SemaTemplate.cpp

Go to the documentation of this file.

//===------- 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/DeclFriend.h"
#include "clang/AST/DeclTemplate.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/AST/TypeVisitor.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/LangOptions.h"
#include "clang/Basic/PartialDiagnostic.h"
#include "clang/Basic/Stack.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/DeclSpec.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/Overload.h"
#include "clang/Sema/ParsedTemplate.h"
#include "clang/Sema/Scope.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "clang/Sema/TemplateDeduction.h"
#include "llvm/ADT/SmallBitVector.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringExtras.h"

#include <iterator>
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 (CXXRecordDecl *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 (ClassTemplateSpecializationDecl *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,
                         MemberOfUnknownSpecialization, SourceLocation(),
                         &AssumedTemplate,
                         /*AllowTypoCorrection=*/!Disambiguation))
    return TNK_Non_template;

  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;
  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;
          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);

    if (SS.isSet() && !SS.isInvalid()) {
      NestedNameSpecifier *Qualifier = SS.getScopeRep();
      Template = Context.getQualifiedTemplateName(Qualifier,
                                                  hasTemplateKeyword, TD);
    } else {
      Template = TemplateName(TD);
    }

    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,
                                ParsedTemplateTy *Template) {
  CXXScopeSpec SS;
  bool MemberOfUnknownSpecialization = false;

  // 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,
                         MemberOfUnknownSpecialization))
    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)
    *Template = TemplateTy::make(TemplateName(TD));
  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,
                              bool &MemberOfUnknownSpecialization,
                              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
  MemberOfUnknownSpecialization = false;
  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->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 &&
        std::all_of(Found.begin(), Found.end(), [](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) {
      MemberOfUnknownSpecialization = true;
      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.isSuppressingDiagnostics()) {
      //   - 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);
}

/// ActOnDependentIdExpression - Handle a dependent id-expression that
/// was just parsed.  This is only possible with an explicit scope
/// specifier naming a dependent type.
ExprResult
Sema::ActOnDependentIdExpression(const CXXScopeSpec &SS,
                                 SourceLocation TemplateKWLoc,
                                 const DeclarationNameInfo &NameInfo,
                                 bool isAddressOfOperand,
                           const TemplateArgumentListInfo *TemplateArgs) {
  DeclContext *DC = getFunctionLevelDeclContext();

  // C++11 [expr.prim.general]p12:
  //   An id-expression that denotes a non-static data member or non-static
  //   member function of a class can only be used:
  //   (...)
  //   - if that id-expression denotes a non-static data member and it
  //     appears in an unevaluated operand.
  //
  // If this might be the case, form a DependentScopeDeclRefExpr instead of a
  // CXXDependentScopeMemberExpr. The former can instantiate to either
  // DeclRefExpr or MemberExpr depending on lookup results, while the latter is
  // always a MemberExpr.
  bool MightBeCxx11UnevalField =
      getLangOpts().CPlusPlus11 && isUnevaluatedContext();

  // Check if the nested name specifier is an enum type.
  bool IsEnum = false;
  if (NestedNameSpecifier *NNS = SS.getScopeRep())
    IsEnum = dyn_cast_or_null<EnumType>(NNS->getAsType());

  if (!MightBeCxx11UnevalField && !isAddressOfOperand && !IsEnum &&
      isa<CXXMethodDecl>(DC) && cast<CXXMethodDecl>(DC)->isInstance()) {
    QualType ThisType = cast<CXXMethodDecl>(DC)->getThisType();

    // Since the 'this' expression is synthesized, we don't need to
    // perform the double-lookup check.
    NamedDecl *FirstQualifierInScope = nullptr;

    return CXXDependentScopeMemberExpr::Create(
        Context, /*This*/ nullptr, ThisType, /*IsArrow*/ true,
        /*Op*/ SourceLocation(), SS.getWithLocInContext(Context), TemplateKWLoc,
        FirstQualifierInScope, 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 QualifierLoc
  NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
  if (!QualifierLoc)
    return ExprError();

  return DependentScopeDeclRefExpr::Create(
      Context, QualifierLoc, TemplateKWLoc, NameInfo, TemplateArgs);
}


/// Determine whether we would be unable to instantiate this template (because
/// it either has no definition, or is in the process of being instantiated).
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 (!hasVisibleDefinition(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;

  llvm::Optional<unsigned> Note;
  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();
      Note = diag::note_explicit_instantiation_here;
    } else {
      assert(isa<TagDecl>(Instantiation) && "Must be a TagDecl!");
      Diag(PointOfInstantiation,
           diag::err_implicit_instantiate_member_undefined)
        << InstantiationTy;
      Note = diag::note_member_declared_at;
    }
  } else {
    if (isa<FunctionDecl>(Instantiation)) {
      Diag(PointOfInstantiation,
           diag::err_explicit_instantiation_undefined_func_template)
        << Pattern;
      Note = diag::note_explicit_instantiation_here;
    } else if (isa<TagDecl>(Instantiation)) {
      Diag(PointOfInstantiation, diag::err_template_instantiate_undefined)
        << (TSK != TSK_ImplicitInstantiation)
        << InstantiationTy;
      Note = diag::note_template_decl_here;
    } 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();
      Note = diag::note_explicit_instantiation_here;
    }
  }
  if (Note) // Diagnostics were emitted.
    Diag(Pattern->getLocation(), Note.getValue());

  // 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;
}

/// DiagnoseTemplateParameterShadow - Produce a diagnostic complaining
/// that the template parameter 'PrevDecl' is being shadowed by a new
/// declaration at location Loc. Returns true to indicate that this is
/// an error, and false otherwise.
void Sema::DiagnoseTemplateParameterShadow(SourceLocation Loc, Decl *PrevDecl) {
  assert(PrevDecl->isTemplateParameter() && "Not a template parameter");

  // C++ [temp.local]p4:
  //   A template-parameter shall not be redeclared within its
  //   scope (including nested scopes).
  //
  // Make this a warning when MSVC compatibility is requested.
  unsigned DiagId = getLangOpts().MSVCCompat ? diag::ext_template_param_shadow
                                             : diag::err_template_param_shadow;
  Diag(Loc, DiagId) << cast<NamedDecl>(PrevDecl)->getDeclName();
  Diag(PrevDecl->getLocation(), diag::note_template_param_here);
}

/// AdjustDeclIfTemplate - If the given decl happens to be a template, reset
/// the parameter D to reference the templated declaration and return a pointer
/// to the template declaration. Otherwise, do nothing to D and return null.
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, Optional<unsigned int>());
    else
      TArg = Template;
    return TemplateArgumentLoc(TArg,
                               Arg.getScopeSpec().getWithLocInContext(
                                                              SemaRef.Context),
                               Arg.getLocation(),
                               Arg.getEllipsisLoc());
  }
  }

  llvm_unreachable("Unhandled parsed template argument");
}

/// Translates template arguments as provided by the parser
/// into template arguments used by semantic analysis.
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,
                                                 IdentifierInfo *Name) {
  NamedDecl *PrevDecl = SemaRef.LookupSingleName(
      S, Name, Loc, Sema::LookupOrdinaryName, Sema::ForVisibleRedeclaration);
  if (PrevDecl && PrevDecl->isTemplateParameter())
    SemaRef.DiagnoseTemplateParameterShadow(Loc, PrevDecl);
}

/// Convert a parsed type into a parsed template argument. This is mostly
/// trivial, except that we may have parsed a C++17 deduced class template
/// specialization type, in which case we should form a template template
/// argument instead of a type template argument.
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();
      if (SS.isSet())
        Name = Context.getQualifiedTemplateName(SS.getScopeRep(),
                                                /*HasTemplateKeyword*/ false,
                                                Name.getAsTemplateDecl());
      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());
}

/// ActOnTypeParameter - Called when a C++ template type parameter
/// (e.g., "typename T") has been parsed. Typename specifies whether
/// the keyword "typename" was used to declare the type parameter
/// (otherwise, "class" was used), and KeyLoc is the location of the
/// "class" or "typename" keyword. ParamName is the name of the
/// parameter (NULL indicates an unnamed template parameter) and
/// ParamNameLoc is the location of the parameter name (if any).
/// If the type parameter has a default argument, it will be added
/// later via ActOnTypeParameterDefault.
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 *LSI = getEnclosingLambda())
      LSI->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(Param, DefaultTInfo)) {
      Param->setInvalidDecl();
      return Param;
    }

    Param->setDefaultArgument(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::ActOnTypeConstraint(const CXXScopeSpec &SS,
                               TemplateIdAnnotation *TypeConstr,
                               TemplateTypeParmDecl *ConstrainedParameter,
                               SourceLocation EllipsisLoc) {
  ConceptDecl *CD =
      cast<ConceptDecl>(TypeConstr->Template.get().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;
  }

  bool WereArgsSpecified = TypeConstr->LAngleLoc.isValid();

  if (!WereArgsSpecified &&
      CD->getTemplateParameters()->getMinRequiredArguments() > 1) {
    Diag(TypeConstr->TemplateNameLoc,
         diag::err_type_constraint_missing_arguments) << CD;
    return true;
  }

  TemplateArgumentListInfo TemplateArgs;
  if (TypeConstr->LAngleLoc.isValid()) {
    TemplateArgs =
        makeTemplateArgumentListInfo(*this, *TypeConstr);
  }
  return AttachTypeConstraint(
      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
      DeclarationNameInfo(DeclarationName(TypeConstr->Name),
                          TypeConstr->TemplateNameLoc), CD,
      TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
      ConstrainedParameter, EllipsisLoc);
}

template<typename ArgumentLocAppender>
static ExprResult formImmediatelyDeclaredConstraint(
    Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
    ConceptDecl *NamedConcept, 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=*/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> && ...)
  return S.BuildCXXFoldExpr(/*LParenLoc=*/SourceLocation(),
                            ImmediatelyDeclaredConstraint.get(), BO_LAnd,
                            EllipsisLoc, /*RHS=*/nullptr,
                            /*RParenLoc=*/SourceLocation(),
                            /*NumExpansions=*/None);
}

/// Attach a type-constraint to a template parameter.
/// \returns true if an error occured. This can happen if the
/// immediately-declared constraint could not be formed (e.g. incorrect number
/// of arguments for the named concept).
bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
                                DeclarationNameInfo NameInfo,
                                ConceptDecl *NamedConcept,
                                const TemplateArgumentListInfo *TemplateArgs,
                                TemplateTypeParmDecl *ConstrainedParameter,
                                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(ConstrainedParameter->getTypeForDecl(), 0);

  ExprResult ImmediatelyDeclaredConstraint =
      formImmediatelyDeclaredConstraint(
          *this, NS, NameInfo, NamedConcept,
          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;

  ConstrainedParameter->setTypeConstraint(NS, NameInfo,
                                          /*FoundDecl=*/NamedConcept,
                                          NamedConcept, ArgsAsWritten,
                                          ImmediatelyDeclaredConstraint.get());
  return false;
}

bool Sema::AttachTypeConstraint(AutoTypeLoc TL, NonTypeTemplateParmDecl *NTTP,
                                SourceLocation EllipsisLoc) {
  if (NTTP->getType() != TL.getType() ||
      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
    Diag(NTTP->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
         diag::err_unsupported_placeholder_constraint)
       << NTTP->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(NTTP, NTTP->getType(), VK_RValue,
                                      NTTP->getLocation());
  if (!Ref)
    return true;
  ExprResult ImmediatelyDeclaredConstraint =
      formImmediatelyDeclaredConstraint(
          *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
          TL.getNamedConcept(), TL.getLAngleLoc(), TL.getRAngleLoc(),
          BuildDecltypeType(Ref, NTTP->getLocation()), NTTP->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;

  NTTP->setPlaceholderTypeConstraint(ImmediatelyDeclaredConstraint.get());
  return false;
}

/// Check that the type of a non-type template parameter is
/// well-formed.
///
/// \returns the (possibly-promoted) parameter type if valid;
/// otherwise, produces a diagnostic and returns a NULL type.
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 = SubstAutoTypeSourceInfo(TSI, Context.DependentTy);
  }

  return CheckNonTypeTemplateParameterType(TSI->getType(), Loc);
}

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() ||
      //   -- reference to object or reference to function,
      T->isReferenceType() ||
      //   -- pointer to member,
      T->isMemberPointerType() ||
      //   -- std::nullptr_t.
      T->isNullPtrType() ||
      // Allow use of auto in template parameter declarations.
      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();

  Diag(Loc, diag::err_template_nontype_parm_bad_type)
    << T;

  return QualType();
}

NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
                                          unsigned Depth,
                                          unsigned Position,
                                          SourceLocation EqualLoc,
                                          Expr *Default) {
  TypeSourceInfo *TInfo = GetTypeForDeclarator(D, S);

  // 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 (TInfo->getType()->isUndeducedType()) {
    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);

  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, D.getEllipsisLoc()))
        Invalid = true;

  if (Invalid)
    Param->setInvalidDecl();

  if (Param->isParameterPack())
    if (auto *LSI = getEnclosingLambda())
      LSI->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;

    TemplateArgument Converted;
    ExprResult DefaultRes =
        CheckTemplateArgument(Param, Param->getType(), Default, Converted);
    if (DefaultRes.isInvalid()) {
      Param->setInvalidDecl();
      return Param;
    }
    Default = DefaultRes.get();

    Param->setDefaultArgument(Default);
  }

  return Param;
}

/// ActOnTemplateTemplateParameter - Called when a C++ template template
/// parameter (e.g. T in template <template <typename> class T> class array)
/// has been parsed. S is the current scope.
NamedDecl *Sema::ActOnTemplateTemplateParameter(Scope* S,
                                           SourceLocation TmpLoc,
                                           TemplateParameterList *Params,
                                           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, Params);
  Param->setAccess(AS_public);

  if (Param->isParameterPack())
    if (auto *LSI = getEnclosingLambda())
      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;
}

/// ActOnTemplateParameterList - Builds a TemplateParameterList, optionally
/// constrained by RequiresClause, that contains the template parameters in
/// Params.
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);

  return TemplateParameterList::Create(
      Context, TemplateLoc, LAngleLoc,
      llvm::makeArrayRef(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));
}

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 != TUK_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 != TTK_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 == TUK_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 == TUK_Friend
                        ? diag::warn_template_qualified_friend_ignored
                        : diag::err_template_qualified_declarator_no_match)
          << SS.getScopeRep() << SS.getRange();
      return TUK != TUK_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;
    } else if (TUK != TUK_Friend && TUK != TUK_Reference)
      diagnoseQualifiedDeclaration(SS, SemanticContext, Name, NameLoc, 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 != TUK_Friend &&
        DiagnoseClassNameShadow(SemanticContext,
                                DeclarationNameInfo(Name, NameLoc)))
      return true;

    LookupName(Previous, S);
  }

  if (Previous.isAmbiguous())
    return true;

  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 && PrevDecl && isa<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 == TUK_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->getUsingDecl()->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 == TUK_Friend && CurContext->isDependentContext()) &&
        !TemplateParameterListsAreEqual(TemplateParams,
                                   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 == TUK_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 == TUK_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 == TUK_Friend && CurContext->isDependentContext()) &&
      CheckTemplateParameterList(
          TemplateParams,
          PrevClassTemplate
              ? PrevClassTemplate->getMostRecentDecl()->getTemplateParameters()
              : nullptr,
          (SS.isSet() && SemanticContext && SemanticContext->isRecord() &&
           SemanticContext->isDependentContext())
              ? TPC_ClassTemplateMember
              : TUK == TUK_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 == TUK_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 == TUK_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::makeArrayRef(OuterTemplateParamLists,
                                    NumOuterTemplateParamLists));

  // Add alignment attributes if necessary; these attributes are checked when
  // the ASTContext lays out the structure.
  if (TUK == TUK_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 != TUK_Friend && NewTemplate->getDeclContext()->isRecord())
    SetMemberAccessSpecifier(NewTemplate, PrevClassTemplate, AS);

  // Set the lexical context of these templates
  NewClass->setLexicalDeclContext(CurContext);
  NewTemplate->setLexicalDeclContext(CurContext);

  if (TUK == TUK_Definition && (!SkipBody || !SkipBody->ShouldSkip))
    NewClass->startDefinition();

  ProcessDeclAttributeList(S, NewClass, Attr);

  if (PrevClassTemplate)
    mergeDeclAttributes(NewClass, PrevClassTemplate->getTemplatedDecl());

  AddPushedVisibilityAttribute(NewClass);
  inferGslOwnerPointerAttribute(NewClass);

  if (TUK != TUK_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)
    CheckRedeclarationModuleOwnership(NewTemplate, PrevClassTemplate);

  if (Invalid) {
    NewTemplate->setInvalidDecl();
    NewClass->setInvalidDecl();
  }

  ActOnDocumentableDecl(NewTemplate);

  if (SkipBody && SkipBody->ShouldSkip)
    return SkipBody->Previous;

  return NewTemplate;
}

namespace {
/// Tree transform to "extract" a transformed type from a class template's
/// constructor to a deduction guide.
class ExtractTypeForDeductionGuide
  : public TreeTransform<ExtractTypeForDeductionGuide> {
  llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs;

public:
  typedef TreeTransform<ExtractTypeForDeductionGuide> Base;
  ExtractTypeForDeductionGuide(
      Sema &SemaRef,
      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs)
      : Base(SemaRef), MaterializedTypedefs(MaterializedTypedefs) {}

  TypeSourceInfo *transform(TypeSourceInfo *TSI) { return TransformType(TSI); }

  QualType TransformTypedefType(TypeLocBuilder &TLB, TypedefTypeLoc TL) {
    ASTContext &Context = SemaRef.getASTContext();
    TypedefNameDecl *OrigDecl = TL.getTypedefNameDecl();
    TypeLocBuilder InnerTLB;
    QualType Transformed =
        TransformType(InnerTLB, OrigDecl->getTypeSourceInfo()->getTypeLoc());
    TypeSourceInfo *TSI =
        TransformType(InnerTLB.getTypeSourceInfo(Context, Transformed));

    TypedefNameDecl *Decl = nullptr;

    if (isa<TypeAliasDecl>(OrigDecl))
      Decl = TypeAliasDecl::Create(
          Context, Context.getTranslationUnitDecl(), OrigDecl->getBeginLoc(),
          OrigDecl->getLocation(), OrigDecl->getIdentifier(), TSI);
    else {
      assert(isa<TypedefDecl>(OrigDecl) && "Not a Type alias or typedef");
      Decl = TypedefDecl::Create(
          Context, Context.getTranslationUnitDecl(), OrigDecl->getBeginLoc(),
          OrigDecl->getLocation(), OrigDecl->getIdentifier(), TSI);
    }

    MaterializedTypedefs.push_back(Decl);

    QualType TDTy = Context.getTypedefType(Decl);
    TypedefTypeLoc TypedefTL = TLB.push<TypedefTypeLoc>(TDTy);
    TypedefTL.setNameLoc(TL.getNameLoc());

    return TDTy;
  }
};

/// Transform to convert portions of a constructor declaration into the
/// corresponding deduction guide, per C++1z [over.match.class.deduct]p1.
struct ConvertConstructorToDeductionGuideTransform {
  ConvertConstructorToDeductionGuideTransform(Sema &S,
                                              ClassTemplateDecl *Template)
      : SemaRef(S), Template(Template) {}

  Sema &SemaRef;
  ClassTemplateDecl *Template;

  DeclContext *DC = Template->getDeclContext();
  CXXRecordDecl *Primary = Template->getTemplatedDecl();
  DeclarationName DeductionGuideName =
      SemaRef.Context.DeclarationNames.getCXXDeductionGuideName(Template);

  QualType DeducedType = SemaRef.Context.getTypeDeclType(Primary);

  // Index adjustment to apply to convert depth-1 template parameters into
  // depth-0 template parameters.
  unsigned Depth1IndexAdjustment = Template->getTemplateParameters()->size();

  /// Transform a constructor declaration into a deduction guide.
  NamedDecl *transformConstructor(FunctionTemplateDecl *FTD,
                                  CXXConstructorDecl *CD) {
    SmallVector<TemplateArgument, 16> SubstArgs;

    LocalInstantiationScope Scope(SemaRef);

    // C++ [over.match.class.deduct]p1:
    // -- For each constructor of the class template designated by the
    //    template-name, a function template with the following properties:

    //    -- The template parameters are the template parameters of the class
    //       template followed by the template parameters (including default
    //       template arguments) of the constructor, if any.
    TemplateParameterList *TemplateParams = Template->getTemplateParameters();
    if (FTD) {
      TemplateParameterList *InnerParams = FTD->getTemplateParameters();
      SmallVector<NamedDecl *, 16> AllParams;
      AllParams.reserve(TemplateParams->size() + InnerParams->size());
      AllParams.insert(AllParams.begin(),
                       TemplateParams->begin(), TemplateParams->end());
      SubstArgs.reserve(InnerParams->size());

      // Later template parameters could refer to earlier ones, so build up
      // a list of substituted template arguments as we go.
      for (NamedDecl *Param : *InnerParams) {
        MultiLevelTemplateArgumentList Args;
        Args.setKind(TemplateSubstitutionKind::Rewrite);
        Args.addOuterTemplateArguments(SubstArgs);
        Args.addOuterRetainedLevel();
        NamedDecl *NewParam = transformTemplateParameter(Param, Args);
        if (!NewParam)
          return nullptr;
        AllParams.push_back(NewParam);
        SubstArgs.push_back(SemaRef.Context.getCanonicalTemplateArgument(
            SemaRef.Context.getInjectedTemplateArg(NewParam)));
      }
      TemplateParams = TemplateParameterList::Create(
          SemaRef.Context, InnerParams->getTemplateLoc(),
          InnerParams->getLAngleLoc(), AllParams, InnerParams->getRAngleLoc(),
          /*FIXME: RequiresClause*/ nullptr);
    }

    // If we built a new template-parameter-list, track that we need to
    // substitute references to the old parameters into references to the
    // new ones.
    MultiLevelTemplateArgumentList Args;
    Args.setKind(TemplateSubstitutionKind::Rewrite);
    if (FTD) {
      Args.addOuterTemplateArguments(SubstArgs);
      Args.addOuterRetainedLevel();
    }

    FunctionProtoTypeLoc FPTL = CD->getTypeSourceInfo()->getTypeLoc()
                                   .getAsAdjusted<FunctionProtoTypeLoc>();
    assert(FPTL && "no prototype for constructor declaration");

    // Transform the type of the function, adjusting the return type and
    // replacing references to the old parameters with references to the
    // new ones.
    TypeLocBuilder TLB;
    SmallVector<ParmVarDecl*, 8> Params;
    SmallVector<TypedefNameDecl *, 4> MaterializedTypedefs;
    QualType NewType = transformFunctionProtoType(TLB, FPTL, Params, Args,
                                                  MaterializedTypedefs);
    if (NewType.isNull())
      return nullptr;
    TypeSourceInfo *NewTInfo = TLB.getTypeSourceInfo(SemaRef.Context, NewType);

    return buildDeductionGuide(TemplateParams, CD->getExplicitSpecifier(),
                               NewTInfo, CD->getBeginLoc(), CD->getLocation(),
                               CD->getEndLoc(), MaterializedTypedefs);
  }

  /// Build a deduction guide with the specified parameter types.
  NamedDecl *buildSimpleDeductionGuide(MutableArrayRef<QualType> ParamTypes) {
    SourceLocation Loc = Template->getLocation();

    // Build the requested type.
    FunctionProtoType::ExtProtoInfo EPI;
    EPI.HasTrailingReturn = true;
    QualType Result = SemaRef.BuildFunctionType(DeducedType, ParamTypes, Loc,
                                                DeductionGuideName, EPI);
    TypeSourceInfo *TSI = SemaRef.Context.getTrivialTypeSourceInfo(Result, Loc);

    FunctionProtoTypeLoc FPTL =
        TSI->getTypeLoc().castAs<FunctionProtoTypeLoc>();

    // Build the parameters, needed during deduction / substitution.
    SmallVector<ParmVarDecl*, 4> Params;
    for (auto T : ParamTypes) {
      ParmVarDecl *NewParam = ParmVarDecl::Create(
          SemaRef.Context, DC, Loc, Loc, nullptr, T,
          SemaRef.Context.getTrivialTypeSourceInfo(T, Loc), SC_None, nullptr);
      NewParam->setScopeInfo(0, Params.size());
      FPTL.setParam(Params.size(), NewParam);
      Params.push_back(NewParam);
    }

    return buildDeductionGuide(Template->getTemplateParameters(),
                               ExplicitSpecifier(), TSI, Loc, Loc, Loc);
  }

private:
  /// Transform a constructor template parameter into a deduction guide template
  /// parameter, rebuilding any internal references to earlier parameters and
  /// renumbering as we go.
  NamedDecl *transformTemplateParameter(NamedDecl *TemplateParam,
                                        MultiLevelTemplateArgumentList &Args) {
    if (auto *TTP = dyn_cast<TemplateTypeParmDecl>(TemplateParam)) {
      // TemplateTypeParmDecl's index cannot be changed after creation, so
      // substitute it directly.
      auto *NewTTP = TemplateTypeParmDecl::Create(
          SemaRef.Context, DC, TTP->getBeginLoc(), TTP->getLocation(),
          /*Depth*/ 0, Depth1IndexAdjustment + TTP->getIndex(),
          TTP->getIdentifier(), TTP->wasDeclaredWithTypename(),
          TTP->isParameterPack(), TTP->hasTypeConstraint(),
          TTP->isExpandedParameterPack() ?
          llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
      if (const auto *TC = TTP->getTypeConstraint()) {
        TemplateArgumentListInfo TransformedArgs;
        const auto *ArgsAsWritten = TC->getTemplateArgsAsWritten();
        if (!ArgsAsWritten ||
            SemaRef.Subst(ArgsAsWritten->getTemplateArgs(),
                          ArgsAsWritten->NumTemplateArgs, TransformedArgs,
                          Args))
          SemaRef.AttachTypeConstraint(
              TC->getNestedNameSpecifierLoc(), TC->getConceptNameInfo(),
              TC->getNamedConcept(), ArgsAsWritten ? &TransformedArgs : nullptr,
              NewTTP,
              NewTTP->isParameterPack()
                 ? cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
                     ->getEllipsisLoc()
                 : SourceLocation());
      }
      if (TTP->hasDefaultArgument()) {
        TypeSourceInfo *InstantiatedDefaultArg =
            SemaRef.SubstType(TTP->getDefaultArgumentInfo(), Args,
                              TTP->getDefaultArgumentLoc(), TTP->getDeclName());
        if (InstantiatedDefaultArg)
          NewTTP->setDefaultArgument(InstantiatedDefaultArg);
      }
      SemaRef.CurrentInstantiationScope->InstantiatedLocal(TemplateParam,
                                                           NewTTP);
      return NewTTP;
    }

    if (auto *TTP = dyn_cast<TemplateTemplateParmDecl>(TemplateParam))
      return transformTemplateParameterImpl(TTP, Args);

    return transformTemplateParameterImpl(
        cast<NonTypeTemplateParmDecl>(TemplateParam), Args);
  }
  template<typename TemplateParmDecl>
  TemplateParmDecl *
  transformTemplateParameterImpl(TemplateParmDecl *OldParam,
                                 MultiLevelTemplateArgumentList &Args) {
    // Ask the template instantiator to do the heavy lifting for us, then adjust
    // the index of the parameter once it's done.
    auto *NewParam =
        cast<TemplateParmDecl>(SemaRef.SubstDecl(OldParam, DC, Args));
    assert(NewParam->getDepth() == 0 && "unexpected template param depth");
    NewParam->setPosition(NewParam->getPosition() + Depth1IndexAdjustment);
    return NewParam;
  }

  QualType transformFunctionProtoType(
      TypeLocBuilder &TLB, FunctionProtoTypeLoc TL,
      SmallVectorImpl<ParmVarDecl *> &Params,
      MultiLevelTemplateArgumentList &Args,
      SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs) {
    SmallVector<QualType, 4> ParamTypes;
    const FunctionProtoType *T = TL.getTypePtr();

    //    -- The types of the function parameters are those of the constructor.
    for (auto *OldParam : TL.getParams()) {
      ParmVarDecl *NewParam =
          transformFunctionTypeParam(OldParam, Args, MaterializedTypedefs);
      if (!NewParam)
        return QualType();
      ParamTypes.push_back(NewParam->getType());
      Params.push_back(NewParam);
    }

    //    -- The return type is the class template specialization designated by
    //       the template-name and template arguments corresponding to the
    //       template parameters obtained from the class template.
    //
    // We use the injected-class-name type of the primary template instead.
    // This has the convenient property that it is different from any type that
    // the user can write in a deduction-guide (because they cannot enter the
    // context of the template), so implicit deduction guides can never collide
    // with explicit ones.
    QualType ReturnType = DeducedType;
    TLB.pushTypeSpec(ReturnType).setNameLoc(Primary->getLocation());

    // Resolving a wording defect, we also inherit the variadicness of the
    // constructor.
    FunctionProtoType::ExtProtoInfo EPI;
    EPI.Variadic = T->isVariadic();
    EPI.HasTrailingReturn = true;

    QualType Result = SemaRef.BuildFunctionType(
        ReturnType, ParamTypes, TL.getBeginLoc(), DeductionGuideName, EPI);
    if (Result.isNull())
      return QualType();

    FunctionProtoTypeLoc NewTL = TLB.push<FunctionProtoTypeLoc>(Result);
    NewTL.setLocalRangeBegin(TL.getLocalRangeBegin());
    NewTL.setLParenLoc(TL.getLParenLoc());
    NewTL.setRParenLoc(TL.getRParenLoc());
    NewTL.setExceptionSpecRange(SourceRange());
    NewTL.setLocalRangeEnd(TL.getLocalRangeEnd());
    for (unsigned I = 0, E = NewTL.getNumParams(); I != E; ++I)
      NewTL.setParam(I, Params[I]);

    return Result;
  }

  ParmVarDecl *transformFunctionTypeParam(
      ParmVarDecl *OldParam, MultiLevelTemplateArgumentList &Args,
      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs) {
    TypeSourceInfo *OldDI = OldParam->getTypeSourceInfo();
    TypeSourceInfo *NewDI;
    if (auto PackTL = OldDI->getTypeLoc().getAs<PackExpansionTypeLoc>()) {
      // Expand out the one and only element in each inner pack.
      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, 0);
      NewDI =
          SemaRef.SubstType(PackTL.getPatternLoc(), Args,
                            OldParam->getLocation(), OldParam->getDeclName());
      if (!NewDI) return nullptr;
      NewDI =
          SemaRef.CheckPackExpansion(NewDI, PackTL.getEllipsisLoc(),
                                     PackTL.getTypePtr()->getNumExpansions());
    } else
      NewDI = SemaRef.SubstType(OldDI, Args, OldParam->getLocation(),
                                OldParam->getDeclName());
    if (!NewDI)
      return nullptr;

    // Extract the type. This (for instance) replaces references to typedef
    // members of the current instantiations with the definitions of those
    // typedefs, avoiding triggering instantiation of the deduced type during
    // deduction.
    NewDI = ExtractTypeForDeductionGuide(SemaRef, MaterializedTypedefs)
                .transform(NewDI);

    // Resolving a wording defect, we also inherit default arguments from the
    // constructor.
    ExprResult NewDefArg;
    if (OldParam->hasDefaultArg()) {
      // We don't care what the value is (we won't use it); just create a
      // placeholder to indicate there is a default argument.
      QualType ParamTy = NewDI->getType();
      NewDefArg = new (SemaRef.Context)
          OpaqueValueExpr(OldParam->getDefaultArg()->getBeginLoc(),
                          ParamTy.getNonLValueExprType(SemaRef.Context),
                          ParamTy->isLValueReferenceType() ? VK_LValue :
                          ParamTy->isRValueReferenceType() ? VK_XValue :
                          VK_RValue);
    }

    ParmVarDecl *NewParam = ParmVarDecl::Create(SemaRef.Context, DC,
                                                OldParam->getInnerLocStart(),
                                                OldParam->getLocation(),
                                                OldParam->getIdentifier(),
                                                NewDI->getType(),
                                                NewDI,
                                                OldParam->getStorageClass(),
                                                NewDefArg.get());
    NewParam->setScopeInfo(OldParam->getFunctionScopeDepth(),
                           OldParam->getFunctionScopeIndex());
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(OldParam, NewParam);
    return NewParam;
  }

  FunctionTemplateDecl *buildDeductionGuide(
      TemplateParameterList *TemplateParams, ExplicitSpecifier ES,
      TypeSourceInfo *TInfo, SourceLocation LocStart, SourceLocation Loc,
      SourceLocation LocEnd,
      llvm::ArrayRef<TypedefNameDecl *> MaterializedTypedefs = {}) {
    DeclarationNameInfo Name(DeductionGuideName, Loc);
    ArrayRef<ParmVarDecl *> Params =
        TInfo->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams();

    // Build the implicit deduction guide template.
    auto *Guide =
        CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, ES, Name,
                                      TInfo->getType(), TInfo, LocEnd);
    Guide->setImplicit();
    Guide->setParams(Params);

    for (auto *Param : Params)
      Param->setDeclContext(Guide);
    for (auto *TD : MaterializedTypedefs)
      TD->setDeclContext(Guide);

    auto *GuideTemplate = FunctionTemplateDecl::Create(
        SemaRef.Context, DC, Loc, DeductionGuideName, TemplateParams, Guide);
    GuideTemplate->setImplicit();
    Guide->setDescribedFunctionTemplate(GuideTemplate);

    if (isa<CXXRecordDecl>(DC)) {
      Guide->setAccess(AS_public);
      GuideTemplate->setAccess(AS_public);
    }

    DC->addDecl(GuideTemplate);
    return GuideTemplate;
  }
};
}

void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
                                          SourceLocation Loc) {
  if (CXXRecordDecl *DefRecord =
          cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
    TemplateDecl *DescribedTemplate = DefRecord->getDescribedClassTemplate();
    Template = DescribedTemplate ? DescribedTemplate : Template;
  }

  DeclContext *DC = Template->getDeclContext();
  if (DC->isDependentContext())
    return;

  ConvertConstructorToDeductionGuideTransform Transform(
      *this, cast<ClassTemplateDecl>(Template));
  if (!isCompleteType(Loc, Transform.DeducedType))
    return;

  // Check whether we've already declared deduction guides for this template.
  // FIXME: Consider storing a flag on the template to indicate this.
  auto Existing = DC->lookup(Transform.DeductionGuideName);
  for (auto *D : Existing)
    if (D->isImplicit())
      return;

  // In case we were expanding a pack when we attempted to declare deduction
  // guides, turn off pack expansion for everything we're about to do.
  ArgumentPackSubstitutionIndexRAII SubstIndex(*this, -1);
  // Create a template instantiation record to track the "instantiation" of
  // constructors into deduction guides.
  // FIXME: Add a kind for this to give more meaningful diagnostics. But can
  // this substitution process actually fail?
  InstantiatingTemplate BuildingDeductionGuides(*this, Loc, Template);
  if (BuildingDeductionGuides.isInvalid())
    return;

  // Convert declared constructors into deduction guide templates.
  // FIXME: Skip constructors for which deduction must necessarily fail (those
  // for which some class template parameter without a default argument never
  // appears in a deduced context).
  bool AddedAny = false;
  for (NamedDecl *D : LookupConstructors(Transform.Primary)) {
    D = D->getUnderlyingDecl();
    if (D->isInvalidDecl() || D->isImplicit())
      continue;
    D = cast<NamedDecl>(D->getCanonicalDecl());

    auto *FTD = dyn_cast<FunctionTemplateDecl>(D);
    auto *CD =
        dyn_cast_or_null<CXXConstructorDecl>(FTD ? FTD->getTemplatedDecl() : D);
    // Class-scope explicit specializations (MS extension) do not result in
    // deduction guides.
    if (!CD || (!FTD && CD->isFunctionTemplateSpecialization()))
      continue;

    Transform.transformConstructor(FTD, CD);
    AddedAny = true;
  }

  // C++17 [over.match.class.deduct]
  //    --  If C is not defined or does not declare any constructors, an
  //    additional function template derived as above from a hypothetical
  //    constructor C().
  if (!AddedAny)
    Transform.buildSimpleDeductionGuide(None);

  //    -- An additional function template derived as above from a hypothetical
  //    constructor C(C), called the copy deduction candidate.
  cast<CXXDeductionGuideDecl>(
      cast<FunctionTemplateDecl>(
          Transform.buildSimpleDeductionGuide(Transform.DeducedType))
          ->getTemplatedDecl())
      ->setIsCopyDeductionCandidate();
}

/// 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;
}

/// Checks the validity of a template parameter list, possibly
/// considering the template parameter list from a previous
/// declaration.
///
/// If an "old" template parameter list is provided, it must be
/// equivalent (per TemplateParameterListsAreEqual) to the "new"
/// template parameter list.
///
/// \param NewParams Template parameter list for a new template
/// declaration. This template parameter list will be updated with any
/// default arguments that are carried through from the previous
/// template parameter list.
///
/// \param OldParams If provided, template parameter list from a
/// previous declaration of the same template. Default template
/// arguments will be merged from the old template parameter list to
/// the new template parameter list.
///
/// \param TPC Describes the context in which we are checking the given
/// template parameter list.
///
/// \param SkipBody If we might have already made a prior merged definition
/// of this template visible, the corresponding body-skipping information.
/// Default argument redefinition is not an error when skipping such a body,
/// because (under the ODR) we can assume the default arguments are the same
/// as the prior merged definition.
///
/// \returns true if an error occurred, false otherwise.
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) {
    // Variables used to diagnose redundant default arguments
    bool RedundantDefaultArg = false;
    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->getDefaultArgumentInfo()->getTypeLoc()
                                                       .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;
        RedundantDefaultArg = true;
        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;
        RedundantDefaultArg = true;
        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;
        RedundantDefaultArg = true;
        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;
    }

    if (RedundantDefaultArg) {
      // C++ [temp.param]p12:
      //   A template-parameter shall not be given default arguments
      //   by two different declarations in the same scope.
      Diag(NewDefaultLoc, diag::err_template_param_default_arg_redefinition);
      Diag(OldDefaultLoc, diag::note_template_param_prev_default_arg);
      Invalid = true;
    } else if (MissingDefaultArg && TPC != TPC_FunctionTemplate) {
      // C++ [temp.param]p11:
      //   If a template-parameter of a class 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 : RecursiveASTVisitor<DependencyChecker> {
  typedef RecursiveASTVisitor<DependencyChecker> super;

  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, DataRecursionQueue *Q = nullptr) {
    // 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 super::TraverseStmt(S, Q);
  }

  bool TraverseTypeLoc(TypeLoc TL) {
    if (IgnoreNonTypeDependent && !TL.isNull() &&
        !TL.getType()->isDependentType())
      return true;
    return super::TraverseTypeLoc(TL);
  }

  bool VisitTemplateTypeParmTypeLoc(TemplateTypeParmTypeLoc TL) {
    return !Matches(TL.getTypePtr()->getDepth(), TL.getNameLoc());
  }

  bool VisitTemplateTypeParmType(const TemplateTypeParmType *T) {
    // For a best-effort search, keep looking until we find a location.
    return IgnoreNonTypeDependent || !Matches(T->getDepth());
  }

  bool TraverseTemplateName(TemplateName N) {
    if (TemplateTemplateParmDecl *PD =
          dyn_cast_or_null<TemplateTemplateParmDecl>(N.getAsTemplateDecl()))
      if (Matches(PD->getDepth()))
        return false;
    return super::TraverseTemplateName(N);
  }

  bool VisitDeclRefExpr(DeclRefExpr *E) {
    if (NonTypeTemplateParmDecl *PD =
          dyn_cast<NonTypeTemplateParmDecl>(E->getDecl()))
      if (Matches(PD->getDepth(), E->getExprLoc()))
        return false;
    return super::VisitDeclRefExpr(E);
  }

  bool VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *T) {
    return TraverseType(T->getReplacementType());
  }

  bool
  VisitSubstTemplateTypeParmPackType(const SubstTemplateTypeParmPackType *T) {
    return TraverseTemplateArgument(T->getArgumentPack());
  }

  bool TraverseInjectedClassNameType(const InjectedClassNameType *T) {
    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();
}

/// Match the given template parameter lists to the given scope
/// specifier, returning the template parameter list that applies to the
/// name.
///
/// \param DeclStartLoc the start of the declaration that has a scope
/// specifier or a template parameter list.
///
/// \param DeclLoc The location of the declaration itself.
///
/// \param SS the scope specifier that will be matched to the given template
/// parameter lists. This scope specifier precedes a qualified name that is
/// being declared.
///
/// \param TemplateId The template-id following the scope specifier, if there
/// is one. Used to check for a missing 'template<>'.
///
/// \param ParamLists the template parameter lists, from the outermost to the
/// innermost template parameter lists.
///
/// \param IsFriend Whether to apply the slightly different rules for
/// matching template parameters to scope specifiers in friend
/// declarations.
///
/// \param IsMemberSpecialization will be set true if the scope specifier
/// denotes a fully-specialized type, and therefore this is a declaration of
/// a member specialization.
///
/// \returns the template parameter list, if any, that corresponds to the
/// name that is preceded by the scope specifier @p SS. This template
/// parameter list may have template parameters (if we're declaring a
/// template) or may have no template parameters (if we're declaring a
/// template specialization), or may be NULL (if what we're declaring isn't
/// itself a template).
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 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 (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(), None,
                                           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::warn_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
checkBuiltinTemplateIdType(Sema &SemaRef, BuiltinTemplateDecl *BTD,
                           const SmallVectorImpl<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>.

    // C++14 [inteseq.intseq]p1:
    //   T shall be an integer type.
    if (!Converted[1].getAsType()->isIntegralType(Context)) {
      SemaRef.Diag(TemplateArgs[1].getLocation(),
                   diag::err_integer_sequence_integral_element_type);
      return QualType();
    }

    // C++14 [inteseq.make]p1:
    //   If N is negative the program is ill-formed.
    TemplateArgument NumArgsArg = Converted[2];
    llvm::APSInt NumArgs = NumArgsArg.getAsIntegral();
    if (NumArgs < 0) {
      SemaRef.Diag(TemplateArgs[2].getLocation(),
                   diag::err_integer_sequence_negative_length);
      return QualType();
    }

    QualType ArgTy = NumArgsArg.getIntegralType();
    TemplateArgumentListInfo SyntheticTemplateArgs;
    // The type argument gets reused as the first template argument in the
    // synthetic template argument list.
    SyntheticTemplateArgs.addArgument(TemplateArgs[1]);
    // Expand N into 0 ... N-1.
    for (llvm::APSInt I(NumArgs.getBitWidth(), NumArgs.isUnsigned());
         I < NumArgs; ++I) {
      TemplateArgument TA(Context, I, ArgTy);
      SyntheticTemplateArgs.addArgument(SemaRef.getTrivialTemplateArgumentLoc(
          TA, ArgTy, TemplateArgs[2].getLocation()));
    }
    // 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");

    // If the Index is out of bounds, the program is ill-formed.
    TemplateArgument IndexArg = Converted[0], Ts = Converted[1];
    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 (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`.
    auto Nth = std::next(Ts.pack_begin(), Index.getExtValue());
    return Nth->getAsType();
  }
  llvm_unreachable("unexpected BuiltinTemplateDecl!");
}

/// Determine whether this alias template is "enable_if_t".
static bool isEnableIfAliasTemplate(TypeAliasTemplateDecl *AliasTemplate) {
  return AliasTemplate->getName().equals("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);
      }
      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(ETK_None,
                                                          DTN->getQualifier(),
                                                          DTN->getIdentifier(),
                                                          TemplateArgs);

  if (Name.getAsAssumedTemplateName() &&
      resolveAssumedTemplateNameAsType(/*Scope*/nullptr, Name, TemplateLoc))
    return QualType();

  TemplateDecl *Template = Name.getAsTemplateDecl();
  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);

    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> Converted;
  if (CheckTemplateArgumentList(Template, TemplateLoc, TemplateArgs,
                                false, Converted,
                                /*UpdateArgsWithConversion=*/true))
    return QualType();

  QualType CanonType;

  bool InstantiationDependent = false;
  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();

    TemplateArgumentList StackTemplateArgs(TemplateArgumentList::OnStack,
                                           Converted);

    // Only substitute for the innermost template argument list.
    MultiLevelTemplateArgumentList TemplateArgLists;
    TemplateArgLists.addOuterTemplateArguments(&StackTemplateArgs);
    TemplateArgLists.addOuterRetainedLevels(
        AliasTemplate->getTemplateParameters()->getDepth());

    LocalInstantiationScope Scope(*this);
    InstantiatingTemplate Inst(*this, TemplateLoc, Template);
    if (Inst.isInvalid())
      return QualType();

    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 (Name.isDependent() ||
             TemplateSpecializationType::anyDependentTemplateArguments(
               TemplateArgs, InstantiationDependent)) {
    // 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, Converted);

    // 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(Converted, 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, Converted, nullptr);
      ClassTemplate->AddSpecialization(Decl, InsertPos);
      if (ClassTemplate->isOutOfLine())
        Decl->setLexicalDeclContext(ClassTemplate->getLexicalDeclContext());
    }

    if (Decl->getSpecializationKind() == TSK_Undeclared) {
      MultiLevelTemplateArgumentList TemplateArgLists;
      TemplateArgLists.addOuterTemplateArguments(Converted);
      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 if (auto *BTD = dyn_cast<BuiltinTemplateDecl>(Template)) {
    CanonType = checkBuiltinTemplateIdType(*this, BTD, Converted, TemplateLoc,
                                           TemplateArgs);
  }

  // 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, 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 = 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, IdentifierInfo *TemplateII,
    SourceLocation TemplateIILoc, SourceLocation LAngleLoc,
    ASTTemplateArgsPtr TemplateArgsIn, SourceLocation RAngleLoc,
    bool IsCtorOrDtorName, bool IsClassName) {
  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)) {
      Diag(SS.getBeginLoc(), diag::err_typename_missing_template)
        << SS.getScopeRep() << TemplateII->getName();
      // Recover as if 'typename' were specified.
      // 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()) {
    QualType T
      = Context.getDependentTemplateSpecializationType(ETK_None,
                                                       DTN->getQualifier(),
                                                       DTN->getIdentifier(),
                                                       TemplateArgs);
    // 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 Result = CheckTemplateIdType(Template, TemplateIILoc, TemplateArgs);
  if (Result.isNull())
    return true;

  // Build type-source information.
  TypeLocBuilder TLB;
  TemplateSpecializationTypeLoc SpecTL
    = TLB.push<TemplateSpecializationTypeLoc>(Result);
  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());

  // NOTE: avoid constructing an ElaboratedTypeLoc if this is a
  // constructor or destructor name (in such a case, the scope specifier
  // will be attached to the enclosing Decl or Expr node).
  if (SS.isNotEmpty() && !IsCtorOrDtorName) {
    // Create an elaborated-type-specifier containing the nested-name-specifier.
    Result = Context.getElaboratedType(ETK_None, SS.getScopeRep(), Result);
    ElaboratedTypeLoc ElabTL = TLB.push<ElaboratedTypeLoc>(Result);
    ElabTL.setElaboratedKeywordLoc(SourceLocation());
    ElabTL.setQualifierLoc(SS.getWithLocInContext(Context));
  }

  return CreateParsedType(Result, TLB.getTypeSourceInfo(Context, Result));
}

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()) {
    QualType T = Context.getDependentTemplateSpecializationType(Keyword,
                                                          DTN->getQualifier(),
                                                          DTN->getIdentifier(),
                                                                TemplateArgs);

    // 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 << 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 == TUK_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::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.Diag(Template->getLocation(), diag::note_template_decl_here);
  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, 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 it.");

  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;
  }

  // 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],
                                        UPPC_PartialSpecialization))
      return true;

  // Check that the template argument list is well-formed for this
  // template.
  SmallVector<TemplateArgument, 4> Converted;
  if (CheckTemplateArgumentList(VarTemplate, TemplateNameLoc, TemplateArgs,
                                false, Converted,
                                /*UpdateArgsWithConversion=*/true))
    return true;

  // Find the variable template (partial) specialization declaration that
  // corresponds to these arguments.
  if (IsPartialSpecialization) {
    if (CheckTemplatePartialSpecializationArgs(TemplateNameLoc, VarTemplate,
                                               TemplateArgs.size(), Converted))
      return true;

    // FIXME: Move these checks to CheckTemplatePartialSpecializationArgs so we
    // also do them during instantiation.
    bool InstantiationDependent;
    if (!Name.isDependent() &&
        !TemplateSpecializationType::anyDependentTemplateArguments(
            TemplateArgs.arguments(),
            InstantiationDependent)) {
      Diag(TemplateNameLoc, diag::err_partial_spec_fully_specialized)
          << VarTemplate->getDeclName();
      IsPartialSpecialization = false;
    }

    if (isSameAsPrimaryTemplate(VarTemplate->getTemplateParameters(),
                                Converted) &&
        (!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(Converted, TemplateParams,
                                                      InsertPos);
  else
    PrevDecl = VarTemplate->findSpecialization(Converted, 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,
            Converted, 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, Converted);
    Specialization->setTemplateArgsInfo(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->setTemplateKeywordLoc(TemplateKWLoc);
  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);

  if (PrevDecl) {
    // Check that this isn't a redefinition of this specialization,
    // merging with previous declarations.
    LookupResult PrevSpec(*this, GetNameForDeclarator(D), LookupOrdinaryName,
                          forRedeclarationInCurContext());
    PrevSpec.addDecl(PrevDecl);
    D.setRedeclaration(CheckVariableDeclaration(Specialization, PrevSpec));
  } 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> Converted;
  if (CheckTemplateArgumentList(
          Template, TemplateNameLoc,
          const_cast<TemplateArgumentListInfo &>(TemplateArgs), false,
          Converted, /*UpdateArgsWithConversion=*/true))
    return true;

  // Find the variable template specialization declaration that
  // corresponds to these arguments.
  void *InsertPos = nullptr;
  if (VarTemplateSpecializationDecl *Spec = Template->findSpecialization(
          Converted, InsertPos)) {
    checkSpecializationVisibility(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();
  TemplateArgumentList TemplateArgList(TemplateArgumentList::OnStack,
                                       Converted);
  TemplateArgumentList *InstantiationArgs = &TemplateArgList;
  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.
  // If any of the template arguments is dependent, then this is probably
  // a placeholder for an incomplete declarative context; which must be
  // complete by instantiation time. Thus, do not search through the partial
  // specializations yet.
  // TODO: Unify with InstantiateClassTemplateSpecialization()?
  //       Perhaps better after unification of DeduceTemplateArguments() and
  //       getMoreSpecializedPartialSpecialization().
  bool InstantiationDependent = false;
  if (!TemplateSpecializationType::anyDependentTemplateArguments(
          TemplateArgs, InstantiationDependent)) {

    SmallVector<VarTemplatePartialSpecializationDecl *, 4> PartialSpecs;
    Template->getPartialSpecializations(PartialSpecs);

    for (unsigned I = 0, N = PartialSpecs.size(); I != N; ++I) {
      VarTemplatePartialSpecializationDecl *Partial = PartialSpecs[I];
      TemplateDeductionInfo Info(FailedCandidates.getLocation());

      if (TemplateDeductionResult Result =
              DeduceTemplateArguments(Partial, TemplateArgList, Info)) {
        // 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.take();
      }
    }

    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;
      InstantiationArgs = 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, *InstantiationArgs, TemplateArgs,
      Converted, TemplateNameLoc, InsertPos /*, 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, InstantiationArgs);

  checkSpecializationVisibility(TemplateNameLoc, Decl);

  assert(Decl && "No variable template specialization?");
  return Decl;
}

ExprResult
Sema::CheckVarTemplateId(const CXXScopeSpec &SS,
                         const DeclarationNameInfo &NameInfo,
                         VarTemplateDecl *Template, SourceLocation TemplateLoc,
                         const TemplateArgumentListInfo *TemplateArgs) {

  DeclResult Decl = CheckVarTemplateId(Template, TemplateLoc, NameInfo.getLoc(),
                                       *TemplateArgs);
  if (Decl.isInvalid())
    return ExprError();

  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=*/nullptr, TemplateArgs);
}

void Sema::diagnoseMissingTemplateArguments(TemplateName Name,
                                            SourceLocation Loc) {
  Diag(Loc, diag::err_template_missing_args)
    << (int)getTemplateNameKindForDiagnostics(Name) << Name;
  if (TemplateDecl *TD = Name.getAsTemplateDecl()) {
    Diag(TD->getLocation(), diag::note_template_decl_here)
      << TD->getTemplateParameters()->getSourceRange();
  }
}

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> Converted;
  if (CheckTemplateArgumentList(NamedConcept, ConceptNameInfo.getLoc(),
                           const_cast<TemplateArgumentListInfo&>(*TemplateArgs),
                                /*PartialTemplateArgs=*/false, Converted,
                                /*UpdateArgsWithConversion=*/false))
    return ExprError();

  ConstraintSatisfaction Satisfaction;
  bool AreArgsDependent = false;
  for (TemplateArgument &Arg : Converted) {
    if (Arg.isDependent()) {
      AreArgsDependent = true;
      break;
    }
  }
  if (!AreArgsDependent &&
      CheckConstraintSatisfaction(NamedConcept,
                                  {NamedConcept->getConstraintExpr()},
                                  Converted,
                                  SourceRange(SS.isSet() ? SS.getBeginLoc() :
                                                       ConceptNameInfo.getLoc(),
                                                TemplateArgs->getRAngleLoc()),
                                    Satisfaction))
      return ExprError();

  return ConceptSpecializationExpr::Create(Context,
      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc{},
      TemplateKWLoc, ConceptNameInfo, FoundDecl, NamedConcept,
      ASTTemplateArgumentListInfo::Create(Context, *TemplateArgs), Converted,
      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(TemplateName(TD), R.getNameLoc());
      return ExprError();
    }
  }

  auto AnyDependentArguments = [&]() -> bool {
    bool InstantiationDependent;
    return TemplateArgs &&
           TemplateSpecializationType::anyDependentTemplateArguments(
               *TemplateArgs, InstantiationDependent);
  };

  // In C++1y, check variable template ids.
  if (R.getAsSingle<VarTemplateDecl>() && !AnyDependentArguments()) {
    return CheckVarTemplateId(SS, R.getLookupNameInfo(),
                              R.getAsSingle<VarTemplateDecl>(),
                              TemplateKWLoc, TemplateArgs);
  }

  if (R.getAsSingle<ConceptDecl>()) {
    return CheckConceptTemplateId(SS, TemplateKWLoc, R.getLookupNameInfo(),
                                  R.getFoundDecl(),
                                  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());

  return ULE;
}

// We actually only call this from template instantiation.
ExprResult
Sema::BuildQualifiedTemplateIdExpr(CXXScopeSpec &SS,
                                   SourceLocation TemplateKWLoc,
                                   const DeclarationNameInfo &NameInfo,
                             const TemplateArgumentListInfo *TemplateArgs) {

  assert(TemplateArgs || TemplateKWLoc.isValid());
  DeclContext *DC;
  if (!(DC = computeDeclContext(SS, false)) ||
      DC->isDependentContext() ||
      RequireCompleteDeclContext(SS, DC))
    return BuildDependentDeclRefExpr(SS, TemplateKWLoc, NameInfo, TemplateArgs);

  bool MemberOfUnknownSpecialization;
  LookupResult R(*this, NameInfo, LookupOrdinaryName);
  if (LookupTemplateName(R, (Scope *)nullptr, SS, QualType(),
                         /*Entering*/false, MemberOfUnknownSpecialization,
                         TemplateKWLoc))
    return ExprError();

  if (R.isAmbiguous())
    return ExprError();

  if (R.empty()) {
    Diag(NameInfo.getLoc(), diag::err_no_member)
      << NameInfo.getName() << DC << SS.getRange();
    return ExprError();
  }

  if (ClassTemplateDecl *Temp = R.getAsSingle<ClassTemplateDecl>()) {
    Diag(NameInfo.getLoc(), diag::err_template_kw_refers_to_class_template)
      << SS.getScopeRep()
      << NameInfo.getName().getAsString() << SS.getRange();
    Diag(Temp->getLocation(), diag::note_referenced_class_template);
    return ExprError();
  }

  return BuildTemplateIdExpr(SS, TemplateKWLoc, R, /*ADL*/ false, TemplateArgs);
}

/// Form a template name from a name that is syntactically required to name a
/// template, either due to use of the 'template' keyword or because a name in
/// this syntactic context is assumed to name a template (C++ [temp.names]p2-4).
///
/// This action forms a template name given the name of the template and its
/// optional scope specifier. This is used when the 'template' keyword is used
/// or when the parsing context unambiguously treats a following '<' as
/// introducing a template argument list. Note that this may produce a
/// non-dependent template name if we can perform the lookup now and identify
/// the named template.
///
/// For example, given "x.MetaFun::template apply", the scope specifier
/// \p SS will be "MetaFun::", \p TemplateKWLoc contains the location
/// of the "template" keyword, and "apply" is the \p Name.
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);
    bool MOUS;
    // 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, MOUS,
                            RTK, 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> &Converted) {
  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);

      if (Result.getAsSingle<TypeDecl>() ||
          Result.getResultKind() ==
              LookupResult::NotFoundInCurrentInstantiation) {
        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 ");
        Diag(Param->getLocation(), diag::note_template_param_here);

        // Recover by synthesizing a type using the location information that we
        // already have.
        ArgType =
            Context.getDependentNameType(ETK_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
    LLVM_FALLTHROUGH;
  }
  default: {
    // 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;
    Diag(Param->getLocation(), diag::note_template_param_here);

    return true;
  }
  }

  if (CheckTemplateArgument(Param, TSI))
    return true;

  // Add the converted template type argument.
  ArgType = Context.getCanonicalType(ArgType);

  // 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);
  }

  Converted.push_back(TemplateArgument(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.
/// \returns the substituted template argument, or NULL if an error occurred.
static TypeSourceInfo *
SubstDefaultTemplateArgument(Sema &SemaRef,
                             TemplateDecl *Template,
                             SourceLocation TemplateLoc,
                             SourceLocation RAngleLoc,
                             TemplateTypeParmDecl *Param,
                             SmallVectorImpl<TemplateArgument> &Converted) {
  TypeSourceInfo *ArgType = Param->getDefaultArgumentInfo();

  // If the argument type is dependent, instantiate it now based
  // on the previously-computed template arguments.
  if (ArgType->getType()->isInstantiationDependentType()) {
    Sema::InstantiatingTemplate Inst(SemaRef, TemplateLoc,
                                     Param, Template, Converted,
                                     SourceRange(TemplateLoc, RAngleLoc));
    if (Inst.isInvalid())
      return nullptr;

    TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);

    // Only substitute for the innermost template argument list.
    MultiLevelTemplateArgumentList TemplateArgLists;
    TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
    for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
      TemplateArgLists.addOuterTemplateArguments(None);

    Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext());
    ArgType =
        SemaRef.SubstType(ArgType, TemplateArgLists,
                          Param->getDefaultArgumentLoc(), Param->getDeclName());
  }

  return ArgType;
}

/// Substitute template arguments into the default template argument for
/// the given non-type template 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 non-type 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.
///
/// \returns the substituted template argument, or NULL if an error occurred.
static ExprResult
SubstDefaultTemplateArgument(Sema &SemaRef,
                             TemplateDecl *Template,
                             SourceLocation TemplateLoc,
                             SourceLocation RAngleLoc,
                             NonTypeTemplateParmDecl *Param,
                        SmallVectorImpl<TemplateArgument> &Converted) {
  Sema::InstantiatingTemplate Inst(SemaRef, TemplateLoc,
                                   Param, Template, Converted,
                                   SourceRange(TemplateLoc, RAngleLoc));
  if (Inst.isInvalid())
    return ExprError();

  TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);

  // Only substitute for the innermost template argument list.
  MultiLevelTemplateArgumentList TemplateArgLists;
  TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
  for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
    TemplateArgLists.addOuterTemplateArguments(None);

  Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext());
  EnterExpressionEvaluationContext ConstantEvaluated(
      SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
  return SemaRef.SubstExpr(Param->getDefaultArgument(), TemplateArgLists);
}

/// Substitute template arguments into the default template argument for
/// the given template template 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 QualifierLoc Will be set to the nested-name-specifier (with
/// source-location information) that precedes the template name.
///
/// \returns the substituted template argument, or NULL if an error occurred.
static TemplateName
SubstDefaultTemplateArgument(Sema &SemaRef,
                             TemplateDecl *Template,
                             SourceLocation TemplateLoc,
                             SourceLocation RAngleLoc,
                             TemplateTemplateParmDecl *Param,
                       SmallVectorImpl<TemplateArgument> &Converted,
                             NestedNameSpecifierLoc &QualifierLoc) {
  Sema::InstantiatingTemplate Inst(
      SemaRef, TemplateLoc, TemplateParameter(Param), Template, Converted,
      SourceRange(TemplateLoc, RAngleLoc));
  if (Inst.isInvalid())
    return TemplateName();

  TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);

  // Only substitute for the innermost template argument list.
  MultiLevelTemplateArgumentList TemplateArgLists;
  TemplateArgLists.addOuterTemplateArguments(&TemplateArgs);
  for (unsigned i = 0, e = Param->getDepth(); i != e; ++i)
    TemplateArgLists.addOuterTemplateArguments(None);

  Sema::ContextRAII SavedContext(SemaRef, Template->getDeclContext());
  // Substitute into the nested-name-specifier first,
  QualifierLoc = Param->getDefaultArgument().getTemplateQualifierLoc();
  if (QualifierLoc) {
    QualifierLoc =
        SemaRef.SubstNestedNameSpecifierLoc(QualifierLoc, TemplateArgLists);
    if (!QualifierLoc)
      return TemplateName();
  }

  return SemaRef.SubstTemplateName(
             QualifierLoc,
             Param->getDefaultArgument().getArgument().getAsTemplate(),
             Param->getDefaultArgument().getTemplateNameLoc(),
             TemplateArgLists);
}

/// If the given template parameter has a default template
/// argument, substitute into that default template argument and
/// return the corresponding template argument.
TemplateArgumentLoc
Sema::SubstDefaultTemplateArgumentIfAvailable(TemplateDecl *Template,
                                              SourceLocation TemplateLoc,
                                              SourceLocation RAngleLoc,
                                              Decl *Param,
                                              SmallVectorImpl<TemplateArgument>
                                                &Converted,
                                              bool &HasDefaultArg) {
  HasDefaultArg = false;

  if (TemplateTypeParmDecl *TypeParm = dyn_cast<TemplateTypeParmDecl>(Param)) {
    if (!hasVisibleDefaultArgument(TypeParm))
      return TemplateArgumentLoc();

    HasDefaultArg = true;
    TypeSourceInfo *DI = SubstDefaultTemplateArgument(*this, Template,
                                                      TemplateLoc,
                                                      RAngleLoc,
                                                      TypeParm,
                                                      Converted);
    if (DI)
      return TemplateArgumentLoc(TemplateArgument(DI->getType()), DI);

    return TemplateArgumentLoc();
  }

  if (NonTypeTemplateParmDecl *NonTypeParm
        = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
    if (!hasVisibleDefaultArgument(NonTypeParm))
      return TemplateArgumentLoc();

    HasDefaultArg = true;
    ExprResult Arg = SubstDefaultTemplateArgument(*this, Template,
                                                  TemplateLoc,
                                                  RAngleLoc,
                                                  NonTypeParm,
                                                  Converted);
    if (Arg.isInvalid())
      return TemplateArgumentLoc();

    Expr *ArgE = Arg.getAs<Expr>();
    return TemplateArgumentLoc(TemplateArgument(ArgE), ArgE);
  }

  TemplateTemplateParmDecl *TempTempParm
    = cast<TemplateTemplateParmDecl>(Param);
  if (!hasVisibleDefaultArgument(TempTempParm))
    return TemplateArgumentLoc();

  HasDefaultArg = true;
  NestedNameSpecifierLoc QualifierLoc;
  TemplateName TName = SubstDefaultTemplateArgument(*this, Template,
                                                    TemplateLoc,
                                                    RAngleLoc,
                                                    TempTempParm,
                                                    Converted,
                                                    QualifierLoc);
  if (TName.isNull())
    return TemplateArgumentLoc();

  return TemplateArgumentLoc(TemplateArgument(TName),
                TempTempParm->getDefaultArgument().getTemplateQualifierLoc(),
                TempTempParm->getDefaultArgument().getTemplateNameLoc());
}

/// Convert a template-argument that we parsed as a type into a template, if
/// possible. C++ permits injected-class-names to perform dual service as
/// template template arguments and as template type arguments.
static TemplateArgumentLoc convertTypeTemplateArgumentToTemplate(TypeLoc TLoc) {
  // Extract and step over any surrounding nested-name-specifier.
  NestedNameSpecifierLoc QualLoc;
  if (auto ETLoc = TLoc.getAs<ElaboratedTypeLoc>()) {
    if (ETLoc.getTypePtr()->getKeyword() != ETK_None)
      return TemplateArgumentLoc();

    QualLoc = ETLoc.getQualifierLoc();
    TLoc = ETLoc.getNamedTypeLoc();
  }

  // If this type was written as an injected-class-name, it can be used as a
  // template template argument.
  if (auto InjLoc = TLoc.getAs<InjectedClassNameTypeLoc>())
    return TemplateArgumentLoc(InjLoc.getTypePtr()->getTemplateName(),
                               QualLoc, InjLoc.getNameLoc());

  // If this type was written as an injected-class-name, it may have been
  // converted to a RecordType during instantiation. If the RecordType is
  // *not* wrapped in a TemplateSpecializationType and denotes a class
  // template specialization, it must have come from an injected-class-name.
  if (auto RecLoc = TLoc.getAs<RecordTypeLoc>())
    if (auto *CTSD =
            dyn_cast<ClassTemplateSpecializationDecl>(RecLoc.getDecl()))
      return TemplateArgumentLoc(TemplateName(CTSD->getSpecializedTemplate()),
                                 QualLoc, RecLoc.getNameLoc());

  return TemplateArgumentLoc();
}

/// Check that the given template argument corresponds to the given
/// template parameter.
///
/// \param Param The template parameter against which the argument will be
/// checked.
///
/// \param Arg The template argument, which may be updated due to conversions.
///
/// \param Template The template in which the template argument resides.
///
/// \param TemplateLoc The location of the template name for the template
/// whose argument list we're matching.
///
/// \param RAngleLoc The location of the right angle bracket ('>') that closes
/// the template argument list.
///
/// \param ArgumentPackIndex The index into the argument pack where this
/// argument will be placed. Only valid if the parameter is a parameter pack.
///
/// \param Converted The checked, converted argument will be added to the
/// end of this small vector.
///
/// \param CTAK Describes how we arrived at this particular template argument:
/// explicitly written, deduced, etc.
///
/// \returns true on error, false otherwise.
bool Sema::CheckTemplateArgument(NamedDecl *Param,
                                 TemplateArgumentLoc &Arg,
                                 NamedDecl *Template,
                                 SourceLocation TemplateLoc,
                                 SourceLocation RAngleLoc,
                                 unsigned ArgumentPackIndex,
                            SmallVectorImpl<TemplateArgument> &Converted,
                                 CheckTemplateArgumentKind CTAK) {
  // Check template type parameters.
  if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
    return CheckTemplateTypeArgument(TTP, Arg, Converted);

  // Check non-type template parameters.
  if (NonTypeTemplateParmDecl *NTTP =dyn_cast<NonTypeTemplateParmDecl>(Param)) {
    // Do substitution on the type of the non-type template parameter
    // with the template arguments we've seen thus far.  But if the
    // template has a dependent context then we cannot substitute yet.
    QualType NTTPType = NTTP->getType();
    if (NTTP->isParameterPack() && NTTP->isExpandedParameterPack())
      NTTPType = NTTP->getExpansionType(ArgumentPackIndex);

    if (NTTPType->isInstantiationDependentType() &&
        !isa<TemplateTemplateParmDecl>(Template) &&
        !Template->getDeclContext()->isDependentContext()) {
      // Do substitution on the type of the non-type template parameter.
      InstantiatingTemplate Inst(*this, TemplateLoc, Template,
                                 NTTP, Converted,
                                 SourceRange(TemplateLoc, RAngleLoc));
      if (Inst.isInvalid())
        return true;

      TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack,
                                        Converted);

      // If the parameter is a pack expansion, expand this slice of the pack.
      if (auto *PET = NTTPType->getAs<PackExpansionType>()) {
        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this,
                                                           ArgumentPackIndex);
        NTTPType = SubstType(PET->getPattern(),
                             MultiLevelTemplateArgumentList(TemplateArgs),
                             NTTP->getLocation(),
                             NTTP->getDeclName());
      } else {
        NTTPType = SubstType(NTTPType,
                             MultiLevelTemplateArgumentList(TemplateArgs),
                             NTTP->getLocation(),
                             NTTP->getDeclName());
      }

      // If that worked, check the non-type template parameter type
      // for validity.
      if (!NTTPType.isNull())
        NTTPType = CheckNonTypeTemplateParameterType(NTTPType,
                                                     NTTP->getLocation());
      if (NTTPType.isNull())
        return true;
    }

    switch (Arg.getArgument().getKind()) {
    case TemplateArgument::Null:
      llvm_unreachable("Should never see a NULL template argument here");

    case TemplateArgument::Expression: {
      TemplateArgument Result;
      unsigned CurSFINAEErrors = NumSFINAEErrors;
      ExprResult Res =
        CheckTemplateArgument(NTTP, NTTPType, Arg.getArgument().getAsExpr(),
                              Result, CTAK);
      if (Res.isInvalid())
        return true;
      // If the current template argument causes an error, give up now.
      if (CurSFINAEErrors < NumSFINAEErrors)
        return true;

      // If the resulting expression is new, then use it in place of the
      // old expression in the template argument.
      if (Res.get() != Arg.getArgument().getAsExpr()) {
        TemplateArgument TA(Res.get());
        Arg = TemplateArgumentLoc(TA, Res.get());
      }

      Converted.push_back(Result);
      break;
    }

    case TemplateArgument::Declaration:
    case TemplateArgument::Integral:
    case TemplateArgument::NullPtr:
      // We've already checked this template argument, so just copy
      // it to the list of converted arguments.
      Converted.push_back(Arg.getArgument());
      break;

    case TemplateArgument::Template:
    case TemplateArgument::TemplateExpansion:
      // We were given a template template argument. It may not be ill-formed;
      // see below.
      if (DependentTemplateName *DTN
            = Arg.getArgument().getAsTemplateOrTemplatePattern()
                                              .getAsDependentTemplateName()) {
        // We have a template argument such as \c T::template X, which we
        // parsed as a template template argument. However, since we now
        // know that we need a non-type template argument, convert this
        // template name into an expression.

        DeclarationNameInfo NameInfo(DTN->getIdentifier(),
                                     Arg.getTemplateNameLoc());

        CXXScopeSpec SS;
        SS.Adopt(Arg.getTemplateQualifierLoc());
        // FIXME: the template-template arg was a DependentTemplateName,
        // so it was provided with a template keyword. However, its source
        // location is not stored in the template argument structure.
        SourceLocation TemplateKWLoc;
        ExprResult E = DependentScopeDeclRefExpr::Create(
            Context, SS.getWithLocInContext(Context), TemplateKWLoc, NameInfo,
            nullptr);

        // If we parsed the template argument as a pack expansion, create a
        // pack expansion expression.
        if (Arg.getArgument().getKind() == TemplateArgument::TemplateExpansion){
          E = ActOnPackExpansion(E.get(), Arg.getTemplateEllipsisLoc());
          if (E.isInvalid())
            return true;
        }

        TemplateArgument Result;
        E = CheckTemplateArgument(NTTP, NTTPType, E.get(), Result);
        if (E.isInvalid())
          return true;

        Converted.push_back(Result);
        break;
      }

      // We have a template argument that actually does refer to a class
      // template, alias template, or template template parameter, and
      // therefore cannot be a non-type template argument.
      Diag(Arg.getLocation(), diag::err_template_arg_must_be_expr)
        << Arg.getSourceRange();

      Diag(Param->getLocation(), diag::note_template_param_here);
      return true;

    case TemplateArgument::Type: {
      // We have a non-type template parameter but the template
      // argument is a type.

      // C++ [temp.arg]p2:
      //   In a template-argument, an ambiguity between a type-id and
      //   an expression is resolved to a type-id, regardless of the
      //   form of the corresponding template-parameter.
      //
      // We warn specifically about this case, since it can be rather
      // confusing for users.
      QualType T = Arg.getArgument().getAsType();
      SourceRange SR = Arg.getSourceRange();
      if (T->isFunctionType())
        Diag(SR.getBegin(), diag::err_template_arg_nontype_ambig) << SR << T;
      else
        Diag(SR.getBegin(), diag::err_template_arg_must_be_expr) << SR;
      Diag(Param->getLocation(), diag::note_template_param_here);
      return true;
    }

    case TemplateArgument::Pack:
      llvm_unreachable("Caller must expand template argument packs");
    }

    return false;
  }


  // Check template template parameters.
  TemplateTemplateParmDecl *TempParm = cast<TemplateTemplateParmDecl>(Param);

  TemplateParameterList *Params = TempParm->getTemplateParameters();
  if (TempParm->isExpandedParameterPack())
    Params = TempParm->getExpansionTemplateParameters(ArgumentPackIndex);

  // Substitute into the template parameter list of the template
  // template parameter, since previously-supplied template arguments
  // may appear within the template template parameter.
  //
  // FIXME: Skip this if the parameters aren't instantiation-dependent.
  {
    // Set up a template instantiation context.
    LocalInstantiationScope Scope(*this);
    InstantiatingTemplate Inst(*this, TemplateLoc, Template,
                               TempParm, Converted,
                               SourceRange(TemplateLoc, RAngleLoc));
    if (Inst.isInvalid())
      return true;

    TemplateArgumentList TemplateArgs(TemplateArgumentList::OnStack, Converted);
    Params = SubstTemplateParams(Params, CurContext,
                                 MultiLevelTemplateArgumentList(TemplateArgs));
    if (!Params)
      return true;
  }

  // C++1z [temp.local]p1: (DR1004)
  //   When [the injected-class-name] is used [...] as a template-argument for
  //   a template template-parameter [...] it refers to the class template
  //   itself.
  if (Arg.getArgument().getKind() == TemplateArgument::Type) {
    TemplateArgumentLoc ConvertedArg = convertTypeTemplateArgumentToTemplate(
        Arg.getTypeSourceInfo()->getTypeLoc());
    if (!ConvertedArg.getArgument().isNull())
      Arg = ConvertedArg;
  }

  switch (Arg.getArgument().getKind()) {
  case TemplateArgument::Null:
    llvm_unreachable("Should never see a NULL template argument here");

  case TemplateArgument::Template:
  case TemplateArgument::TemplateExpansion:
    if (CheckTemplateTemplateArgument(TempParm, Params, Arg))
      return true;

    Converted.push_back(Arg.getArgument());
    break;

  case TemplateArgument::Expression:
  case TemplateArgument::Type:
    // We have a template template parameter but the template
    // argument does not refer to a template.
    Diag(Arg.getLocation(), diag::err_template_arg_must_be_template)
      << getLangOpts().CPlusPlus11;
    return true;

  case TemplateArgument::Declaration:
    llvm_unreachable("Declaration argument with template template parameter");
  case TemplateArgument::Integral:
    llvm_unreachable("Integral argument with template template parameter");
  case TemplateArgument::NullPtr:
    llvm_unreachable("Null pointer argument with template template parameter");

  case TemplateArgument::Pack:
    llvm_unreachable("Caller must expand template argument packs");
  }

  return false;
}

/// Check whether the template parameter is a pack expansion, and if so,
/// determine the number of parameters produced by that expansion. For instance:
///
/// \code
/// template<typename ...Ts> struct A {
///   template<Ts ...NTs, template<Ts> class ...TTs, typename ...Us> struct B;
/// };
/// \endcode
///
/// In \c A<int,int>::B, \c NTs and \c TTs have expanded pack size 2, and \c Us
/// is not a pack expansion, so returns an empty Optional.
static Optional<unsigned> getExpandedPackSize(NamedDecl *Param) {
  if (TemplateTypeParmDecl *TTP
        = dyn_cast<TemplateTypeParmDecl>(Param)) {
    if (TTP->isExpandedParameterPack())
      return TTP->getNumExpansionParameters();
  }

  if (NonTypeTemplateParmDecl *NTTP
        = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
    if (NTTP->isExpandedParameterPack())
      return NTTP->getNumExpansionTypes();
  }

  if (TemplateTemplateParmDecl *TTP
        = dyn_cast<TemplateTemplateParmDecl>(Param)) {
    if (TTP->isExpandedParameterPack())
      return TTP->getNumExpansionTemplateParameters();
  }

  return None;
}

/// Diagnose a missing template argument.
template<typename TemplateParmDecl>
static bool diagnoseMissingArgument(Sema &S, SourceLocation Loc,
                                    TemplateDecl *TD,
                                    const TemplateParmDecl *D,
                                    TemplateArgumentListInfo &Args) {
  // Dig out the most recent declaration of the template parameter; there may be
  // declarations of the template that are more recent than TD.
  D = cast<TemplateParmDecl>(cast<TemplateDecl>(TD->getMostRecentDecl())
                                 ->getTemplateParameters()
                                 ->getParam(D->getIndex()));

  // If there's a default argument that's not visible, diagnose that we're
  // missing a module import.
  llvm::SmallVector<Module*, 8> Modules;
  if (D->hasDefaultArgument() && !S.hasVisibleDefaultArgument(D, &Modules)) {
    S.diagnoseMissingImport(Loc, cast<NamedDecl>(TD),
                            D->getDefaultArgumentLoc(), Modules,
                            Sema::MissingImportKind::DefaultArgument,
                            /*Recover*/true);
    return true;
  }

  // FIXME: If there's a more recent default argument that *is* visible,
  // diagnose that it was declared too late.

  TemplateParameterList *Params = TD->getTemplateParameters();

  S.Diag(Loc, diag::err_template_arg_list_different_arity)
    << /*not enough args*/0
    << (int)S.getTemplateNameKindForDiagnostics(TemplateName(TD))
    << TD;
  S.Diag(TD->getLocation(), diag::note_template_decl_here)
    << Params->getSourceRange();
  return true;
}

/// Check that the given template argument list is well-formed
/// for specializing the given template.
bool Sema::CheckTemplateArgumentList(
    TemplateDecl *Template, SourceLocation TemplateLoc,
    TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
    SmallVectorImpl<TemplateArgument> &Converted,
    bool UpdateArgsWithConversions, bool *ConstraintsNotSatisfied) {

  if (ConstraintsNotSatisfied)
    *ConstraintsNotSatisfied = false;

  // Make a copy of the template arguments for processing.  Only make the
  // changes at the end when successful in matching the arguments to the
  // template.
  TemplateArgumentListInfo NewArgs = TemplateArgs;

  // Make sure we get the template parameter list from the most
  // recentdeclaration, since that is the only one that has is guaranteed to
  // have all the default template argument information.
  TemplateParameterList *Params =
      cast<TemplateDecl>(Template->getMostRecentDecl())
          ->getTemplateParameters();

  SourceLocation RAngleLoc = NewArgs.getRAngleLoc();

  // C++ [temp.arg]p1:
  //   [...] The type and form of each template-argument specified in
  //   a template-id shall match the type and form specified for the
  //   corresponding parameter declared by the template in its
  //   template-parameter-list.
  bool isTemplateTemplateParameter = isa<TemplateTemplateParmDecl>(Template);
  SmallVector<TemplateArgument, 2> ArgumentPack;
  unsigned ArgIdx = 0, NumArgs = NewArgs.size();
  LocalInstantiationScope InstScope(*this, true);
  for (TemplateParameterList::iterator Param = Params->begin(),
                                       ParamEnd = Params->end();
       Param != ParamEnd; /* increment in loop */) {
    // If we have an expanded parameter pack, make sure we don't have too
    // many arguments.
    if (Optional<unsigned> Expansions = getExpandedPackSize(*Param)) {
      if (*Expansions == ArgumentPack.size()) {
        // We're done with this parameter pack. Pack up its arguments and add
        // them to the list.
        Converted.push_back(
            TemplateArgument::CreatePackCopy(Context, ArgumentPack));
        ArgumentPack.clear();

        // This argument is assigned to the next parameter.
        ++Param;
        continue;
      } else if (ArgIdx == NumArgs && !PartialTemplateArgs) {
        // Not enough arguments for this parameter pack.
        Diag(TemplateLoc, diag::err_template_arg_list_different_arity)
          << /*not enough args*/0
          << (int)getTemplateNameKindForDiagnostics(TemplateName(Template))
          << Template;
        Diag(Template->getLocation(), diag::note_template_decl_here)
          << Params->getSourceRange();
        return true;
      }
    }

    if (ArgIdx < NumArgs) {
      // Check the template argument we were given.
      if (CheckTemplateArgument(*Param, NewArgs[ArgIdx], Template,
                                TemplateLoc, RAngleLoc,
                                ArgumentPack.size(), Converted))
        return true;

      bool PackExpansionIntoNonPack =
          NewArgs[ArgIdx].getArgument().isPackExpansion() &&
          (!(*Param)->isTemplateParameterPack() || getExpandedPackSize(*Param));
      if (PackExpansionIntoNonPack && (isa<TypeAliasTemplateDecl>(Template) ||
                                       isa<ConceptDecl>(Template))) {
        // Core issue 1430: we have a pack expansion as an argument to an
        // alias template, and it's not part of a parameter pack. This
        // can't be canonicalized, so reject it now.
        // As for concepts - we cannot normalize constraints where this
        // situation exists.
        Diag(NewArgs[ArgIdx].getLocation(),
             diag::err_template_expansion_into_fixed_list)
          << (isa<ConceptDecl>(Template) ? 1 : 0)
          << NewArgs[ArgIdx].getSourceRange();
        Diag((*Param)->getLocation(), diag::note_template_param_here);
        return true;
      }

      // We're now done with this argument.
      ++ArgIdx;

      if ((*Param)->isTemplateParameterPack()) {
        // The template parameter was a template parameter pack, so take the
        // deduced argument and place it on the argument pack. Note that we
        // stay on the same template parameter so that we can deduce more
        // arguments.
        ArgumentPack.push_back(Converted.pop_back_val());
      } else {
        // Move to the next template parameter.
        ++Param;
      }

      // If we just saw a pack expansion into a non-pack, then directly convert
      // the remaining arguments, because we don't know what parameters they'll
      // match up with.
      if (PackExpansionIntoNonPack) {
        if (!ArgumentPack.empty()) {
          // If we were part way through filling in an expanded parameter pack,
          // fall back to just producing individual arguments.
          Converted.insert(Converted.end(),
                           ArgumentPack.begin(), ArgumentPack.end());
          ArgumentPack.clear();
        }

        while (ArgIdx < NumArgs) {
          Converted.push_back(NewArgs[ArgIdx].getArgument());
          ++ArgIdx;
        }

        return false;
      }

      continue;
    }

    // If we're checking a partial template argument list, we're done.
    if (PartialTemplateArgs) {
      if ((*Param)->isTemplateParameterPack() && !ArgumentPack.empty())
        Converted.push_back(
            TemplateArgument::CreatePackCopy(Context, ArgumentPack));
      return false;
    }

    // If we have a template parameter pack with no more corresponding
    // arguments, just break out now and we'll fill in the argument pack below.
    if ((*Param)->isTemplateParameterPack()) {
      assert(!getExpandedPackSize(*Param) &&
             "Should have dealt with this already");

      // A non-expanded parameter pack before the end of the parameter list
      // only occurs for an ill-formed template parameter list, unless we've
      // got a partial argument list for a function template, so just bail out.
      if (Param + 1 != ParamEnd)
        return true;

      Converted.push_back(
          TemplateArgument::CreatePackCopy(Context, ArgumentPack));
      ArgumentPack.clear();

      ++Param;
      continue;
    }

    // Check whether we have a default argument.
    TemplateArgumentLoc Arg;

    // Retrieve the default template argument from the template
    // parameter. For each kind of template parameter, we substitute the
    // template arguments provided thus far and any "outer" template arguments
    // (when the template parameter was part of a nested template) into
    // the default argument.
    if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(*Param)) {
      if (!hasVisibleDefaultArgument(TTP))
        return diagnoseMissingArgument(*this, TemplateLoc, Template, TTP,
                                       NewArgs);

      TypeSourceInfo *ArgType = SubstDefaultTemplateArgument(*this,
                                                             Template,
                                                             TemplateLoc,
                                                             RAngleLoc,
                                                             TTP,
                                                             Converted);
      if (!ArgType)
        return true;

      Arg = TemplateArgumentLoc(TemplateArgument(ArgType->getType()),
                                ArgType);
    } else if (NonTypeTemplateParmDecl *NTTP
                 = dyn_cast<NonTypeTemplateParmDecl>(*Param)) {
      if (!hasVisibleDefaultArgument(NTTP))
        return diagnoseMissingArgument(*this, TemplateLoc, Template, NTTP,
                                       NewArgs);

      ExprResult E = SubstDefaultTemplateArgument(*this, Template,
                                                              TemplateLoc,
                                                              RAngleLoc,
                                                              NTTP,
                                                              Converted);
      if (E.isInvalid())
        return true;

      Expr *Ex = E.getAs<Expr>();
      Arg = TemplateArgumentLoc(TemplateArgument(Ex), Ex);
    } else {
      TemplateTemplateParmDecl *TempParm
        = cast<TemplateTemplateParmDecl>(*Param);

      if (!hasVisibleDefaultArgument(TempParm))
        return diagnoseMissingArgument(*this, TemplateLoc, Template, TempParm,
                                       NewArgs);

      NestedNameSpecifierLoc QualifierLoc;
      TemplateName Name = SubstDefaultTemplateArgument(*this, Template,
                                                       TemplateLoc,
                                                       RAngleLoc,
                                                       TempParm,