplate, PrevClassTemplate, AS);
 
  // Set the lexical context of these templates
  NewClass->setLexicalDeclContext(CurContext);
  NewTemplate->setLexicalDeclContext(CurContext);
 
  if (TUK == TagUseKind::Definition && (!SkipBody || !SkipBody->ShouldSkip))
    NewClass->startDefinition();
 
  ProcessDeclAttributeList(S, NewClass, Attr);
  ProcessAPINotes(NewClass);
 
  if (PrevClassTemplate)
    mergeDeclAttributes(NewClass, PrevClassTemplate->getTemplatedDecl());
 
  AddPushedVisibilityAttribute(NewClass);
  inferGslOwnerPointerAttribute(NewClass);
  inferNullableClassAttribute(NewClass);
 
  if (TUK != TagUseKind::Friend) {
    // Per C++ [basic.scope.temp]p2, skip the template parameter scopes.
    Scope *Outer = S;
    while ((Outer->getFlags() & Scope::TemplateParamScope) != 0)
      Outer = Outer->getParent();
    PushOnScopeChains(NewTemplate, Outer);
  } else {
    if (PrevClassTemplate && PrevClassTemplate->getAccess() != AS_none) {
      NewTemplate->setAccess(PrevClassTemplate->getAccess());
      NewClass->setAccess(PrevClassTemplate->getAccess());
    }
 
    NewTemplate->setObjectOfFriendDecl();
 
    // Friend templates are visible in fairly strange ways.
    if (!CurContext->isDependentContext()) {
      DeclContext *DC = SemanticContext->getRedeclContext();
      DC->makeDeclVisibleInContext(NewTemplate);
      if (Scope *EnclosingScope = getScopeForDeclContext(S, DC))
        PushOnScopeChains(NewTemplate, EnclosingScope,
                          /* AddToContext = */ false);
    }
 
    FriendDecl *Friend = FriendDecl::Create(
        Context, CurContext, NewClass->getLocation(), NewTemplate, FriendLoc);
    Friend->setAccess(AS_public);
    CurContext->addDecl(Friend);
  }
 
  if (PrevClassTemplate)
    CheckRedeclarationInModule(NewTemplate, PrevClassTemplate);
 
  if (Invalid) {
    NewTemplate->setInvalidDecl();
    NewClass->setInvalidDecl();
  }
 
  ActOnDocumentableDecl(NewTemplate);
 
  if (SkipBody && SkipBody->ShouldSkip)
    return SkipBody->Previous;
 
  return NewTemplate;
}
 
/// Diagnose the presence of a default template argument on a
/// template parameter, which is ill-formed in certain contexts.
///
/// \returns true if the default template argument should be dropped.
static bool DiagnoseDefaultTemplateArgument(Sema &S,
                                            Sema::TemplateParamListContext TPC,
                                            SourceLocation ParamLoc,
                                            SourceRange DefArgRange) {
  switch (TPC) {
  case Sema::TPC_ClassTemplate:
  case Sema::TPC_VarTemplate:
  case Sema::TPC_TypeAliasTemplate:
    return false;
 
  case Sema::TPC_FunctionTemplate:
  case Sema::TPC_FriendFunctionTemplateDefinition:
    // C++ [temp.param]p9:
    //   A default template-argument shall not be specified in a
    //   function template declaration or a function template
    //   definition [...]
    //   If a friend function template declaration specifies a default
    //   template-argument, that declaration shall be a definition and shall be
    //   the only declaration of the function template in the translation unit.
    // (C++98/03 doesn't have this wording; see DR226).
    S.Diag(ParamLoc, S.getLangOpts().CPlusPlus11 ?
         diag::warn_cxx98_compat_template_parameter_default_in_function_template
           : diag::ext_template_parameter_default_in_function_template)
      << DefArgRange;
    return false;
 
  case Sema::TPC_ClassTemplateMember:
    // C++0x [temp.param]p9:
    //   A default template-argument shall not be specified in the
    //   template-parameter-lists of the definition of a member of a
    //   class template that appears outside of the member's class.
    S.Diag(ParamLoc, diag::err_template_parameter_default_template_member)
      << DefArgRange;
    return true;
 
  case Sema::TPC_FriendClassTemplate:
  case Sema::TPC_FriendFunctionTemplate:
    // C++ [temp.param]p9:
    //   A default template-argument shall not be specified in a
    //   friend template declaration.
    S.Diag(ParamLoc, diag::err_template_parameter_default_friend_template)
      << DefArgRange;
    return true;
 
    // FIXME: C++0x [temp.param]p9 allows default template-arguments
    // for friend function templates if there is only a single
    // declaration (and it is a definition). Strange!
  }
 
  llvm_unreachable("Invalid TemplateParamListContext!");
}
 
/// Check for unexpanded parameter packs within the template parameters
/// of a template template parameter, recursively.
static bool DiagnoseUnexpandedParameterPacks(Sema &S,
                                             TemplateTemplateParmDecl *TTP) {
  // A template template parameter which is a parameter pack is also a pack
  // expansion.
  if (TTP->isParameterPack())
    return false;
 
  TemplateParameterList *Params = TTP->getTemplateParameters();
  for (unsigned I = 0, N = Params->size(); I != N; ++I) {
    NamedDecl *P = Params->getParam(I);
    if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(P)) {
      if (!TTP->isParameterPack())
        if (const TypeConstraint *TC = TTP->getTypeConstraint())
          if (TC->hasExplicitTemplateArgs())
            for (auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
              if (S.DiagnoseUnexpandedParameterPack(ArgLoc,
                                                    Sema::UPPC_TypeConstraint))
                return true;
      continue;
    }
 
    if (NonTypeTemplateParmDecl *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P)) {
      if (!NTTP->isParameterPack() &&
          S.DiagnoseUnexpandedParameterPack(NTTP->getLocation(),
                                            NTTP->getTypeSourceInfo(),
                                      Sema::UPPC_NonTypeTemplateParameterType))
        return true;
 
      continue;
    }
 
    if (TemplateTemplateParmDecl *InnerTTP
                                        = dyn_cast<TemplateTemplateParmDecl>(P))
      if (DiagnoseUnexpandedParameterPacks(S, InnerTTP))
        return true;
  }
 
  return false;
}
 
bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
                                      TemplateParameterList *OldParams,
                                      TemplateParamListContext TPC,
                                      SkipBodyInfo *SkipBody) {
  bool Invalid = false;
 
  // C++ [temp.param]p10:
  //   The set of default template-arguments available for use with a
  //   template declaration or definition is obtained by merging the
  //   default arguments from the definition (if in scope) and all
  //   declarations in scope in the same way default function
  //   arguments are (8.3.6).
  bool SawDefaultArgument = false;
  SourceLocation PreviousDefaultArgLoc;
 
  // Dummy initialization to avoid warnings.
  TemplateParameterList::iterator OldParam = NewParams->end();
  if (OldParams)
    OldParam = OldParams->begin();
 
  bool RemoveDefaultArguments = false;
  for (TemplateParameterList::iterator NewParam = NewParams->begin(),
                                    NewParamEnd = NewParams->end();
       NewParam != NewParamEnd; ++NewParam) {
    // Whether we've seen a duplicate default argument in the same translation
    // unit.
    bool RedundantDefaultArg = false;
    // Whether we've found inconsis inconsitent default arguments in different
    // translation unit.
    bool InconsistentDefaultArg = false;
    // The name of the module which contains the inconsistent default argument.
    std::string PrevModuleName;
 
    SourceLocation OldDefaultLoc;
    SourceLocation NewDefaultLoc;
 
    // Variable used to diagnose missing default arguments
    bool MissingDefaultArg = false;
 
    // Variable used to diagnose non-final parameter packs
    bool SawParameterPack = false;
 
    if (TemplateTypeParmDecl *NewTypeParm
          = dyn_cast<TemplateTypeParmDecl>(*NewParam)) {
      // Check the presence of a default argument here.
      if (NewTypeParm->hasDefaultArgument() &&
          DiagnoseDefaultTemplateArgument(
              *this, TPC, NewTypeParm->getLocation(),
              NewTypeParm->getDefaultArgument().getSourceRange()))
        NewTypeParm->removeDefaultArgument();
 
      // Merge default arguments for template type parameters.
      TemplateTypeParmDecl *OldTypeParm
          = OldParams? cast<TemplateTypeParmDecl>(*OldParam) : nullptr;
      if (NewTypeParm->isParameterPack()) {
        assert(!NewTypeParm->hasDefaultArgument() &&
               "Parameter packs can't have a default argument!");
        SawParameterPack = true;
      } else if (OldTypeParm && hasVisibleDefaultArgument(OldTypeParm) &&
                 NewTypeParm->hasDefaultArgument() &&
                 (!SkipBody || !SkipBody->ShouldSkip)) {
        OldDefaultLoc = OldTypeParm->getDefaultArgumentLoc();
        NewDefaultLoc = NewTypeParm->getDefaultArgumentLoc();
        SawDefaultArgument = true;
 
        if (!OldTypeParm->getOwningModule())
          RedundantDefaultArg = true;
        else if (!getASTContext().isSameDefaultTemplateArgument(OldTypeParm,
                                                                NewTypeParm)) {
          InconsistentDefaultArg = true;
          PrevModuleName =
              OldTypeParm->getImportedOwningModule()->getFullModuleName();
        }
        PreviousDefaultArgLoc = NewDefaultLoc;
      } else if (OldTypeParm && OldTypeParm->hasDefaultArgument()) {
        // Merge the default argument from the old declaration to the
        // new declaration.
        NewTypeParm->setInheritedDefaultArgument(Context, OldTypeParm);
        PreviousDefaultArgLoc = OldTypeParm->getDefaultArgumentLoc();
      } else if (NewTypeParm->hasDefaultArgument()) {
        SawDefaultArgument = true;
        PreviousDefaultArgLoc = NewTypeParm->getDefaultArgumentLoc();
      } else if (SawDefaultArgument)
        MissingDefaultArg = true;
    } else if (NonTypeTemplateParmDecl *NewNonTypeParm
               = dyn_cast<NonTypeTemplateParmDecl>(*NewParam)) {
      // Check for unexpanded parameter packs.
      if (!NewNonTypeParm->isParameterPack() &&
          DiagnoseUnexpandedParameterPack(NewNonTypeParm->getLocation(),
                                          NewNonTypeParm->getTypeSourceInfo(),
                                          UPPC_NonTypeTemplateParameterType)) {
        Invalid = true;
        continue;
      }
 
      // Check the presence of a default argument here.
      if (NewNonTypeParm->hasDefaultArgument() &&
          DiagnoseDefaultTemplateArgument(
              *this, TPC, NewNonTypeParm->getLocation(),
              NewNonTypeParm->getDefaultArgument().getSourceRange())) {
        NewNonTypeParm->removeDefaultArgument();
      }
 
      // Merge default arguments for non-type template parameters
      NonTypeTemplateParmDecl *OldNonTypeParm
        = OldParams? cast<NonTypeTemplateParmDecl>(*OldParam) : nullptr;
      if (NewNonTypeParm->isParameterPack()) {
        assert(!NewNonTypeParm->hasDefaultArgument() &&
               "Parameter packs can't have a default argument!");
        if (!NewNonTypeParm->isPackExpansion())
          SawParameterPack = true;
      } else if (OldNonTypeParm && hasVisibleDefaultArgument(OldNonTypeParm) &&
                 NewNonTypeParm->hasDefaultArgument() &&
                 (!SkipBody || !SkipBody->ShouldSkip)) {
        OldDefaultLoc = OldNonTypeParm->getDefaultArgumentLoc();
        NewDefaultLoc = NewNonTypeParm->getDefaultArgumentLoc();
        SawDefaultArgument = true;
        if (!OldNonTypeParm->getOwningModule())
          RedundantDefaultArg = true;
        else if (!getASTContext().isSameDefaultTemplateArgument(
                     OldNonTypeParm, NewNonTypeParm)) {
          InconsistentDefaultArg = true;
          PrevModuleName =
              OldNonTypeParm->getImportedOwningModule()->getFullModuleName();
        }
        PreviousDefaultArgLoc = NewDefaultLoc;
      } else if (OldNonTypeParm && OldNonTypeParm->hasDefaultArgument()) {
        // Merge the default argument from the old declaration to the
        // new declaration.
        NewNonTypeParm->setInheritedDefaultArgument(Context, OldNonTypeParm);
        PreviousDefaultArgLoc = OldNonTypeParm->getDefaultArgumentLoc();
      } else if (NewNonTypeParm->hasDefaultArgument()) {
        SawDefaultArgument = true;
        PreviousDefaultArgLoc = NewNonTypeParm->getDefaultArgumentLoc();
      } else if (SawDefaultArgument)
        MissingDefaultArg = true;
    } else {
      TemplateTemplateParmDecl *NewTemplateParm
        = cast<TemplateTemplateParmDecl>(*NewParam);
 
      // Check for unexpanded parameter packs, recursively.
      if (::DiagnoseUnexpandedParameterPacks(*this, NewTemplateParm)) {
        Invalid = true;
        continue;
      }
 
      // Check the presence of a default argument here.
      if (NewTemplateParm->hasDefaultArgument() &&
          DiagnoseDefaultTemplateArgument(*this, TPC,
                                          NewTemplateParm->getLocation(),
                     NewTemplateParm->getDefaultArgument().getSourceRange()))
        NewTemplateParm->removeDefaultArgument();
 
      // Merge default arguments for template template parameters
      TemplateTemplateParmDecl *OldTemplateParm
        = OldParams? cast<TemplateTemplateParmDecl>(*OldParam) : nullptr;
      if (NewTemplateParm->isParameterPack()) {
        assert(!NewTemplateParm->hasDefaultArgument() &&
               "Parameter packs can't have a default argument!");
        if (!NewTemplateParm->isPackExpansion())
          SawParameterPack = true;
      } else if (OldTemplateParm &&
                 hasVisibleDefaultArgument(OldTemplateParm) &&
                 NewTemplateParm->hasDefaultArgument() &&
                 (!SkipBody || !SkipBody->ShouldSkip)) {
        OldDefaultLoc = OldTemplateParm->getDefaultArgument().getLocation();
        NewDefaultLoc = NewTemplateParm->getDefaultArgument().getLocation();
        SawDefaultArgument = true;
        if (!OldTemplateParm->getOwningModule())
          RedundantDefaultArg = true;
        else if (!getASTContext().isSameDefaultTemplateArgument(
                     OldTemplateParm, NewTemplateParm)) {
          InconsistentDefaultArg = true;
          PrevModuleName =
              OldTemplateParm->getImportedOwningModule()->getFullModuleName();
        }
        PreviousDefaultArgLoc = NewDefaultLoc;
      } else if (OldTemplateParm && OldTemplateParm->hasDefaultArgument()) {
        // Merge the default argument from the old declaration to the
        // new declaration.
        NewTemplateParm->setInheritedDefaultArgument(Context, OldTemplateParm);
        PreviousDefaultArgLoc
          = OldTemplateParm->getDefaultArgument().getLocation();
      } else if (NewTemplateParm->hasDefaultArgument()) {
        SawDefaultArgument = true;
        PreviousDefaultArgLoc
          = NewTemplateParm->getDefaultArgument().getLocation();
      } else if (SawDefaultArgument)
        MissingDefaultArg = true;
    }
 
    // C++11 [temp.param]p11:
    //   If a template parameter of a primary class template or alias template
    //   is a template parameter pack, it shall be the last template parameter.
    if (SawParameterPack && (NewParam + 1) != NewParamEnd &&
        (TPC == TPC_ClassTemplate || TPC == TPC_VarTemplate ||
         TPC == TPC_TypeAliasTemplate)) {
      Diag((*NewParam)->getLocation(),
           diag::err_template_param_pack_must_be_last_template_parameter);
      Invalid = true;
    }
 
    // [basic.def.odr]/13:
    //     There can be more than one definition of a
    //     ...
    //     default template argument
    //     ...
    //     in a program provided that each definition appears in a different
    //     translation unit and the definitions satisfy the [same-meaning
    //     criteria of the ODR].
    //
    // Simply, the design of modules allows the definition of template default
    // argument to be repeated across translation unit. Note that the ODR is
    // checked elsewhere. But it is still not allowed to repeat template default
    // argument in the same translation unit.
    if (RedundantDefaultArg) {
      Diag(NewDefaultLoc, diag::err_template_param_default_arg_redefinition);
      Diag(OldDefaultLoc, diag::note_template_param_prev_default_arg);
      Invalid = true;
    } else if (InconsistentDefaultArg) {
      // We could only diagnose about the case that the OldParam is imported.
      // The case NewParam is imported should be handled in ASTReader.
      Diag(NewDefaultLoc,
           diag::err_template_param_default_arg_inconsistent_redefinition);
      Diag(OldDefaultLoc,
           diag::note_template_param_prev_default_arg_in_other_module)
          << PrevModuleName;
      Invalid = true;
    } else if (MissingDefaultArg &&
               (TPC == TPC_ClassTemplate || TPC == TPC_FriendClassTemplate ||
                TPC == TPC_VarTemplate || TPC == TPC_TypeAliasTemplate)) {
      // C++ 23[temp.param]p14:
      // If a template-parameter of a class template, variable template, or
      // alias template has a default template argument, each subsequent
      // template-parameter shall either have a default template argument
      // supplied or be a template parameter pack.
      Diag((*NewParam)->getLocation(),
           diag::err_template_param_default_arg_missing);
      Diag(PreviousDefaultArgLoc, diag::note_template_param_prev_default_arg);
      Invalid = true;
      RemoveDefaultArguments = true;
    }
 
    // If we have an old template parameter list that we're merging
    // in, move on to the next parameter.
    if (OldParams)
      ++OldParam;
  }
 
  // We were missing some default arguments at the end of the list, so remove
  // all of the default arguments.
  if (RemoveDefaultArguments) {
    for (TemplateParameterList::iterator NewParam = NewParams->begin(),
                                      NewParamEnd = NewParams->end();
         NewParam != NewParamEnd; ++NewParam) {
      if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(*NewParam))
        TTP->removeDefaultArgument();
      else if (NonTypeTemplateParmDecl *NTTP
                                = dyn_cast<NonTypeTemplateParmDecl>(*NewParam))
        NTTP->removeDefaultArgument();
      else
        cast<TemplateTemplateParmDecl>(*NewParam)->removeDefaultArgument();
    }
  }
 
  return Invalid;
}
 
namespace {
 
/// A class which looks for a use of a certain level of template
/// parameter.
struct DependencyChecker : DynamicRecursiveASTVisitor {
  unsigned Depth;
 
  // Whether we're looking for a use of a template parameter that makes the
  // overall construct type-dependent / a dependent type. This is strictly
  // best-effort for now; we may fail to match at all for a dependent type
  // in some cases if this is set.
  bool IgnoreNonTypeDependent;
 
  bool Match;
  SourceLocation MatchLoc;
 
  DependencyChecker(unsigned Depth, bool IgnoreNonTypeDependent)
      : Depth(Depth), IgnoreNonTypeDependent(IgnoreNonTypeDependent),
        Match(false) {}
 
  DependencyChecker(TemplateParameterList *Params, bool IgnoreNonTypeDependent)
      : IgnoreNonTypeDependent(IgnoreNonTypeDependent), Match(false) {
    NamedDecl *ND = Params->getParam(0);
    if (TemplateTypeParmDecl *PD = dyn_cast<TemplateTypeParmDecl>(ND)) {
      Depth = PD->getDepth();
    } else if (NonTypeTemplateParmDecl *PD =
                 dyn_cast<NonTypeTemplateParmDecl>(ND)) {
      Depth = PD->getDepth();
    } else {
      Depth = cast<TemplateTemplateParmDecl>(ND)->getDepth();
    }
  }
 
  bool Matches(unsigned ParmDepth, SourceLocation Loc = SourceLocation()) {
    if (ParmDepth >= Depth) {
      Match = true;
      MatchLoc = Loc;
      return true;
    }
    return false;
  }
 
  bool TraverseStmt(Stmt *S) override {
    // Prune out non-type-dependent expressions if requested. This can
    // sometimes result in us failing to find a template parameter reference
    // (if a value-dependent expression creates a dependent type), but this
    // mode is best-effort only.
    if (auto *E = dyn_cast_or_null<Expr>(S))
      if (IgnoreNonTypeDependent && !E->isTypeDependent())
        return true;
    return DynamicRecursiveASTVisitor::TraverseStmt(S);
  }
 
  bool TraverseTypeLoc(TypeLoc TL) override {
    if (IgnoreNonTypeDependent && !TL.isNull() &&
        !TL.getType()->isDependentType())
      return true;
    return DynamicRecursiveASTVisitor::TraverseTypeLoc(TL);
  }
 
  bool VisitTemplateTypeParmTypeLoc(TemplateTypeParmTypeLoc TL) override {
    return !Matches(TL.getTypePtr()->getDepth(), TL.getNameLoc());
  }
 
  bool VisitTemplateTypeParmType(TemplateTypeParmType *T) override {
    // For a best-effort search, keep looking until we find a location.
    return IgnoreNonTypeDependent || !Matches(T->getDepth());
  }
 
  bool TraverseTemplateName(TemplateName N) override {
    if (TemplateTemplateParmDecl *PD =
          dyn_cast_or_null<TemplateTemplateParmDecl>(N.getAsTemplateDecl()))
      if (Matches(PD->getDepth()))
        return false;
    return DynamicRecursiveASTVisitor::TraverseTemplateName(N);
  }
 
  bool VisitDeclRefExpr(DeclRefExpr *E) override {
    if (NonTypeTemplateParmDecl *PD =
          dyn_cast<NonTypeTemplateParmDecl>(E->getDecl()))
      if (Matches(PD->getDepth(), E->getExprLoc()))
        return false;
    return DynamicRecursiveASTVisitor::VisitDeclRefExpr(E);
  }
 
  bool VisitSubstTemplateTypeParmType(SubstTemplateTypeParmType *T) override {
    return TraverseType(T->getReplacementType());
  }
 
  bool VisitSubstTemplateTypeParmPackType(
      SubstTemplateTypeParmPackType *T) override {
    return TraverseTemplateArgument(T->getArgumentPack());
  }
 
  bool TraverseInjectedClassNameType(InjectedClassNameType *T) override {
    return TraverseType(T->getInjectedSpecializationType());
  }
};
} // end anonymous namespace
 
/// Determines whether a given type depends on the given parameter
/// list.
static bool
DependsOnTemplateParameters(QualType T, TemplateParameterList *Params) {
  if (!Params->size())
    return false;
 
  DependencyChecker Checker(Params, /*IgnoreNonTypeDependent*/false);
  Checker.TraverseType(T);
  return Checker.Match;
}
 
// Find the source range corresponding to the named type in the given
// nested-name-specifier, if any.
static SourceRange getRangeOfTypeInNestedNameSpecifier(ASTContext &Context,
                                                       QualType T,
                                                       const CXXScopeSpec &SS) {
  NestedNameSpecifierLoc NNSLoc(SS.getScopeRep(), SS.location_data());
  while (NestedNameSpecifier *NNS = NNSLoc.getNestedNameSpecifier()) {
    if (const Type *CurType = NNS->getAsType()) {
      if (Context.hasSameUnqualifiedType(T, QualType(CurType, 0)))
        return NNSLoc.getTypeLoc().getSourceRange();
    } else
      break;
 
    NNSLoc = NNSLoc.getPrefix();
  }
 
  return SourceRange();
}
 
TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
    SourceLocation DeclStartLoc, SourceLocation DeclLoc, const CXXScopeSpec &SS,
    TemplateIdAnnotation *TemplateId,
    ArrayRef<TemplateParameterList *> ParamLists, bool IsFriend,
    bool &IsMemberSpecialization, bool &Invalid, bool SuppressDiagnostic) {
  IsMemberSpecialization = false;
  Invalid = false;
 
  // The sequence of nested types to which we will match up the template
  // parameter lists. We first build this list by starting with the type named
  // by the nested-name-specifier and walking out until we run out of types.
  SmallVector<QualType, 4> NestedTypes;
  QualType T;
  if (SS.getScopeRep()) {
    if (CXXRecordDecl *Record
              = dyn_cast_or_null<CXXRecordDecl>(computeDeclContext(SS, true)))
      T = Context.getTypeDeclType(Record);
    else
      T = QualType(SS.getScopeRep()->getAsType(), 0);
  }
 
  // If we found an explicit specialization that prevents us from needing
  // 'template<>' headers, this will be set to the location of that
  // explicit specialization.
  SourceLocation ExplicitSpecLoc;
 
  while (!T.isNull()) {
    NestedTypes.push_back(T);
 
    // Retrieve the parent of a record type.
    if (CXXRecordDecl *Record = T->getAsCXXRecordDecl()) {
      // If this type is an explicit specialization, we're done.
      if (ClassTemplateSpecializationDecl *Spec
          = dyn_cast<ClassTemplateSpecializationDecl>(Record)) {
        if (!isa<ClassTemplatePartialSpecializationDecl>(Spec) &&
            Spec->getSpecializationKind() == TSK_ExplicitSpecialization) {
          ExplicitSpecLoc = Spec->getLocation();
          break;
        }
      } else if (Record->getTemplateSpecializationKind()
                                                == TSK_ExplicitSpecialization) {
        ExplicitSpecLoc = Record->getLocation();
        break;
      }
 
      if (TypeDecl *Parent = dyn_cast<TypeDecl>(Record->getParent()))
        T = Context.getTypeDeclType(Parent);
      else
        T = QualType();
      continue;
    }
 
    if (const TemplateSpecializationType *TST
                                     = T->getAs<TemplateSpecializationType>()) {
      if (TemplateDecl *Template = TST->getTemplateName().getAsTemplateDecl()) {
        if (TypeDecl *Parent = dyn_cast<TypeDecl>(Template->getDeclContext()))
          T = Context.getTypeDeclType(Parent);
        else
          T = QualType();
        continue;
      }
    }
 
    // Look one step prior in a dependent template specialization type.
    if (const DependentTemplateSpecializationType *DependentTST
                          = T->getAs<DependentTemplateSpecializationType>()) {
      if (NestedNameSpecifier *NNS = DependentTST->getQualifier())
        T = QualType(NNS->getAsType(), 0);
      else
        T = QualType();
      continue;
    }
 
    // Look one step prior in a dependent name type.
    if (const DependentNameType *DependentName = T->getAs<DependentNameType>()){
      if (NestedNameSpecifier *NNS = DependentName->getQualifier())
        T = QualType(NNS->getAsType(), 0);
      else
        T = QualType();
      continue;
    }
 
    // Retrieve the parent of an enumeration type.
    if (const EnumType *EnumT = T->getAs<EnumType>()) {
      // FIXME: Forward-declared enums require a TSK_ExplicitSpecialization
      // check here.
      EnumDecl *Enum = EnumT->getDecl();
 
      // Get to the parent type.
      if (TypeDecl *Parent = dyn_cast<TypeDecl>(Enum->getParent()))
        T = Context.getTypeDeclType(Parent);
      else
        T = QualType();
      continue;
    }
 
    T = QualType();
  }
  // Reverse the nested types list, since we want to traverse from the outermost
  // to the innermost while checking template-parameter-lists.
  std::reverse(NestedTypes.begin(), NestedTypes.end());
 
  // C++0x [temp.expl.spec]p17:
  //   A member or a member template may be nested within many
  //   enclosing class templates. In an explicit specialization for
  //   such a member, the member declaration shall be preceded by a
  //   template<> for each enclosing class template that is
  //   explicitly specialized.
  bool SawNonEmptyTemplateParameterList = false;
 
  auto CheckExplicitSpecialization = [&](SourceRange Range, bool Recovery) {
    if (SawNonEmptyTemplateParameterList) {
      if (!SuppressDiagnostic)
        Diag(DeclLoc, diag::err_specialize_member_of_template)
          << !Recovery << Range;
      Invalid = true;
      IsMemberSpecialization = false;
      return true;
    }
 
    return false;
  };
 
  auto DiagnoseMissingExplicitSpecialization = [&] (SourceRange Range) {
    // Check that we can have an explicit specialization here.
    if (CheckExplicitSpecialization(Range, true))
      return true;
 
    // We don't have a template header, but we should.
    SourceLocation ExpectedTemplateLoc;
    if (!ParamLists.empty())
      ExpectedTemplateLoc = ParamLists[0]->getTemplateLoc();
    else
      ExpectedTemplateLoc = DeclStartLoc;
 
    if (!SuppressDiagnostic)
      Diag(DeclLoc, diag::err_template_spec_needs_header)
        << Range
        << FixItHint::CreateInsertion(ExpectedTemplateLoc, "template<> ");
    return false;
  };
 
  unsigned ParamIdx = 0;
  for (unsigned TypeIdx = 0, NumTypes = NestedTypes.size(); TypeIdx != NumTypes;
       ++TypeIdx) {
    T = NestedTypes[TypeIdx];
 
    // Whether we expect a 'template<>' header.
    bool NeedEmptyTemplateHeader = false;
 
    // Whether we expect a template header with parameters.
    bool NeedNonemptyTemplateHeader = false;
 
    // For a dependent type, the set of template parameters that we
    // expect to see.
    TemplateParameterList *ExpectedTemplateParams = nullptr;
 
    // C++0x [temp.expl.spec]p15:
    //   A member or a member template may be nested within many enclosing
    //   class templates. In an explicit specialization for such a member, the
    //   member declaration shall be preceded by a template<> for each
    //   enclosing class template that is explicitly specialized.
    if (CXXRecordDecl *Record = T->getAsCXXRecordDecl()) {
      if (ClassTemplatePartialSpecializationDecl *Partial
            = dyn_cast<ClassTemplatePartialSpecializationDecl>(Record)) {
        ExpectedTemplateParams = Partial->getTemplateParameters();
        NeedNonemptyTemplateHeader = true;
      } else if (Record->isDependentType()) {
        if (Record->getDescribedClassTemplate()) {
          ExpectedTemplateParams = Record->getDescribedClassTemplate()
                                                      ->getTemplateParameters();
          NeedNonemptyTemplateHeader = true;
        }
      } else if (ClassTemplateSpecializationDecl *Spec
                     = dyn_cast<ClassTemplateSpecializationDecl>(Record)) {
        // C++0x [temp.expl.spec]p4:
        //   Members of an explicitly specialized class template are defined
        //   in the same manner as members of normal classes, and not using
        //   the template<> syntax.
        if (Spec->getSpecializationKind() != TSK_ExplicitSpecialization)
          NeedEmptyTemplateHeader = true;
        else
          continue;
      } else if (Record->getTemplateSpecializationKind()) {
        if (Record->getTemplateSpecializationKind()
                                                != TSK_ExplicitSpecialization &&
            TypeIdx == NumTypes - 1)
          IsMemberSpecialization = true;
 
        continue;
      }
    } else if (const TemplateSpecializationType *TST
                                     = T->getAs<TemplateSpecializationType>()) {
      if (TemplateDecl *Template = TST->getTemplateName().getAsTemplateDecl()) {
        ExpectedTemplateParams = Template->getTemplateParameters();
        NeedNonemptyTemplateHeader = true;
      }
    } else if (T->getAs<DependentTemplateSpecializationType>()) {
      // FIXME:  We actually could/should check the template arguments here
      // against the corresponding template parameter list.
      NeedNonemptyTemplateHeader = false;
    }
 
    // C++ [temp.expl.spec]p16:
    //   In an explicit specialization declaration for a member of a class
    //   template or a member template that appears in namespace scope, the
    //   member template and some of its enclosing class templates may remain
    //   unspecialized, except that the declaration shall not explicitly
    //   specialize a class member template if its enclosing class templates
    //   are not explicitly specialized as well.
    if (ParamIdx < ParamLists.size()) {
      if (ParamLists[ParamIdx]->size() == 0) {
        if (CheckExplicitSpecialization(ParamLists[ParamIdx]->getSourceRange(),
                                        false))
          return nullptr;
      } else
        SawNonEmptyTemplateParameterList = true;
    }
 
    if (NeedEmptyTemplateHeader) {
      // If we're on the last of the types, and we need a 'template<>' header
      // here, then it's a member specialization.
      if (TypeIdx == NumTypes - 1)
        IsMemberSpecialization = true;
 
      if (ParamIdx < ParamLists.size()) {
        if (ParamLists[ParamIdx]->size() > 0) {
          // The header has template parameters when it shouldn't. Complain.
          if (!SuppressDiagnostic)
            Diag(ParamLists[ParamIdx]->getTemplateLoc(),
                 diag::err_template_param_list_matches_nontemplate)
              << T
              << SourceRange(ParamLists[ParamIdx]->getLAngleLoc(),
                             ParamLists[ParamIdx]->getRAngleLoc())
              << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
          Invalid = true;
          return nullptr;
        }
 
        // Consume this template header.
        ++ParamIdx;
        continue;
      }
 
      if (!IsFriend)
        if (DiagnoseMissingExplicitSpecialization(
                getRangeOfTypeInNestedNameSpecifier(Context, T, SS)))
          return nullptr;
 
      continue;
    }
 
    if (NeedNonemptyTemplateHeader) {
      // In friend declarations we can have template-ids which don't
      // depend on the corresponding template parameter lists.  But
      // assume that empty parameter lists are supposed to match this
      // template-id.
      if (IsFriend && T->isDependentType()) {
        if (ParamIdx < ParamLists.size() &&
            DependsOnTemplateParameters(T, ParamLists[ParamIdx]))
          ExpectedTemplateParams = nullptr;
        else
          continue;
      }
 
      if (ParamIdx < ParamLists.size()) {
        // Check the template parameter list, if we can.
        if (ExpectedTemplateParams &&
            !TemplateParameterListsAreEqual(ParamLists[ParamIdx],
                                            ExpectedTemplateParams,
                                            !SuppressDiagnostic, TPL_TemplateMatch))
          Invalid = true;
 
        if (!Invalid &&
            CheckTemplateParameterList(ParamLists[ParamIdx], nullptr,
                                       TPC_ClassTemplateMember))
          Invalid = true;
 
        ++ParamIdx;
        continue;
      }
 
      if (!SuppressDiagnostic)
        Diag(DeclLoc, diag::err_template_spec_needs_template_parameters)
          << T
          << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
      Invalid = true;
      continue;
    }
  }
 
  // If there were at least as many template-ids as there were template
  // parameter lists, then there are no template parameter lists remaining for
  // the declaration itself.
  if (ParamIdx >= ParamLists.size()) {
    if (TemplateId && !IsFriend) {
      // We don't have a template header for the declaration itself, but we
      // should.
      DiagnoseMissingExplicitSpecialization(SourceRange(TemplateId->LAngleLoc,
                                                        TemplateId->RAngleLoc));
 
      // Fabricate an empty template parameter list for the invented header.
      return TemplateParameterList::Create(Context, SourceLocation(),
                                           SourceLocation(), {},
                                           SourceLocation(), nullptr);
    }
 
    return nullptr;
  }
 
  // If there were too many template parameter lists, complain about that now.
  if (ParamIdx < ParamLists.size() - 1) {
    bool HasAnyExplicitSpecHeader = false;
    bool AllExplicitSpecHeaders = true;
    for (unsigned I = ParamIdx, E = ParamLists.size() - 1; I != E; ++I) {
      if (ParamLists[I]->size() == 0)
        HasAnyExplicitSpecHeader = true;
      else
        AllExplicitSpecHeaders = false;
    }
 
    if (!SuppressDiagnostic)
      Diag(ParamLists[ParamIdx]->getTemplateLoc(),
           AllExplicitSpecHeaders ? diag::ext_template_spec_extra_headers
                                  : diag::err_template_spec_extra_headers)
          << SourceRange(ParamLists[ParamIdx]->getTemplateLoc(),
                         ParamLists[ParamLists.size() - 2]->getRAngleLoc());
 
    // If there was a specialization somewhere, such that 'template<>' is
    // not required, and there were any 'template<>' headers, note where the
    // specialization occurred.
    if (ExplicitSpecLoc.isValid() && HasAnyExplicitSpecHeader &&
        !SuppressDiagnostic)
      Diag(ExplicitSpecLoc,
           diag::note_explicit_template_spec_does_not_need_header)
        << NestedTypes.back();
 
    // We have a template parameter list with no corresponding scope, which
    // means that the resulting template declaration can't be instantiated
    // properly (we'll end up with dependent nodes when we shouldn't).
    if (!AllExplicitSpecHeaders)
      Invalid = true;
  }
 
  // C++ [temp.expl.spec]p16:
  //   In an explicit specialization declaration for a member of a class
  //   template or a member template that ap- pears in namespace scope, the
  //   member template and some of its enclosing class templates may remain
  //   unspecialized, except that the declaration shall not explicitly
  //   specialize a class member template if its en- closing class templates
  //   are not explicitly specialized as well.
  if (ParamLists.back()->size() == 0 &&
      CheckExplicitSpecialization(ParamLists[ParamIdx]->getSourceRange(),
                                  false))
    return nullptr;
 
  // Return the last template parameter list, which corresponds to the
  // entity being declared.
  return ParamLists.back();
}
 
void Sema::NoteAllFoundTemplates(TemplateName Name) {
  if (TemplateDecl *Template = Name.getAsTemplateDecl()) {
    Diag(Template->getLocation(), diag::note_template_declared_here)
        << (isa<FunctionTemplateDecl>(Template)
                ? 0
                : isa<ClassTemplateDecl>(Template)
                      ? 1
                      : isa<VarTemplateDecl>(Template)
                            ? 2
                            : isa<TypeAliasTemplateDecl>(Template) ? 3 : 4)
        << Template->getDeclName();
    return;
  }
 
  if (OverloadedTemplateStorage *OST = Name.getAsOverloadedTemplate()) {
    for (OverloadedTemplateStorage::iterator I = OST->begin(),
                                          IEnd = OST->end();
         I != IEnd; ++I)
      Diag((*I)->getLocation(), diag::note_template_declared_here)
        << 0 << (*I)->getDeclName();
 
    return;
  }
}
 
static QualType builtinCommonTypeImpl(Sema &S, TemplateName BaseTemplate,
                                      SourceLocation TemplateLoc,
                                      ArrayRef<TemplateArgument> Ts) {
  auto lookUpCommonType = [&](TemplateArgument T1,
                              TemplateArgument T2) -> QualType {
    // Don't bother looking for other specializations if both types are
    // builtins - users aren't allowed to specialize for them
    if (T1.getAsType()->isBuiltinType() && T2.getAsType()->isBuiltinType())
      return builtinCommonTypeImpl(S, BaseTemplate, TemplateLoc, {T1, T2});
 
    TemplateArgumentListInfo Args;
    Args.addArgument(TemplateArgumentLoc(
        T1, S.Context.getTrivialTypeSourceInfo(T1.getAsType())));
    Args.addArgument(TemplateArgumentLoc(
        T2, S.Context.getTrivialTypeSourceInfo(T2.getAsType())));
 
    EnterExpressionEvaluationContext UnevaluatedContext(
        S, Sema::ExpressionEvaluationContext::Unevaluated);
    Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
    Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
 
    QualType BaseTemplateInst =
        S.CheckTemplateIdType(BaseTemplate, TemplateLoc, Args);
 
    if (SFINAE.hasErrorOccurred())
      return QualType();
 
    return BaseTemplateInst;
  };
 
  // Note A: For the common_type trait applied to a template parameter pack T of
  // types, the member type shall be either defined or not present as follows:
  switch (Ts.size()) {
 
  // If sizeof...(T) is zero, there shall be no member type.
  case 0:
    return QualType();
 
  // If sizeof...(T) is one, let T0 denote the sole type constituting the
  // pack T. The member typedef-name type shall denote the same type, if any, as
  // common_type_t<T0, T0>; otherwise there shall be no member type.
  case 1:
    return lookUpCommonType(Ts[0], Ts[0]);
 
  // If sizeof...(T) is two, let the first and second types constituting T be
  // denoted by T1 and T2, respectively, and let D1 and D2 denote the same types
  // as decay_t<T1> and decay_t<T2>, respectively.
  case 2: {
    QualType T1 = Ts[0].getAsType();
    QualType T2 = Ts[1].getAsType();
    QualType D1 = S.BuiltinDecay(T1, {});
    QualType D2 = S.BuiltinDecay(T2, {});
 
    // If is_same_v<T1, D1> is false or is_same_v<T2, D2> is false, let C denote
    // the same type, if any, as common_type_t<D1, D2>.
    if (!S.Context.hasSameType(T1, D1) || !S.Context.hasSameType(T2, D2))
      return lookUpCommonType(D1, D2);
 
    // Otherwise, if decay_t<decltype(false ? declval<D1>() : declval<D2>())>
    // denotes a valid type, let C denote that type.
    {
      auto CheckConditionalOperands = [&](bool ConstRefQual) -> QualType {
        EnterExpressionEvaluationContext UnevaluatedContext(
            S, Sema::ExpressionEvaluationContext::Unevaluated);
        Sema::SFINAETrap SFINAE(S, /*AccessCheckingSFINAE=*/true);
        Sema::ContextRAII TUContext(S, S.Context.getTranslationUnitDecl());
 
        // false
        OpaqueValueExpr CondExpr(SourceLocation(), S.Context.BoolTy,
                                 VK_PRValue);
        ExprResult Cond = &CondExpr;
 
        auto EVK = ConstRefQual ? VK_LValue : VK_PRValue;
        if (ConstRefQual) {
          D1.addConst();
          D2.addConst();
        }
 
        // declval<D1>()
        OpaqueValueExpr LHSExpr(TemplateLoc, D1, EVK);
        ExprResult LHS = &LHSExpr;
 
        // declval<D2>()
        OpaqueValueExpr RHSExpr(TemplateLoc, D2, EVK);
        ExprResult RHS = &RHSExpr;
 
        ExprValueKind VK = VK_PRValue;
        ExprObjectKind OK = OK_Ordinary;
 
        // decltype(false ? declval<D1>() : declval<D2>())
        QualType Result =
            S.CheckConditionalOperands(Cond, LHS, RHS, VK, OK, TemplateLoc);
 
        if (Result.isNull() || SFINAE.hasErrorOccurred())
          return QualType();
 
        // decay_t<decltype(false ? declval<D1>() : declval<D2>())>
        return S.BuiltinDecay(Result, TemplateLoc);
      };
 
      if (auto Res = CheckConditionalOperands(false); !Res.isNull())
        return Res;
 
      // Let:
      // CREF(A) be add_lvalue_reference_t<const remove_reference_t<A>>,
      // COND-RES(X, Y) be
      //   decltype(false ? declval<X(&)()>()() : declval<Y(&)()>()()).
 
      // C++20 only
      // Otherwise, if COND-RES(CREF(D1), CREF(D2)) denotes a type, let C denote
      // the type decay_t<COND-RES(CREF(D1), CREF(D2))>.
      if (!S.Context.getLangOpts().CPlusPlus20)
        return QualType();
      return CheckConditionalOperands(true);
    }
  }
 
  // If sizeof...(T) is greater than two, let T1, T2, and R, respectively,
  // denote the first, second, and (pack of) remaining types constituting T. Let
  // C denote the same type, if any, as common_type_t<T1, T2>. If there is such
  // a type C, the member typedef-name type shall denote the same type, if any,
  // as common_type_t<C, R...>. Otherwise, there shall be no member type.
  default: {
    QualType Result = Ts.front().getAsType();
    for (auto T : llvm::drop_begin(Ts)) {
      Result = lookUpCommonType(Result, T.getAsType());
      if (Result.isNull())
        return QualType();
    }
    return Result;
  }
  }
}
 
static QualType
checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
                           ArrayRef<TemplateArgument> Converted,
                           SourceLocation TemplateLoc,
                           TemplateArgumentListInfo &TemplateArgs) {
  ASTContext &Context = SemaRef.getASTContext();
 
  switch (BTD->getBuiltinTemplateKind()) {
  case BTK__make_integer_seq: {
    // Specializations of __make_integer_seq<S, T, N> are treated like
    // S<T, 0, ..., N-1>.
 
    QualType OrigType = Converted[1].getAsType();
    // C++14 [inteseq.intseq]p1:
    //   T shall be an integer type.
    if (!OrigType->isDependentType() && !OrigType->isIntegralType(Context)) {
      SemaRef.Diag(TemplateArgs[1].getLocation(),
                   diag::err_integer_sequence_integral_element_type);
      return QualType();
    }
 
    TemplateArgument NumArgsArg = Converted[2];
    if (NumArgsArg.isDependent())
      return Context.getCanonicalTemplateSpecializationType(TemplateName(BTD),
                                                            Converted);
 
    TemplateArgumentListInfo SyntheticTemplateArgs;
    // The type argument, wrapped in substitution sugar, gets reused as the
    // first template argument in the synthetic template argument list.
    SyntheticTemplateArgs.addArgument(
        TemplateArgumentLoc(TemplateArgument(OrigType),
                            SemaRef.Context.getTrivialTypeSourceInfo(
                                OrigType, TemplateArgs[1].getLocation())));
 
    if (llvm::APSInt NumArgs = NumArgsArg.getAsIntegral(); NumArgs >= 0) {
      // Expand N into 0 ... N-1.
      for (llvm::APSInt I(NumArgs.getBitWidth(), NumArgs.isUnsigned());
           I < NumArgs; ++I) {
        TemplateArgument TA(Context, I, OrigType);
        SyntheticTemplateArgs.addArgument(SemaRef.getTrivialTemplateArgumentLoc(
            TA, OrigType, TemplateArgs[2].getLocation()));
      }
    } else {
      // C++14 [inteseq.make]p1:
      //   If N is negative the program is ill-formed.
      SemaRef.Diag(TemplateArgs[2].getLocation(),
                   diag::err_integer_sequence_negative_length);
      return QualType();
    }
 
    // The first template argument will be reused as the template decl that
    // our synthetic template arguments will be applied to.
    return SemaRef.CheckTemplateIdType(Converted[0].getAsTemplate(),
                                       TemplateLoc, SyntheticTemplateArgs);
  }
 
  case BTK__type_pack_element: {
    // Specializations of
    //    __type_pack_element<Index, T_1, ..., T_N>
    // are treated like T_Index.
    assert(Converted.size() == 2 &&
      "__type_pack_element should be given an index and a parameter pack");
 
    TemplateArgument IndexArg = Converted[0], Ts = Converted[1];
    if (IndexArg.isDependent() || Ts.isDependent())
      return Context.getCanonicalTemplateSpecializationType(TemplateName(BTD),
                                                            Converted);
 
    llvm::APSInt Index = IndexArg.getAsIntegral();
    assert(Index >= 0 && "the index used with __type_pack_element should be of "
                         "type std::size_t, and hence be non-negative");
    // If the Index is out of bounds, the program is ill-formed.
    if (Index >= Ts.pack_size()) {
      SemaRef.Diag(TemplateArgs[0].getLocation(),
                   diag::err_type_pack_element_out_of_bounds);
      return QualType();
    }
 
    // We simply return the type at index `Index`.
    int64_t N = Index.getExtValue();
    return Ts.getPackAsArray()[N].getAsType();
  }
 
  case BTK__builtin_common_type: {
    assert(Converted.size() == 4);
    if (llvm::any_of(Converted, [](auto &C) { return C.isDependent(); }))
      return Context.getCanonicalTemplateSpecializationType(TemplateName(BTD),
                                                            Converted);
 
    TemplateName BaseTemplate = Converted[0].getAsTemplate();
    TemplateName HasTypeMember = Converted[1].getAsTemplate();
    QualType HasNoTypeMember = Converted[2].getAsType();
    ArrayRef<TemplateArgument> Ts = Converted[3].getPackAsArray();
    if (auto CT = builtinCommonTypeImpl(SemaRef, BaseTemplate, TemplateLoc, Ts);
        !CT.isNull()) {
      TemplateArgumentListInfo TAs;
      TAs.addArgument(TemplateArgumentLoc(
          TemplateArgument(CT), SemaRef.Context.getTrivialTypeSourceInfo(
                                    CT, TemplateArgs[1].getLocation())));
 
      return SemaRef.CheckTemplateIdType(HasTypeMember, TemplateLoc, TAs);
    }
    return HasNoTypeMember;
  }
  }
  llvm_unreachable("unexpected BuiltinTemplateDecl!");
}
 
/// Determine whether this alias template is "enable_if_t".
/// libc++ >=14 uses "__enable_if_t" in C++11 mode.
static bool isEnableIfAliasTemplate(TypeAliasTemplateDecl *AliasTemplate) {
  return AliasTemplate->getName() == "enable_if_t" ||
         AliasTemplate->getName() == "__enable_if_t";
}
 
/// Collect all of the separable terms in the given condition, which
/// might be a conjunction.
///
/// FIXME: The right answer is to convert the logical expression into
/// disjunctive normal form, so we can find the first failed term
/// within each possible clause.
static void collectConjunctionTerms(Expr *Clause,
                                    SmallVectorImpl<Expr *> &Terms) {
  if (auto BinOp = dyn_cast<BinaryOperator>(Clause->IgnoreParenImpCasts())) {
    if (BinOp->getOpcode() == BO_LAnd) {
      collectConjunctionTerms(BinOp->getLHS(), Terms);
      collectConjunctionTerms(BinOp->getRHS(), Terms);
      return;
    }
  }
 
  Terms.push_back(Clause);
}
 
// The ranges-v3 library uses an odd pattern of a top-level "||" with
// a left-hand side that is value-dependent but never true. Identify
// the idiom and ignore that term.
static Expr *lookThroughRangesV3Condition(Preprocessor &PP, Expr *Cond) {
  // Top-level '||'.
  auto *BinOp = dyn_cast<BinaryOperator>(Cond->IgnoreParenImpCasts());
  if (!BinOp) return Cond;
 
  if (BinOp->getOpcode() != BO_LOr) return Cond;
 
  // With an inner '==' that has a literal on the right-hand side.
  Expr *LHS = BinOp->getLHS();
  auto *InnerBinOp = dyn_cast<BinaryOperator>(LHS->IgnoreParenImpCasts());
  if (!InnerBinOp) return Cond;
 
  if (InnerBinOp->getOpcode() != BO_EQ ||
      !isa<IntegerLiteral>(InnerBinOp->getRHS()))
    return Cond;
 
  // If the inner binary operation came from a macro expansion named
  // CONCEPT_REQUIRES or CONCEPT_REQUIRES_, return the right-hand side
  // of the '||', which is the real, user-provided condition.
  SourceLocation Loc = InnerBinOp->getExprLoc();
  if (!Loc.isMacroID()) return Cond;
 
  StringRef MacroName = PP.getImmediateMacroName(Loc);
  if (MacroName == "CONCEPT_REQUIRES" || MacroName == "CONCEPT_REQUIRES_")
    return BinOp->getRHS();
 
  return Cond;
}
 
namespace {
 
// A PrinterHelper that prints more helpful diagnostics for some sub-expressions
// within failing boolean expression, such as substituting template parameters
// for actual types.
class FailedBooleanConditionPrinterHelper : public PrinterHelper {
public:
  explicit FailedBooleanConditionPrinterHelper(const PrintingPolicy &P)
      : Policy(P) {}
 
  bool handledStmt(Stmt *E, raw_ostream &OS) override {
    const auto *DR = dyn_cast<DeclRefExpr>(E);
    if (DR && DR->getQualifier()) {
      // If this is a qualified name, expand the template arguments in nested
      // qualifiers.
      DR->getQualifier()->print(OS, Policy, true);
      // Then print the decl itself.
      const ValueDecl *VD = DR->getDecl();
      OS << VD->getName();
      if (const auto *IV = dyn_cast<VarTemplateSpecializationDecl>(VD)) {
        // This is a template variable, print the expanded template arguments.
        printTemplateArgumentList(
            OS, IV->getTemplateArgs().asArray(), Policy,
            IV->getSpecializedTemplate()->getTemplateParameters());
      }
      return true;
    }
    return false;
  }
 
private:
  const PrintingPolicy Policy;
};
 
} // end anonymous namespace
 
std::pair<Expr *, std::string>
Sema::findFailedBooleanCondition(Expr *Cond) {
  Cond = lookThroughRangesV3Condition(PP, Cond);
 
  // Separate out all of the terms in a conjunction.
  SmallVector<Expr *, 4> Terms;
  collectConjunctionTerms(Cond, Terms);
 
  // Determine which term failed.
  Expr *FailedCond = nullptr;
  for (Expr *Term : Terms) {
    Expr *TermAsWritten = Term->IgnoreParenImpCasts();
 
    // Literals are uninteresting.
    if (isa<CXXBoolLiteralExpr>(TermAsWritten) ||
        isa<IntegerLiteral>(TermAsWritten))
      continue;
 
    // The initialization of the parameter from the argument is
    // a constant-evaluated context.
    EnterExpressionEvaluationContext ConstantEvaluated(
      *this, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
    bool Succeeded;
    if (Term->EvaluateAsBooleanCondition(Succeeded, Context) &&
        !Succeeded) {
      FailedCond = TermAsWritten;
      break;
    }
  }
  if (!FailedCond)
    FailedCond = Cond->IgnoreParenImpCasts();
 
  std::string Description;
  {
    llvm::raw_string_ostream Out(Description);
    PrintingPolicy Policy = getPrintingPolicy();
    Policy.PrintCanonicalTypes = true;
    FailedBooleanConditionPrinterHelper Helper(Policy);
    FailedCond->printPretty(Out, &Helper, Policy, 0, "\n", nullptr);
  }
  return { FailedCond, Description };
}
 
QualType Sema::CheckTemplateIdType(TemplateName Name,
                                   SourceLocation TemplateLoc,
                                   TemplateArgumentListInfo &TemplateArgs) {
  DependentTemplateName *DTN =
      Name.getUnderlying().getAsDependentTemplateName();
  if (DTN && DTN->isIdentifier())
    // When building a template-id where the template-name is dependent,
    // assume the template is a type template. Either our assumption is
    // correct, or the code is ill-formed and will be diagnosed when the
    // dependent name is substituted.
    return Context.getDependentTemplateSpecializationType(
        ElaboratedTypeKeyword::None, DTN->getQualifier(), DTN->getIdentifier(),
        TemplateArgs.arguments());
 
  if (Name.getAsAssumedTemplateName() &&
      resolveAssumedTemplateNameAsType(/*Scope=*/nullptr, Name, TemplateLoc))
    return QualType();
 
  auto [Template, DefaultArgs] = Name.getTemplateDeclAndDefaultArgs();
 
  if (!Template || isa<FunctionTemplateDecl>(Template) ||
      isa<VarTemplateDecl>(Template) || isa<ConceptDecl>(Template)) {
    // We might have a substituted template template parameter pack. If so,
    // build a template specialization type for it.
    if (Name.getAsSubstTemplateTemplateParmPack())
      return Context.getTemplateSpecializationType(Name,
                                                   TemplateArgs.arguments());
 
    Diag(TemplateLoc, diag::err_template_id_not_a_type)
      << Name;
    NoteAllFoundTemplates(Name);
    return QualType();
  }
 
  // Check that the template argument list is well-formed for this
  // template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (CheckTemplateArgumentList(Template, TemplateLoc, TemplateArgs,
                                DefaultArgs, false, SugaredConverted,
                                CanonicalConverted,
                                /*UpdateArgsWithConversions=*/true))
    return QualType();
 
  QualType CanonType;
 
  if (TypeAliasTemplateDecl *AliasTemplate =
          dyn_cast<TypeAliasTemplateDecl>(Template)) {
 
    // Find the canonical type for this type alias template specialization.
    TypeAliasDecl *Pattern = AliasTemplate->getTemplatedDecl();
    if (Pattern->isInvalidDecl())
      return QualType();
 
    // Only substitute for the innermost template argument list.  NOTE: Some
    // external resugarers rely on leaving a Subst* node here.  Make the
    // substitution non-final in that case.  Note that these external resugarers
    // will still miss some information in this representation, because we don't
    // provide enough context in the Subst* nodes in order to tell different
    // template type alias specializations apart.
    MultiLevelTemplateArgumentList TemplateArgLists;
    TemplateArgLists.addOuterTemplateArguments(
        Template, SugaredConverted,
        /*Final=*/!getLangOpts().RetainSubstTemplateTypeParmTypeAstNodes);
    TemplateArgLists.addOuterRetainedLevels(
        AliasTemplate->getTemplateParameters()->getDepth());
 
    LocalInstantiationScope Scope(*this);
    InstantiatingTemplate Inst(
        *this, /*PointOfInstantiation=*/TemplateLoc,
        /*Entity=*/AliasTemplate,
        /*TemplateArgs=*/TemplateArgLists.getInnermost());
 
    // Diagnose uses of this alias.
    (void)DiagnoseUseOfDecl(AliasTemplate, TemplateLoc);
 
    if (Inst.isInvalid())
      return QualType();
 
    std::optional<ContextRAII> SavedContext;
    if (!AliasTemplate->getDeclContext()->isFileContext())
      SavedContext.emplace(*this, AliasTemplate->getDeclContext());
 
    CanonType =
        SubstType(Pattern->getUnderlyingType(), TemplateArgLists,
                  AliasTemplate->getLocation(), AliasTemplate->getDeclName());
    if (CanonType.isNull()) {
      // If this was enable_if and we failed to find the nested type
      // within enable_if in a SFINAE context, dig out the specific
      // enable_if condition that failed and present that instead.
      if (isEnableIfAliasTemplate(AliasTemplate)) {
        if (auto DeductionInfo = isSFINAEContext()) {
          if (*DeductionInfo &&
              (*DeductionInfo)->hasSFINAEDiagnostic() &&
              (*DeductionInfo)->peekSFINAEDiagnostic().second.getDiagID() ==
                diag::err_typename_nested_not_found_enable_if &&
              TemplateArgs[0].getArgument().getKind()
                == TemplateArgument::Expression) {
            Expr *FailedCond;
            std::string FailedDescription;
            std::tie(FailedCond, FailedDescription) =
              findFailedBooleanCondition(TemplateArgs[0].getSourceExpression());
 
            // Remove the old SFINAE diagnostic.
            PartialDiagnosticAt OldDiag =
              {SourceLocation(), PartialDiagnostic::NullDiagnostic()};
            (*DeductionInfo)->takeSFINAEDiagnostic(OldDiag);
 
            // Add a new SFINAE diagnostic specifying which condition
            // failed.
            (*DeductionInfo)->addSFINAEDiagnostic(
              OldDiag.first,
              PDiag(diag::err_typename_nested_not_found_requirement)
                << FailedDescription
                << FailedCond->getSourceRange());
          }
        }
      }
 
      return QualType();
    }
  } else if (auto *BTD = dyn_cast<BuiltinTemplateDecl>(Template)) {
    CanonType = checkBuiltinTemplateIdType(*this, BTD, SugaredConverted,
                                           TemplateLoc, TemplateArgs);
  } else if (Name.isDependent() ||
             TemplateSpecializationType::anyDependentTemplateArguments(
                 TemplateArgs, CanonicalConverted)) {
    // This class template specialization is a dependent
    // type. Therefore, its canonical type is another class template
    // specialization type that contains all of the converted
    // arguments in canonical form. This ensures that, e.g., A<T> and
    // A<T, T> have identical types when A is declared as:
    //
    //   template<typename T, typename U = T> struct A;
    CanonType = Context.getCanonicalTemplateSpecializationType(
        Name, CanonicalConverted);
 
    // This might work out to be a current instantiation, in which
    // case the canonical type needs to be the InjectedClassNameType.
    //
    // TODO: in theory this could be a simple hashtable lookup; most
    // changes to CurContext don't change the set of current
    // instantiations.
    if (isa<ClassTemplateDecl>(Template)) {
      for (DeclContext *Ctx = CurContext; Ctx; Ctx = Ctx->getLookupParent()) {
        // If we get out to a namespace, we're done.
        if (Ctx->isFileContext()) break;
 
        // If this isn't a record, keep looking.
        CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(Ctx);
        if (!Record) continue;
 
        // Look for one of the two cases with InjectedClassNameTypes
        // and check whether it's the same template.
        if (!isa<ClassTemplatePartialSpecializationDecl>(Record) &&
            !Record->getDescribedClassTemplate())
          continue;
 
        // Fetch the injected class name type and check whether its
        // injected type is equal to the type we just built.
        QualType ICNT = Context.getTypeDeclType(Record);
        QualType Injected = cast<InjectedClassNameType>(ICNT)
          ->getInjectedSpecializationType();
 
        if (CanonType != Injected->getCanonicalTypeInternal())
          continue;
 
        // If so, the canonical type of this TST is the injected
        // class name type of the record we just found.
        assert(ICNT.isCanonical());
        CanonType = ICNT;
        break;
      }
    }
  } else if (ClassTemplateDecl *ClassTemplate =
                 dyn_cast<ClassTemplateDecl>(Template)) {
    // Find the class template specialization declaration that
    // corresponds to these arguments.
    void *InsertPos = nullptr;
    ClassTemplateSpecializationDecl *Decl =
        ClassTemplate->findSpecialization(CanonicalConverted, InsertPos);
    if (!Decl) {
      // This is the first time we have referenced this class template
      // specialization. Create the canonical declaration and add it to
      // the set of specializations.
      Decl = ClassTemplateSpecializationDecl::Create(
          Context, ClassTemplate->getTemplatedDecl()->getTagKind(),
          ClassTemplate->getDeclContext(),
          ClassTemplate->getTemplatedDecl()->getBeginLoc(),
          ClassTemplate->getLocation(), ClassTemplate, CanonicalConverted,
          nullptr);
      ClassTemplate->AddSpecialization(Decl, InsertPos);
      if (ClassTemplate->isOutOfLine())
        Decl->setLexicalDeclContext(ClassTemplate->getLexicalDeclContext());
    }
 
    if (Decl->getSpecializationKind() == TSK_Undeclared &&
        ClassTemplate->getTemplatedDecl()->hasAttrs()) {
      InstantiatingTemplate Inst(*this, TemplateLoc, Decl);
      if (!Inst.isInvalid()) {
        MultiLevelTemplateArgumentList TemplateArgLists(Template,
                                                        CanonicalConverted,
                                                        /*Final=*/false);
        InstantiateAttrsForDecl(TemplateArgLists,
                                ClassTemplate->getTemplatedDecl(), Decl);
      }
    }
 
    // Diagnose uses of this specialization.
    (void)DiagnoseUseOfDecl(Decl, TemplateLoc);
 
    CanonType = Context.getTypeDeclType(Decl);
    assert(isa<RecordType>(CanonType) &&
           "type of non-dependent specialization is not a RecordType");
  } else {
    llvm_unreachable("Unhandled template kind");
  }
 
  // Build the fully-sugared type for this class template
  // specialization, which refers back to the class template
  // specialization we created or found.
  return Context.getTemplateSpecializationType(Name, TemplateArgs.arguments(),
                                               CanonType);
}
 
void Sema::ActOnUndeclaredTypeTemplateName(Scope *S, TemplateTy &ParsedName,
                                           TemplateNameKind &TNK,
                                           SourceLocation NameLoc,
                                           IdentifierInfo *&II) {
  assert(TNK == TNK_Undeclared_template && "not an undeclared template name");
 
  TemplateName Name = ParsedName.get();
  auto *ATN = Name.getAsAssumedTemplateName();
  assert(ATN && "not an assumed template name");
  II = ATN->getDeclName().getAsIdentifierInfo();
 
  if (!resolveAssumedTemplateNameAsType(S, Name, NameLoc, /*Diagnose*/false)) {
    // Resolved to a type template name.
    ParsedName = TemplateTy::make(Name);
    TNK = TNK_Type_template;
  }
}
 
bool Sema::resolveAssumedTemplateNameAsType(Scope *S, TemplateName &Name,
                                            SourceLocation NameLoc,
                                            bool Diagnose) {
  // We assumed this undeclared identifier to be an (ADL-only) function
  // template name, but it was used in a context where a type was required.
  // Try to typo-correct it now.
  AssumedTemplateStorage *ATN = Name.getAsAssumedTemplateName();
  assert(ATN && "not an assumed template name");
 
  LookupResult R(*this, ATN->getDeclName(), NameLoc, LookupOrdinaryName);
  struct CandidateCallback : CorrectionCandidateCallback {
    bool ValidateCandidate(const TypoCorrection &TC) override {
      return TC.getCorrectionDecl() &&
             getAsTypeTemplateDecl(TC.getCorrectionDecl());
    }
    std::unique_ptr<CorrectionCandidateCallback> clone() override {
      return std::make_unique<CandidateCallback>(*this);
    }
  } FilterCCC;
 
  TypoCorrection Corrected =
      CorrectTypo(R.getLookupNameInfo(), R.getLookupKind(), S, nullptr,
                  FilterCCC, CTK_ErrorRecovery);
  if (Corrected && Corrected.getFoundDecl()) {
    diagnoseTypo(Corrected, PDiag(diag::err_no_template_suggest)
                                << ATN->getDeclName());
    Name = Context.getQualifiedTemplateName(
        /*NNS=*/nullptr, /*TemplateKeyword=*/false,
        TemplateName(Corrected.getCorrectionDeclAs<TemplateDecl>()));
    return false;
  }
 
  if (Diagnose)
    Diag(R.getNameLoc(), diag::err_no_template) << R.getLookupName();
  return true;
}
 
TypeResult Sema::ActOnTemplateIdType(
    Scope *S, CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
    TemplateTy TemplateD, const IdentifierInfo *TemplateII,
    SourceLocation TemplateIILoc, SourceLocation LAngleLoc,
    ASTTemplateArgsPtr TemplateArgsIn, SourceLocation RAngleLoc,
    bool IsCtorOrDtorName, bool IsClassName,
    ImplicitTypenameContext AllowImplicitTypename) {
  if (SS.isInvalid())
    return true;
 
  if (!IsCtorOrDtorName && !IsClassName && SS.isSet()) {
    DeclContext *LookupCtx = computeDeclContext(SS, /*EnteringContext*/false);
 
    // 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).
    if (!LookupCtx && isDependentScopeSpecifier(SS)) {
      // C++2a relaxes some of those restrictions in [temp.res]p5.
      if (AllowImplicitTypename == ImplicitTypenameContext::Yes) {
        if (getLangOpts().CPlusPlus20)
          Diag(SS.getBeginLoc(), diag::warn_cxx17_compat_implicit_typename);
        else
          Diag(SS.getBeginLoc(), diag::ext_implicit_typename)
              << SS.getScopeRep() << TemplateII->getName()
              << FixItHint::CreateInsertion(SS.getBeginLoc(), "typename ");
      } else
        Diag(SS.getBeginLoc(), diag::err_typename_missing_template)
            << SS.getScopeRep() << TemplateII->getName();
 
      // FIXME: This is not quite correct recovery as we don't transform SS
      // into the corresponding dependent form (and we don't diagnose missing
      // 'template' keywords within SS as a result).
      return ActOnTypenameType(nullptr, SourceLocation(), SS, TemplateKWLoc,
                               TemplateD, TemplateII, TemplateIILoc, LAngleLoc,
                               TemplateArgsIn, RAngleLoc);
    }
 
    // Per C++ [class.qual]p2, if the template-id was an injected-class-name,
    // it's not actually allowed to be used as a type in most cases. Because
    // we annotate it before we know whether it's valid, we have to check for
    // this case here.
    auto *LookupRD = dyn_cast_or_null<CXXRecordDecl>(LookupCtx);
    if (LookupRD && LookupRD->getIdentifier() == TemplateII) {
      Diag(TemplateIILoc,
           TemplateKWLoc.isInvalid()
               ? diag::err_out_of_line_qualified_id_type_names_constructor
               : diag::ext_out_of_line_qualified_id_type_names_constructor)
        << TemplateII << 0 /*injected-class-name used as template name*/
        << 1 /*if any keyword was present, it was 'template'*/;
    }
  }
 
  TemplateName Template = TemplateD.get();
  if (Template.getAsAssumedTemplateName() &&
      resolveAssumedTemplateNameAsType(S, Template, TemplateIILoc))
    return true;
 
  // Translate the parser's template argument list in our AST format.
  TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
  translateTemplateArguments(TemplateArgsIn, TemplateArgs);
 
  if (DependentTemplateName *DTN = Template.getAsDependentTemplateName()) {
    assert(SS.getScopeRep() == DTN->getQualifier());
    QualType T = Context.getDependentTemplateSpecializationType(
        ElaboratedTypeKeyword::None, DTN->getQualifier(), DTN->getIdentifier(),
        TemplateArgs.arguments());
    // Build type-source information.
    TypeLocBuilder TLB;
    DependentTemplateSpecializationTypeLoc SpecTL
      = TLB.push<DependentTemplateSpecializationTypeLoc>(T);
    SpecTL.setElaboratedKeywordLoc(SourceLocation());
    SpecTL.setQualifierLoc(SS.getWithLocInContext(Context));
    SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
    SpecTL.setTemplateNameLoc(TemplateIILoc);
    SpecTL.setLAngleLoc(LAngleLoc);
    SpecTL.setRAngleLoc(RAngleLoc);
    for (unsigned I = 0, N = SpecTL.getNumArgs(); I != N; ++I)
      SpecTL.setArgLocInfo(I, TemplateArgs[I].getLocInfo());
    return CreateParsedType(T, TLB.getTypeSourceInfo(Context, T));
  }
 
  QualType SpecTy = CheckTemplateIdType(Template, TemplateIILoc, TemplateArgs);
  if (SpecTy.isNull())
    return true;
 
  // Build type-source information.
  TypeLocBuilder TLB;
  TemplateSpecializationTypeLoc SpecTL =
      TLB.push<TemplateSpecializationTypeLoc>(SpecTy);
  SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
  SpecTL.setTemplateNameLoc(TemplateIILoc);
  SpecTL.setLAngleLoc(LAngleLoc);
  SpecTL.setRAngleLoc(RAngleLoc);
  for (unsigned i = 0, e = SpecTL.getNumArgs(); i != e; ++i)
    SpecTL.setArgLocInfo(i, TemplateArgs[i].getLocInfo());
 
  // Create an elaborated-type-specifier containing the nested-name-specifier.
  QualType ElTy =
      getElaboratedType(ElaboratedTypeKeyword::None,
                        !IsCtorOrDtorName ? SS : CXXScopeSpec(), SpecTy);
  ElaboratedTypeLoc ElabTL = TLB.push<ElaboratedTypeLoc>(ElTy);
  ElabTL.setElaboratedKeywordLoc(SourceLocation());
  if (!ElabTL.isEmpty())
    ElabTL.setQualifierLoc(SS.getWithLocInContext(Context));
  return CreateParsedType(ElTy, TLB.getTypeSourceInfo(Context, ElTy));
}
 
TypeResult Sema::ActOnTagTemplateIdType(TagUseKind TUK,
                                        TypeSpecifierType TagSpec,
                                        SourceLocation TagLoc,
                                        CXXScopeSpec &SS,
                                        SourceLocation TemplateKWLoc,
                                        TemplateTy TemplateD,
                                        SourceLocation TemplateLoc,
                                        SourceLocation LAngleLoc,
                                        ASTTemplateArgsPtr TemplateArgsIn,
                                        SourceLocation RAngleLoc) {
  if (SS.isInvalid())
    return TypeResult(true);
 
  TemplateName Template = TemplateD.get();
 
  // Translate the parser's template argument list in our AST format.
  TemplateArgumentListInfo TemplateArgs(LAngleLoc, RAngleLoc);
  translateTemplateArguments(TemplateArgsIn, TemplateArgs);
 
  // Determine the tag kind
  TagTypeKind TagKind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
  ElaboratedTypeKeyword Keyword
    = TypeWithKeyword::getKeywordForTagTypeKind(TagKind);
 
  if (DependentTemplateName *DTN = Template.getAsDependentTemplateName()) {
    assert(SS.getScopeRep() == DTN->getQualifier());
    QualType T = Context.getDependentTemplateSpecializationType(
        Keyword, DTN->getQualifier(), DTN->getIdentifier(),
        TemplateArgs.arguments());
 
    // Build type-source information.
    TypeLocBuilder TLB;
    DependentTemplateSpecializationTypeLoc SpecTL
      = TLB.push<DependentTemplateSpecializationTypeLoc>(T);
    SpecTL.setElaboratedKeywordLoc(TagLoc);
    SpecTL.setQualifierLoc(SS.getWithLocInContext(Context));
    SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
    SpecTL.setTemplateNameLoc(TemplateLoc);
    SpecTL.setLAngleLoc(LAngleLoc);
    SpecTL.setRAngleLoc(RAngleLoc);
    for (unsigned I = 0, N = SpecTL.getNumArgs(); I != N; ++I)
      SpecTL.setArgLocInfo(I, TemplateArgs[I].getLocInfo());
    return CreateParsedType(T, TLB.getTypeSourceInfo(Context, T));
  }
 
  if (TypeAliasTemplateDecl *TAT =
        dyn_cast_or_null<TypeAliasTemplateDecl>(Template.getAsTemplateDecl())) {
    // C++0x [dcl.type.elab]p2:
    //   If the identifier resolves to a typedef-name or the simple-template-id
    //   resolves to an alias template specialization, the
    //   elaborated-type-specifier is ill-formed.
    Diag(TemplateLoc, diag::err_tag_reference_non_tag)
        << TAT << NTK_TypeAliasTemplate << llvm::to_underlying(TagKind);
    Diag(TAT->getLocation(), diag::note_declared_at);
  }
 
  QualType Result = CheckTemplateIdType(Template, TemplateLoc, TemplateArgs);
  if (Result.isNull())
    return TypeResult(true);
 
  // Check the tag kind
  if (const RecordType *RT = Result->getAs<RecordType>()) {
    RecordDecl *D = RT->getDecl();
 
    IdentifierInfo *Id = D->getIdentifier();
    assert(Id && "templated class must have an identifier");
 
    if (!isAcceptableTagRedeclaration(D, TagKind, TUK == TagUseKind::Definition,
                                      TagLoc, Id)) {
      Diag(TagLoc, diag::err_use_with_wrong_tag)
        << Result
        << FixItHint::CreateReplacement(SourceRange(TagLoc), D->getKindName());
      Diag(D->getLocation(), diag::note_previous_use);
    }
  }
 
  // Provide source-location information for the template specialization.
  TypeLocBuilder TLB;
  TemplateSpecializationTypeLoc SpecTL
    = TLB.push<TemplateSpecializationTypeLoc>(Result);
  SpecTL.setTemplateKeywordLoc(TemplateKWLoc);
  SpecTL.setTemplateNameLoc(TemplateLoc);
  SpecTL.setLAngleLoc(LAngleLoc);
  SpecTL.setRAngleLoc(RAngleLoc);
  for (unsigned i = 0, e = SpecTL.getNumArgs(); i != e; ++i)
    SpecTL.setArgLocInfo(i, TemplateArgs[i].getLocInfo());
 
  // Construct an elaborated type containing the nested-name-specifier (if any)
  // and tag keyword.
  Result = Context.getElaboratedType(Keyword, SS.getScopeRep(), Result);
  ElaboratedTypeLoc ElabTL = TLB.push<ElaboratedTypeLoc>(Result);
  ElabTL.setElaboratedKeywordLoc(TagLoc);
  ElabTL.setQualifierLoc(SS.getWithLocInContext(Context));
  return CreateParsedType(Result, TLB.getTypeSourceInfo(Context, Result));
}
 
static bool CheckTemplateSpecializationScope(Sema &S, NamedDecl *Specialized,
                                             NamedDecl *PrevDecl,
                                             SourceLocation Loc,
                                             bool IsPartialSpecialization);
 
static TemplateSpecializationKind getTemplateSpecializationKind(Decl *D);
 
static bool isTemplateArgumentTemplateParameter(
    const TemplateArgument &Arg, unsigned Depth, unsigned Index) {
  switch (Arg.getKind()) {
  case TemplateArgument::Null:
  case TemplateArgument::NullPtr:
  case TemplateArgument::Integral:
  case TemplateArgument::Declaration:
  case TemplateArgument::StructuralValue:
  case TemplateArgument::Pack:
  case TemplateArgument::TemplateExpansion:
    return false;
 
  case TemplateArgument::Type: {
    QualType Type = Arg.getAsType();
    const TemplateTypeParmType *TPT =
        Arg.getAsType()->getAs<TemplateTypeParmType>();
    return TPT && !Type.hasQualifiers() &&
           TPT->getDepth() == Depth && TPT->getIndex() == Index;
  }
 
  case TemplateArgument::Expression: {
    DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(Arg.getAsExpr());
    if (!DRE || !DRE->getDecl())
      return false;
    const NonTypeTemplateParmDecl *NTTP =
        dyn_cast<NonTypeTemplateParmDecl>(DRE->getDecl());
    return NTTP && NTTP->getDepth() == Depth && NTTP->getIndex() == Index;
  }
 
  case TemplateArgument::Template:
    const TemplateTemplateParmDecl *TTP =
        dyn_cast_or_null<TemplateTemplateParmDecl>(
            Arg.getAsTemplateOrTemplatePattern().getAsTemplateDecl());
    return TTP && TTP->getDepth() == Depth && TTP->getIndex() == Index;
  }
  llvm_unreachable("unexpected kind of template argument");
}
 
static bool isSameAsPrimaryTemplate(TemplateParameterList *Params,
                                    ArrayRef<TemplateArgument> Args) {
  if (Params->size() != Args.size())
    return false;
 
  unsigned Depth = Params->getDepth();
 
  for (unsigned I = 0, N = Args.size(); I != N; ++I) {
    TemplateArgument Arg = Args[I];
 
    // If the parameter is a pack expansion, the argument must be a pack
    // whose only element is a pack expansion.
    if (Params->getParam(I)->isParameterPack()) {
      if (Arg.getKind() != TemplateArgument::Pack || Arg.pack_size() != 1 ||
          !Arg.pack_begin()->isPackExpansion())
        return false;
      Arg = Arg.pack_begin()->getPackExpansionPattern();
    }
 
    if (!isTemplateArgumentTemplateParameter(Arg, Depth, I))
      return false;
  }
 
  return true;
}
 
template<typename PartialSpecDecl>
static void checkMoreSpecializedThanPrimary(Sema &S, PartialSpecDecl *Partial) {
  if (Partial->getDeclContext()->isDependentContext())
    return;
 
  // FIXME: Get the TDK from deduction in order to provide better diagnostics
  // for non-substitution-failure issues?
  TemplateDeductionInfo Info(Partial->getLocation());
  if (S.isMoreSpecializedThanPrimary(Partial, Info))
    return;
 
  auto *Template = Partial->getSpecializedTemplate();
  S.Diag(Partial->getLocation(),
         diag::ext_partial_spec_not_more_specialized_than_primary)
      << isa<VarTemplateDecl>(Template);
 
  if (Info.hasSFINAEDiagnostic()) {
    PartialDiagnosticAt Diag = {SourceLocation(),
                                PartialDiagnostic::NullDiagnostic()};
    Info.takeSFINAEDiagnostic(Diag);
    SmallString<128> SFINAEArgString;
    Diag.second.EmitToString(S.getDiagnostics(), SFINAEArgString);
    S.Diag(Diag.first,
           diag::note_partial_spec_not_more_specialized_than_primary)
      << SFINAEArgString;
  }
 
  S.NoteTemplateLocation(*Template);
  SmallVector<const Expr *, 3> PartialAC, TemplateAC;
  Template->getAssociatedConstraints(TemplateAC);
  Partial->getAssociatedConstraints(PartialAC);
  S.MaybeEmitAmbiguousAtomicConstraintsDiagnostic(Partial, PartialAC, Template,
                                                  TemplateAC);
}
 
static void
noteNonDeducibleParameters(Sema &S, TemplateParameterList *TemplateParams,
                           const llvm::SmallBitVector &DeducibleParams) {
  for (unsigned I = 0, N = DeducibleParams.size(); I != N; ++I) {
    if (!DeducibleParams[I]) {
      NamedDecl *Param = TemplateParams->getParam(I);
      if (Param->getDeclName())
        S.Diag(Param->getLocation(), diag::note_non_deducible_parameter)
            << Param->getDeclName();
      else
        S.Diag(Param->getLocation(), diag::note_non_deducible_parameter)
            << "(anonymous)";
    }
  }
}
 
 
template<typename PartialSpecDecl>
static void checkTemplatePartialSpecialization(Sema &S,
                                               PartialSpecDecl *Partial) {
  // C++1z [temp.class.spec]p8: (DR1495)
  //   - The specialization shall be more specialized than the primary
  //     template (14.5.5.2).
  checkMoreSpecializedThanPrimary(S, Partial);
 
  // C++ [temp.class.spec]p8: (DR1315)
  //   - Each template-parameter shall appear at least once in the
  //     template-id outside a non-deduced context.
  // C++1z [temp.class.spec.match]p3 (P0127R2)
  //   If the template arguments of a partial specialization cannot be
  //   deduced because of the structure of its template-parameter-list
  //   and the template-id, the program is ill-formed.
  auto *TemplateParams = Partial->getTemplateParameters();
  llvm::SmallBitVector DeducibleParams(TemplateParams->size());
  S.MarkUsedTemplateParameters(Partial->getTemplateArgs(), true,
                               TemplateParams->getDepth(), DeducibleParams);
 
  if (!DeducibleParams.all()) {
    unsigned NumNonDeducible = DeducibleParams.size() - DeducibleParams.count();
    S.Diag(Partial->getLocation(), diag::ext_partial_specs_not_deducible)
      << isa<VarTemplatePartialSpecializationDecl>(Partial)
      << (NumNonDeducible > 1)
      << SourceRange(Partial->getLocation(),
                     Partial->getTemplateArgsAsWritten()->RAngleLoc);
    noteNonDeducibleParameters(S, TemplateParams, DeducibleParams);
  }
}
 
void Sema::CheckTemplatePartialSpecialization(
    ClassTemplatePartialSpecializationDecl *Partial) {
  checkTemplatePartialSpecialization(*this, Partial);
}
 
void Sema::CheckTemplatePartialSpecialization(
    VarTemplatePartialSpecializationDecl *Partial) {
  checkTemplatePartialSpecialization(*this, Partial);
}
 
void Sema::CheckDeductionGuideTemplate(FunctionTemplateDecl *TD) {
  // C++1z [temp.param]p11:
  //   A template parameter of a deduction guide template that does not have a
  //   default-argument shall be deducible from the parameter-type-list of the
  //   deduction guide template.
  auto *TemplateParams = TD->getTemplateParameters();
  llvm::SmallBitVector DeducibleParams(TemplateParams->size());
  MarkDeducedTemplateParameters(TD, DeducibleParams);
  for (unsigned I = 0; I != TemplateParams->size(); ++I) {
    // A parameter pack is deducible (to an empty pack).
    auto *Param = TemplateParams->getParam(I);
    if (Param->isParameterPack() || hasVisibleDefaultArgument(Param))
      DeducibleParams[I] = true;
  }
 
  if (!DeducibleParams.all()) {
    unsigned NumNonDeducible = DeducibleParams.size() - DeducibleParams.count();
    Diag(TD->getLocation(), diag::err_deduction_guide_template_not_deducible)
      << (NumNonDeducible > 1);
    noteNonDeducibleParameters(*this, TemplateParams, DeducibleParams);
  }
}
 
DeclResult Sema::ActOnVarTemplateSpecialization(
    Scope *S, Declarator &D, TypeSourceInfo *DI, LookupResult &Previous,
    SourceLocation TemplateKWLoc, TemplateParameterList *TemplateParams,
    StorageClass SC, bool IsPartialSpecialization) {
  // D must be variable template id.
  assert(D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId &&
         "Variable template specialization is declared with a template id.");
 
  TemplateIdAnnotation *TemplateId = D.getName().TemplateId;
  TemplateArgumentListInfo TemplateArgs =
      makeTemplateArgumentListInfo(*this, *TemplateId);
  SourceLocation TemplateNameLoc = D.getIdentifierLoc();
  SourceLocation LAngleLoc = TemplateId->LAngleLoc;
  SourceLocation RAngleLoc = TemplateId->RAngleLoc;
 
  TemplateName Name = TemplateId->Template.get();
 
  // The template-id must name a variable template.
  VarTemplateDecl *VarTemplate =
      dyn_cast_or_null<VarTemplateDecl>(Name.getAsTemplateDecl());
  if (!VarTemplate) {
    NamedDecl *FnTemplate;
    if (auto *OTS = Name.getAsOverloadedTemplate())
      FnTemplate = *OTS->begin();
    else
      FnTemplate = dyn_cast_or_null<FunctionTemplateDecl>(Name.getAsTemplateDecl());
    if (FnTemplate)
      return Diag(D.getIdentifierLoc(), diag::err_var_spec_no_template_but_method)
               << FnTemplate->getDeclName();
    return Diag(D.getIdentifierLoc(), diag::err_var_spec_no_template)
             << IsPartialSpecialization;
  }
 
  if (const auto *DSA = VarTemplate->getAttr<NoSpecializationsAttr>()) {
    auto Message = DSA->getMessage();
    Diag(TemplateNameLoc, diag::warn_invalid_specialization)
        << VarTemplate << !Message.empty() << Message;
    Diag(DSA->getLoc(), diag::note_marked_here) << DSA;
  }
 
  // Check for unexpanded parameter packs in any of the template arguments.
  for (unsigned I = 0, N = TemplateArgs.size(); I != N; ++I)
    if (DiagnoseUnexpandedParameterPack(TemplateArgs[I],
                                        IsPartialSpecialization
                                            ? UPPC_PartialSpecialization
                                            : UPPC_ExplicitSpecialization))
      return true;
 
  // Check that the template argument list is well-formed for this
  // template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (CheckTemplateArgumentList(VarTemplate, TemplateNameLoc, TemplateArgs,
                                /*DefaultArgs=*/{}, false, SugaredConverted,
                                CanonicalConverted,
                                /*UpdateArgsWithConversions=*/true))
    return true;
 
  // Find the variable template (partial) specialization declaration that
  // corresponds to these arguments.
  if (IsPartialSpecialization) {
    if (CheckTemplatePartialSpecializationArgs(TemplateNameLoc, VarTemplate,
                                               TemplateArgs.size(),
                                               CanonicalConverted))
      return true;
 
    // FIXME: Move these checks to CheckTemplatePartialSpecializationArgs so we
    // also do them during instantiation.
    if (!Name.isDependent() &&
        !TemplateSpecializationType::anyDependentTemplateArguments(
            TemplateArgs, CanonicalConverted)) {
      Diag(TemplateNameLoc, diag::err_partial_spec_fully_specialized)
          << VarTemplate->getDeclName();
      IsPartialSpecialization = false;
    }
 
    if (isSameAsPrimaryTemplate(VarTemplate->getTemplateParameters(),
                                CanonicalConverted) &&
        (!Context.getLangOpts().CPlusPlus20 ||
         !TemplateParams->hasAssociatedConstraints())) {
      // C++ [temp.class.spec]p9b3:
      //
      //   -- The argument list of the specialization shall not be identical
      //      to the implicit argument list of the primary template.
      Diag(TemplateNameLoc, diag::err_partial_spec_args_match_primary_template)
        << /*variable template*/ 1
        << /*is definition*/(SC != SC_Extern && !CurContext->isRecord())
        << FixItHint::CreateRemoval(SourceRange(LAngleLoc, RAngleLoc));
      // FIXME: Recover from this by treating the declaration as a redeclaration
      // of the primary template.
      return true;
    }
  }
 
  void *InsertPos = nullptr;
  VarTemplateSpecializationDecl *PrevDecl = nullptr;
 
  if (IsPartialSpecialization)
    PrevDecl = VarTemplate->findPartialSpecialization(
        CanonicalConverted, TemplateParams, InsertPos);
  else
    PrevDecl = VarTemplate->findSpecialization(CanonicalConverted, InsertPos);
 
  VarTemplateSpecializationDecl *Specialization = nullptr;
 
  // Check whether we can declare a variable template specialization in
  // the current scope.
  if (CheckTemplateSpecializationScope(*this, VarTemplate, PrevDecl,
                                       TemplateNameLoc,
                                       IsPartialSpecialization))
    return true;
 
  if (PrevDecl && PrevDecl->getSpecializationKind() == TSK_Undeclared) {
    // Since the only prior variable template specialization with these
    // arguments was referenced but not declared,  reuse that
    // declaration node as our own, updating its source location and
    // the list of outer template parameters to reflect our new declaration.
    Specialization = PrevDecl;
    Specialization->setLocation(TemplateNameLoc);
    PrevDecl = nullptr;
  } else if (IsPartialSpecialization) {
    // Create a new class template partial specialization declaration node.
    VarTemplatePartialSpecializationDecl *PrevPartial =
        cast_or_null<VarTemplatePartialSpecializationDecl>(PrevDecl);
    VarTemplatePartialSpecializationDecl *Partial =
        VarTemplatePartialSpecializationDecl::Create(
            Context, VarTemplate->getDeclContext(), TemplateKWLoc,
            TemplateNameLoc, TemplateParams, VarTemplate, DI->getType(), DI, SC,
            CanonicalConverted);
    Partial->setTemplateArgsAsWritten(TemplateArgs);
 
    if (!PrevPartial)
      VarTemplate->AddPartialSpecialization(Partial, InsertPos);
    Specialization = Partial;
 
    // If we are providing an explicit specialization of a member variable
    // template specialization, make a note of that.
    if (PrevPartial && PrevPartial->getInstantiatedFromMember())
      PrevPartial->setMemberSpecialization();
 
    CheckTemplatePartialSpecialization(Partial);
  } else {
    // Create a new class template specialization declaration node for
    // this explicit specialization or friend declaration.
    Specialization = VarTemplateSpecializationDecl::Create(
        Context, VarTemplate->getDeclContext(), TemplateKWLoc, TemplateNameLoc,
        VarTemplate, DI->getType(), DI, SC, CanonicalConverted);
    Specialization->setTemplateArgsAsWritten(TemplateArgs);
 
    if (!PrevDecl)
      VarTemplate->AddSpecialization(Specialization, InsertPos);
  }
 
  // C++ [temp.expl.spec]p6:
  //   If a template, a member template or the member of a class template is
  //   explicitly specialized then that specialization shall be declared
  //   before the first use of that specialization that would cause an implicit
  //   instantiation to take place, in every translation unit in which such a
  //   use occurs; no diagnostic is required.
  if (PrevDecl && PrevDecl->getPointOfInstantiation().isValid()) {
    bool Okay = false;
    for (Decl *Prev = PrevDecl; Prev; Prev = Prev->getPreviousDecl()) {
      // Is there any previous explicit specialization declaration?
      if (getTemplateSpecializationKind(Prev) == TSK_ExplicitSpecialization) {
        Okay = true;
        break;
      }
    }
 
    if (!Okay) {
      SourceRange Range(TemplateNameLoc, RAngleLoc);
      Diag(TemplateNameLoc, diag::err_specialization_after_instantiation)
          << Name << Range;
 
      Diag(PrevDecl->getPointOfInstantiation(),
           diag::note_instantiation_required_here)
          << (PrevDecl->getTemplateSpecializationKind() !=
              TSK_ImplicitInstantiation);
      return true;
    }
  }
 
  Specialization->setLexicalDeclContext(CurContext);
 
  // Add the specialization into its lexical context, so that it can
  // be seen when iterating through the list of declarations in that
  // context. However, specializations are not found by name lookup.
  CurContext->addDecl(Specialization);
 
  // Note that this is an explicit specialization.
  Specialization->setSpecializationKind(TSK_ExplicitSpecialization);
 
  Previous.clear();
  if (PrevDecl)
    Previous.addDecl(PrevDecl);
  else if (Specialization->isStaticDataMember() &&
           Specialization->isOutOfLine())
    Specialization->setAccess(VarTemplate->getAccess());
 
  return Specialization;
}
 
namespace {
/// A partial specialization whose template arguments have matched
/// a given template-id.
struct PartialSpecMatchResult {
  VarTemplatePartialSpecializationDecl *Partial;
  TemplateArgumentList *Args;
};
} // end anonymous namespace
 
DeclResult
Sema::CheckVarTemplateId(VarTemplateDecl *Template, SourceLocation TemplateLoc,
                         SourceLocation TemplateNameLoc,
                         const TemplateArgumentListInfo &TemplateArgs) {
  assert(Template && "A variable template id without template?");
 
  // Check that the template argument list is well-formed for this template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (CheckTemplateArgumentList(
          Template, TemplateNameLoc,
          const_cast<TemplateArgumentListInfo &>(TemplateArgs),
          /*DefaultArgs=*/{}, false, SugaredConverted, CanonicalConverted,
          /*UpdateArgsWithConversions=*/true))
    return true;
 
  // Produce a placeholder value if the specialization is dependent.
  if (Template->getDeclContext()->isDependentContext() ||
      TemplateSpecializationType::anyDependentTemplateArguments(
          TemplateArgs, CanonicalConverted))
    return DeclResult();
 
  // Find the variable template specialization declaration that
  // corresponds to these arguments.
  void *InsertPos = nullptr;
  if (VarTemplateSpecializationDecl *Spec =
          Template->findSpecialization(CanonicalConverted, InsertPos)) {
    checkSpecializationReachability(TemplateNameLoc, Spec);
    // If we already have a variable template specialization, return it.
    return Spec;
  }
 
  // This is the first time we have referenced this variable template
  // specialization. Create the canonical declaration and add it to
  // the set of specializations, based on the closest partial specialization
  // that it represents. That is,
  VarDecl *InstantiationPattern = Template->getTemplatedDecl();
  const TemplateArgumentList *PartialSpecArgs = nullptr;
  bool AmbiguousPartialSpec = false;
  typedef PartialSpecMatchResult MatchResult;
  SmallVector<MatchResult, 4> Matched;
  SourceLocation PointOfInstantiation = TemplateNameLoc;
  TemplateSpecCandidateSet FailedCandidates(PointOfInstantiation,
                                            /*ForTakingAddress=*/false);
 
  // 1. Attempt to find the closest partial specialization that this
  // specializes, if any.
  // TODO: Unify with InstantiateClassTemplateSpecialization()?
  //       Perhaps better after unification of DeduceTemplateArguments() and
  //       getMoreSpecializedPartialSpecialization().
  SmallVector<VarTemplatePartialSpecializationDecl *, 4> PartialSpecs;
  Template->getPartialSpecializations(PartialSpecs);
 
  for (VarTemplatePartialSpecializationDecl *Partial : PartialSpecs) {
    // C++ [temp.spec.partial.member]p2:
    //   If the primary member template is explicitly specialized for a given
    //   (implicit) specialization of the enclosing class template, the partial
    //   specializations of the member template are ignored for this
    //   specialization of the enclosing class template. If a partial
    //   specialization of the member template is explicitly specialized for a
    //   given (implicit) specialization of the enclosing class template, the
    //   primary member template and its other partial specializations are still
    //   considered for this specialization of the enclosing class template.
    if (Template->getMostRecentDecl()->isMemberSpecialization() &&
        !Partial->getMostRecentDecl()->isMemberSpecialization())
      continue;
 
    TemplateDeductionInfo Info(FailedCandidates.getLocation());
 
    if (TemplateDeductionResult Result =
            DeduceTemplateArguments(Partial, SugaredConverted, Info);
        Result != TemplateDeductionResult::Success) {
      // Store the failed-deduction information for use in diagnostics, later.
      // TODO: Actually use the failed-deduction info?
      FailedCandidates.addCandidate().set(
          DeclAccessPair::make(Template, AS_public), Partial,
          MakeDeductionFailureInfo(Context, Result, Info));
      (void)Result;
    } else {
      Matched.push_back(PartialSpecMatchResult());
      Matched.back().Partial = Partial;
      Matched.back().Args = Info.takeSugared();
    }
  }
 
  if (Matched.size() >= 1) {
    SmallVector<MatchResult, 4>::iterator Best = Matched.begin();
    if (Matched.size() == 1) {
      //   -- If exactly one matching specialization is found, the
      //      instantiation is generated from that specialization.
      // We don't need to do anything for this.
    } else {
      //   -- If more than one matching specialization is found, the
      //      partial order rules (14.5.4.2) are used to determine
      //      whether one of the specializations is more specialized
      //      than the others. If none of the specializations is more
      //      specialized than all of the other matching
      //      specializations, then the use of the variable template is
      //      ambiguous and the program is ill-formed.
      for (SmallVector<MatchResult, 4>::iterator P = Best + 1,
                                                 PEnd = Matched.end();
           P != PEnd; ++P) {
        if (getMoreSpecializedPartialSpecialization(P->Partial, Best->Partial,
                                                    PointOfInstantiation) ==
            P->Partial)
          Best = P;
      }
 
      // Determine if the best partial specialization is more specialized than
      // the others.
      for (SmallVector<MatchResult, 4>::iterator P = Matched.begin(),
                                                 PEnd = Matched.end();
           P != PEnd; ++P) {
        if (P != Best && getMoreSpecializedPartialSpecialization(
                             P->Partial, Best->Partial,
                             PointOfInstantiation) != Best->Partial) {
          AmbiguousPartialSpec = true;
          break;
        }
      }
    }
 
    // Instantiate using the best variable template partial specialization.
    InstantiationPattern = Best->Partial;
    PartialSpecArgs = Best->Args;
  } else {
    //   -- If no match is found, the instantiation is generated
    //      from the primary template.
    // InstantiationPattern = Template->getTemplatedDecl();
  }
 
  // 2. Create the canonical declaration.
  // Note that we do not instantiate a definition until we see an odr-use
  // in DoMarkVarDeclReferenced().
  // FIXME: LateAttrs et al.?
  VarTemplateSpecializationDecl *Decl = BuildVarTemplateInstantiation(
      Template, InstantiationPattern, PartialSpecArgs, TemplateArgs,
      CanonicalConverted, TemplateNameLoc /*, LateAttrs, StartingScope*/);
  if (!Decl)
    return true;
 
  if (AmbiguousPartialSpec) {
    // Partial ordering did not produce a clear winner. Complain.
    Decl->setInvalidDecl();
    Diag(PointOfInstantiation, diag::err_partial_spec_ordering_ambiguous)
        << Decl;
 
    // Print the matching partial specializations.
    for (MatchResult P : Matched)
      Diag(P.Partial->getLocation(), diag::note_partial_spec_match)
          << getTemplateArgumentBindingsText(P.Partial->getTemplateParameters(),
                                             *P.Args);
    return true;
  }
 
  if (VarTemplatePartialSpecializationDecl *D =
          dyn_cast<VarTemplatePartialSpecializationDecl>(InstantiationPattern))
    Decl->setInstantiationOf(D, PartialSpecArgs);
 
  checkSpecializationReachability(TemplateNameLoc, Decl);
 
  assert(Decl && "No variable template specialization?");
  return Decl;
}
 
ExprResult Sema::CheckVarTemplateId(
    const CXXScopeSpec &SS, const DeclarationNameInfo &NameInfo,
    VarTemplateDecl *Template, NamedDecl *FoundD, SourceLocation TemplateLoc,
    const TemplateArgumentListInfo *TemplateArgs) {
 
  DeclResult Decl = CheckVarTemplateId(Template, TemplateLoc, NameInfo.getLoc(),
                                       *TemplateArgs);
  if (Decl.isInvalid())
    return ExprError();
 
  if (!Decl.get())
    return ExprResult();
 
  VarDecl *Var = cast<VarDecl>(Decl.get());
  if (!Var->getTemplateSpecializationKind())
    Var->setTemplateSpecializationKind(TSK_ImplicitInstantiation,
                                       NameInfo.getLoc());
 
  // Build an ordinary singleton decl ref.
  return BuildDeclarationNameExpr(SS, NameInfo, Var, FoundD, TemplateArgs);
}
 
void Sema::diagnoseMissingTemplateArguments(TemplateName Name,
                                            SourceLocation Loc) {
  Diag(Loc, diag::err_template_missing_args)
    << (int)getTemplateNameKindForDiagnostics(Name) << Name;
  if (TemplateDecl *TD = Name.getAsTemplateDecl()) {
    NoteTemplateLocation(*TD, TD->getTemplateParameters()->getSourceRange());
  }
}
 
void Sema::diagnoseMissingTemplateArguments(const CXXScopeSpec &SS,
                                            bool TemplateKeyword,
                                            TemplateDecl *TD,
                                            SourceLocation Loc) {
  TemplateName Name = Context.getQualifiedTemplateName(
      SS.getScopeRep(), TemplateKeyword, TemplateName(TD));
  diagnoseMissingTemplateArguments(Name, Loc);
}
 
ExprResult
Sema::CheckConceptTemplateId(const CXXScopeSpec &SS,
                             SourceLocation TemplateKWLoc,
                             const DeclarationNameInfo &ConceptNameInfo,
                             NamedDecl *FoundDecl,
                             ConceptDecl *NamedConcept,
                             const TemplateArgumentListInfo *TemplateArgs) {
  assert(NamedConcept && "A concept template id without a template?");
 
  llvm::SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (CheckTemplateArgumentList(
          NamedConcept, ConceptNameInfo.getLoc(),
          const_cast<TemplateArgumentListInfo &>(*TemplateArgs),
          /*DefaultArgs=*/{},
          /*PartialTemplateArgs=*/false, SugaredConverted, CanonicalConverted,
          /*UpdateArgsWithConversions=*/false))
    return ExprError();
 
  DiagnoseUseOfDecl(NamedConcept, ConceptNameInfo.getLoc());
 
  auto *CSD = ImplicitConceptSpecializationDecl::Create(
      Context, NamedConcept->getDeclContext(), NamedConcept->getLocation(),
      CanonicalConverted);
  ConstraintSatisfaction Satisfaction;
  bool AreArgsDependent =
      TemplateSpecializationType::anyDependentTemplateArguments(
          *TemplateArgs, CanonicalConverted);
  MultiLevelTemplateArgumentList MLTAL(NamedConcept, CanonicalConverted,
                                       /*Final=*/false);
  LocalInstantiationScope Scope(*this);
 
  EnterExpressionEvaluationContext EECtx{
      *this, ExpressionEvaluationContext::Unevaluated, CSD};
 
  if (!AreArgsDependent &&
      CheckConstraintSatisfaction(
          NamedConcept, {NamedConcept->getConstraintExpr()}, MLTAL,
          SourceRange(SS.isSet() ? SS.getBeginLoc() : ConceptNameInfo.getLoc(),
                      TemplateArgs->getRAngleLoc()),
          Satisfaction))
    return ExprError();
  auto *CL = ConceptReference::Create(
      Context,
      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc{},
      TemplateKWLoc, ConceptNameInfo, FoundDecl, NamedConcept,
      ASTTemplateArgumentListInfo::Create(Context, *TemplateArgs));
  return ConceptSpecializationExpr::Create(
      Context, CL, CSD, AreArgsDependent ? nullptr : &Satisfaction);
}
 
ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
                                     SourceLocation TemplateKWLoc,
                                     LookupResult &R,
                                     bool RequiresADL,
                                 const TemplateArgumentListInfo *TemplateArgs) {
  // FIXME: Can we do any checking at this point? I guess we could check the
  // template arguments that we have against the template name, if the template
  // name refers to a single template. That's not a terribly common case,
  // though.
  // foo<int> could identify a single function unambiguously
  // This approach does NOT work, since f<int>(1);
  // gets resolved prior to resorting to overload resolution
  // i.e., template<class T> void f(double);
  //       vs template<class T, class U> void f(U);
 
  // These should be filtered out by our callers.
  assert(!R.isAmbiguous() && "ambiguous lookup when building templateid");
 
  // Non-function templates require a template argument list.
  if (auto *TD = R.getAsSingle<TemplateDecl>()) {
    if (!TemplateArgs && !isa<FunctionTemplateDecl>(TD)) {
      diagnoseMissingTemplateArguments(
          SS, /*TemplateKeyword=*/TemplateKWLoc.isValid(), TD, R.getNameLoc());
      return ExprError();
    }
  }
  bool KnownDependent = false;
  // In C++1y, check variable template ids.
  if (R.getAsSingle<VarTemplateDecl>()) {
    ExprResult Res = CheckVarTemplateId(
        SS, R.getLookupNameInfo(), R.getAsSingle<VarTemplateDecl>(),
        R.getRepresentativeDecl(), TemplateKWLoc, TemplateArgs);
    if (Res.isInvalid() || Res.isUsable())
      return Res;
    // Result is dependent. Carry on to build an UnresolvedLookupExpr.
    KnownDependent = true;
  }
 
  if (R.getAsSingle<ConceptDecl>()) {
    return CheckConceptTemplateId(SS, TemplateKWLoc, R.getLookupNameInfo(),
                                  R.getRepresentativeDecl(),
                                  R.getAsSingle<ConceptDecl>(), TemplateArgs);
  }
 
  // We don't want lookup warnings at this point.
  R.suppressDiagnostics();
 
  UnresolvedLookupExpr *ULE = UnresolvedLookupExpr::Create(
      Context, R.getNamingClass(), SS.getWithLocInContext(Context),
      TemplateKWLoc, R.getLookupNameInfo(), RequiresADL, TemplateArgs,
      R.begin(), R.end(), KnownDependent,
      /*KnownInstantiationDependent=*/false);
 
  // Model the templates with UnresolvedTemplateTy. The expression should then
  // either be transformed in an instantiation or be diagnosed in
  // CheckPlaceholderExpr.
  if (ULE->getType() == Context.OverloadTy && R.isSingleResult() &&
      !R.getFoundDecl()->getAsFunction())
    ULE->setType(Context.UnresolvedTemplateTy);
 
  return ULE;
}
 
ExprResult Sema::BuildQualifiedTemplateIdExpr(
    CXXScopeSpec &SS, SourceLocation TemplateKWLoc,
    const DeclarationNameInfo &NameInfo,
    const TemplateArgumentListInfo *TemplateArgs, bool IsAddressOfOperand) {
  assert(TemplateArgs || TemplateKWLoc.isValid());
 
  LookupResult R(*this, NameInfo, LookupOrdinaryName);
  if (LookupTemplateName(R, /*S=*/nullptr, SS, /*ObjectType=*/QualType(),
                         /*EnteringContext=*/false, TemplateKWLoc))
    return ExprError();
 
  if (R.isAmbiguous())
    return ExprError();
 
  if (R.wasNotFoundInCurrentInstantiation() || SS.isInvalid())
    return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);
 
  if (R.empty()) {
    DeclContext *DC = computeDeclContext(SS);
    Diag(NameInfo.getLoc(), diag::err_no_member)
      << NameInfo.getName() << DC << SS.getRange();
    return ExprError();
  }
 
  // If necessary, build an implicit class member access.
  if (isPotentialImplicitMemberAccess(SS, R, IsAddressOfOperand))
    return BuildPossibleImplicitMemberExpr(SS, TemplateKWLoc, R, TemplateArgs,
                                           /*S=*/nullptr);
 
  return BuildTemplateIdExpr(SS, TemplateKWLoc, R, /*ADL=*/false, TemplateArgs);
}
 
TemplateNameKind Sema::ActOnTemplateName(Scope *S,
                                         CXXScopeSpec &SS,
                                         SourceLocation TemplateKWLoc,
                                         const UnqualifiedId &Name,
                                         ParsedType ObjectType,
                                         bool EnteringContext,
                                         TemplateTy &Result,
                                         bool AllowInjectedClassName) {
  if (TemplateKWLoc.isValid() && S && !S->getTemplateParamParent())
    Diag(TemplateKWLoc,
         getLangOpts().CPlusPlus11 ?
           diag::warn_cxx98_compat_template_outside_of_template :
           diag::ext_template_outside_of_template)
      << FixItHint::CreateRemoval(TemplateKWLoc);
 
  if (SS.isInvalid())
    return TNK_Non_template;
 
  // Figure out where isTemplateName is going to look.
  DeclContext *LookupCtx = nullptr;
  if (SS.isNotEmpty())
    LookupCtx = computeDeclContext(SS, EnteringContext);
  else if (ObjectType)
    LookupCtx = computeDeclContext(GetTypeFromParser(ObjectType));
 
  // C++0x [temp.names]p5:
  //   If a name prefixed by the keyword template is not the name of
  //   a template, the program is ill-formed. [Note: the keyword
  //   template may not be applied to non-template members of class
  //   templates. -end note ] [ Note: as is the case with the
  //   typename prefix, the template prefix is allowed in cases
  //   where it is not strictly necessary; i.e., when the
  //   nested-name-specifier or the expression on the left of the ->
  //   or . is not dependent on a template-parameter, or the use
  //   does not appear in the scope of a template. -end note]
  //
  // Note: C++03 was more strict here, because it banned the use of
  // the "template" keyword prior to a template-name that was not a
  // dependent name. C++ DR468 relaxed this requirement (the
  // "template" keyword is now permitted). We follow the C++0x
  // rules, even in C++03 mode with a warning, retroactively applying the DR.
  bool MemberOfUnknownSpecialization;
  TemplateNameKind TNK = isTemplateName(S, SS, TemplateKWLoc.isValid(), Name,
                                        ObjectType, EnteringContext, Result,
                                        MemberOfUnknownSpecialization);
  if (TNK != TNK_Non_template) {
    // We resolved this to a (non-dependent) template name. Return it.
    auto *LookupRD = dyn_cast_or_null<CXXRecordDecl>(LookupCtx);
    if (!AllowInjectedClassName && SS.isNotEmpty() && LookupRD &&
        Name.getKind() == UnqualifiedIdKind::IK_Identifier &&
        Name.Identifier && LookupRD->getIdentifier() == Name.Identifier) {
      // C++14 [class.qual]p2:
      //   In a lookup in which function names are not ignored and the
      //   nested-name-specifier nominates a class C, if the name specified
      //   [...] is the injected-class-name of C, [...] the name is instead
      //   considered to name the constructor
      //
      // We don't get here if naming the constructor would be valid, so we
      // just reject immediately and recover by treating the
      // injected-class-name as naming the template.
      Diag(Name.getBeginLoc(),
           diag::ext_out_of_line_qualified_id_type_names_constructor)
          << Name.Identifier
          << 0 /*injected-class-name used as template name*/
          << TemplateKWLoc.isValid();
    }
    return TNK;
  }
 
  if (!MemberOfUnknownSpecialization) {
    // Didn't find a template name, and the lookup wasn't dependent.
    // Do the lookup again to determine if this is a "nothing found" case or
    // a "not a template" case. FIXME: Refactor isTemplateName so we don't
    // need to do this.
    DeclarationNameInfo DNI = GetNameFromUnqualifiedId(Name);
    LookupResult R(*this, DNI.getName(), Name.getBeginLoc(),
                   LookupOrdinaryName);
    // Tell LookupTemplateName that we require a template so that it diagnoses
    // cases where it finds a non-template.
    RequiredTemplateKind RTK = TemplateKWLoc.isValid()
                                   ? RequiredTemplateKind(TemplateKWLoc)
                                   : TemplateNameIsRequired;
    if (!LookupTemplateName(R, S, SS, ObjectType.get(), EnteringContext, RTK,
                            /*ATK=*/nullptr, /*AllowTypoCorrection=*/false) &&
        !R.isAmbiguous()) {
      if (LookupCtx)
        Diag(Name.getBeginLoc(), diag::err_no_member)
            << DNI.getName() << LookupCtx << SS.getRange();
      else
        Diag(Name.getBeginLoc(), diag::err_undeclared_use)
            << DNI.getName() << SS.getRange();
    }
    return TNK_Non_template;
  }
 
  NestedNameSpecifier *Qualifier = SS.getScopeRep();
 
  switch (Name.getKind()) {
  case UnqualifiedIdKind::IK_Identifier:
    Result = TemplateTy::make(
        Context.getDependentTemplateName(Qualifier, Name.Identifier));
    return TNK_Dependent_template_name;
 
  case UnqualifiedIdKind::IK_OperatorFunctionId:
    Result = TemplateTy::make(Context.getDependentTemplateName(
        Qualifier, Name.OperatorFunctionId.Operator));
    return TNK_Function_template;
 
  case UnqualifiedIdKind::IK_LiteralOperatorId:
    // This is a kind of template name, but can never occur in a dependent
    // scope (literal operators can only be declared at namespace scope).
    break;
 
  default:
    break;
  }
 
  // This name cannot possibly name a dependent template. Diagnose this now
  // rather than building a dependent template name that can never be valid.
  Diag(Name.getBeginLoc(),
       diag::err_template_kw_refers_to_dependent_non_template)
      << GetNameFromUnqualifiedId(Name).getName() << Name.getSourceRange()
      << TemplateKWLoc.isValid() << TemplateKWLoc;
  return TNK_Non_template;
}
 
bool Sema::CheckTemplateTypeArgument(
    TemplateTypeParmDecl *Param, TemplateArgumentLoc &AL,
    SmallVectorImpl<TemplateArgument> &SugaredConverted,
    SmallVectorImpl<TemplateArgument> &CanonicalConverted) {
  const TemplateArgument &Arg = AL.getArgument();
  QualType ArgType;
  TypeSourceInfo *TSI = nullptr;
 
  // Check template type parameter.
  switch(Arg.getKind()) {
  case TemplateArgument::Type:
    // C++ [temp.arg.type]p1:
    //   A template-argument for a template-parameter which is a
    //   type shall be a type-id.
    ArgType = Arg.getAsType();
    TSI = AL.getTypeSourceInfo();
    break;
  case TemplateArgument::Template:
  case TemplateArgument::TemplateExpansion: {
    // We have a template type parameter but the template argument
    // is a template without any arguments.
    SourceRange SR = AL.getSourceRange();
    TemplateName Name = Arg.getAsTemplateOrTemplatePattern();
    diagnoseMissingTemplateArguments(Name, SR.getEnd());
    return true;
  }
  case TemplateArgument::Expression: {
    // We have a template type parameter but the template argument is an
    // expression; see if maybe it is missing the "typename" keyword.
    CXXScopeSpec SS;
    DeclarationNameInfo NameInfo;
 
   if (DependentScopeDeclRefExpr *ArgExpr =
               dyn_cast<DependentScopeDeclRefExpr>(Arg.getAsExpr())) {
      SS.Adopt(ArgExpr->getQualifierLoc());
      NameInfo = ArgExpr->getNameInfo();
    } else if (CXXDependentScopeMemberExpr *ArgExpr =
               dyn_cast<CXXDependentScopeMemberExpr>(Arg.getAsExpr())) {
      if (ArgExpr->isImplicitAccess()) {
        SS.Adopt(ArgExpr->getQualifierLoc());
        NameInfo = ArgExpr->getMemberNameInfo();
      }
    }
 
    if (auto *II = NameInfo.getName().getAsIdentifierInfo()) {
      LookupResult Result(*this, NameInfo, LookupOrdinaryName);
      LookupParsedName(Result, CurScope, &SS, /*ObjectType=*/QualType());
 
      if (Result.getAsSingle<TypeDecl>() ||
          Result.wasNotFoundInCurrentInstantiation()) {
        assert(SS.getScopeRep() && "dependent scope expr must has a scope!");
        // Suggest that the user add 'typename' before the NNS.
        SourceLocation Loc = AL.getSourceRange().getBegin();
        Diag(Loc, getLangOpts().MSVCCompat
                      ? diag::ext_ms_template_type_arg_missing_typename
                      : diag::err_template_arg_must_be_type_suggest)
            << FixItHint::CreateInsertion(Loc, "typename ");
        NoteTemplateParameterLocation(*Param);
 
        // Recover by synthesizing a type using the location information that we
        // already have.
        ArgType = Context.getDependentNameType(ElaboratedTypeKeyword::Typename,
                                               SS.getScopeRep(), II);
        TypeLocBuilder TLB;
        DependentNameTypeLoc TL = TLB.push<DependentNameTypeLoc>(ArgType);
        TL.setElaboratedKeywordLoc(SourceLocation(/*synthesized*/));
        TL.setQualifierLoc(SS.getWithLocInContext(Context));
        TL.setNameLoc(NameInfo.getLoc());
        TSI = TLB.getTypeSourceInfo(Context, ArgType);
 
        // Overwrite our input TemplateArgumentLoc so that we can recover
        // properly.
        AL = TemplateArgumentLoc(TemplateArgument(ArgType),
                                 TemplateArgumentLocInfo(TSI));
 
        break;
      }
    }
    // fallthrough
    [[fallthrough]];
  }
  default: {
    // We allow instantiateing a template with template argument packs when
    // building deduction guides.
    if (Arg.getKind() == TemplateArgument::Pack &&
        CodeSynthesisContexts.back().Kind ==
            Sema::CodeSynthesisContext::BuildingDeductionGuides) {
      SugaredConverted.push_back(Arg);
      CanonicalConverted.push_back(Arg);
      return false;
    }
    // We have a template type parameter but the template argument
    // is not a type.
    SourceRange SR = AL.getSourceRange();
    Diag(SR.getBegin(), diag::err_template_arg_must_be_type) << SR;
    NoteTemplateParameterLocation(*Param);
 
    return true;
  }
  }
 
  if (CheckTemplateArgument(TSI))
    return true;
 
  // Objective-C ARC:
  //   If an explicitly-specified template argument type is a lifetime type
  //   with no lifetime qualifier, the __strong lifetime qualifier is inferred.
  if (getLangOpts().ObjCAutoRefCount &&
      ArgType->isObjCLifetimeType() &&
      !ArgType.getObjCLifetime()) {
    Qualifiers Qs;
    Qs.setObjCLifetime(Qualifiers::OCL_Strong);
    ArgType = Context.getQualifiedType(ArgType, Qs);
  }
 
  SugaredConverted.push_back(TemplateArgument(ArgType));
  CanonicalConverted.push_back(
      TemplateArgument(Context.getCanonicalType(ArgType)));
  return false;
}
 
/// Substitute template arguments into the default template argument for
/// the given template type parameter.
///
/// \param SemaRef the semantic analysis object for which we are performing
/// the substitution.
///
/// \param Template the template that we are synthesizing template arguments
/// for.
///
/// \param TemplateLoc the location of the template name that started the
/// template-id we are checking.
///
/// \param RAngleLoc the location of the right angle bracket ('>') that
/// terminates the template-id.
///
/// \param Param the template template parameter whose default we are
/// substituting into.
///
/// \param Converted the list of template arguments provided for template
/// parameters that precede \p Param in the template parameter list.
///
/// \param Output the resulting substituted template argument.
///
/// \returns true if an error occurred.
static bool SubstDefaultTemplateArgument(
    Sema &SemaRef, TemplateDecl *Template, SourceLocation TemplateLoc,
    SourceLocation RAngleLoc, TemplateTypeParmDecl *Param,