text: //===------- SemaTemplateInstantiate.cpp - C++ Template Instantiation ------===/
  //
  // 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 C++ template instantiation.
  //
  //===----------------------------------------------------------------------===/
  
  #include "clang/Sema/SemaInternal.h"
  #include "TreeTransform.h"
  #include "clang/AST/ASTConsumer.h"
  #include "clang/AST/ASTContext.h"
  #include "clang/AST/ASTLambda.h"
  #include "clang/AST/ASTMutationListener.h"
  #include "clang/AST/DeclTemplate.h"
  #include "clang/AST/Expr.h"
  #include "clang/AST/PrettyDeclStackTrace.h"
  #include "clang/Basic/LangOptions.h"
  #include "clang/Sema/DeclSpec.h"
  #include "clang/Sema/Initialization.h"
  #include "clang/Sema/Lookup.h"
  #include "clang/Sema/Template.h"
  #include "clang/Sema/TemplateDeduction.h"
  #include "clang/Sema/TemplateInstCallback.h"
  #include "llvm/Support/TimeProfiler.h"
  
  using namespace clang;
  using namespace sema;
  
  //===----------------------------------------------------------------------===/
  // Template Instantiation Support
  //===----------------------------------------------------------------------===/
  
  /// Retrieve the template argument list(s) that should be used to
  /// instantiate the definition of the given declaration.
  ///
  /// \param D the declaration for which we are computing template instantiation
  /// arguments.
  ///
  /// \param Innermost if non-NULL, the innermost template argument list.
  ///
  /// \param RelativeToPrimary true if we should get the template
  /// arguments relative to the primary template, even when we're
  /// dealing with a specialization. This is only relevant for function
  /// template specializations.
  ///
  /// \param Pattern If non-NULL, indicates the pattern from which we will be
  /// instantiating the definition of the given declaration, \p D. This is
  /// used to determine the proper set of template instantiation arguments for
  /// friend function template specializations.
  MultiLevelTemplateArgumentList
  Sema::getTemplateInstantiationArgs(NamedDecl *D,
                                     const TemplateArgumentList *Innermost,
                                     bool RelativeToPrimary,
                                     const FunctionDecl *Pattern) {
    // Accumulate the set of template argument lists in this structure.
    MultiLevelTemplateArgumentList Result;
  
    if (Innermost)
      Result.addOuterTemplateArguments(Innermost);
  
    DeclContext *Ctx = dyn_cast<DeclContext>(D);
    if (!Ctx) {
      Ctx = D->getDeclContext();
  
      // Add template arguments from a variable template instantiation. For a
      // class-scope explicit specialization, there are no template arguments
      // at this level, but there may be enclosing template arguments.
      VarTemplateSpecializationDecl *Spec =
          dyn_cast<VarTemplateSpecializationDecl>(D);
      if (Spec && !Spec->isClassScopeExplicitSpecialization()) {
        // We're done when we hit an explicit specialization.
        if (Spec->getSpecializationKind() == TSK_ExplicitSpecialization &&
            !isa<VarTemplatePartialSpecializationDecl>(Spec))
          return Result;
  
        Result.addOuterTemplateArguments(&Spec->getTemplateInstantiationArgs());
  
        // If this variable template specialization was instantiated from a
        // specialized member that is a variable template, we're done.
        assert(Spec->getSpecializedTemplate() && "No variable template?");
        llvm::PointerUnion<VarTemplateDecl*,
                           VarTemplatePartialSpecializationDecl*> Specialized
                               = Spec->getSpecializedTemplateOrPartial();
        if (VarTemplatePartialSpecializationDecl *Partial =
                Specialized.dyn_cast<VarTemplatePartialSpecializationDecl *>()) {
          if (Partial->isMemberSpecialization())
            return Result;
        } else {
          VarTemplateDecl *Tmpl = Specialized.get<VarTemplateDecl *>();
          if (Tmpl->isMemberSpecialization())
            return Result;
        }
      }
  
      // If we have a template template parameter with translation unit context,
      // then we're performing substitution into a default template argument of
      // this template template parameter before we've constructed the template
      // that will own this template template parameter. In this case, we
      // use empty template parameter lists for all of the outer templates
      // to avoid performing any substitutions.
      if (Ctx->isTranslationUnit()) {
        if (TemplateTemplateParmDecl *TTP
                                        = dyn_cast<TemplateTemplateParmDecl>(D)) {
          for (unsigned I = 0, N = TTP->getDepth() + 1; I != N; ++I)
            Result.addOuterTemplateArguments(None);
          return Result;
        }
      }
    }
  
    while (!Ctx->isFileContext()) {
      // Add template arguments from a class template instantiation.
      ClassTemplateSpecializationDecl *Spec
            = dyn_cast<ClassTemplateSpecializationDecl>(Ctx);
      if (Spec && !Spec->isClassScopeExplicitSpecialization()) {
        // We're done when we hit an explicit specialization.
        if (Spec->getSpecializationKind() == TSK_ExplicitSpecialization &&
            !isa<ClassTemplatePartialSpecializationDecl>(Spec))
          break;
  
        Result.addOuterTemplateArguments(&Spec->getTemplateInstantiationArgs());
  
        // If this class template specialization was instantiated from a
        // specialized member that is a class template, we're done.
        assert(Spec->getSpecializedTemplate() && "No class template?");
        if (Spec->getSpecializedTemplate()->isMemberSpecialization())
          break;
      }
      // Add template arguments from a function template specialization.
      else if (FunctionDecl *Function = dyn_cast<FunctionDecl>(Ctx)) {
        if (!RelativeToPrimary &&
            Function->getTemplateSpecializationKindForInstantiation() ==
                TSK_ExplicitSpecialization)
          break;
  
        if (const TemplateArgumentList *TemplateArgs
              = Function->getTemplateSpecializationArgs()) {
          // Add the template arguments for this specialization.
          Result.addOuterTemplateArguments(TemplateArgs);
  
          // If this function was instantiated from a specialized member that is
          // a function template, we're done.
          assert(Function->getPrimaryTemplate() && "No function template?");
          if (Function->getPrimaryTemplate()->isMemberSpecialization())
            break;
  
          // If this function is a generic lambda specialization, we are done.
          if (isGenericLambdaCallOperatorSpecialization(Function))
            break;
  
        } else if (FunctionTemplateDecl *FunTmpl
                                     = Function->getDescribedFunctionTemplate()) {
          // Add the "injected" template arguments.
          Result.addOuterTemplateArguments(FunTmpl->getInjectedTemplateArgs());
        }
  
        // If this is a friend declaration and it declares an entity at
        // namespace scope, take arguments from its lexical parent
        // instead of its semantic parent, unless of course the pattern we're
        // instantiating actually comes from the file's context!
        if (Function->getFriendObjectKind() &&
            Function->getDeclContext()->isFileContext() &&
            (!Pattern || !Pattern->getLexicalDeclContext()->isFileContext())) {
          Ctx = Function->getLexicalDeclContext();
          RelativeToPrimary = false;
          continue;
        }
      } else if (CXXRecordDecl *Rec = dyn_cast<CXXRecordDecl>(Ctx)) {
        if (ClassTemplateDecl *ClassTemplate = Rec->getDescribedClassTemplate()) {
          QualType T = ClassTemplate->getInjectedClassNameSpecialization();
          const TemplateSpecializationType *TST =
              cast<TemplateSpecializationType>(Context.getCanonicalType(T));
          Result.addOuterTemplateArguments(
              llvm::makeArrayRef(TST->getArgs(), TST->getNumArgs()));
          if (ClassTemplate->isMemberSpecialization())
            break;
        }
      }
  
      Ctx = Ctx->getParent();
      RelativeToPrimary = false;
    }
  
    return Result;
  }
  
  bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
    switch (Kind) {
    case TemplateInstantiation:
    case ExceptionSpecInstantiation:
    case DefaultTemplateArgumentInstantiation:
    case DefaultFunctionArgumentInstantiation:
    case ExplicitTemplateArgumentSubstitution:
    case DeducedTemplateArgumentSubstitution:
    case PriorTemplateArgumentSubstitution:
      return true;
  
    case DefaultTemplateArgumentChecking:
    case DeclaringSpecialMember:
    case DefiningSynthesizedFunction:
    case ExceptionSpecEvaluation:
      return false;
  
    // This function should never be called when Kind's value is Memoization.
    case Memoization:
      break;
    }
  
    llvm_unreachable("Invalid SynthesisKind!");
  }
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, CodeSynthesisContext::SynthesisKind Kind,
      SourceLocation PointOfInstantiation, SourceRange InstantiationRange,
      Decl *Entity, NamedDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
      sema::TemplateDeductionInfo *DeductionInfo)
      : SemaRef(SemaRef) {
    // Don't allow further instantiation if a fatal error and an uncompilable
    // error have occurred. Any diagnostics we might have raised will not be
    // visible, and we do not need to construct a correct AST.
    if (SemaRef.Diags.hasFatalErrorOccurred() &&
        SemaRef.Diags.hasUncompilableErrorOccurred()) {
      Invalid = true;
      return;
    }
    Invalid = CheckInstantiationDepth(PointOfInstantiation, InstantiationRange);
    if (!Invalid) {
      CodeSynthesisContext Inst;
      Inst.Kind = Kind;
      Inst.PointOfInstantiation = PointOfInstantiation;
      Inst.Entity = Entity;
      Inst.Template = Template;
      Inst.TemplateArgs = TemplateArgs.data();
      Inst.NumTemplateArgs = TemplateArgs.size();
      Inst.DeductionInfo = DeductionInfo;
      Inst.InstantiationRange = InstantiationRange;
      SemaRef.pushCodeSynthesisContext(Inst);
  
      AlreadyInstantiating =
          !SemaRef.InstantiatingSpecializations
               .insert(std::make_pair(Inst.Entity->getCanonicalDecl(), Inst.Kind))
               .second;
      atTemplateBegin(SemaRef.TemplateInstCallbacks, SemaRef, Inst);
    }
  }
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, Decl *Entity,
      SourceRange InstantiationRange)
      : InstantiatingTemplate(SemaRef,
                              CodeSynthesisContext::TemplateInstantiation,
                              PointOfInstantiation, InstantiationRange, Entity) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, FunctionDecl *Entity,
      ExceptionSpecification, SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef, CodeSynthesisContext::ExceptionSpecInstantiation,
            PointOfInstantiation, InstantiationRange, Entity) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, TemplateParameter Param,
      TemplateDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
      SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::DefaultTemplateArgumentInstantiation,
            PointOfInstantiation, InstantiationRange, getAsNamedDecl(Param),
            Template, TemplateArgs) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation,
      FunctionTemplateDecl *FunctionTemplate,
      ArrayRef<TemplateArgument> TemplateArgs,
      CodeSynthesisContext::SynthesisKind Kind,
      sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
      : InstantiatingTemplate(SemaRef, Kind, PointOfInstantiation,
                              InstantiationRange, FunctionTemplate, nullptr,
                              TemplateArgs, &DeductionInfo) {
    assert(
      Kind == CodeSynthesisContext::ExplicitTemplateArgumentSubstitution ||
      Kind == CodeSynthesisContext::DeducedTemplateArgumentSubstitution);
  }
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation,
      TemplateDecl *Template,
      ArrayRef<TemplateArgument> TemplateArgs,
      sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::DeducedTemplateArgumentSubstitution,
            PointOfInstantiation, InstantiationRange, Template, nullptr,
            TemplateArgs, &DeductionInfo) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation,
      ClassTemplatePartialSpecializationDecl *PartialSpec,
      ArrayRef<TemplateArgument> TemplateArgs,
      sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::DeducedTemplateArgumentSubstitution,
            PointOfInstantiation, InstantiationRange, PartialSpec, nullptr,
            TemplateArgs, &DeductionInfo) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation,
      VarTemplatePartialSpecializationDecl *PartialSpec,
      ArrayRef<TemplateArgument> TemplateArgs,
      sema::TemplateDeductionInfo &DeductionInfo, SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::DeducedTemplateArgumentSubstitution,
            PointOfInstantiation, InstantiationRange, PartialSpec, nullptr,
            TemplateArgs, &DeductionInfo) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, ParmVarDecl *Param,
      ArrayRef<TemplateArgument> TemplateArgs, SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::DefaultFunctionArgumentInstantiation,
            PointOfInstantiation, InstantiationRange, Param, nullptr,
            TemplateArgs) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, NamedDecl *Template,
      NonTypeTemplateParmDecl *Param, ArrayRef<TemplateArgument> TemplateArgs,
      SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::PriorTemplateArgumentSubstitution,
            PointOfInstantiation, InstantiationRange, Param, Template,
            TemplateArgs) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, NamedDecl *Template,
      TemplateTemplateParmDecl *Param, ArrayRef<TemplateArgument> TemplateArgs,
      SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef,
            CodeSynthesisContext::PriorTemplateArgumentSubstitution,
            PointOfInstantiation, InstantiationRange, Param, Template,
            TemplateArgs) {}
  
  Sema::InstantiatingTemplate::InstantiatingTemplate(
      Sema &SemaRef, SourceLocation PointOfInstantiation, TemplateDecl *Template,
      NamedDecl *Param, ArrayRef<TemplateArgument> TemplateArgs,
      SourceRange InstantiationRange)
      : InstantiatingTemplate(
            SemaRef, CodeSynthesisContext::DefaultTemplateArgumentChecking,
            PointOfInstantiation, InstantiationRange, Param, Template,
            TemplateArgs) {}
  
  void Sema::pushCodeSynthesisContext(CodeSynthesisContext Ctx) {
    Ctx.SavedInNonInstantiationSFINAEContext = InNonInstantiationSFINAEContext;
    InNonInstantiationSFINAEContext = false;
  
    CodeSynthesisContexts.push_back(Ctx);
  
    if (!Ctx.isInstantiationRecord())
      ++NonInstantiationEntries;
  }
  
  void Sema::popCodeSynthesisContext() {
    auto &Active = CodeSynthesisContexts.back();
    if (!Active.isInstantiationRecord()) {
      assert(NonInstantiationEntries > 0);
      --NonInstantiationEntries;
    }
  
    InNonInstantiationSFINAEContext = Active.SavedInNonInstantiationSFINAEContext;
  
    // Name lookup no longer looks in this template's defining module.
    assert(CodeSynthesisContexts.size() >=
               CodeSynthesisContextLookupModules.size() &&
           "forgot to remove a lookup module for a template instantiation");
    if (CodeSynthesisContexts.size() ==
        CodeSynthesisContextLookupModules.size()) {
      if (Module *M = CodeSynthesisContextLookupModules.back())
        LookupModulesCache.erase(M);
      CodeSynthesisContextLookupModules.pop_back();
    }
  
    // If we've left the code synthesis context for the current context stack,
    // stop remembering that we've emitted that stack.
    if (CodeSynthesisContexts.size() ==
        LastEmittedCodeSynthesisContextDepth)
      LastEmittedCodeSynthesisContextDepth = 0;
  
    CodeSynthesisContexts.pop_back();
  }
  
  void Sema::InstantiatingTemplate::Clear() {
    if (!Invalid) {
      if (!AlreadyInstantiating) {
        auto &Active = SemaRef.CodeSynthesisContexts.back();
        SemaRef.InstantiatingSpecializations.erase(
            std::make_pair(Active.Entity, Active.Kind));
      }
  
      atTemplateEnd(SemaRef.TemplateInstCallbacks, SemaRef,
                    SemaRef.CodeSynthesisContexts.back());
  
      SemaRef.popCodeSynthesisContext();
      Invalid = true;
    }
  }
  
  bool Sema::InstantiatingTemplate::CheckInstantiationDepth(
                                          SourceLocation PointOfInstantiation,
                                             SourceRange InstantiationRange) {
    assert(SemaRef.NonInstantiationEntries <=
           SemaRef.CodeSynthesisContexts.size());
    if ((SemaRef.CodeSynthesisContexts.size() -
            SemaRef.NonInstantiationEntries)
          <= SemaRef.getLangOpts().InstantiationDepth)
      return false;
  
    SemaRef.Diag(PointOfInstantiation,
                 diag::err_template_recursion_depth_exceeded)
      << SemaRef.getLangOpts().InstantiationDepth
      << InstantiationRange;
    SemaRef.Diag(PointOfInstantiation, diag::note_template_recursion_depth)
      << SemaRef.getLangOpts().InstantiationDepth;
    return true;
  }
  
  /// Prints the current instantiation stack through a series of
  /// notes.
  void Sema::PrintInstantiationStack() {
    // Determine which template instantiations to skip, if any.
    unsigned SkipStart = CodeSynthesisContexts.size(), SkipEnd = SkipStart;
    unsigned Limit = Diags.getTemplateBacktraceLimit();
    if (Limit && Limit < CodeSynthesisContexts.size()) {
      SkipStart = Limit / 2 + Limit % 2;
      SkipEnd = CodeSynthesisContexts.size() - Limit / 2;
    }
  
    // FIXME: In all of these cases, we need to show the template arguments
    unsigned InstantiationIdx = 0;
    for (SmallVectorImpl<CodeSynthesisContext>::reverse_iterator
           Active = CodeSynthesisContexts.rbegin(),
           ActiveEnd = CodeSynthesisContexts.rend();
         Active != ActiveEnd;
         ++Active, ++InstantiationIdx) {
      // Skip this instantiation?
      if (InstantiationIdx >= SkipStart && InstantiationIdx < SkipEnd) {
        if (InstantiationIdx == SkipStart) {
          // Note that we're skipping instantiations.
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_instantiation_contexts_suppressed)
            << unsigned(CodeSynthesisContexts.size() - Limit);
        }
        continue;
      }
  
      switch (Active->Kind) {
      case CodeSynthesisContext::TemplateInstantiation: {
        Decl *D = Active->Entity;
        if (CXXRecordDecl *Record = dyn_cast<CXXRecordDecl>(D)) {
          unsigned DiagID = diag::note_template_member_class_here;
          if (isa<ClassTemplateSpecializationDecl>(Record))
            DiagID = diag::note_template_class_instantiation_here;
          Diags.Report(Active->PointOfInstantiation, DiagID)
            << Record << Active->InstantiationRange;
        } else if (FunctionDecl *Function = dyn_cast<FunctionDecl>(D)) {
          unsigned DiagID;
          if (Function->getPrimaryTemplate())
            DiagID = diag::note_function_template_spec_here;
          else
            DiagID = diag::note_template_member_function_here;
          Diags.Report(Active->PointOfInstantiation, DiagID)
            << Function
            << Active->InstantiationRange;
        } else if (VarDecl *VD = dyn_cast<VarDecl>(D)) {
          Diags.Report(Active->PointOfInstantiation,
                       VD->isStaticDataMember()?
                         diag::note_template_static_data_member_def_here
                       : diag::note_template_variable_def_here)
            << VD
            << Active->InstantiationRange;
        } else if (EnumDecl *ED = dyn_cast<EnumDecl>(D)) {
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_template_enum_def_here)
            << ED
            << Active->InstantiationRange;
        } else if (FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_template_nsdmi_here)
              << FD << Active->InstantiationRange;
        } else {
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_template_type_alias_instantiation_here)
            << cast<TypeAliasTemplateDecl>(D)
            << Active->InstantiationRange;
        }
        break;
      }
  
      case CodeSynthesisContext::DefaultTemplateArgumentInstantiation: {
        TemplateDecl *Template = cast<TemplateDecl>(Active->Template);
        SmallVector<char, 128> TemplateArgsStr;
        llvm::raw_svector_ostream OS(TemplateArgsStr);
        Template->printName(OS);
        printTemplateArgumentList(OS, Active->template_arguments(),
                                  getPrintingPolicy());
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_default_arg_instantiation_here)
          << OS.str()
          << Active->InstantiationRange;
        break;
      }
  
      case CodeSynthesisContext::ExplicitTemplateArgumentSubstitution: {
        FunctionTemplateDecl *FnTmpl = cast<FunctionTemplateDecl>(Active->Entity);
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_explicit_template_arg_substitution_here)
          << FnTmpl
          << getTemplateArgumentBindingsText(FnTmpl->getTemplateParameters(),
                                             Active->TemplateArgs,
                                             Active->NumTemplateArgs)
          << Active->InstantiationRange;
        break;
      }
  
      case CodeSynthesisContext::DeducedTemplateArgumentSubstitution: {
        if (FunctionTemplateDecl *FnTmpl =
                dyn_cast<FunctionTemplateDecl>(Active->Entity)) {
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_function_template_deduction_instantiation_here)
            << FnTmpl
            << getTemplateArgumentBindingsText(FnTmpl->getTemplateParameters(),
                                               Active->TemplateArgs,
                                               Active->NumTemplateArgs)
            << Active->InstantiationRange;
        } else {
          bool IsVar = isa<VarTemplateDecl>(Active->Entity) ||
                       isa<VarTemplateSpecializationDecl>(Active->Entity);
          bool IsTemplate = false;
          TemplateParameterList *Params;
          if (auto *D = dyn_cast<TemplateDecl>(Active->Entity)) {
            IsTemplate = true;
            Params = D->getTemplateParameters();
          } else if (auto *D = dyn_cast<ClassTemplatePartialSpecializationDecl>(
                         Active->Entity)) {
            Params = D->getTemplateParameters();
          } else if (auto *D = dyn_cast<VarTemplatePartialSpecializationDecl>(
                         Active->Entity)) {
            Params = D->getTemplateParameters();
          } else {
            llvm_unreachable("unexpected template kind");
          }
  
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_deduced_template_arg_substitution_here)
            << IsVar << IsTemplate << cast<NamedDecl>(Active->Entity)
            << getTemplateArgumentBindingsText(Params, Active->TemplateArgs,
                                               Active->NumTemplateArgs)
            << Active->InstantiationRange;
        }
        break;
      }
  
      case CodeSynthesisContext::DefaultFunctionArgumentInstantiation: {
        ParmVarDecl *Param = cast<ParmVarDecl>(Active->Entity);
        FunctionDecl *FD = cast<FunctionDecl>(Param->getDeclContext());
  
        SmallVector<char, 128> TemplateArgsStr;
        llvm::raw_svector_ostream OS(TemplateArgsStr);
        FD->printName(OS);
        printTemplateArgumentList(OS, Active->template_arguments(),
                                  getPrintingPolicy());
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_default_function_arg_instantiation_here)
          << OS.str()
          << Active->InstantiationRange;
        break;
      }
  
      case CodeSynthesisContext::PriorTemplateArgumentSubstitution: {
        NamedDecl *Parm = cast<NamedDecl>(Active->Entity);
        std::string Name;
        if (!Parm->getName().empty())
          Name = std::string(" '") + Parm->getName().str() + "'";
  
        TemplateParameterList *TemplateParams = nullptr;
        if (TemplateDecl *Template = dyn_cast<TemplateDecl>(Active->Template))
          TemplateParams = Template->getTemplateParameters();
        else
          TemplateParams =
            cast<ClassTemplatePartialSpecializationDecl>(Active->Template)
                                                        ->getTemplateParameters();
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_prior_template_arg_substitution)
          << isa<TemplateTemplateParmDecl>(Parm)
          << Name
          << getTemplateArgumentBindingsText(TemplateParams,
                                             Active->TemplateArgs,
                                             Active->NumTemplateArgs)
          << Active->InstantiationRange;
        break;
      }
  
      case CodeSynthesisContext::DefaultTemplateArgumentChecking: {
        TemplateParameterList *TemplateParams = nullptr;
        if (TemplateDecl *Template = dyn_cast<TemplateDecl>(Active->Template))
          TemplateParams = Template->getTemplateParameters();
        else
          TemplateParams =
            cast<ClassTemplatePartialSpecializationDecl>(Active->Template)
                                                        ->getTemplateParameters();
  
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_template_default_arg_checking)
          << getTemplateArgumentBindingsText(TemplateParams,
                                             Active->TemplateArgs,
                                             Active->NumTemplateArgs)
          << Active->InstantiationRange;
        break;
      }
  
      case CodeSynthesisContext::ExceptionSpecEvaluation:
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_evaluating_exception_spec_here)
            << cast<FunctionDecl>(Active->Entity);
        break;
  
      case CodeSynthesisContext::ExceptionSpecInstantiation:
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_template_exception_spec_instantiation_here)
          << cast<FunctionDecl>(Active->Entity)
          << Active->InstantiationRange;
        break;
  
      case CodeSynthesisContext::DeclaringSpecialMember:
        Diags.Report(Active->PointOfInstantiation,
                     diag::note_in_declaration_of_implicit_special_member)
          << cast<CXXRecordDecl>(Active->Entity) << Active->SpecialMember;
        break;
  
      case CodeSynthesisContext::DefiningSynthesizedFunction: {
        // FIXME: For synthesized members other than special members, produce a note.
        auto *MD = dyn_cast<CXXMethodDecl>(Active->Entity);
        auto CSM = MD ? getSpecialMember(MD) : CXXInvalid;
        if (CSM != CXXInvalid) {
          Diags.Report(Active->PointOfInstantiation,
                       diag::note_member_synthesized_at)
            << CSM << Context.getTagDeclType(MD->getParent());
        }
        break;
      }
  
      case CodeSynthesisContext::Memoization:
        break;
      }
    }
  }
  
  Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
    if (InNonInstantiationSFINAEContext)
      return Optional<TemplateDeductionInfo *>(nullptr);
  
    for (SmallVectorImpl<CodeSynthesisContext>::const_reverse_iterator
           Active = CodeSynthesisContexts.rbegin(),
           ActiveEnd = CodeSynthesisContexts.rend();
         Active != ActiveEnd;
         ++Active)
    {
      switch (Active->Kind) {
      case CodeSynthesisContext::TemplateInstantiation:
        // An instantiation of an alias template may or may not be a SFINAE
        // context, depending on what else is on the stack.
        if (isa<TypeAliasTemplateDecl>(Active->Entity))
          break;
        LLVM_FALLTHROUGH;
      case CodeSynthesisContext::DefaultFunctionArgumentInstantiation:
      case CodeSynthesisContext::ExceptionSpecInstantiation:
        // This is a template instantiation, so there is no SFINAE.
        return None;
  
      case CodeSynthesisContext::DefaultTemplateArgumentInstantiation:
      case CodeSynthesisContext::PriorTemplateArgumentSubstitution:
      case CodeSynthesisContext::DefaultTemplateArgumentChecking:
        // A default template argument instantiation and substitution into
        // template parameters with arguments for prior parameters may or may
        // not be a SFINAE context; look further up the stack.
        break;
  
      case CodeSynthesisContext::ExplicitTemplateArgumentSubstitution:
      case CodeSynthesisContext::DeducedTemplateArgumentSubstitution:
        // We're either substitution explicitly-specified template arguments
        // or deduced template arguments, so SFINAE applies.
        assert(Active->DeductionInfo && "Missing deduction info pointer");
        return Active->DeductionInfo;
  
      case CodeSynthesisContext::DeclaringSpecialMember:
      case CodeSynthesisContext::DefiningSynthesizedFunction:
        // This happens in a context unrelated to template instantiation, so
        // there is no SFINAE.
        return None;
  
      case CodeSynthesisContext::ExceptionSpecEvaluation:
        // FIXME: This should not be treated as a SFINAE context, because
        // we will cache an incorrect exception specification. However, clang
        // bootstrap relies this! See PR31692.
        break;
  
      case CodeSynthesisContext::Memoization:
        break;
      }
  
      // The inner context was transparent for SFINAE. If it occurred within a
      // non-instantiation SFINAE context, then SFINAE applies.
      if (Active->SavedInNonInstantiationSFINAEContext)
        return Optional<TemplateDeductionInfo *>(nullptr);
    }
  
    return None;
  }
  
  //===----------------------------------------------------------------------===/
  // Template Instantiation for Types
  //===----------------------------------------------------------------------===/
  namespace {
    class TemplateInstantiator : public TreeTransform<TemplateInstantiator> {
      const MultiLevelTemplateArgumentList &TemplateArgs;
      SourceLocation Loc;
      DeclarationName Entity;
  
    public:
      typedef TreeTransform<TemplateInstantiator> inherited;
  
      TemplateInstantiator(Sema &SemaRef,
                           const MultiLevelTemplateArgumentList &TemplateArgs,
                           SourceLocation Loc,
                           DeclarationName Entity)
        : inherited(SemaRef), TemplateArgs(TemplateArgs), Loc(Loc),
          Entity(Entity) { }
  
      /// Determine whether the given type \p T has already been
      /// transformed.
      ///
      /// For the purposes of template instantiation, a type has already been
      /// transformed if it is NULL or if it is not dependent.
      bool AlreadyTransformed(QualType T);
  
      /// Returns the location of the entity being instantiated, if known.
      SourceLocation getBaseLocation() { return Loc; }
  
      /// Returns the name of the entity being instantiated, if any.
      DeclarationName getBaseEntity() { return Entity; }
  
      /// Sets the "base" location and entity when that
      /// information is known based on another transformation.
      void setBase(SourceLocation Loc, DeclarationName Entity) {
        this->Loc = Loc;
        this->Entity = Entity;
      }
  
      bool TryExpandParameterPacks(SourceLocation EllipsisLoc,
                                   SourceRange PatternRange,
                                   ArrayRef<UnexpandedParameterPack> Unexpanded,
                                   bool &ShouldExpand, bool &RetainExpansion,
                                   Optional<unsigned> &NumExpansions) {
        return getSema().CheckParameterPacksForExpansion(EllipsisLoc,
                                                         PatternRange, Unexpanded,
                                                         TemplateArgs,
                                                         ShouldExpand,
                                                         RetainExpansion,
                                                         NumExpansions);
      }
  
      void ExpandingFunctionParameterPack(ParmVarDecl *Pack) {
        SemaRef.CurrentInstantiationScope->MakeInstantiatedLocalArgPack(Pack);
      }
  
      TemplateArgument ForgetPartiallySubstitutedPack() {
        TemplateArgument Result;
        if (NamedDecl *PartialPack
              = SemaRef.CurrentInstantiationScope->getPartiallySubstitutedPack()){
          MultiLevelTemplateArgumentList &TemplateArgs
            = const_cast<MultiLevelTemplateArgumentList &>(this->TemplateArgs);
          unsigned Depth, Index;
          std::tie(Depth, Index) = getDepthAndIndex(PartialPack);
          if (TemplateArgs.hasTemplateArgument(Depth, Index)) {
            Result = TemplateArgs(Depth, Index);
            TemplateArgs.setArgument(Depth, Index, TemplateArgument());
          }
        }
  
        return Result;
      }
  
      void RememberPartiallySubstitutedPack(TemplateArgument Arg) {
        if (Arg.isNull())
          return;
  
        if (NamedDecl *PartialPack
              = SemaRef.CurrentInstantiationScope->getPartiallySubstitutedPack()){
          MultiLevelTemplateArgumentList &TemplateArgs
          = const_cast<MultiLevelTemplateArgumentList &>(this->TemplateArgs);
          unsigned Depth, Index;
          std::tie(Depth, Index) = getDepthAndIndex(PartialPack);
          TemplateArgs.setArgument(Depth, Index, Arg);
        }
      }
  
      /// Transform the given declaration by instantiating a reference to
      /// this declaration.
      Decl *TransformDecl(SourceLocation Loc, Decl *D);
  
      void transformAttrs(Decl *Old, Decl *New) {
        SemaRef.InstantiateAttrs(TemplateArgs, Old, New);
      }
  
      void transformedLocalDecl(Decl *Old, ArrayRef<Decl *> NewDecls) {
        if (Old->isParameterPack()) {
          SemaRef.CurrentInstantiationScope->MakeInstantiatedLocalArgPack(Old);
          for (auto *New : NewDecls)
            SemaRef.CurrentInstantiationScope->InstantiatedLocalPackArg(
                Old, cast<VarDecl>(New));
          return;
        }
  
        assert(NewDecls.size() == 1 &&
               "should only have multiple expansions for a pack");
        Decl *New = NewDecls.front();
  
        // If we've instantiated the call operator of a lambda or the call
        // operator template of a generic lambda, update the "instantiation of"
        // information.
        auto *NewMD = dyn_cast<CXXMethodDecl>(New);
        if (NewMD && isLambdaCallOperator(NewMD)) {
          auto *OldMD = dyn_cast<CXXMethodDecl>(Old);
          if (auto *NewTD = NewMD->getDescribedFunctionTemplate())
            NewTD->setInstantiatedFromMemberTemplate(
                OldMD->getDescribedFunctionTemplate());
          else
            NewMD->setInstantiationOfMemberFunction(OldMD,
                                                    TSK_ImplicitInstantiation);
        }
  
        SemaRef.CurrentInstantiationScope->InstantiatedLocal(Old, New);
  
        // We recreated a local declaration, but not by instantiating it. There
        // may be pending dependent diagnostics to produce.
        if (auto *DC = dyn_cast<DeclContext>(Old))
          SemaRef.PerformDependentDiagnostics(DC, TemplateArgs);
      }
  
      /// Transform the definition of the given declaration by
      /// instantiating it.
      Decl *TransformDefinition(SourceLocation Loc, Decl *D);
  
      /// Transform the first qualifier within a scope by instantiating the
      /// declaration.
      NamedDecl *TransformFirstQualifierInScope(NamedDecl *D, SourceLocation Loc);
  
      /// Rebuild the exception declaration and register the declaration
      /// as an instantiated local.
      VarDecl *RebuildExceptionDecl(VarDecl *ExceptionDecl,
                                    TypeSourceInfo *Declarator,
                                    SourceLocation StartLoc,
                                    SourceLocation NameLoc,
                                    IdentifierInfo *Name);
  
      /// Rebuild the Objective-C exception declaration and register the
      /// declaration as an instantiated local.
      VarDecl *RebuildObjCExceptionDecl(VarDecl *ExceptionDecl,
                                        TypeSourceInfo *TSInfo, QualType T);
  
      /// Check for tag mismatches when instantiating an
      /// elaborated type.
      QualType RebuildElaboratedType(SourceLocation KeywordLoc,
                                     ElaboratedTypeKeyword Keyword,
                                     NestedNameSpecifierLoc QualifierLoc,
                                     QualType T);
  
      TemplateName
      TransformTemplateName(CXXScopeSpec &SS, TemplateName Name,
                            SourceLocation NameLoc,
                            QualType ObjectType = QualType(),
                            NamedDecl *FirstQualifierInScope = nullptr,
                            bool AllowInjectedClassName = false);
  
      const LoopHintAttr *TransformLoopHintAttr(const LoopHintAttr *LH);
  
      ExprResult TransformPredefinedExpr(PredefinedExpr *E);
      ExprResult TransformDeclRefExpr(DeclRefExpr *E);
      ExprResult TransformCXXDefaultArgExpr(CXXDefaultArgExpr *E);
  
      ExprResult TransformTemplateParmRefExpr(DeclRefExpr *E,
                                              NonTypeTemplateParmDecl *D);
      ExprResult TransformSubstNonTypeTemplateParmPackExpr(
                                             SubstNonTypeTemplateParmPackExpr *E);
  
      /// Rebuild a DeclRefExpr for a VarDecl reference.
      ExprResult RebuildVarDeclRefExpr(VarDecl *PD, SourceLocation Loc);
  
      /// Transform a reference to a function or init-capture parameter pack.
      ExprResult TransformFunctionParmPackRefExpr(DeclRefExpr *E, VarDecl *PD);
  
      /// Transform a FunctionParmPackExpr which was built when we couldn't
      /// expand a function parameter pack reference which refers to an expanded
      /// pack.
      ExprResult TransformFunctionParmPackExpr(FunctionParmPackExpr *E);
  
      QualType TransformFunctionProtoType(TypeLocBuilder &TLB,
                                          FunctionProtoTypeLoc TL) {
        // Call the base version; it will forward to our overridden version below.
        return inherited::TransformFunctionProtoType(TLB, TL);
      }
  
      template<typename Fn>
      QualType TransformFunctionProtoType(TypeLocBuilder &TLB,
                                          FunctionProtoTypeLoc TL,
                                          CXXRecordDecl *ThisContext,
                                          Qualifiers ThisTypeQuals,
                                          Fn TransformExceptionSpec);
  
      ParmVarDecl *TransformFunctionTypeParam(ParmVarDecl *OldParm,
                                              int indexAdjustment,
                                              Optional<unsigned> NumExpansions,
                                              bool ExpectParameterPack);
  
      /// Transforms a template type parameter type by performing
      /// substitution of the corresponding template type argument.
      QualType TransformTemplateTypeParmType(TypeLocBuilder &TLB,
                                             TemplateTypeParmTypeLoc TL);
  
      /// Transforms an already-substituted template type parameter pack
      /// into either itself (if we aren't substituting into its pack expansion)
      /// or the appropriate substituted argument.
      QualType TransformSubstTemplateTypeParmPackType(TypeLocBuilder &TLB,
                                             SubstTemplateTypeParmPackTypeLoc TL);
  
      ExprResult TransformLambdaExpr(LambdaExpr *E) {
        LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
        return TreeTransform<TemplateInstantiator>::TransformLambdaExpr(E);
      }
  
      TemplateParameterList *TransformTemplateParameterList(
                                TemplateParameterList *OrigTPL)  {
        if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
  
        DeclContext *Owner = OrigTPL->getParam(0)->getDeclContext();
        TemplateDeclInstantiator  DeclInstantiator(getSema(),
                          /* DeclContext *Owner */ Owner, TemplateArgs);
        return DeclInstantiator.SubstTemplateParams(OrigTPL);
      }
    private:
      ExprResult transformNonTypeTemplateParmRef(NonTypeTemplateParmDecl *parm,
                                                 SourceLocation loc,
                                                 TemplateArgument arg);
    };
  }
  
  bool TemplateInstantiator::AlreadyTransformed(QualType T) {
    if (T.isNull())
      return true;
  
    if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
      return false;
  
    getSema().MarkDeclarationsReferencedInType(Loc, T);
    return true;
  }
  
  static TemplateArgument
  getPackSubstitutedTemplateArgument(Sema &S, TemplateArgument Arg) {
    assert(S.ArgumentPackSubstitutionIndex >= 0);
    assert(S.ArgumentPackSubstitutionIndex < (int)Arg.pack_size());
    Arg = Arg.pack_begin()[S.ArgumentPackSubstitutionIndex];
    if (Arg.isPackExpansion())
      Arg = Arg.getPackExpansionPattern();
    return Arg;
  }
  
  Decl *TemplateInstantiator::TransformDecl(SourceLocation Loc, Decl *D) {
    if (!D)
      return nullptr;
  
    if (TemplateTemplateParmDecl *TTP = dyn_cast<TemplateTemplateParmDecl>(D)) {
      if (TTP->getDepth() < TemplateArgs.getNumLevels()) {
        // If the corresponding template argument is NULL or non-existent, it's
        // because we are performing instantiation from explicitly-specified
        // template arguments in a function template, but there were some
        // arguments left unspecified.
        if (!TemplateArgs.hasTemplateArgument(TTP->getDepth(),
                                              TTP->getPosition()))
          return D;
  
        TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getPosition());
  
        if (TTP->isParameterPack()) {
          assert(Arg.getKind() == TemplateArgument::Pack &&
                 "Missing argument pack");
          Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
        }
  
        TemplateName Template = Arg.getAsTemplate().getNameToSubstitute();
        assert(!Template.isNull() && Template.getAsTemplateDecl() &&
               "Wrong kind of template template argument");
        return Template.getAsTemplateDecl();
      }
  
      // Fall through to find the instantiated declaration for this template
      // template parameter.
    }
  
    return SemaRef.FindInstantiatedDecl(Loc, cast<NamedDecl>(D), TemplateArgs);
  }
  
  Decl *TemplateInstantiator::TransformDefinition(SourceLocation Loc, Decl *D) {
    Decl *Inst = getSema().SubstDecl(D, getSema().CurContext, TemplateArgs);
    if (!Inst)
      return nullptr;
  
    getSema().CurrentInstantiationScope->InstantiatedLocal(D, Inst);
    return Inst;
  }
  
  NamedDecl *
  TemplateInstantiator::TransformFirstQualifierInScope(NamedDecl *D,
                                                       SourceLocation Loc) {
    // If the first part of the nested-name-specifier was a template type
    // parameter, instantiate that type parameter down to a tag type.
    if (TemplateTypeParmDecl *TTPD = dyn_cast_or_null<TemplateTypeParmDecl>(D)) {
      const TemplateTypeParmType *TTP
        = cast<TemplateTypeParmType>(getSema().Context.getTypeDeclType(TTPD));
  
      if (TTP->getDepth() < TemplateArgs.getNumLevels()) {
        // FIXME: This needs testing w/ member access expressions.
        TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getIndex());
  
        if (TTP->isParameterPack()) {
          assert(Arg.getKind() == TemplateArgument::Pack &&
                 "Missing argument pack");
  
          if (getSema().ArgumentPackSubstitutionIndex == -1)
            return nullptr;
  
          Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
        }
  
        QualType T = Arg.getAsType();
        if (T.isNull())
          return cast_or_null<NamedDecl>(TransformDecl(Loc, D));
  
        if (const TagType *Tag = T->getAs<TagType>())
          return Tag->getDecl();
  
        // The resulting type is not a tag; complain.
        getSema().Diag(Loc, diag::err_nested_name_spec_non_tag) << T;
        return nullptr;
      }
    }
  
    return cast_or_null<NamedDecl>(TransformDecl(Loc, D));
  }
  
  VarDecl *
  TemplateInstantiator::RebuildExceptionDecl(VarDecl *ExceptionDecl,
                                             TypeSourceInfo *Declarator,
                                             SourceLocation StartLoc,
                                             SourceLocation NameLoc,
                                             IdentifierInfo *Name) {
    VarDecl *Var = inherited::RebuildExceptionDecl(ExceptionDecl, Declarator,
                                                   StartLoc, NameLoc, Name);
    if (Var)
      getSema().CurrentInstantiationScope->InstantiatedLocal(ExceptionDecl, Var);
    return Var;
  }
  
  VarDecl *TemplateInstantiator::RebuildObjCExceptionDecl(VarDecl *ExceptionDecl,
                                                          TypeSourceInfo *TSInfo,
                                                          QualType T) {
    VarDecl *Var = inherited::RebuildObjCExceptionDecl(ExceptionDecl, TSInfo, T);
    if (Var)
      getSema().CurrentInstantiationScope->InstantiatedLocal(ExceptionDecl, Var);
    return Var;
  }
  
  QualType
  TemplateInstantiator::RebuildElaboratedType(SourceLocation KeywordLoc,
                                              ElaboratedTypeKeyword Keyword,
                                              NestedNameSpecifierLoc QualifierLoc,
                                              QualType T) {
    if (const TagType *TT = T->getAs<TagType>()) {
      TagDecl* TD = TT->getDecl();
  
      SourceLocation TagLocation = KeywordLoc;
  
      IdentifierInfo *Id = TD->getIdentifier();
  
      // TODO: should we even warn on struct/class mismatches for this?  Seems
      // like it's likely to produce a lot of spurious errors.
      if (Id && Keyword != ETK_None && Keyword != ETK_Typename) {
        TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForKeyword(Keyword);
        if (!SemaRef.isAcceptableTagRedeclaration(TD, Kind, /*isDefinition*/false,
                                                  TagLocation, Id)) {
          SemaRef.Diag(TagLocation, diag::err_use_with_wrong_tag)
            << Id
            << FixItHint::CreateReplacement(SourceRange(TagLocation),
                                            TD->getKindName());
          SemaRef.Diag(TD->getLocation(), diag::note_previous_use);
        }
      }
    }
  
    return TreeTransform<TemplateInstantiator>::RebuildElaboratedType(KeywordLoc,
                                                                      Keyword,
                                                                    QualifierLoc,
                                                                      T);
  }
  
  TemplateName TemplateInstantiator::TransformTemplateName(
      CXXScopeSpec &SS, TemplateName Name, SourceLocation NameLoc,
      QualType ObjectType, NamedDecl *FirstQualifierInScope,
      bool AllowInjectedClassName) {
    if (TemplateTemplateParmDecl *TTP
         = dyn_cast_or_null<TemplateTemplateParmDecl>(Name.getAsTemplateDecl())) {
      if (TTP->getDepth() < TemplateArgs.getNumLevels()) {
        // If the corresponding template argument is NULL or non-existent, it's
        // because we are performing instantiation from explicitly-specified
        // template arguments in a function template, but there were some
        // arguments left unspecified.
        if (!TemplateArgs.hasTemplateArgument(TTP->getDepth(),
                                              TTP->getPosition()))
          return Name;
  
        TemplateArgument Arg = TemplateArgs(TTP->getDepth(), TTP->getPosition());
  
        if (TTP->isParameterPack()) {
          assert(Arg.getKind() == TemplateArgument::Pack &&
                 "Missing argument pack");
  
          if (getSema().ArgumentPackSubstitutionIndex == -1) {
            // We have the template argument pack to substitute, but we're not
            // actually expanding the enclosing pack expansion yet. So, just
            // keep the entire argument pack.
            return getSema().Context.getSubstTemplateTemplateParmPack(TTP, Arg);
          }
  
          Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
        }
  
        TemplateName Template = Arg.getAsTemplate().getNameToSubstitute();
        assert(!Template.isNull() && "Null template template argument");
        assert(!Template.getAsQualifiedTemplateName() &&
               "template decl to substitute is qualified?");
  
        Template = getSema().Context.getSubstTemplateTemplateParm(TTP, Template);
        return Template;
      }
    }
  
    if (SubstTemplateTemplateParmPackStorage *SubstPack
        = Name.getAsSubstTemplateTemplateParmPack()) {
      if (getSema().ArgumentPackSubstitutionIndex == -1)
        return Name;
  
      TemplateArgument Arg = SubstPack->getArgumentPack();
      Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
      return Arg.getAsTemplate().getNameToSubstitute();
    }
  
    return inherited::TransformTemplateName(SS, Name, NameLoc, ObjectType,
                                            FirstQualifierInScope,
                                            AllowInjectedClassName);
  }
  
  ExprResult
  TemplateInstantiator::TransformPredefinedExpr(PredefinedExpr *E) {
    if (!E->isTypeDependent())
      return E;
  
    if (E->getIdentKind() == PredefinedExpr::UniqueStableNameExpr) {
      EnterExpressionEvaluationContext Unevaluated(
          SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
  
      ExprResult SubExpr = getDerived().TransformExpr(E->getExpr());
      if (SubExpr.isInvalid())
        return ExprError();
  
      if (!getDerived().AlwaysRebuild() && SubExpr.get() == E->getExpr())
        return E;
  
      return getSema().BuildUniqueStableName(E->getLocation(), SubExpr.get());
    }
  
    if (E->getIdentKind() == PredefinedExpr::UniqueStableNameType) {
      TypeSourceInfo *Info = getDerived().TransformType(E->getTypeSourceInfo());
      if (!Info)
        return ExprError();
  
      if (!getDerived().AlwaysRebuild() && Info == E->getTypeSourceInfo())
        return E;
      return getSema().BuildUniqueStableName(E->getLocation(), Info);
    }
  
    return getSema().BuildPredefinedExpr(E->getLocation(), E->getIdentKind());
  }
  
  ExprResult
  TemplateInstantiator::TransformTemplateParmRefExpr(DeclRefExpr *E,
                                                 NonTypeTemplateParmDecl *NTTP) {
    // If the corresponding template argument is NULL or non-existent, it's
    // because we are performing instantiation from explicitly-specified
    // template arguments in a function template, but there were some
    // arguments left unspecified.
    if (!TemplateArgs.hasTemplateArgument(NTTP->getDepth(),
                                          NTTP->getPosition()))
      return E;
  
    TemplateArgument Arg = TemplateArgs(NTTP->getDepth(), NTTP->getPosition());
  
    if (TemplateArgs.getNumLevels() != TemplateArgs.getNumSubstitutedLevels()) {
      // We're performing a partial substitution, so the substituted argument
      // could be dependent. As a result we can't create a SubstNonType*Expr
      // node now, since that represents a fully-substituted argument.
      // FIXME: We should have some AST representation for this.
      if (Arg.getKind() == TemplateArgument::Pack) {
        // FIXME: This won't work for alias templates.
        assert(Arg.pack_size() == 1 && Arg.pack_begin()->isPackExpansion() &&
               "unexpected pack arguments in partial substitution");
        Arg = Arg.pack_begin()->getPackExpansionPattern();
      }
      assert(Arg.getKind() == TemplateArgument::Expression &&
             "unexpected nontype template argument kind in partial substitution");
      return Arg.getAsExpr();
    }
  
    if (NTTP->isParameterPack()) {
      assert(Arg.getKind() == TemplateArgument::Pack &&
             "Missing argument pack");
  
      if (getSema().ArgumentPackSubstitutionIndex == -1) {
        // We have an argument pack, but we can't select a particular argument
        // out of it yet. Therefore, we'll build an expression to hold on to that
        // argument pack.
        QualType TargetType = SemaRef.SubstType(NTTP->getType(), TemplateArgs,
                                                E->getLocation(),
                                                NTTP->getDeclName());
        if (TargetType.isNull())
          return ExprError();
  
        return new (SemaRef.Context) SubstNonTypeTemplateParmPackExpr(
            TargetType.getNonLValueExprType(SemaRef.Context),
            TargetType->isReferenceType() ? VK_LValue : VK_RValue, NTTP,
            E->getLocation(), Arg);
      }
  
      Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
    }
  
    return transformNonTypeTemplateParmRef(NTTP, E->getLocation(), Arg);
  }
  
  const LoopHintAttr *
  TemplateInstantiator::TransformLoopHintAttr(const LoopHintAttr *LH) {
    Expr *TransformedExpr = getDerived().TransformExpr(LH->getValue()).get();
  
    if (TransformedExpr == LH->getValue())
      return LH;
  
    // Generate error if there is a problem with the value.
    if (getSema().CheckLoopHintExpr(TransformedExpr, LH->getLocation()))
      return LH;
  
    // Create new LoopHintValueAttr with integral expression in place of the
    // non-type template parameter.
    return LoopHintAttr::CreateImplicit(
        getSema().Context, LH->getSemanticSpelling(), LH->getOption(),
        LH->getState(), TransformedExpr, LH->getRange());
  }
  
  ExprResult TemplateInstantiator::transformNonTypeTemplateParmRef(
                                                   NonTypeTemplateParmDecl *parm,
                                                   SourceLocation loc,
                                                   TemplateArgument arg) {
    ExprResult result;
    QualType type;
  
    // The template argument itself might be an expression, in which
    // case we just return that expression.
    if (arg.getKind() == TemplateArgument::Expression) {
      Expr *argExpr = arg.getAsExpr();
      result = argExpr;
      type = argExpr->getType();
  
    } else if (arg.getKind() == TemplateArgument::Declaration ||
               arg.getKind() == TemplateArgument::NullPtr) {
      ValueDecl *VD;
      if (arg.getKind() == TemplateArgument::Declaration) {
        VD = arg.getAsDecl();
  
        // Find the instantiation of the template argument.  This is
        // required for nested templates.
        VD = cast_or_null<ValueDecl>(
               getSema().FindInstantiatedDecl(loc, VD, TemplateArgs));
        if (!VD)
          return ExprError();
      } else {
        // Propagate NULL template argument.
        VD = nullptr;
      }
  
      // Derive the type we want the substituted decl to have.  This had
      // better be non-dependent, or these checks will have serious problems.
      if (parm->isExpandedParameterPack()) {
        type = parm->getExpansionType(SemaRef.ArgumentPackSubstitutionIndex);
      } else if (parm->isParameterPack() &&
                 isa<PackExpansionType>(parm->getType())) {
        type = SemaRef.SubstType(
                          cast<PackExpansionType>(parm->getType())->getPattern(),
                                       TemplateArgs, loc, parm->getDeclName());
      } else {
        type = SemaRef.SubstType(VD ? arg.getParamTypeForDecl() : arg.getNullPtrType(),
                                 TemplateArgs, loc, parm->getDeclName());
      }
      assert(!type.isNull() && "type substitution failed for param type");
      assert(!type->isDependentType() && "param type still dependent");
      result = SemaRef.BuildExpressionFromDeclTemplateArgument(arg, type, loc);
  
      if (!result.isInvalid()) type = result.get()->getType();
    } else {
      result = SemaRef.BuildExpressionFromIntegralTemplateArgument(arg, loc);
  
      // Note that this type can be different from the type of 'result',
      // e.g. if it's an enum type.
      type = arg.getIntegralType();
    }
    if (result.isInvalid()) return ExprError();
  
    Expr *resultExpr = result.get();
    return new (SemaRef.Context) SubstNonTypeTemplateParmExpr(
        type, resultExpr->getValueKind(), loc, parm, resultExpr);
  }
  
  ExprResult
  TemplateInstantiator::TransformSubstNonTypeTemplateParmPackExpr(
                                            SubstNonTypeTemplateParmPackExpr *E) {
    if (getSema().ArgumentPackSubstitutionIndex == -1) {
      // We aren't expanding the parameter pack, so just return ourselves.
      return E;
    }
  
    TemplateArgument Arg = E->getArgumentPack();
    Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
    return transformNonTypeTemplateParmRef(E->getParameterPack(),
                                           E->getParameterPackLocation(),
                                           Arg);
  }
  
  ExprResult TemplateInstantiator::RebuildVarDeclRefExpr(VarDecl *PD,
                                                         SourceLocation Loc) {
    DeclarationNameInfo NameInfo(PD->getDeclName(), Loc);
    return getSema().BuildDeclarationNameExpr(CXXScopeSpec(), NameInfo, PD);
  }
  
  ExprResult
  TemplateInstantiator::TransformFunctionParmPackExpr(FunctionParmPackExpr *E) {
    if (getSema().ArgumentPackSubstitutionIndex != -1) {
      // We can expand this parameter pack now.
      VarDecl *D = E->getExpansion(getSema().ArgumentPackSubstitutionIndex);
      VarDecl *VD = cast_or_null<VarDecl>(TransformDecl(E->getExprLoc(), D));
      if (!VD)
        return ExprError();
      return RebuildVarDeclRefExpr(VD, E->getExprLoc());
    }
  
    QualType T = TransformType(E->getType());
    if (T.isNull())
      return ExprError();
  
    // Transform each of the parameter expansions into the corresponding
    // parameters in the instantiation of the function decl.
    SmallVector<VarDecl *, 8> Vars;
    Vars.reserve(E->getNumExpansions());
    for (FunctionParmPackExpr::iterator I = E->begin(), End = E->end();
         I != End; ++I) {
      VarDecl *D = cast_or_null<VarDecl>(TransformDecl(E->getExprLoc(), *I));
      if (!D)
        return ExprError();
      Vars.push_back(D);
    }
  
    auto *PackExpr =
        FunctionParmPackExpr::Create(getSema().Context, T, E->getParameterPack(),
                                     E->getParameterPackLocation(), Vars);
    getSema().MarkFunctionParmPackReferenced(PackExpr);
    return PackExpr;
  }
  
  ExprResult
  TemplateInstantiator::TransformFunctionParmPackRefExpr(DeclRefExpr *E,
                                                         VarDecl *PD) {
    typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
    llvm::PointerUnion<Decl *, DeclArgumentPack *> *Found
      = getSema().CurrentInstantiationScope->findInstantiationOf(PD);
    assert(Found && "no instantiation for parameter pack");
  
    Decl *TransformedDecl;
    if (DeclArgumentPack *Pack = Found->dyn_cast<DeclArgumentPack *>()) {
      // If this is a reference to a function parameter pack which we can
      // substitute but can't yet expand, build a FunctionParmPackExpr for it.
      if (getSema().ArgumentPackSubstitutionIndex == -1) {
        QualType T = TransformType(E->getType());
        if (T.isNull())
          return ExprError();
        auto *PackExpr = FunctionParmPackExpr::Create(getSema().Context, T, PD,
                                                      E->getExprLoc(), *Pack);
        getSema().MarkFunctionParmPackReferenced(PackExpr);
        return PackExpr;
      }
  
      TransformedDecl = (*Pack)[getSema().ArgumentPackSubstitutionIndex];
    } else {
      TransformedDecl = Found->get<Decl*>();
    }
  
    // We have either an unexpanded pack or a specific expansion.
    return RebuildVarDeclRefExpr(cast<VarDecl>(TransformedDecl), E->getExprLoc());
  }
  
  ExprResult
  TemplateInstantiator::TransformDeclRefExpr(DeclRefExpr *E) {
    NamedDecl *D = E->getDecl();
  
    // Handle references to non-type template parameters and non-type template
    // parameter packs.
    if (NonTypeTemplateParmDecl *NTTP = dyn_cast<NonTypeTemplateParmDecl>(D)) {
      if (NTTP->getDepth() < TemplateArgs.getNumLevels())
        return TransformTemplateParmRefExpr(E, NTTP);
  
      // We have a non-type template parameter that isn't fully substituted;
      // FindInstantiatedDecl will find it in the local instantiation scope.
    }
  
    // Handle references to function parameter packs.
    if (VarDecl *PD = dyn_cast<VarDecl>(D))
      if (PD->isParameterPack())
        return TransformFunctionParmPackRefExpr(E, PD);
  
    return TreeTransform<TemplateInstantiator>::TransformDeclRefExpr(E);
  }
  
  ExprResult TemplateInstantiator::TransformCXXDefaultArgExpr(
      CXXDefaultArgExpr *E) {
    assert(!cast<FunctionDecl>(E->getParam()->getDeclContext())->
               getDescribedFunctionTemplate() &&
           "Default arg expressions are never formed in dependent cases.");
    return SemaRef.BuildCXXDefaultArgExpr(E->getUsedLocation(),
                             cast<FunctionDecl>(E->getParam()->getDeclContext()),
                                          E->getParam());
  }
  
  template<typename Fn>
  QualType TemplateInstantiator::TransformFunctionProtoType(TypeLocBuilder &TLB,
                                   FunctionProtoTypeLoc TL,
                                   CXXRecordDecl *ThisContext,
                                   Qualifiers ThisTypeQuals,
                                   Fn TransformExceptionSpec) {
    // We need a local instantiation scope for this function prototype.
    LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
    return inherited::TransformFunctionProtoType(
        TLB, TL, ThisContext, ThisTypeQuals, TransformExceptionSpec);
  }
  
  ParmVarDecl *
  TemplateInstantiator::TransformFunctionTypeParam(ParmVarDecl *OldParm,
                                                   int indexAdjustment,
                                                 Optional<unsigned> NumExpansions,
                                                   bool ExpectParameterPack) {
    return SemaRef.SubstParmVarDecl(OldParm, TemplateArgs, indexAdjustment,
                                    NumExpansions, ExpectParameterPack);
  }
  
  QualType
  TemplateInstantiator::TransformTemplateTypeParmType(TypeLocBuilder &TLB,
                                                  TemplateTypeParmTypeLoc TL) {
    const TemplateTypeParmType *T = TL.getTypePtr();
    if (T->getDepth() < TemplateArgs.getNumLevels()) {
      // Replace the template type parameter with its corresponding
      // template argument.
  
      // If the corresponding template argument is NULL or doesn't exist, it's
      // because we are performing instantiation from explicitly-specified
      // template arguments in a function template class, but there were some
      // arguments left unspecified.
      if (!TemplateArgs.hasTemplateArgument(T->getDepth(), T->getIndex())) {
        TemplateTypeParmTypeLoc NewTL
          = TLB.push<TemplateTypeParmTypeLoc>(TL.getType());
        NewTL.setNameLoc(TL.getNameLoc());
        return TL.getType();
      }
  
      TemplateArgument Arg = TemplateArgs(T->getDepth(), T->getIndex());
  
      if (T->isParameterPack()) {
        assert(Arg.getKind() == TemplateArgument::Pack &&
               "Missing argument pack");
  
        if (getSema().ArgumentPackSubstitutionIndex == -1) {
          // We have the template argument pack, but we're not expanding the
          // enclosing pack expansion yet. Just save the template argument
          // pack for later substitution.
          QualType Result
            = getSema().Context.getSubstTemplateTypeParmPackType(T, Arg);
          SubstTemplateTypeParmPackTypeLoc NewTL
            = TLB.push<SubstTemplateTypeParmPackTypeLoc>(Result);
          NewTL.setNameLoc(TL.getNameLoc());
          return Result;
        }
  
        Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
      }
  
      assert(Arg.getKind() == TemplateArgument::Type &&
             "Template argument kind mismatch");
  
      QualType Replacement = Arg.getAsType();
  
      // TODO: only do this uniquing once, at the start of instantiation.
      QualType Result
        = getSema().Context.getSubstTemplateTypeParmType(T, Replacement);
      SubstTemplateTypeParmTypeLoc NewTL
        = TLB.push<SubstTemplateTypeParmTypeLoc>(Result);
      NewTL.setNameLoc(TL.getNameLoc());
      return Result;
    }
  
    // The template type parameter comes from an inner template (e.g.,
    // the template parameter list of a member template inside the
    // template we are instantiating). Create a new template type
    // parameter with the template "level" reduced by one.
    TemplateTypeParmDecl *NewTTPDecl = nullptr;
    if (TemplateTypeParmDecl *OldTTPDecl = T->getDecl())
      NewTTPDecl = cast_or_null<TemplateTypeParmDecl>(
                                    TransformDecl(TL.getNameLoc(), OldTTPDecl));
  
    QualType Result = getSema().Context.getTemplateTypeParmType(
        T->getDepth() - TemplateArgs.getNumSubstitutedLevels(), T->getIndex(),
        T->isParameterPack(), NewTTPDecl);
    TemplateTypeParmTypeLoc NewTL = TLB.push<TemplateTypeParmTypeLoc>(Result);
    NewTL.setNameLoc(TL.getNameLoc());
    return Result;
  }
  
  QualType
  TemplateInstantiator::TransformSubstTemplateTypeParmPackType(
                                                              TypeLocBuilder &TLB,
                                           SubstTemplateTypeParmPackTypeLoc TL) {
    if (getSema().ArgumentPackSubstitutionIndex == -1) {
      // We aren't expanding the parameter pack, so just return ourselves.
      SubstTemplateTypeParmPackTypeLoc NewTL
        = TLB.push<SubstTemplateTypeParmPackTypeLoc>(TL.getType());
      NewTL.setNameLoc(TL.getNameLoc());
      return TL.getType();
    }
  
    TemplateArgument Arg = TL.getTypePtr()->getArgumentPack();
    Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
    QualType Result = Arg.getAsType();
  
    Result = getSema().Context.getSubstTemplateTypeParmType(
                                        TL.getTypePtr()->getReplacedParameter(),
                                                            Result);
    SubstTemplateTypeParmTypeLoc NewTL
      = TLB.push<SubstTemplateTypeParmTypeLoc>(Result);
    NewTL.setNameLoc(TL.getNameLoc());
    return Result;
  }
  
  /// Perform substitution on the type T with a given set of template
  /// arguments.
  ///
  /// This routine substitutes the given template arguments into the
  /// type T and produces the instantiated type.
  ///
  /// \param T the type into which the template arguments will be
  /// substituted. If this type is not dependent, it will be returned
  /// immediately.
  ///
  /// \param Args the template arguments that will be
  /// substituted for the top-level template parameters within T.
  ///
  /// \param Loc the location in the source code where this substitution
  /// is being performed. It will typically be the location of the
  /// declarator (if we're instantiating the type of some declaration)
  /// or the location of the type in the source code (if, e.g., we're
  /// instantiating the type of a cast expression).
  ///
  /// \param Entity the name of the entity associated with a declaration
  /// being instantiated (if any). May be empty to indicate that there
  /// is no such entity (if, e.g., this is a type that occurs as part of
  /// a cast expression) or that the entity has no name (e.g., an
  /// unnamed function parameter).
  ///
  /// \param AllowDeducedTST Whether a DeducedTemplateSpecializationType is
  /// acceptable as the top level type of the result.
  ///
  /// \returns If the instantiation succeeds, the instantiated
  /// type. Otherwise, produces diagnostics and returns a NULL type.
  TypeSourceInfo *Sema::SubstType(TypeSourceInfo *T,
                                  const MultiLevelTemplateArgumentList &Args,
                                  SourceLocation Loc,
                                  DeclarationName Entity,
                                  bool AllowDeducedTST) {
    assert(!CodeSynthesisContexts.empty() &&
           "Cannot perform an instantiation without some context on the "
           "instantiation stack");
  
    if (!T->getType()->isInstantiationDependentType() &&
        !T->getType()->isVariablyModifiedType())
      return T;
  
    TemplateInstantiator Instantiator(*this, Args, Loc, Entity);
    return AllowDeducedTST ? Instantiator.TransformTypeWithDeducedTST(T)
                           : Instantiator.TransformType(T);
  }
  
  TypeSourceInfo *Sema::SubstType(TypeLoc TL,
                                  const MultiLevelTemplateArgumentList &Args,
                                  SourceLocation Loc,
                                  DeclarationName Entity) {
    assert(!CodeSynthesisContexts.empty() &&
           "Cannot perform an instantiation without some context on the "
           "instantiation stack");
  
    if (TL.getType().isNull())
      return nullptr;
  
    if (!TL.getType()->isInstantiationDependentType() &&
        !TL.getType()->isVariablyModifiedType()) {
      // FIXME: Make a copy of the TypeLoc data here, so that we can
      // return a new TypeSourceInfo. Inefficient!
      TypeLocBuilder TLB;
      TLB.pushFullCopy(TL);
      return TLB.getTypeSourceInfo(Context, TL.getType());
    }
  
    TemplateInstantiator Instantiator(*this, Args, Loc, Entity);
    TypeLocBuilder TLB;
    TLB.reserve(TL.getFullDataSize());
    QualType Result = Instantiator.TransformType(TLB, TL);
    if (Result.isNull())
      return nullptr;
  
    return TLB.getTypeSourceInfo(Context, Result);
  }
  
  /// Deprecated form of the above.
  QualType Sema::SubstType(QualType T,
                           const MultiLevelTemplateArgumentList &TemplateArgs,
                           SourceLocation Loc, DeclarationName Entity) {
    assert(!CodeSynthesisContexts.empty() &&
           "Cannot perform an instantiation without some context on the "
           "instantiation stack");
  
    // If T is not a dependent type or a variably-modified type, there
    // is nothing to do.
    if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType())
      return T;
  
    TemplateInstantiator Instantiator(*this, TemplateArgs, Loc, Entity);
    return Instantiator.TransformType(T);
  }
  
  static bool NeedsInstantiationAsFunctionType(TypeSourceInfo *T) {
    if (T->getType()->isInstantiationDependentType() ||
        T->getType()->isVariablyModifiedType())
      return true;
  
    TypeLoc TL = T->getTypeLoc().IgnoreParens();
    if (!TL.getAs<FunctionProtoTypeLoc>())
      return false;
  
    FunctionProtoTypeLoc FP = TL.castAs<FunctionProtoTypeLoc>();
    for (ParmVarDecl *P : FP.getParams()) {
      // This must be synthesized from a typedef.
      if (!P) continue;
  
      // If there are any parameters, a new TypeSourceInfo that refers to the
      // instantiated parameters must be built.
      return true;
    }
  
    return false;
  }
  
  /// A form of SubstType intended specifically for instantiating the
  /// type of a FunctionDecl.  Its purpose is solely to force the
  /// instantiation of default-argument expressions and to avoid
  /// instantiating an exception-specification.
  TypeSourceInfo *Sema::SubstFunctionDeclType(TypeSourceInfo *T,
                                  const MultiLevelTemplateArgumentList &Args,
                                  SourceLocation Loc,
                                  DeclarationName Entity,
                                  CXXRecordDecl *ThisContext,
                                  Qualifiers ThisTypeQuals) {
    assert(!CodeSynthesisContexts.empty() &&
           "Cannot perform an instantiation without some context on the "
           "instantiation stack");
  
    if (!NeedsInstantiationAsFunctionType(T))
      return T;
  
    TemplateInstantiator Instantiator(*this, Args, Loc, Entity);
  
    TypeLocBuilder TLB;
  
    TypeLoc TL = T->getTypeLoc();
    TLB.reserve(TL.getFullDataSize());
  
    QualType Result;
  
    if (FunctionProtoTypeLoc Proto =
            TL.IgnoreParens().getAs<FunctionProtoTypeLoc>()) {
      // Instantiate the type, other than its exception specification. The
      // exception specification is instantiated in InitFunctionInstantiation
      // once we've built the FunctionDecl.
      // FIXME: Set the exception specification to EST_Uninstantiated here,
      // instead of rebuilding the function type again later.
      Result = Instantiator.TransformFunctionProtoType(
          TLB, Proto, ThisContext, ThisTypeQuals,
          [](FunctionProtoType::ExceptionSpecInfo &ESI,
             bool &Changed) { return false; });
    } else {
      Result = Instantiator.TransformType(TLB, TL);
    }
    if (Result.isNull())
      return nullptr;
  
    return TLB.getTypeSourceInfo(Context, Result);
  }
  
  bool Sema::SubstExceptionSpec(SourceLocation Loc,
                                FunctionProtoType::ExceptionSpecInfo &ESI,
                                SmallVectorImpl<QualType> &ExceptionStorage,
                                const MultiLevelTemplateArgumentList &Args) {
    assert(ESI.Type != EST_Uninstantiated);
  
    bool Changed = false;
    TemplateInstantiator Instantiator(*this, Args, Loc, DeclarationName());
    return Instantiator.TransformExceptionSpec(Loc, ESI, ExceptionStorage,
                                               Changed);
  }
  
  void Sema::SubstExceptionSpec(FunctionDecl *New, const FunctionProtoType *Proto,
                                const MultiLevelTemplateArgumentList &Args) {
    FunctionProtoType::ExceptionSpecInfo ESI =
        Proto->getExtProtoInfo().ExceptionSpec;
  
    SmallVector<QualType, 4> ExceptionStorage;
    if (SubstExceptionSpec(New->getTypeSourceInfo()->getTypeLoc().getEndLoc(),
                           ESI, ExceptionStorage, Args))
      // On error, recover by dropping the exception specification.
      ESI.Type = EST_None;
  
    UpdateExceptionSpec(New, ESI);
  }
  
  ParmVarDecl *Sema::SubstParmVarDecl(ParmVarDecl *OldParm,
                              const MultiLevelTemplateArgumentList &TemplateArgs,
                                      int indexAdjustment,
                                      Optional<unsigned> NumExpansions,
                                      bool ExpectParameterPack) {
    TypeSourceInfo *OldDI = OldParm->getTypeSourceInfo();
    TypeSourceInfo *NewDI = nullptr;
  
    TypeLoc OldTL = OldDI->getTypeLoc();
    if (PackExpansionTypeLoc ExpansionTL = OldTL.getAs<PackExpansionTypeLoc>()) {
  
      // We have a function parameter pack. Substitute into the pattern of the
      // expansion.
      NewDI = SubstType(ExpansionTL.getPatternLoc(), TemplateArgs,
                        OldParm->getLocation(), OldParm->getDeclName());
      if (!NewDI)
        return nullptr;
  
      if (NewDI->getType()->containsUnexpandedParameterPack()) {
        // We still have unexpanded parameter packs, which means that
        // our function parameter is still a function parameter pack.
        // Therefore, make its type a pack expansion type.
        NewDI = CheckPackExpansion(NewDI, ExpansionTL.getEllipsisLoc(),
                                   NumExpansions);
      } else if (ExpectParameterPack) {
        // We expected to get a parameter pack but didn't (because the type
        // itself is not a pack expansion type), so complain. This can occur when
        // the substitution goes through an alias template that "loses" the
        // pack expansion.
        Diag(OldParm->getLocation(),
             diag::err_function_parameter_pack_without_parameter_packs)
          << NewDI->getType();
        return nullptr;
      }
    } else {
      NewDI = SubstType(OldDI, TemplateArgs, OldParm->getLocation(),
                        OldParm->getDeclName());
    }
  
    if (!NewDI)
      return nullptr;
  
    if (NewDI->getType()->isVoidType()) {
      Diag(OldParm->getLocation(), diag::err_param_with_void_type);
      return nullptr;
    }
  
    ParmVarDecl *NewParm = CheckParameter(Context.getTranslationUnitDecl(),
                                          OldParm->getInnerLocStart(),
                                          OldParm->getLocation(),
                                          OldParm->getIdentifier(),
                                          NewDI->getType(), NewDI,
                                          OldParm->getStorageClass());
    if (!NewParm)
      return nullptr;
  
    // Mark the (new) default argument as uninstantiated (if any).
    if (OldParm->hasUninstantiatedDefaultArg()) {
      Expr *Arg = OldParm->getUninstantiatedDefaultArg();
      NewParm->setUninstantiatedDefaultArg(Arg);
    } else if (OldParm->hasUnparsedDefaultArg()) {
      NewParm->setUnparsedDefaultArg();
      UnparsedDefaultArgInstantiations[OldParm].push_back(NewParm);
    } else if (Expr *Arg = OldParm->getDefaultArg()) {
      FunctionDecl *OwningFunc = cast<FunctionDecl>(OldParm->getDeclContext());
      if (OwningFunc->isLexicallyWithinFunctionOrMethod()) {
        // Instantiate default arguments for methods of local classes (DR1484)
        // and non-defining declarations.
        Sema::ContextRAII SavedContext(*this, OwningFunc);
        LocalInstantiationScope Local(*this, true);
        ExprResult NewArg = SubstExpr(Arg, TemplateArgs);
        if (NewArg.isUsable()) {
          // It would be nice if we still had this.
          SourceLocation EqualLoc = NewArg.get()->getBeginLoc();
          SetParamDefaultArgument(NewParm, NewArg.get(), EqualLoc);
        }
      } else {
        // FIXME: if we non-lazily instantiated non-dependent default args for
        // non-dependent parameter types we could remove a bunch of duplicate
        // conversion warnings for such arguments.
        NewParm->setUninstantiatedDefaultArg(Arg);
      }
    }
  
    NewParm->setHasInheritedDefaultArg(OldParm->hasInheritedDefaultArg());
  
    if (OldParm->isParameterPack() && !NewParm->isParameterPack()) {
      // Add the new parameter to the instantiated parameter pack.
      CurrentInstantiationScope->InstantiatedLocalPackArg(OldParm, NewParm);
    } else {
      // Introduce an Old -> New mapping
      CurrentInstantiationScope->InstantiatedLocal(OldParm, NewParm);
    }
  
    // FIXME: OldParm may come from a FunctionProtoType, in which case CurContext
    // can be anything, is this right ?
    NewParm->setDeclContext(CurContext);
  
    NewParm->setScopeInfo(OldParm->getFunctionScopeDepth(),
                          OldParm->getFunctionScopeIndex() + indexAdjustment);
  
    InstantiateAttrs(TemplateArgs, OldParm, NewParm);
  
    return NewParm;
  }
  
  /// Substitute the given template arguments into the given set of
  /// parameters, producing the set of parameter types that would be generated
  /// from such a substitution.
  bool Sema::SubstParmTypes(
      SourceLocation Loc, ArrayRef<ParmVarDecl *> Params,
      const FunctionProtoType::ExtParameterInfo *ExtParamInfos,
      const MultiLevelTemplateArgumentList &TemplateArgs,
      SmallVectorImpl<QualType> &ParamTypes,
      SmallVectorImpl<ParmVarDecl *> *OutParams,
      ExtParameterInfoBuilder &ParamInfos) {
    assert(!CodeSynthesisContexts.empty() &&
           "Cannot perform an instantiation without some context on the "
           "instantiation stack");
  
    TemplateInstantiator Instantiator(*this, TemplateArgs, Loc,
                                      DeclarationName());
    return Instantiator.TransformFunctionTypeParams(
        Loc, Params, nullptr, ExtParamInfos, ParamTypes, OutParams, ParamInfos);
  }
  
  /// Perform substitution on the base class specifiers of the
  /// given class template specialization.
  ///
  /// Produces a diagnostic and returns true on error, returns false and
  /// attaches the instantiated base classes to the class template
  /// specialization if successful.
  bool
  Sema::SubstBaseSpecifiers(CXXRecordDecl *Instantiation,
                            CXXRecordDecl *Pattern,
                            const MultiLevelTemplateArgumentList &TemplateArgs) {
    bool Invalid = false;
    SmallVector<CXXBaseSpecifier*, 4> InstantiatedBases;
    for (const auto &Base : Pattern->bases()) {
      if (!Base.getType()->isDependentType()) {
        if (const CXXRecordDecl *RD = Base.getType()->getAsCXXRecordDecl()) {
          if (RD->isInvalidDecl())
            Instantiation->setInvalidDecl();
        }
        InstantiatedBases.push_back(new (Context) CXXBaseSpecifier(Base));
        continue;
      }
  
      SourceLocation EllipsisLoc;
      TypeSourceInfo *BaseTypeLoc;
      if (Base.isPackExpansion()) {
        // This is a pack expansion. See whether we should expand it now, or
        // wait until later.
        SmallVector<UnexpandedParameterPack, 2> Unexpanded;
        collectUnexpandedParameterPacks(Base.getTypeSourceInfo()->getTypeLoc(),
                                        Unexpanded);
        bool ShouldExpand = false;
        bool RetainExpansion = false;
        Optional<unsigned> NumExpansions;
        if (CheckParameterPacksForExpansion(Base.getEllipsisLoc(),
                                            Base.getSourceRange(),
                                            Unexpanded,
                                            TemplateArgs, ShouldExpand,
                                            RetainExpansion,
                                            NumExpansions)) {
          Invalid = true;
          continue;
        }
  
        // If we should expand this pack expansion now, do so.
        if (ShouldExpand) {
          for (unsigned I = 0; I != *NumExpansions; ++I) {
              Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this, I);
  
            TypeSourceInfo *BaseTypeLoc = SubstType(Base.getTypeSourceInfo(),
                                                    TemplateArgs,
                                                Base.getSourceRange().getBegin(),
                                                    DeclarationName());
            if (!BaseTypeLoc) {
              Invalid = true;
              continue;
            }
  
            if (CXXBaseSpecifier *InstantiatedBase
                  = CheckBaseSpecifier(Instantiation,
                                       Base.getSourceRange(),
                                       Base.isVirtual(),
                                       Base.getAccessSpecifierAsWritten(),
                                       BaseTypeLoc,
                                       SourceLocation()))
              InstantiatedBases.push_back(InstantiatedBase);
            else
              Invalid = true;
          }
  
          continue;
        }
  
        // The resulting base specifier will (still) be a pack expansion.
        EllipsisLoc = Base.getEllipsisLoc();
        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this, -1);
        BaseTypeLoc = SubstType(Base.getTypeSourceInfo(),
                                TemplateArgs,
                                Base.getSourceRange().getBegin(),
                                DeclarationName());
      } else {
        BaseTypeLoc = SubstType(Base.getTypeSourceInfo(),
                                TemplateArgs,
                                Base.getSourceRange().getBegin(),
                                DeclarationName());
      }
  
      if (!BaseTypeLoc) {
        Invalid = true;
        continue;
      }
  
      if (CXXBaseSpecifier *InstantiatedBase
            = CheckBaseSpecifier(Instantiation,
                                 Base.getSourceRange(),
                                 Base.isVirtual(),
                                 Base.getAccessSpecifierAsWritten(),
                                 BaseTypeLoc,
                                 EllipsisLoc))
        InstantiatedBases.push_back(InstantiatedBase);
      else
        Invalid = true;
    }
  
    if (!Invalid && AttachBaseSpecifiers(Instantiation, InstantiatedBases))
      Invalid = true;
  
    return Invalid;
  }
  
  // Defined via #include from SemaTemplateInstantiateDecl.cpp
  namespace clang {
    namespace sema {
      Attr *instantiateTemplateAttribute(const Attr *At, ASTContext &C, Sema &S,
                              const MultiLevelTemplateArgumentList &TemplateArgs);
      Attr *instantiateTemplateAttributeForDecl(
          const Attr *At, ASTContext &C, Sema &S,
          const MultiLevelTemplateArgumentList &TemplateArgs);
    }
  }
  
  /// Instantiate the definition of a class from a given pattern.
  ///
  /// \param PointOfInstantiation The point of instantiation within the
  /// source code.
  ///
  /// \param Instantiation is the declaration whose definition is being
  /// instantiated. This will be either a class template specialization
  /// or a member class of a class template specialization.
  ///
  /// \param Pattern is the pattern from which the instantiation
  /// occurs. This will be either the declaration of a class template or
  /// the declaration of a member class of a class template.
  ///
  /// \param TemplateArgs The template arguments to be substituted into
  /// the pattern.
  ///
  /// \param TSK the kind of implicit or explicit instantiation to perform.
  ///
  /// \param Complain whether to complain if the class cannot be instantiated due
  /// to the lack of a definition.
  ///
  /// \returns true if an error occurred, false otherwise.
  bool
  Sema::InstantiateClass(SourceLocation PointOfInstantiation,
                         CXXRecordDecl *Instantiation, CXXRecordDecl *Pattern,
                         const MultiLevelTemplateArgumentList &TemplateArgs,
                         TemplateSpecializationKind TSK,
                         bool Complain) {
    CXXRecordDecl *PatternDef
      = cast_or_null<CXXRecordDecl>(Pattern->getDefinition());
    if (DiagnoseUninstantiableTemplate(PointOfInstantiation, Instantiation,
                                  Instantiation->getInstantiatedFromMemberClass(),
                                       Pattern, PatternDef, TSK, Complain))
      return true;
  
    llvm::TimeTraceScope TimeScope("InstantiateClass", [&]() {
      std::string Name;
      llvm::raw_string_ostream OS(Name);
      Instantiation->getNameForDiagnostic(OS, getPrintingPolicy(),
                                          /*Qualified=*/true);
      return Name;
    });
  
    Pattern = PatternDef;
  
    // Record the point of instantiation.
    if (MemberSpecializationInfo *MSInfo
          = Instantiation->getMemberSpecializationInfo()) {
      MSInfo->setTemplateSpecializationKind(TSK);
      MSInfo->setPointOfInstantiation(PointOfInstantiation);
    } else if (ClassTemplateSpecializationDecl *Spec
          = dyn_cast<ClassTemplateSpecializationDecl>(Instantiation)) {
      Spec->setTemplateSpecializationKind(TSK);
      Spec->setPointOfInstantiation(PointOfInstantiation);
    }
  
    InstantiatingTemplate Inst(*this, PointOfInstantiation, Instantiation);
    if (Inst.isInvalid())
      return true;
    assert(!Inst.isAlreadyInstantiating() && "should have been caught by caller");
    PrettyDeclStackTraceEntry CrashInfo(Context, Instantiation, SourceLocation(),
                                        "instantiating class definition");
  
    // Enter the scope of this instantiation. We don't use
    // PushDeclContext because we don't have a scope.
    ContextRAII SavedContext(*this, Instantiation);
    EnterExpressionEvaluationContext EvalContext(
        *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
  
    // If this is an instantiation of a local class, merge this local
    // instantiation scope with the enclosing scope. Otherwise, every
    // instantiation of a class has its own local instantiation scope.
    bool MergeWithParentScope = !Instantiation->isDefinedOutsideFunctionOrMethod();
    LocalInstantiationScope Scope(*this, MergeWithParentScope);
  
    // Some class state isn't processed immediately but delayed till class
    // instantiation completes. We may not be ready to handle any delayed state
    // already on the stack as it might correspond to a different class, so save
    // it now and put it back later.
    SavePendingParsedClassStateRAII SavedPendingParsedClassState(*this);
  
    // Pull attributes from the pattern onto the instantiation.
    InstantiateAttrs(TemplateArgs, Pattern, Instantiation);
  
    // Start the definition of this instantiation.
    Instantiation->startDefinition();
  
    // The instantiation is visible here, even if it was first declared in an
    // unimported module.
    Instantiation->setVisibleDespiteOwningModule();
  
    // FIXME: This loses the as-written tag kind for an explicit instantiation.
    Instantiation->setTagKind(Pattern->getTagKind());
  
    // Do substitution on the base class specifiers.
    if (SubstBaseSpecifiers(Instantiation, Pattern, TemplateArgs))
      Instantiation->setInvalidDecl();
  
    TemplateDeclInstantiator Instantiator(*this, Instantiation, TemplateArgs);
    SmallVector<Decl*, 4> Fields;
    // Delay instantiation of late parsed attributes.
    LateInstantiatedAttrVec LateAttrs;
    Instantiator.enableLateAttributeInstantiation(&LateAttrs);
  
    bool MightHaveConstexprVirtualFunctions = false;
    for (auto *Member : Pattern->decls()) {
      // Don't instantiate members not belonging in this semantic context.
      // e.g. for:
      // @code
      //    template <int i> class A {
      //      class B *g;
      //    };
      // @endcode
      // 'class B' has the template as lexical context but semantically it is
      // introduced in namespace scope.
      if (Member->getDeclContext() != Pattern)
        continue;
  
      // BlockDecls can appear in a default-member-initializer. They must be the
      // child of a BlockExpr, so we only know how to instantiate them from there.
      if (isa<BlockDecl>(Member))
        continue;
  
      if (Member->isInvalidDecl()) {
        Instantiation->setInvalidDecl();
        continue;
      }
  
      Decl *NewMember = Instantiator.Visit(Member);
      if (NewMember) {
        if (FieldDecl *Field = dyn_cast<FieldDecl>(NewMember)) {
          Fields.push_back(Field);
        } else if (EnumDecl *Enum = dyn_cast<EnumDecl>(NewMember)) {
          // C++11 [temp.inst]p1: The implicit instantiation of a class template
          // specialization causes the implicit instantiation of the definitions
          // of unscoped member enumerations.
          // Record a point of instantiation for this implicit instantiation.
          if (TSK == TSK_ImplicitInstantiation && !Enum->isScoped() &&
              Enum->isCompleteDefinition()) {
            MemberSpecializationInfo *MSInfo =Enum->getMemberSpecializationInfo();
            assert(MSInfo && "no spec info for member enum specialization");
            MSInfo->setTemplateSpecializationKind(TSK_ImplicitInstantiation);
            MSInfo->setPointOfInstantiation(PointOfInstantiation);
          }
        } else if (StaticAssertDecl *SA = dyn_cast<StaticAssertDecl>(NewMember)) {
          if (SA->isFailed()) {
            // A static_assert failed. Bail out; instantiating this
            // class is probably not meaningful.
            Instantiation->setInvalidDecl();
            break;
          }
        } else if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewMember)) {
          if (MD->isConstexpr() && !MD->getFriendObjectKind() &&
              (MD->isVirtualAsWritten() || Instantiation->getNumBases()))
            MightHaveConstexprVirtualFunctions = true;
        }
  
        if (NewMember->isInvalidDecl())
          Instantiation->setInvalidDecl();
      } else {
        // FIXME: Eventually, a NULL return will mean that one of the
        // instantiations was a semantic disaster, and we'll want to mark the
        // declaration invalid.
        // For now, we expect to skip some members that we can't yet handle.
      }
    }
  
    // Finish checking fields.
    ActOnFields(nullptr, Instantiation->getLocation(), Instantiation, Fields,
                SourceLocation(), SourceLocation(), ParsedAttributesView());
    CheckCompletedCXXClass(Instantiation);
  
    // Default arguments are parsed, if not instantiated. We can go instantiate
    // default arg exprs for default constructors if necessary now.
    ActOnFinishCXXNonNestedClass(Instantiation);
  
    // Instantiate late parsed attributes, and attach them to their decls.
    // See Sema::InstantiateAttrs
    for (LateInstantiatedAttrVec::iterator I = LateAttrs.begin(),
         E = LateAttrs.end(); I != E; ++I) {
      assert(CurrentInstantiationScope == Instantiator.getStartingScope());
      CurrentInstantiationScope = I->Scope;
  
      // Allow 'this' within late-parsed attributes.
      NamedDecl *ND = dyn_cast<NamedDecl>(I->NewDecl);
      CXXRecordDecl *ThisContext =
          dyn_cast_or_null<CXXRecordDecl>(ND->getDeclContext());
      CXXThisScopeRAII ThisScope(*this, ThisContext, Qualifiers(),
                                 ND && ND->isCXXInstanceMember());
  
      Attr *NewAttr =
        instantiateTemplateAttribute(I->TmplAttr, Context, *this, TemplateArgs);
      I->NewDecl->addAttr(NewAttr);
      LocalInstantiationScope::deleteScopes(I->Scope,
                                            Instantiator.getStartingScope());
    }
    Instantiator.disableLateAttributeInstantiation();
    LateAttrs.clear();
  
    ActOnFinishDelayedMemberInitializers(Instantiation);
  
    // FIXME: We should do something similar for explicit instantiations so they
    // end up in the right module.
    if (TSK == TSK_ImplicitInstantiation) {
      Instantiation->setLocation(Pattern->getLocation());
      Instantiation->setLocStart(Pattern->getInnerLocStart());
      Instantiation->setBraceRange(Pattern->getBraceRange());
    }
  
    if (!Instantiation->isInvalidDecl()) {
      // Perform any dependent diagnostics from the pattern.
      PerformDependentDiagnostics(Pattern, TemplateArgs);
  
      // Instantiate any out-of-line class template partial
      // specializations now.
      for (TemplateDeclInstantiator::delayed_partial_spec_iterator
                P = Instantiator.delayed_partial_spec_begin(),
             PEnd = Instantiator.delayed_partial_spec_end();
           P != PEnd; ++P) {
        if (!Instantiator.InstantiateClassTemplatePartialSpecialization(
                P->first, P->second)) {
          Instantiation->setInvalidDecl();
          break;
        }
      }
  
      // Instantiate any out-of-line variable template partial
      // specializations now.
      for (TemplateDeclInstantiator::delayed_var_partial_spec_iterator
                P = Instantiator.delayed_var_partial_spec_begin(),
             PEnd = Instantiator.delayed_var_partial_spec_end();
           P != PEnd; ++P) {
        if (!Instantiator.InstantiateVarTemplatePartialSpecialization(
                P->first, P->second)) {
          Instantiation->setInvalidDecl();
          break;
        }
      }
    }
  
    // Exit the scope of this instantiation.
    SavedContext.pop();
  
    if (!Instantiation->isInvalidDecl()) {
      Consumer.HandleTagDeclDefinition(Instantiation);
  
      // Always emit the vtable for an explicit instantiation definition
      // of a polymorphic class template specialization. Otherwise, eagerly
      // instantiate only constexpr virtual functions in preparation for their use
      // in constant evaluation.
      if (TSK == TSK_ExplicitInstantiationDefinition)
        MarkVTableUsed(PointOfInstantiation, Instantiation, true);
      else if (MightHaveConstexprVirtualFunctions)
        MarkVirtualMembersReferenced(PointOfInstantiation, Instantiation,
                                     /*ConstexprOnly*/ true);
    }
  
    return Instantiation->isInvalidDecl();
  }
  
  /// Instantiate the definition of an enum from a given pattern.
  ///
  /// \param PointOfInstantiation The point of instantiation within the
  ///        source code.
  /// \param Instantiation is the declaration whose definition is being
  ///        instantiated. This will be a member enumeration of a class
  ///        temploid specialization, or a local enumeration within a
  ///        function temploid specialization.
  /// \param Pattern The templated declaration from which the instantiation
  ///        occurs.
  /// \param TemplateArgs The template arguments to be substituted into
  ///        the pattern.
  /// \param TSK The kind of implicit or explicit instantiation to perform.
  ///
  /// \return \c true if an error occurred, \c false otherwise.
  bool Sema::InstantiateEnum(SourceLocation PointOfInstantiation,
                             EnumDecl *Instantiation, EnumDecl *Pattern,
                             const MultiLevelTemplateArgumentList &TemplateArgs,
                             TemplateSpecializationKind TSK) {
    EnumDecl *PatternDef = Pattern->getDefinition();
    if (DiagnoseUninstantiableTemplate(PointOfInstantiation, Instantiation,
                                   Instantiation->getInstantiatedFromMemberEnum(),
                                       Pattern, PatternDef, TSK,/*Complain*/true))
      return true;
    Pattern = PatternDef;
  
    // Record the point of instantiation.
    if (MemberSpecializationInfo *MSInfo
          = Instantiation->getMemberSpecializationInfo()) {
      MSInfo->setTemplateSpecializationKind(TSK);
      MSInfo->setPointOfInstantiation(PointOfInstantiation);
    }
  
    InstantiatingTemplate Inst(*this, PointOfInstantiation, Instantiation);
    if (Inst.isInvalid())
      return true;
    if (Inst.isAlreadyInstantiating())
      return false;
    PrettyDeclStackTraceEntry CrashInfo(Context, Instantiation, SourceLocation(),
                                        "instantiating enum definition");
  
    // The instantiation is visible here, even if it was first declared in an
    // unimported module.
    Instantiation->setVisibleDespiteOwningModule();
  
    // Enter the scope of this instantiation. We don't use
    // PushDeclContext because we don't have a scope.
    ContextRAII SavedContext(*this, Instantiation);
    EnterExpressionEvaluationContext EvalContext(
        *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
  
    LocalInstantiationScope Scope(*this, /*MergeWithParentScope*/true);
  
    // Pull attributes from the pattern onto the instantiation.
    InstantiateAttrs(TemplateArgs, Pattern, Instantiation);
  
    TemplateDeclInstantiator Instantiator(*this, Instantiation, TemplateArgs);
    Instantiator.InstantiateEnumDefinition(Instantiation, Pattern);
  
    // Exit the scope of this instantiation.
    SavedContext.pop();
  
    return Instantiation->isInvalidDecl();
  }
  
  
  /// Instantiate the definition of a field from the given pattern.
  ///
  /// \param PointOfInstantiation The point of instantiation within the
  ///        source code.
  /// \param Instantiation is the declaration whose definition is being
  ///        instantiated. This will be a class of a class temploid
  ///        specialization, or a local enumeration within a function temploid
  ///        specialization.
  /// \param Pattern The templated declaration from which the instantiation
  ///        occurs.
  /// \param TemplateArgs The template arguments to be substituted into
  ///        the pattern.
  ///
  /// \return \c true if an error occurred, \c false otherwise.
  bool Sema::InstantiateInClassInitializer(
      SourceLocation PointOfInstantiation, FieldDecl *Instantiation,
      FieldDecl *Pattern, const MultiLevelTemplateArgumentList &TemplateArgs) {
    // If there is no initializer, we don't need to do anything.
    if (!Pattern->hasInClassInitializer())
      return false;
  
    assert(Instantiation->getInClassInitStyle() ==
               Pattern->getInClassInitStyle() &&
           "pattern and instantiation disagree about init style");
  
    // Error out if we haven't parsed the initializer of the pattern yet because
    // we are waiting for the closing brace of the outer class.
    Expr *OldInit = Pattern->getInClassInitializer();
    if (!OldInit) {
      RecordDecl *PatternRD = Pattern->getParent();
      RecordDecl *OutermostClass = PatternRD->getOuterLexicalRecordContext();
      Diag(PointOfInstantiation,
           diag::err_in_class_initializer_not_yet_parsed)
          << OutermostClass << Pattern;
      Diag(Pattern->getEndLoc(), diag::note_in_class_initializer_not_yet_parsed);
      Instantiation->setInvalidDecl();
      return true;
    }
  
    InstantiatingTemplate Inst(*this, PointOfInstantiation, Instantiation);
    if (Inst.isInvalid())
      return true;
    if (Inst.isAlreadyInstantiating()) {
      // Error out if we hit an instantiation cycle for this initializer.
      Diag(PointOfInstantiation, diag::err_in_class_initializer_cycle)
        << Instantiation;
      return true;
    }
    PrettyDeclStackTraceEntry CrashInfo(Context, Instantiation, SourceLocation(),
                                        "instantiating default member init");
  
    // Enter the scope of this instantiation. We don't use PushDeclContext because
    // we don't have a scope.
    ContextRAII SavedContext(*this, Instantiation->getParent());
    EnterExpressionEvaluationContext EvalContext(
        *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
  
    LocalInstantiationScope Scope(*this, true);
  
    // Instantiate the initializer.
    ActOnStartCXXInClassMemberInitializer();
    CXXThisScopeRAII ThisScope(*this, Instantiation->getParent(), Qualifiers());
  
    ExprResult NewInit = SubstInitializer(OldInit, TemplateArgs,
                                          /*CXXDirectInit=*/false);
    Expr *Init = NewInit.get();
    assert((!Init || !isa<ParenListExpr>(Init)) && "call-style init in class");
    ActOnFinishCXXInClassMemberInitializer(
        Instantiation, Init ? Init->getBeginLoc() : SourceLocation(), Init);
  
    if (auto *L = getASTMutationListener())
      L->DefaultMemberInitializerInstantiated(Instantiation);
  
    // Return true if the in-class initializer is still missing.
    return !Instantiation->getInClassInitializer();
  }
  
  namespace {
    /// A partial specialization whose template arguments have matched
    /// a given template-id.
    struct PartialSpecMatchResult {
      ClassTemplatePartialSpecializationDecl *Partial;
      TemplateArgumentList *Args;
    };
  }
  
  bool Sema::usesPartialOrExplicitSpecialization(
      SourceLocation Loc, ClassTemplateSpecializationDecl *ClassTemplateSpec) {
    if (ClassTemplateSpec->getTemplateSpecializationKind() ==
        TSK_ExplicitSpecialization)
      return true;
  
    SmallVector<ClassTemplatePartialSpecializationDecl *, 4> PartialSpecs;
    ClassTemplateSpec->getSpecializedTemplate()
                     ->getPartialSpecializations(PartialSpecs);
    for (unsigned I = 0, N = PartialSpecs.size(); I != N; ++I) {
      TemplateDeductionInfo Info(Loc);
      if (!DeduceTemplateArguments(PartialSpecs[I],
                                   ClassTemplateSpec->getTemplateArgs(), Info))
        return true;
    }
  
    return false;
  }
  
  /// Get the instantiation pattern to use to instantiate the definition of a
  /// given ClassTemplateSpecializationDecl (either the pattern of the primary
  /// template or of a partial specialization).
  static CXXRecordDecl *
  getPatternForClassTemplateSpecialization(
      Sema &S, SourceLocation PointOfInstantiation,
      ClassTemplateSpecializationDecl *ClassTemplateSpec,
      TemplateSpecializationKind TSK, bool Complain) {
    Sema::InstantiatingTemplate Inst(S, PointOfInstantiation, ClassTemplateSpec);
    if (Inst.isInvalid() || Inst.isAlreadyInstantiating())
      return nullptr;
  
    llvm::PointerUnion<ClassTemplateDecl *,
                       ClassTemplatePartialSpecializationDecl *>
        Specialized = ClassTemplateSpec->getSpecializedTemplateOrPartial();
    if (!Specialized.is<ClassTemplatePartialSpecializationDecl *>()) {
      // Find best matching specialization.
      ClassTemplateDecl *Template = ClassTemplateSpec->getSpecializedTemplate();
  
      // C++ [temp.class.spec.match]p1:
      //   When a class template is used in a context that requires an
      //   instantiation of the class, it is necessary to determine
      //   whether the instantiation is to be generated using the primary
      //   template or one of the partial specializations. This is done by
      //   matching the template arguments of the class template
      //   specialization with the template argument lists of the partial
      //   specializations.
      typedef PartialSpecMatchResult MatchResult;
      SmallVector<MatchResult, 4> Matched;
      SmallVector<ClassTemplatePartialSpecializationDecl *, 4> PartialSpecs;
      Template->getPartialSpecializations(PartialSpecs);
      TemplateSpecCandidateSet FailedCandidates(PointOfInstantiation);
      for (unsigned I = 0, N = PartialSpecs.size(); I != N; ++I) {
        ClassTemplatePartialSpecializationDecl *Partial = PartialSpecs[I];
        TemplateDeductionInfo Info(FailedCandidates.getLocation());
        if (Sema::TemplateDeductionResult Result = S.DeduceTemplateArguments(
                Partial, ClassTemplateSpec->getTemplateArgs(), 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(S.Context, Result, Info));
          (void)Result;
        } else {
          Matched.push_back(PartialSpecMatchResult());
          Matched.back().Partial = Partial;
          Matched.back().Args = Info.take();
        }
      }
  
      // If we're dealing with a member template where the template parameters
      // have been instantiated, this provides the original template parameters
      // from which the member template's parameters were instantiated.
  
      if (Matched.size() >= 1) {
        SmallVectorImpl<MatchResult>::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 class template is
          //      ambiguous and the program is ill-formed.
          for (SmallVectorImpl<MatchResult>::iterator P = Best + 1,
                                                   PEnd = Matched.end();
               P != PEnd; ++P) {
            if (S.getMoreSpecializedPartialSpecialization(
                    P->Partial, Best->Partial, PointOfInstantiation) ==
                P->Partial)
              Best = P;
          }
  
          // Determine if the best partial specialization is more specialized than
          // the others.
          bool Ambiguous = false;
          for (SmallVectorImpl<MatchResult>::iterator P = Matched.begin(),
                                                   PEnd = Matched.end();
               P != PEnd; ++P) {
            if (P != Best && S.getMoreSpecializedPartialSpecialization(
                                 P->Partial, Best->Partial,
                                 PointOfInstantiation) != Best->Partial) {
              Ambiguous = true;
              break;
            }
          }
  
          if (Ambiguous) {
            // Partial ordering did not produce a clear winner. Complain.
            Inst.Clear();
            ClassTemplateSpec->setInvalidDecl();
            S.Diag(PointOfInstantiation,
                   diag::err_partial_spec_ordering_ambiguous)
                << ClassTemplateSpec;
  
            // Print the matching partial specializations.
            for (SmallVectorImpl<MatchResult>::iterator P = Matched.begin(),
                                                     PEnd = Matched.end();
                 P != PEnd; ++P)
              S.Diag(P->Partial->getLocation(), diag::note_partial_spec_match)
                  << S.getTemplateArgumentBindingsText(
                         P->Partial->getTemplateParameters(), *P->Args);
  
            return nullptr;
          }
        }
  
        ClassTemplateSpec->setInstantiationOf(Best->Partial, Best->Args);
      } else {
        //   -- If no matches are found, the instantiation is generated
        //      from the primary template.
      }
    }
  
    CXXRecordDecl *Pattern = nullptr;
    Specialized = ClassTemplateSpec->getSpecializedTemplateOrPartial();
    if (auto *PartialSpec =
            Specialized.dyn_cast<ClassTemplatePartialSpecializationDecl *>()) {
      // Instantiate using the best class template partial specialization.
      while (PartialSpec->getInstantiatedFromMember()) {
        // If we've found an explicit specialization of this class template,
        // stop here and use that as the pattern.
        if (PartialSpec->isMemberSpecialization())
          break;
  
        PartialSpec = PartialSpec->getInstantiatedFromMember();
      }
      Pattern = PartialSpec;
    } else {
      ClassTemplateDecl *Template = ClassTemplateSpec->getSpecializedTemplate();
      while (Template->getInstantiatedFromMemberTemplate()) {
        // If we've found an explicit specialization of this class template,
        // stop here and use that as the pattern.
        if (Template->isMemberSpecialization())
          break;
  
        Template = Template->getInstantiatedFromMemberTemplate();
      }
      Pattern = Template->getTemplatedDecl();
    }
  
    return Pattern;
  }
  
  bool Sema::InstantiateClassTemplateSpecialization(
      SourceLocation PointOfInstantiation,
      ClassTemplateSpecializationDecl *ClassTemplateSpec,
      TemplateSpecializationKind TSK, bool Complain) {
    // Perform the actual instantiation on the canonical declaration.
    ClassTemplateSpec = cast<ClassTemplateSpecializationDecl>(
        ClassTemplateSpec->getCanonicalDecl());
    if (ClassTemplateSpec->isInvalidDecl())
      return true;
  
    CXXRecordDecl *Pattern = getPatternForClassTemplateSpecialization(
        *this, PointOfInstantiation, ClassTemplateSpec, TSK, Complain);
    if (!Pattern)
      return true;
  
    return InstantiateClass(PointOfInstantiation, ClassTemplateSpec, Pattern,
                            getTemplateInstantiationArgs(ClassTemplateSpec), TSK,
                            Complain);
  }
  
  /// Instantiates the definitions of all of the member
  /// of the given class, which is an instantiation of a class template
  /// or a member class of a template.
  void
  Sema::InstantiateClassMembers(SourceLocation PointOfInstantiation,
                                CXXRecordDecl *Instantiation,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
                                TemplateSpecializationKind TSK) {
    // FIXME: We need to notify the ASTMutationListener that we did all of these
    // things, in case we have an explicit instantiation definition in a PCM, a
    // module, or preamble, and the declaration is in an imported AST.
    assert(
        (TSK == TSK_ExplicitInstantiationDefinition ||
         TSK == TSK_ExplicitInstantiationDeclaration ||
         (TSK == TSK_ImplicitInstantiation && Instantiation->isLocalClass())) &&
        "Unexpected template specialization kind!");
    for (auto *D : Instantiation->decls()) {
      bool SuppressNew = false;
      if (auto *Function = dyn_cast<FunctionDecl>(D)) {
        if (FunctionDecl *Pattern =
                Function->getInstantiatedFromMemberFunction()) {
  
          if (Function->hasAttr<ExcludeFromExplicitInstantiationAttr>())
            continue;
  
          MemberSpecializationInfo *MSInfo =
              Function->getMemberSpecializationInfo();
          assert(MSInfo && "No member specialization information?");
          if (MSInfo->getTemplateSpecializationKind()
                                                   == TSK_ExplicitSpecialization)
            continue;
  
          if (CheckSpecializationInstantiationRedecl(PointOfInstantiation, TSK,
                                                     Function,
                                          MSInfo->getTemplateSpecializationKind(),
                                                MSInfo->getPointOfInstantiation(),
                                                     SuppressNew) ||
              SuppressNew)
            continue;
  
          // C++11 [temp.explicit]p8:
          //   An explicit instantiation definition that names a class template
          //   specialization explicitly instantiates the class template
          //   specialization and is only an explicit instantiation definition
          //   of members whose definition is visible at the point of
          //   instantiation.
          if (TSK == TSK_ExplicitInstantiationDefinition && !Pattern->isDefined())
            continue;
  
          Function->setTemplateSpecializationKind(TSK, PointOfInstantiation);
  
          if (Function->isDefined()) {
            // Let the ASTConsumer know that this function has been explicitly
            // instantiated now, and its linkage might have changed.
            Consumer.HandleTopLevelDecl(DeclGroupRef(Function));
          } else if (TSK == TSK_ExplicitInstantiationDefinition) {
            InstantiateFunctionDefinition(PointOfInstantiation, Function);
          } else if (TSK == TSK_ImplicitInstantiation) {
            PendingLocalImplicitInstantiations.push_back(
                std::make_pair(Function, PointOfInstantiation));
          }
        }
      } else if (auto *Var = dyn_cast<VarDecl>(D)) {
        if (isa<VarTemplateSpecializationDecl>(Var))
          continue;
  
        if (Var->isStaticDataMember()) {
          if (Var->hasAttr<ExcludeFromExplicitInstantiationAttr>())
            continue;
  
          MemberSpecializationInfo *MSInfo = Var->getMemberSpecializationInfo();
          assert(MSInfo && "No member specialization information?");
          if (MSInfo->getTemplateSpecializationKind()
                                                   == TSK_ExplicitSpecialization)
            continue;
  
          if (CheckSpecializationInstantiationRedecl(PointOfInstantiation, TSK,
                                                     Var,
                                          MSInfo->getTemplateSpecializationKind(),
                                                MSInfo->getPointOfInstantiation(),
                                                     SuppressNew) ||
              SuppressNew)
            continue;
  
          if (TSK == TSK_ExplicitInstantiationDefinition) {
            // C++0x [temp.explicit]p8:
            //   An explicit instantiation definition that names a class template
            //   specialization explicitly instantiates the class template
            //   specialization and is only an explicit instantiation definition
            //   of members whose definition is visible at the point of
            //   instantiation.
            if (!Var->getInstantiatedFromStaticDataMember()->getDefinition())
              continue;
  
            Var->setTemplateSpecializationKind(TSK, PointOfInstantiation);
            InstantiateVariableDefinition(PointOfInstantiation, Var);
          } else {
            Var->setTemplateSpecializationKind(TSK, PointOfInstantiation);
          }
        }
      } else if (auto *Record = dyn_cast<CXXRecordDecl>(D)) {
        if (Record->hasAttr<ExcludeFromExplicitInstantiationAttr>())
          continue;
  
        // Always skip the injected-class-name, along with any
        // redeclarations of nested classes, since both would cause us
        // to try to instantiate the members of a class twice.
        // Skip closure types; they'll get instantiated when we instantiate
        // the corresponding lambda-expression.
        if (Record->isInjectedClassName() || Record->getPreviousDecl() ||
            Record->isLambda())
          continue;
  
        MemberSpecializationInfo *MSInfo = Record->getMemberSpecializationInfo();
        assert(MSInfo && "No member specialization information?");
  
        if (MSInfo->getTemplateSpecializationKind()
                                                  == TSK_ExplicitSpecialization)
          continue;
  
        if (Context.getTargetInfo().getTriple().isOSWindows() &&
            TSK == TSK_ExplicitInstantiationDeclaration) {
          // On Windows, explicit instantiation decl of the outer class doesn't
          // affect the inner class. Typically extern template declarations are
          // used in combination with dll import/export annotations, but those
          // are not propagated from the outer class templates to inner classes.
          // Therefore, do not instantiate inner classes on this platform, so
          // that users don't end up with undefined symbols during linking.
          continue;
        }
  
        if (CheckSpecializationInstantiationRedecl(PointOfInstantiation, TSK,
                                                   Record,
                                          MSInfo->getTemplateSpecializationKind(),
                                                MSInfo->getPointOfInstantiation(),
                                                   SuppressNew) ||
            SuppressNew)
          continue;
  
        CXXRecordDecl *Pattern = Record->getInstantiatedFromMemberClass();
        assert(Pattern && "Missing instantiated-from-template information");
  
        if (!Record->getDefinition()) {
          if (!Pattern->getDefinition()) {
            // C++0x [temp.explicit]p8:
            //   An explicit instantiation definition that names a class template
            //   specialization explicitly instantiates the class template
            //   specialization and is only an explicit instantiation definition
            //   of members whose definition is visible at the point of
            //   instantiation.
            if (TSK == TSK_ExplicitInstantiationDeclaration) {
              MSInfo->setTemplateSpecializationKind(TSK);
              MSInfo->setPointOfInstantiation(PointOfInstantiation);
            }
  
            continue;
          }
  
          InstantiateClass(PointOfInstantiation, Record, Pattern,
                           TemplateArgs,
                           TSK);
        } else {
          if (TSK == TSK_ExplicitInstantiationDefinition &&
              Record->getTemplateSpecializationKind() ==
                  TSK_ExplicitInstantiationDeclaration) {
            Record->setTemplateSpecializationKind(TSK);
            MarkVTableUsed(PointOfInstantiation, Record, true);
          }
        }
  
        Pattern = cast_or_null<CXXRecordDecl>(Record->getDefinition());
        if (Pattern)
          InstantiateClassMembers(PointOfInstantiation, Pattern, TemplateArgs,
                                  TSK);
      } else if (auto *Enum = dyn_cast<EnumDecl>(D)) {
        MemberSpecializationInfo *MSInfo = Enum->getMemberSpecializationInfo();
        assert(MSInfo && "No member specialization information?");
  
        if (MSInfo->getTemplateSpecializationKind()
              == TSK_ExplicitSpecialization)
          continue;
  
        if (CheckSpecializationInstantiationRedecl(
              PointOfInstantiation, TSK, Enum,
              MSInfo->getTemplateSpecializationKind(),
              MSInfo->getPointOfInstantiation(), SuppressNew) ||
            SuppressNew)
          continue;
  
        if (Enum->getDefinition())
          continue;
  
        EnumDecl *Pattern = Enum->getTemplateInstantiationPattern();
        assert(Pattern && "Missing instantiated-from-template information");
  
        if (TSK == TSK_ExplicitInstantiationDefinition) {
          if (!Pattern->getDefinition())
            continue;
  
          InstantiateEnum(PointOfInstantiation, Enum, Pattern, TemplateArgs, TSK);
        } else {
          MSInfo->setTemplateSpecializationKind(TSK);
          MSInfo->setPointOfInstantiation(PointOfInstantiation);
        }
      } else if (auto *Field = dyn_cast<FieldDecl>(D)) {
        // No need to instantiate in-class initializers during explicit
        // instantiation.
        if (Field->hasInClassInitializer() && TSK == TSK_ImplicitInstantiation) {
          CXXRecordDecl *ClassPattern =
              Instantiation->getTemplateInstantiationPattern();
          DeclContext::lookup_result Lookup =
              ClassPattern->lookup(Field->getDeclName());
          FieldDecl *Pattern = cast<FieldDecl>(Lookup.front());
          InstantiateInClassInitializer(PointOfInstantiation, Field, Pattern,
                                        TemplateArgs);
        }
      }
    }
  }
  
  /// Instantiate the definitions of all of the members of the
  /// given class template specialization, which was named as part of an
  /// explicit instantiation.
  void
  Sema::InstantiateClassTemplateSpecializationMembers(
                                             SourceLocation PointOfInstantiation,
                              ClassTemplateSpecializationDecl *ClassTemplateSpec,
                                                 TemplateSpecializationKind TSK) {
    // C++0x [temp.explicit]p7:
    //   An explicit instantiation that names a class template
    //   specialization is an explicit instantion of the same kind
    //   (declaration or definition) of each of its members (not
    //   including members inherited from base classes) that has not
    //   been previously explicitly specialized in the translation unit
    //   containing the explicit instantiation, except as described
    //   below.
    InstantiateClassMembers(PointOfInstantiation, ClassTemplateSpec,
                            getTemplateInstantiationArgs(ClassTemplateSpec),
                            TSK);
  }
  
  StmtResult
  Sema::SubstStmt(Stmt *S, const MultiLevelTemplateArgumentList &TemplateArgs) {
    if (!S)
      return S;
  
    TemplateInstantiator Instantiator(*this, TemplateArgs,
                                      SourceLocation(),
                                      DeclarationName());
    return Instantiator.TransformStmt(S);
  }
  
  ExprResult
  Sema::SubstExpr(Expr *E, const MultiLevelTemplateArgumentList &TemplateArgs) {
    if (!E)
      return E;
  
    TemplateInstantiator Instantiator(*this, TemplateArgs,
                                      SourceLocation(),
                                      DeclarationName());
    return Instantiator.TransformExpr(E);
  }
  
  ExprResult Sema::SubstInitializer(Expr *Init,
                            const MultiLevelTemplateArgumentList &TemplateArgs,
                            bool CXXDirectInit) {
    TemplateInstantiator Instantiator(*this, TemplateArgs,
                                      SourceLocation(),
                                      DeclarationName());
    return Instantiator.TransformInitializer(Init, CXXDirectInit);
  }
  
  bool Sema::SubstExprs(ArrayRef<Expr *> Exprs, bool IsCall,
                        const MultiLevelTemplateArgumentList &TemplateArgs,
                        SmallVectorImpl<Expr *> &Outputs) {
    if (Exprs.empty())
      return false;
  
    TemplateInstantiator Instantiator(*this, TemplateArgs,
                                      SourceLocation(),
                                      DeclarationName());
    return Instantiator.TransformExprs(Exprs.data(), Exprs.size(),
                                       IsCall, Outputs);
  }
  
  NestedNameSpecifierLoc
  Sema::SubstNestedNameSpecifierLoc(NestedNameSpecifierLoc NNS,
                          const MultiLevelTemplateArgumentList &TemplateArgs) {
    if (!NNS)
      return NestedNameSpecifierLoc();
  
    TemplateInstantiator Instantiator(*this, TemplateArgs, NNS.getBeginLoc(),
                                      DeclarationName());
    return Instantiator.TransformNestedNameSpecifierLoc(NNS);
  }
  
  /// Do template substitution on declaration name info.
  DeclarationNameInfo
  Sema::SubstDeclarationNameInfo(const DeclarationNameInfo &NameInfo,
                           const MultiLevelTemplateArgumentList &TemplateArgs) {
    TemplateInstantiator Instantiator(*this, TemplateArgs, NameInfo.getLoc(),
                                      NameInfo.getName());
    return Instantiator.TransformDeclarationNameInfo(NameInfo);
  }
  
  TemplateName
  Sema::SubstTemplateName(NestedNameSpecifierLoc QualifierLoc,
                          TemplateName Name, SourceLocation Loc,
                          const MultiLevelTemplateArgumentList &TemplateArgs) {
    TemplateInstantiator Instantiator(*this, TemplateArgs, Loc,
                                      DeclarationName());
    CXXScopeSpec SS;
    SS.Adopt(QualifierLoc);
    return Instantiator.TransformTemplateName(SS, Name, Loc);
  }
  
  bool Sema::Subst(const TemplateArgumentLoc *Args, unsigned NumArgs,
                   TemplateArgumentListInfo &Result,
                   const MultiLevelTemplateArgumentList &TemplateArgs) {
    TemplateInstantiator Instantiator(*this, TemplateArgs, SourceLocation(),
                                      DeclarationName());
  
    return Instantiator.TransformTemplateArguments(Args, NumArgs, Result);
  }
  
  static const Decl *getCanonicalParmVarDecl(const Decl *D) {
    // When storing ParmVarDecls in the local instantiation scope, we always
    // want to use the ParmVarDecl from the canonical function declaration,
    // since the map is then valid for any redeclaration or definition of that
    // function.
    if (const ParmVarDecl *PV = dyn_cast<ParmVarDecl>(D)) {
      if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(PV->getDeclContext())) {
        unsigned i = PV->getFunctionScopeIndex();
        // This parameter might be from a freestanding function type within the
        // function and isn't necessarily referring to one of FD's parameters.
        if (i < FD->getNumParams() && FD->getParamDecl(i) == PV)
          return FD->getCanonicalDecl()->getParamDecl(i);
      }
    }
    return D;
  }
  
  
  llvm::PointerUnion<Decl *, LocalInstantiationScope::DeclArgumentPack *> *
  LocalInstantiationScope::findInstantiationOf(const Decl *D) {
    D = getCanonicalParmVarDecl(D);
    for (LocalInstantiationScope *Current = this; Current;
         Current = Current->Outer) {
  
      // Check if we found something within this scope.
      const Decl *CheckD = D;
      do {
        LocalDeclsMap::iterator Found = Current->LocalDecls.find(CheckD);
        if (Found != Current->LocalDecls.end())
          return &Found->second;
  
        // If this is a tag declaration, it's possible that we need to look for
        // a previous declaration.
        if (const TagDecl *Tag = dyn_cast<TagDecl>(CheckD))
          CheckD = Tag->getPreviousDecl();
        else
          CheckD = nullptr;
      } while (CheckD);
  
      // If we aren't combined with our outer scope, we're done.
      if (!Current->CombineWithOuterScope)
        break;
    }
  
    // If we're performing a partial substitution during template argument
    // deduction, we may not have values for template parameters yet.
    if (isa<NonTypeTemplateParmDecl>(D) || isa<TemplateTypeParmDecl>(D) ||
        isa<TemplateTemplateParmDecl>(D))
      return nullptr;
  
    // Local types referenced prior to definition may require instantiation.
    if (const CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(D))
      if (RD->isLocalClass())
        return nullptr;
  
    // Enumeration types referenced prior to definition may appear as a result of
    // error recovery.
    if (isa<EnumDecl>(D))
      return nullptr;
  
    // If we didn't find the decl, then we either have a sema bug, or we have a
    // forward reference to a label declaration.  Return null to indicate that
    // we have an uninstantiated label.
    assert(isa<LabelDecl>(D) && "declaration not instantiated in this scope");
    return nullptr;
  }
  
  void LocalInstantiationScope::InstantiatedLocal(const Decl *D, Decl *Inst) {
    D = getCanonicalParmVarDecl(D);
    llvm::PointerUnion<Decl *, DeclArgumentPack *> &Stored = LocalDecls[D];
    if (Stored.isNull()) {
  #ifndef NDEBUG
      // It should not be present in any surrounding scope either.
      LocalInstantiationScope *Current = this;
      while (Current->CombineWithOuterScope && Current->Outer) {
        Current = Current->Outer;
        assert(Current->LocalDecls.find(D) == Current->LocalDecls.end() &&
               "Instantiated local in inner and outer scopes");
      }
  #endif
      Stored = Inst;
    } else if (DeclArgumentPack *Pack = Stored.dyn_cast<DeclArgumentPack *>()) {
      Pack->push_back(cast<VarDecl>(Inst));
    } else {
      assert(Stored.get<Decl *>() == Inst && "Already instantiated this local");
    }
  }
  
  void LocalInstantiationScope::InstantiatedLocalPackArg(const Decl *D,
                                                         VarDecl *Inst) {
    D = getCanonicalParmVarDecl(D);
    DeclArgumentPack *Pack = LocalDecls[D].get<DeclArgumentPack *>();
    Pack->push_back(Inst);
  }
  
  void LocalInstantiationScope::MakeInstantiatedLocalArgPack(const Decl *D) {
  #ifndef NDEBUG
    // This should be the first time we've been told about this decl.
    for (LocalInstantiationScope *Current = this;
         Current && Current->CombineWithOuterScope; Current = Current->Outer)
      assert(Current->LocalDecls.find(D) == Current->LocalDecls.end() &&
             "Creating local pack after instantiation of local");
  #endif
  
    D = getCanonicalParmVarDecl(D);
    llvm::PointerUnion<Decl *, DeclArgumentPack *> &Stored = LocalDecls[D];
    DeclArgumentPack *Pack = new DeclArgumentPack;
    Stored = Pack;
    ArgumentPacks.push_back(Pack);
  }
  
  void LocalInstantiationScope::SetPartiallySubstitutedPack(NamedDecl *Pack,
                                            const TemplateArgument *ExplicitArgs,
                                                      unsigned NumExplicitArgs) {
    assert((!PartiallySubstitutedPack || PartiallySubstitutedPack == Pack) &&
           "Already have a partially-substituted pack");
    assert((!PartiallySubstitutedPack
            || NumArgsInPartiallySubstitutedPack == NumExplicitArgs) &&
           "Wrong number of arguments in partially-substituted pack");
    PartiallySubstitutedPack = Pack;
    ArgsInPartiallySubstitutedPack = ExplicitArgs;
    NumArgsInPartiallySubstitutedPack = NumExplicitArgs;
  }
  
  NamedDecl *LocalInstantiationScope::getPartiallySubstitutedPack(
                                           const TemplateArgument **ExplicitArgs,
                                                unsigned *NumExplicitArgs) const {
    if (ExplicitArgs)
      *ExplicitArgs = nullptr;
    if (NumExplicitArgs)
      *NumExplicitArgs = 0;
  
    for (const LocalInstantiationScope *Current = this; Current;
         Current = Current->Outer) {
      if (Current->PartiallySubstitutedPack) {
        if (ExplicitArgs)
          *ExplicitArgs = Current->ArgsInPartiallySubstitutedPack;
        if (NumExplicitArgs)
          *NumExplicitArgs = Current->NumArgsInPartiallySubstitutedPack;
  
        return Current->PartiallySubstitutedPack;
      }
  
      if (!Current->CombineWithOuterScope)
        break;
    }
  
    return nullptr;
  }
  