SemaTemplateInstantiateDecl.cpp

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//===--- SemaTemplateInstantiateDecl.cpp - C++ Template Decl 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 for declarations.
//
//===----------------------------------------------------------------------===/
 
#include "TreeTransform.h"
#include "clang/AST/ASTConsumer.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/ASTMutationListener.h"
#include "clang/AST/DeclTemplate.h"
#include "clang/AST/DeclVisitor.h"
#include "clang/AST/DependentDiagnostic.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprCXX.h"
#include "clang/AST/PrettyDeclStackTrace.h"
#include "clang/AST/TypeLoc.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Sema/Initialization.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/ScopeInfo.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "clang/Sema/TemplateInstCallback.h"
#include "llvm/Support/TimeProfiler.h"
#include <optional>
 
using namespace clang;
 
static bool isDeclWithinFunction(const Decl *D) {
  const DeclContext *DC = D->getDeclContext();
  if (DC->isFunctionOrMethod())
    return true;
 
  if (DC->isRecord())
    return cast<CXXRecordDecl>(DC)->isLocalClass();
 
  return false;
}
 
template<typename DeclT>
static bool SubstQualifier(Sema &SemaRef, const DeclT *OldDecl, DeclT *NewDecl,
                           const MultiLevelTemplateArgumentList &TemplateArgs) {
  if (!OldDecl->getQualifierLoc())
    return false;
 
  assert((NewDecl->getFriendObjectKind() ||
          !OldDecl->getLexicalDeclContext()->isDependentContext()) &&
         "non-friend with qualified name defined in dependent context");
  Sema::ContextRAII SavedContext(
      SemaRef,
      const_cast<DeclContext *>(NewDecl->getFriendObjectKind()
                                    ? NewDecl->getLexicalDeclContext()
                                    : OldDecl->getLexicalDeclContext()));
 
  NestedNameSpecifierLoc NewQualifierLoc
      = SemaRef.SubstNestedNameSpecifierLoc(OldDecl->getQualifierLoc(),
                                            TemplateArgs);
 
  if (!NewQualifierLoc)
    return true;
 
  NewDecl->setQualifierInfo(NewQualifierLoc);
  return false;
}
 
bool TemplateDeclInstantiator::SubstQualifier(const DeclaratorDecl *OldDecl,
                                              DeclaratorDecl *NewDecl) {
  return ::SubstQualifier(SemaRef, OldDecl, NewDecl, TemplateArgs);
}
 
bool TemplateDeclInstantiator::SubstQualifier(const TagDecl *OldDecl,
                                              TagDecl *NewDecl) {
  return ::SubstQualifier(SemaRef, OldDecl, NewDecl, TemplateArgs);
}
 
// Include attribute instantiation code.
#include "clang/Sema/AttrTemplateInstantiate.inc"
 
static void instantiateDependentAlignedAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AlignedAttr *Aligned, Decl *New, bool IsPackExpansion) {
  if (Aligned->isAlignmentExpr()) {
    // The alignment expression is a constant expression.
    EnterExpressionEvaluationContext Unevaluated(
        S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
    ExprResult Result = S.SubstExpr(Aligned->getAlignmentExpr(), TemplateArgs);
    if (!Result.isInvalid())
      S.AddAlignedAttr(New, *Aligned, Result.getAs<Expr>(), IsPackExpansion);
  } else {
    TypeSourceInfo *Result = S.SubstType(Aligned->getAlignmentType(),
                                         TemplateArgs, Aligned->getLocation(),
                                         DeclarationName());
    if (Result)
      S.AddAlignedAttr(New, *Aligned, Result, IsPackExpansion);
  }
}
 
static void instantiateDependentAlignedAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AlignedAttr *Aligned, Decl *New) {
  if (!Aligned->isPackExpansion()) {
    instantiateDependentAlignedAttr(S, TemplateArgs, Aligned, New, false);
    return;
  }
 
  SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  if (Aligned->isAlignmentExpr())
    S.collectUnexpandedParameterPacks(Aligned->getAlignmentExpr(),
                                      Unexpanded);
  else
    S.collectUnexpandedParameterPacks(Aligned->getAlignmentType()->getTypeLoc(),
                                      Unexpanded);
  assert(!Unexpanded.empty() && "Pack expansion without parameter packs?");
 
  // Determine whether we can expand this attribute pack yet.
  bool Expand = true, RetainExpansion = false;
  std::optional<unsigned> NumExpansions;
  // FIXME: Use the actual location of the ellipsis.
  SourceLocation EllipsisLoc = Aligned->getLocation();
  if (S.CheckParameterPacksForExpansion(EllipsisLoc, Aligned->getRange(),
                                        Unexpanded, TemplateArgs, Expand,
                                        RetainExpansion, NumExpansions))
    return;
 
  if (!Expand) {
    Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(S, -1);
    instantiateDependentAlignedAttr(S, TemplateArgs, Aligned, New, true);
  } else {
    for (unsigned I = 0; I != *NumExpansions; ++I) {
      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(S, I);
      instantiateDependentAlignedAttr(S, TemplateArgs, Aligned, New, false);
    }
  }
}
 
static void instantiateDependentAssumeAlignedAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AssumeAlignedAttr *Aligned, Decl *New) {
  // The alignment expression is a constant expression.
  EnterExpressionEvaluationContext Unevaluated(
      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
  Expr *E, *OE = nullptr;
  ExprResult Result = S.SubstExpr(Aligned->getAlignment(), TemplateArgs);
  if (Result.isInvalid())
    return;
  E = Result.getAs<Expr>();
 
  if (Aligned->getOffset()) {
    Result = S.SubstExpr(Aligned->getOffset(), TemplateArgs);
    if (Result.isInvalid())
      return;
    OE = Result.getAs<Expr>();
  }
 
  S.AddAssumeAlignedAttr(New, *Aligned, E, OE);
}
 
static void instantiateDependentAlignValueAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AlignValueAttr *Aligned, Decl *New) {
  // The alignment expression is a constant expression.
  EnterExpressionEvaluationContext Unevaluated(
      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
  ExprResult Result = S.SubstExpr(Aligned->getAlignment(), TemplateArgs);
  if (!Result.isInvalid())
    S.AddAlignValueAttr(New, *Aligned, Result.getAs<Expr>());
}
 
static void instantiateDependentAllocAlignAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AllocAlignAttr *Align, Decl *New) {
  Expr *Param = IntegerLiteral::Create(
      S.getASTContext(),
      llvm::APInt(64, Align->getParamIndex().getSourceIndex()),
      S.getASTContext().UnsignedLongLongTy, Align->getLocation());
  S.AddAllocAlignAttr(New, *Align, Param);
}
 
static void instantiateDependentAnnotationAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AnnotateAttr *Attr, Decl *New) {
  EnterExpressionEvaluationContext Unevaluated(
      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
  // If the attribute has delayed arguments it will have to instantiate those
  // and handle them as new arguments for the attribute.
  bool HasDelayedArgs = Attr->delayedArgs_size();
 
  ArrayRef<Expr *> ArgsToInstantiate =
      HasDelayedArgs
          ? ArrayRef<Expr *>{Attr->delayedArgs_begin(), Attr->delayedArgs_end()}
          : ArrayRef<Expr *>{Attr->args_begin(), Attr->args_end()};
 
  SmallVector<Expr *, 4> Args;
  if (S.SubstExprs(ArgsToInstantiate,
                   /*IsCall=*/false, TemplateArgs, Args))
    return;
 
  StringRef Str = Attr->getAnnotation();
  if (HasDelayedArgs) {
    if (Args.size() < 1) {
      S.Diag(Attr->getLoc(), diag::err_attribute_too_few_arguments)
          << Attr << 1;
      return;
    }
 
    if (!S.checkStringLiteralArgumentAttr(*Attr, Args[0], Str))
      return;
 
    llvm::SmallVector<Expr *, 4> ActualArgs;
    ActualArgs.insert(ActualArgs.begin(), Args.begin() + 1, Args.end());
    std::swap(Args, ActualArgs);
  }
  S.AddAnnotationAttr(New, *Attr, Str, Args);
}
 
static Expr *instantiateDependentFunctionAttrCondition(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const Attr *A, Expr *OldCond, const Decl *Tmpl, FunctionDecl *New) {
  Expr *Cond = nullptr;
  {
    Sema::ContextRAII SwitchContext(S, New);
    EnterExpressionEvaluationContext Unevaluated(
        S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
    ExprResult Result = S.SubstExpr(OldCond, TemplateArgs);
    if (Result.isInvalid())
      return nullptr;
    Cond = Result.getAs<Expr>();
  }
  if (!Cond->isTypeDependent()) {
    ExprResult Converted = S.PerformContextuallyConvertToBool(Cond);
    if (Converted.isInvalid())
      return nullptr;
    Cond = Converted.get();
  }
 
  SmallVector<PartialDiagnosticAt, 8> Diags;
  if (OldCond->isValueDependent() && !Cond->isValueDependent() &&
      !Expr::isPotentialConstantExprUnevaluated(Cond, New, Diags)) {
    S.Diag(A->getLocation(), diag::err_attr_cond_never_constant_expr) << A;
    for (const auto &P : Diags)
      S.Diag(P.first, P.second);
    return nullptr;
  }
  return Cond;
}
 
static void instantiateDependentEnableIfAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const EnableIfAttr *EIA, const Decl *Tmpl, FunctionDecl *New) {
  Expr *Cond = instantiateDependentFunctionAttrCondition(
      S, TemplateArgs, EIA, EIA->getCond(), Tmpl, New);
 
  if (Cond)
    New->addAttr(new (S.getASTContext()) EnableIfAttr(S.getASTContext(), *EIA,
                                                      Cond, EIA->getMessage()));
}
 
static void instantiateDependentDiagnoseIfAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const DiagnoseIfAttr *DIA, const Decl *Tmpl, FunctionDecl *New) {
  Expr *Cond = instantiateDependentFunctionAttrCondition(
      S, TemplateArgs, DIA, DIA->getCond(), Tmpl, New);
 
  if (Cond)
    New->addAttr(new (S.getASTContext()) DiagnoseIfAttr(
        S.getASTContext(), *DIA, Cond, DIA->getMessage(),
        DIA->getDiagnosticType(), DIA->getArgDependent(), New));
}
 
// Constructs and adds to New a new instance of CUDALaunchBoundsAttr using
// template A as the base and arguments from TemplateArgs.
static void instantiateDependentCUDALaunchBoundsAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const CUDALaunchBoundsAttr &Attr, Decl *New) {
  // The alignment expression is a constant expression.
  EnterExpressionEvaluationContext Unevaluated(
      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
  ExprResult Result = S.SubstExpr(Attr.getMaxThreads(), TemplateArgs);
  if (Result.isInvalid())
    return;
  Expr *MaxThreads = Result.getAs<Expr>();
 
  Expr *MinBlocks = nullptr;
  if (Attr.getMinBlocks()) {
    Result = S.SubstExpr(Attr.getMinBlocks(), TemplateArgs);
    if (Result.isInvalid())
      return;
    MinBlocks = Result.getAs<Expr>();
  }
 
  S.AddLaunchBoundsAttr(New, Attr, MaxThreads, MinBlocks);
}
 
static void
instantiateDependentModeAttr(Sema &S,
                             const MultiLevelTemplateArgumentList &TemplateArgs,
                             const ModeAttr &Attr, Decl *New) {
  S.AddModeAttr(New, Attr, Attr.getMode(),
                /*InInstantiation=*/true);
}
 
/// Instantiation of 'declare simd' attribute and its arguments.
static void instantiateOMPDeclareSimdDeclAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const OMPDeclareSimdDeclAttr &Attr, Decl *New) {
  // Allow 'this' in clauses with varlists.
  if (auto *FTD = dyn_cast<FunctionTemplateDecl>(New))
    New = FTD->getTemplatedDecl();
  auto *FD = cast<FunctionDecl>(New);
  auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(FD->getDeclContext());
  SmallVector<Expr *, 4> Uniforms, Aligneds, Alignments, Linears, Steps;
  SmallVector<unsigned, 4> LinModifiers;
 
  auto SubstExpr = [&](Expr *E) -> ExprResult {
    if (auto *DRE = dyn_cast<DeclRefExpr>(E->IgnoreParenImpCasts()))
      if (auto *PVD = dyn_cast<ParmVarDecl>(DRE->getDecl())) {
        Sema::ContextRAII SavedContext(S, FD);
        LocalInstantiationScope Local(S);
        if (FD->getNumParams() > PVD->getFunctionScopeIndex())
          Local.InstantiatedLocal(
              PVD, FD->getParamDecl(PVD->getFunctionScopeIndex()));
        return S.SubstExpr(E, TemplateArgs);
      }
    Sema::CXXThisScopeRAII ThisScope(S, ThisContext, Qualifiers(),
                                     FD->isCXXInstanceMember());
    return S.SubstExpr(E, TemplateArgs);
  };
 
  // Substitute a single OpenMP clause, which is a potentially-evaluated
  // full-expression.
  auto Subst = [&](Expr *E) -> ExprResult {
    EnterExpressionEvaluationContext Evaluated(
        S, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
    ExprResult Res = SubstExpr(E);
    if (Res.isInvalid())
      return Res;
    return S.ActOnFinishFullExpr(Res.get(), false);
  };
 
  ExprResult Simdlen;
  if (auto *E = Attr.getSimdlen())
    Simdlen = Subst(E);
 
  if (Attr.uniforms_size() > 0) {
    for(auto *E : Attr.uniforms()) {
      ExprResult Inst = Subst(E);
      if (Inst.isInvalid())
        continue;
      Uniforms.push_back(Inst.get());
    }
  }
 
  auto AI = Attr.alignments_begin();
  for (auto *E : Attr.aligneds()) {
    ExprResult Inst = Subst(E);
    if (Inst.isInvalid())
      continue;
    Aligneds.push_back(Inst.get());
    Inst = ExprEmpty();
    if (*AI)
      Inst = S.SubstExpr(*AI, TemplateArgs);
    Alignments.push_back(Inst.get());
    ++AI;
  }
 
  auto SI = Attr.steps_begin();
  for (auto *E : Attr.linears()) {
    ExprResult Inst = Subst(E);
    if (Inst.isInvalid())
      continue;
    Linears.push_back(Inst.get());
    Inst = ExprEmpty();
    if (*SI)
      Inst = S.SubstExpr(*SI, TemplateArgs);
    Steps.push_back(Inst.get());
    ++SI;
  }
  LinModifiers.append(Attr.modifiers_begin(), Attr.modifiers_end());
  (void)S.ActOnOpenMPDeclareSimdDirective(
      S.ConvertDeclToDeclGroup(New), Attr.getBranchState(), Simdlen.get(),
      Uniforms, Aligneds, Alignments, Linears, LinModifiers, Steps,
      Attr.getRange());
}
 
/// Instantiation of 'declare variant' attribute and its arguments.
static void instantiateOMPDeclareVariantAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const OMPDeclareVariantAttr &Attr, Decl *New) {
  // Allow 'this' in clauses with varlists.
  if (auto *FTD = dyn_cast<FunctionTemplateDecl>(New))
    New = FTD->getTemplatedDecl();
  auto *FD = cast<FunctionDecl>(New);
  auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(FD->getDeclContext());
 
  auto &&SubstExpr = [FD, ThisContext, &S, &TemplateArgs](Expr *E) {
    if (auto *DRE = dyn_cast<DeclRefExpr>(E->IgnoreParenImpCasts()))
      if (auto *PVD = dyn_cast<ParmVarDecl>(DRE->getDecl())) {
        Sema::ContextRAII SavedContext(S, FD);
        LocalInstantiationScope Local(S);
        if (FD->getNumParams() > PVD->getFunctionScopeIndex())
          Local.InstantiatedLocal(
              PVD, FD->getParamDecl(PVD->getFunctionScopeIndex()));
        return S.SubstExpr(E, TemplateArgs);
      }
    Sema::CXXThisScopeRAII ThisScope(S, ThisContext, Qualifiers(),
                                     FD->isCXXInstanceMember());
    return S.SubstExpr(E, TemplateArgs);
  };
 
  // Substitute a single OpenMP clause, which is a potentially-evaluated
  // full-expression.
  auto &&Subst = [&SubstExpr, &S](Expr *E) {
    EnterExpressionEvaluationContext Evaluated(
        S, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
    ExprResult Res = SubstExpr(E);
    if (Res.isInvalid())
      return Res;
    return S.ActOnFinishFullExpr(Res.get(), false);
  };
 
  ExprResult VariantFuncRef;
  if (Expr *E = Attr.getVariantFuncRef()) {
    // Do not mark function as is used to prevent its emission if this is the
    // only place where it is used.
    EnterExpressionEvaluationContext Unevaluated(
        S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
    VariantFuncRef = Subst(E);
  }
 
  // Copy the template version of the OMPTraitInfo and run substitute on all
  // score and condition expressiosn.
  OMPTraitInfo &TI = S.getASTContext().getNewOMPTraitInfo();
  TI = *Attr.getTraitInfos();
 
  // Try to substitute template parameters in score and condition expressions.
  auto SubstScoreOrConditionExpr = [&S, Subst](Expr *&E, bool) {
    if (E) {
      EnterExpressionEvaluationContext Unevaluated(
          S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
      ExprResult ER = Subst(E);
      if (ER.isUsable())
        E = ER.get();
      else
        return true;
    }
    return false;
  };
  if (TI.anyScoreOrCondition(SubstScoreOrConditionExpr))
    return;
 
  Expr *E = VariantFuncRef.get();
 
  // Check function/variant ref for `omp declare variant` but not for `omp
  // begin declare variant` (which use implicit attributes).
  std::optional<std::pair<FunctionDecl *, Expr *>> DeclVarData =
      S.checkOpenMPDeclareVariantFunction(S.ConvertDeclToDeclGroup(New), E, TI,
                                          Attr.appendArgs_size(),
                                          Attr.getRange());
 
  if (!DeclVarData)
    return;
 
  E = DeclVarData->second;
  FD = DeclVarData->first;
 
  if (auto *VariantDRE = dyn_cast<DeclRefExpr>(E->IgnoreParenImpCasts())) {
    if (auto *VariantFD = dyn_cast<FunctionDecl>(VariantDRE->getDecl())) {
      if (auto *VariantFTD = VariantFD->getDescribedFunctionTemplate()) {
        if (!VariantFTD->isThisDeclarationADefinition())
          return;
        Sema::TentativeAnalysisScope Trap(S);
        const TemplateArgumentList *TAL = TemplateArgumentList::CreateCopy(
            S.Context, TemplateArgs.getInnermost());
 
        auto *SubstFD = S.InstantiateFunctionDeclaration(VariantFTD, TAL,
                                                         New->getLocation());
        if (!SubstFD)
          return;
        QualType NewType = S.Context.mergeFunctionTypes(
            SubstFD->getType(), FD->getType(),
            /* OfBlockPointer */ false,
            /* Unqualified */ false, /* AllowCXX */ true);
        if (NewType.isNull())
          return;
        S.InstantiateFunctionDefinition(
            New->getLocation(), SubstFD, /* Recursive */ true,
            /* DefinitionRequired */ false, /* AtEndOfTU */ false);
        SubstFD->setInstantiationIsPending(!SubstFD->isDefined());
        E = DeclRefExpr::Create(S.Context, NestedNameSpecifierLoc(),
                                SourceLocation(), SubstFD,
                                /* RefersToEnclosingVariableOrCapture */ false,
                                /* NameLoc */ SubstFD->getLocation(),
                                SubstFD->getType(), ExprValueKind::VK_PRValue);
      }
    }
  }
 
  SmallVector<Expr *, 8> NothingExprs;
  SmallVector<Expr *, 8> NeedDevicePtrExprs;
  SmallVector<OMPInteropInfo, 4> AppendArgs;
 
  for (Expr *E : Attr.adjustArgsNothing()) {
    ExprResult ER = Subst(E);
    if (ER.isInvalid())
      continue;
    NothingExprs.push_back(ER.get());
  }
  for (Expr *E : Attr.adjustArgsNeedDevicePtr()) {
    ExprResult ER = Subst(E);
    if (ER.isInvalid())
      continue;
    NeedDevicePtrExprs.push_back(ER.get());
  }
  for (OMPInteropInfo &II : Attr.appendArgs()) {
    // When prefer_type is implemented for append_args handle them here too.
    AppendArgs.emplace_back(II.IsTarget, II.IsTargetSync);
  }
 
  S.ActOnOpenMPDeclareVariantDirective(
      FD, E, TI, NothingExprs, NeedDevicePtrExprs, AppendArgs, SourceLocation(),
      SourceLocation(), Attr.getRange());
}
 
static void instantiateDependentAMDGPUFlatWorkGroupSizeAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AMDGPUFlatWorkGroupSizeAttr &Attr, Decl *New) {
  // Both min and max expression are constant expressions.
  EnterExpressionEvaluationContext Unevaluated(
      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
  ExprResult Result = S.SubstExpr(Attr.getMin(), TemplateArgs);
  if (Result.isInvalid())
    return;
  Expr *MinExpr = Result.getAs<Expr>();
 
  Result = S.SubstExpr(Attr.getMax(), TemplateArgs);
  if (Result.isInvalid())
    return;
  Expr *MaxExpr = Result.getAs<Expr>();
 
  S.addAMDGPUFlatWorkGroupSizeAttr(New, Attr, MinExpr, MaxExpr);
}
 
static ExplicitSpecifier
instantiateExplicitSpecifier(Sema &S,
                             const MultiLevelTemplateArgumentList &TemplateArgs,
                             ExplicitSpecifier ES, FunctionDecl *New) {
  if (!ES.getExpr())
    return ES;
  Expr *OldCond = ES.getExpr();
  Expr *Cond = nullptr;
  {
    EnterExpressionEvaluationContext Unevaluated(
        S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
    ExprResult SubstResult = S.SubstExpr(OldCond, TemplateArgs);
    if (SubstResult.isInvalid()) {
      return ExplicitSpecifier::Invalid();
    }
    Cond = SubstResult.get();
  }
  ExplicitSpecifier Result(Cond, ES.getKind());
  if (!Cond->isTypeDependent())
    S.tryResolveExplicitSpecifier(Result);
  return Result;
}
 
static void instantiateDependentAMDGPUWavesPerEUAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const AMDGPUWavesPerEUAttr &Attr, Decl *New) {
  // Both min and max expression are constant expressions.
  EnterExpressionEvaluationContext Unevaluated(
      S, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
  ExprResult Result = S.SubstExpr(Attr.getMin(), TemplateArgs);
  if (Result.isInvalid())
    return;
  Expr *MinExpr = Result.getAs<Expr>();
 
  Expr *MaxExpr = nullptr;
  if (auto Max = Attr.getMax()) {
    Result = S.SubstExpr(Max, TemplateArgs);
    if (Result.isInvalid())
      return;
    MaxExpr = Result.getAs<Expr>();
  }
 
  S.addAMDGPUWavesPerEUAttr(New, Attr, MinExpr, MaxExpr);
}
 
// This doesn't take any template parameters, but we have a custom action that
// needs to happen when the kernel itself is instantiated. We need to run the
// ItaniumMangler to mark the names required to name this kernel.
static void instantiateDependentSYCLKernelAttr(
    Sema &S, const MultiLevelTemplateArgumentList &TemplateArgs,
    const SYCLKernelAttr &Attr, Decl *New) {
  New->addAttr(Attr.clone(S.getASTContext()));
}
 
/// Determine whether the attribute A might be relevant to the declaration D.
/// If not, we can skip instantiating it. The attribute may or may not have
/// been instantiated yet.
static bool isRelevantAttr(Sema &S, const Decl *D, const Attr *A) {
  // 'preferred_name' is only relevant to the matching specialization of the
  // template.
  if (const auto *PNA = dyn_cast<PreferredNameAttr>(A)) {
    QualType T = PNA->getTypedefType();
    const auto *RD = cast<CXXRecordDecl>(D);
    if (!T->isDependentType() && !RD->isDependentContext() &&
        !declaresSameEntity(T->getAsCXXRecordDecl(), RD))
      return false;
    for (const auto *ExistingPNA : D->specific_attrs<PreferredNameAttr>())
      if (S.Context.hasSameType(ExistingPNA->getTypedefType(),
                                PNA->getTypedefType()))
        return false;
    return true;
  }
 
  if (const auto *BA = dyn_cast<BuiltinAttr>(A)) {
    const FunctionDecl *FD = dyn_cast<FunctionDecl>(D);
    switch (BA->getID()) {
    case Builtin::BIforward:
      // Do not treat 'std::forward' as a builtin if it takes an rvalue reference
      // type and returns an lvalue reference type. The library implementation
      // will produce an error in this case; don't get in its way.
      if (FD && FD->getNumParams() >= 1 &&
          FD->getParamDecl(0)->getType()->isRValueReferenceType() &&
          FD->getReturnType()->isLValueReferenceType()) {
        return false;
      }
      [[fallthrough]];
    case Builtin::BImove:
    case Builtin::BImove_if_noexcept:
      // HACK: Super-old versions of libc++ (3.1 and earlier) provide
      // std::forward and std::move overloads that sometimes return by value
      // instead of by reference when building in C++98 mode. Don't treat such
      // cases as builtins.
      if (FD && !FD->getReturnType()->isReferenceType())
        return false;
      break;
    }
  }
 
  return true;
}
 
void Sema::InstantiateAttrsForDecl(
    const MultiLevelTemplateArgumentList &TemplateArgs, const Decl *Tmpl,
    Decl *New, LateInstantiatedAttrVec *LateAttrs,
    LocalInstantiationScope *OuterMostScope) {
  if (NamedDecl *ND = dyn_cast<NamedDecl>(New)) {
    // FIXME: This function is called multiple times for the same template
    // specialization. We should only instantiate attributes that were added
    // since the previous instantiation.
    for (const auto *TmplAttr : Tmpl->attrs()) {
      if (!isRelevantAttr(*this, New, TmplAttr))
        continue;
 
      // FIXME: If any of the special case versions from InstantiateAttrs become
      // applicable to template declaration, we'll need to add them here.
      CXXThisScopeRAII ThisScope(
          *this, dyn_cast_or_null<CXXRecordDecl>(ND->getDeclContext()),
          Qualifiers(), ND->isCXXInstanceMember());
 
      Attr *NewAttr = sema::instantiateTemplateAttributeForDecl(
          TmplAttr, Context, *this, TemplateArgs);
      if (NewAttr && isRelevantAttr(*this, New, NewAttr))
        New->addAttr(NewAttr);
    }
  }
}
 
static Sema::RetainOwnershipKind
attrToRetainOwnershipKind(const Attr *A) {
  switch (A->getKind()) {
  case clang::attr::CFConsumed:
    return Sema::RetainOwnershipKind::CF;
  case clang::attr::OSConsumed:
    return Sema::RetainOwnershipKind::OS;
  case clang::attr::NSConsumed:
    return Sema::RetainOwnershipKind::NS;
  default:
    llvm_unreachable("Wrong argument supplied");
  }
}
 
void Sema::InstantiateAttrs(const MultiLevelTemplateArgumentList &TemplateArgs,
                            const Decl *Tmpl, Decl *New,
                            LateInstantiatedAttrVec *LateAttrs,
                            LocalInstantiationScope *OuterMostScope) {
  for (const auto *TmplAttr : Tmpl->attrs()) {
    if (!isRelevantAttr(*this, New, TmplAttr))
      continue;
 
    // FIXME: This should be generalized to more than just the AlignedAttr.
    const AlignedAttr *Aligned = dyn_cast<AlignedAttr>(TmplAttr);
    if (Aligned && Aligned->isAlignmentDependent()) {
      instantiateDependentAlignedAttr(*this, TemplateArgs, Aligned, New);
      continue;
    }
 
    if (const auto *AssumeAligned = dyn_cast<AssumeAlignedAttr>(TmplAttr)) {
      instantiateDependentAssumeAlignedAttr(*this, TemplateArgs, AssumeAligned, New);
      continue;
    }
 
    if (const auto *AlignValue = dyn_cast<AlignValueAttr>(TmplAttr)) {
      instantiateDependentAlignValueAttr(*this, TemplateArgs, AlignValue, New);
      continue;
    }
 
    if (const auto *AllocAlign = dyn_cast<AllocAlignAttr>(TmplAttr)) {
      instantiateDependentAllocAlignAttr(*this, TemplateArgs, AllocAlign, New);
      continue;
    }
 
    if (const auto *Annotate = dyn_cast<AnnotateAttr>(TmplAttr)) {
      instantiateDependentAnnotationAttr(*this, TemplateArgs, Annotate, New);
      continue;
    }
 
    if (const auto *EnableIf = dyn_cast<EnableIfAttr>(TmplAttr)) {
      instantiateDependentEnableIfAttr(*this, TemplateArgs, EnableIf, Tmpl,
                                       cast<FunctionDecl>(New));
      continue;
    }
 
    if (const auto *DiagnoseIf = dyn_cast<DiagnoseIfAttr>(TmplAttr)) {
      instantiateDependentDiagnoseIfAttr(*this, TemplateArgs, DiagnoseIf, Tmpl,
                                         cast<FunctionDecl>(New));
      continue;
    }
 
    if (const auto *CUDALaunchBounds =
            dyn_cast<CUDALaunchBoundsAttr>(TmplAttr)) {
      instantiateDependentCUDALaunchBoundsAttr(*this, TemplateArgs,
                                               *CUDALaunchBounds, New);
      continue;
    }
 
    if (const auto *Mode = dyn_cast<ModeAttr>(TmplAttr)) {
      instantiateDependentModeAttr(*this, TemplateArgs, *Mode, New);
      continue;
    }
 
    if (const auto *OMPAttr = dyn_cast<OMPDeclareSimdDeclAttr>(TmplAttr)) {
      instantiateOMPDeclareSimdDeclAttr(*this, TemplateArgs, *OMPAttr, New);
      continue;
    }
 
    if (const auto *OMPAttr = dyn_cast<OMPDeclareVariantAttr>(TmplAttr)) {
      instantiateOMPDeclareVariantAttr(*this, TemplateArgs, *OMPAttr, New);
      continue;
    }
 
    if (const auto *AMDGPUFlatWorkGroupSize =
            dyn_cast<AMDGPUFlatWorkGroupSizeAttr>(TmplAttr)) {
      instantiateDependentAMDGPUFlatWorkGroupSizeAttr(
          *this, TemplateArgs, *AMDGPUFlatWorkGroupSize, New);
    }
 
    if (const auto *AMDGPUFlatWorkGroupSize =
            dyn_cast<AMDGPUWavesPerEUAttr>(TmplAttr)) {
      instantiateDependentAMDGPUWavesPerEUAttr(*this, TemplateArgs,
                                               *AMDGPUFlatWorkGroupSize, New);
    }
 
    // Existing DLL attribute on the instantiation takes precedence.
    if (TmplAttr->getKind() == attr::DLLExport ||
        TmplAttr->getKind() == attr::DLLImport) {
      if (New->hasAttr<DLLExportAttr>() || New->hasAttr<DLLImportAttr>()) {
        continue;
      }
    }
 
    if (const auto *ABIAttr = dyn_cast<ParameterABIAttr>(TmplAttr)) {
      AddParameterABIAttr(New, *ABIAttr, ABIAttr->getABI());
      continue;
    }
 
    if (isa<NSConsumedAttr>(TmplAttr) || isa<OSConsumedAttr>(TmplAttr) ||
        isa<CFConsumedAttr>(TmplAttr)) {
      AddXConsumedAttr(New, *TmplAttr, attrToRetainOwnershipKind(TmplAttr),
                       /*template instantiation=*/true);
      continue;
    }
 
    if (auto *A = dyn_cast<PointerAttr>(TmplAttr)) {
      if (!New->hasAttr<PointerAttr>())
        New->addAttr(A->clone(Context));
      continue;
    }
 
    if (auto *A = dyn_cast<OwnerAttr>(TmplAttr)) {
      if (!New->hasAttr<OwnerAttr>())
        New->addAttr(A->clone(Context));
      continue;
    }
 
    if (auto *A = dyn_cast<SYCLKernelAttr>(TmplAttr)) {
      instantiateDependentSYCLKernelAttr(*this, TemplateArgs, *A, New);
      continue;
    }
 
    assert(!TmplAttr->isPackExpansion());
    if (TmplAttr->isLateParsed() && LateAttrs) {
      // Late parsed attributes must be instantiated and attached after the
      // enclosing class has been instantiated.  See Sema::InstantiateClass.
      LocalInstantiationScope *Saved = nullptr;
      if (CurrentInstantiationScope)
        Saved = CurrentInstantiationScope->cloneScopes(OuterMostScope);
      LateAttrs->push_back(LateInstantiatedAttribute(TmplAttr, Saved, New));
    } else {
      // Allow 'this' within late-parsed attributes.
      auto *ND = cast<NamedDecl>(New);
      auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(ND->getDeclContext());
      CXXThisScopeRAII ThisScope(*this, ThisContext, Qualifiers(),
                                 ND->isCXXInstanceMember());
 
      Attr *NewAttr = sema::instantiateTemplateAttribute(TmplAttr, Context,
                                                         *this, TemplateArgs);
      if (NewAttr && isRelevantAttr(*this, New, TmplAttr))
        New->addAttr(NewAttr);
    }
  }
}
 
/// Update instantiation attributes after template was late parsed.
///
/// Some attributes are evaluated based on the body of template. If it is
/// late parsed, such attributes cannot be evaluated when declaration is
/// instantiated. This function is used to update instantiation attributes when
/// template definition is ready.
void Sema::updateAttrsForLateParsedTemplate(const Decl *Pattern, Decl *Inst) {
  for (const auto *Attr : Pattern->attrs()) {
    if (auto *A = dyn_cast<StrictFPAttr>(Attr)) {
      if (!Inst->hasAttr<StrictFPAttr>())
        Inst->addAttr(A->clone(getASTContext()));
      continue;
    }
  }
}
 
/// In the MS ABI, we need to instantiate default arguments of dllexported
/// default constructors along with the constructor definition. This allows IR
/// gen to emit a constructor closure which calls the default constructor with
/// its default arguments.
void Sema::InstantiateDefaultCtorDefaultArgs(CXXConstructorDecl *Ctor) {
  assert(Context.getTargetInfo().getCXXABI().isMicrosoft() &&
         Ctor->isDefaultConstructor());
  unsigned NumParams = Ctor->getNumParams();
  if (NumParams == 0)
    return;
  DLLExportAttr *Attr = Ctor->getAttr<DLLExportAttr>();
  if (!Attr)
    return;
  for (unsigned I = 0; I != NumParams; ++I) {
    (void)CheckCXXDefaultArgExpr(Attr->getLocation(), Ctor,
                                   Ctor->getParamDecl(I));
    CleanupVarDeclMarking();
  }
}
 
/// Get the previous declaration of a declaration for the purposes of template
/// instantiation. If this finds a previous declaration, then the previous
/// declaration of the instantiation of D should be an instantiation of the
/// result of this function.
template<typename DeclT>
static DeclT *getPreviousDeclForInstantiation(DeclT *D) {
  DeclT *Result = D->getPreviousDecl();
 
  // If the declaration is within a class, and the previous declaration was
  // merged from a different definition of that class, then we don't have a
  // previous declaration for the purpose of template instantiation.
  if (Result && isa<CXXRecordDecl>(D->getDeclContext()) &&
      D->getLexicalDeclContext() != Result->getLexicalDeclContext())
    return nullptr;
 
  return Result;
}
 
Decl *
TemplateDeclInstantiator::VisitTranslationUnitDecl(TranslationUnitDecl *D) {
  llvm_unreachable("Translation units cannot be instantiated");
}
 
Decl *TemplateDeclInstantiator::VisitHLSLBufferDecl(HLSLBufferDecl *Decl) {
  llvm_unreachable("HLSL buffer declarations cannot be instantiated");
}
 
Decl *
TemplateDeclInstantiator::VisitPragmaCommentDecl(PragmaCommentDecl *D) {
  llvm_unreachable("pragma comment cannot be instantiated");
}
 
Decl *TemplateDeclInstantiator::VisitPragmaDetectMismatchDecl(
    PragmaDetectMismatchDecl *D) {
  llvm_unreachable("pragma comment cannot be instantiated");
}
 
Decl *
TemplateDeclInstantiator::VisitExternCContextDecl(ExternCContextDecl *D) {
  llvm_unreachable("extern \"C\" context cannot be instantiated");
}
 
Decl *TemplateDeclInstantiator::VisitMSGuidDecl(MSGuidDecl *D) {
  llvm_unreachable("GUID declaration cannot be instantiated");
}
 
Decl *TemplateDeclInstantiator::VisitUnnamedGlobalConstantDecl(
    UnnamedGlobalConstantDecl *D) {
  llvm_unreachable("UnnamedGlobalConstantDecl cannot be instantiated");
}
 
Decl *TemplateDeclInstantiator::VisitTemplateParamObjectDecl(
    TemplateParamObjectDecl *D) {
  llvm_unreachable("template parameter objects cannot be instantiated");
}
 
Decl *
TemplateDeclInstantiator::VisitLabelDecl(LabelDecl *D) {
  LabelDecl *Inst = LabelDecl::Create(SemaRef.Context, Owner, D->getLocation(),
                                      D->getIdentifier());
  Owner->addDecl(Inst);
  return Inst;
}
 
Decl *
TemplateDeclInstantiator::VisitNamespaceDecl(NamespaceDecl *D) {
  llvm_unreachable("Namespaces cannot be instantiated");
}
 
Decl *
TemplateDeclInstantiator::VisitNamespaceAliasDecl(NamespaceAliasDecl *D) {
  NamespaceAliasDecl *Inst
    = NamespaceAliasDecl::Create(SemaRef.Context, Owner,
                                 D->getNamespaceLoc(),
                                 D->getAliasLoc(),
                                 D->getIdentifier(),
                                 D->getQualifierLoc(),
                                 D->getTargetNameLoc(),
                                 D->getNamespace());
  Owner->addDecl(Inst);
  return Inst;
}
 
Decl *TemplateDeclInstantiator::InstantiateTypedefNameDecl(TypedefNameDecl *D,
                                                           bool IsTypeAlias) {
  bool Invalid = false;
  TypeSourceInfo *DI = D->getTypeSourceInfo();
  if (DI->getType()->isInstantiationDependentType() ||
      DI->getType()->isVariablyModifiedType()) {
    DI = SemaRef.SubstType(DI, TemplateArgs,
                           D->getLocation(), D->getDeclName());
    if (!DI) {
      Invalid = true;
      DI = SemaRef.Context.getTrivialTypeSourceInfo(SemaRef.Context.IntTy);
    }
  } else {
    SemaRef.MarkDeclarationsReferencedInType(D->getLocation(), DI->getType());
  }
 
  // HACK: 2012-10-23 g++ has a bug where it gets the value kind of ?: wrong.
  // libstdc++ relies upon this bug in its implementation of common_type.  If we
  // happen to be processing that implementation, fake up the g++ ?:
  // semantics. See LWG issue 2141 for more information on the bug.  The bugs
  // are fixed in g++ and libstdc++ 4.9.0 (2014-04-22).
  const DecltypeType *DT = DI->getType()->getAs<DecltypeType>();
  CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(D->getDeclContext());
  if (DT && RD && isa<ConditionalOperator>(DT->getUnderlyingExpr()) &&
      DT->isReferenceType() &&
      RD->getEnclosingNamespaceContext() == SemaRef.getStdNamespace() &&
      RD->getIdentifier() && RD->getIdentifier()->isStr("common_type") &&
      D->getIdentifier() && D->getIdentifier()->isStr("type") &&
      SemaRef.getSourceManager().isInSystemHeader(D->getBeginLoc()))
    // Fold it to the (non-reference) type which g++ would have produced.
    DI = SemaRef.Context.getTrivialTypeSourceInfo(
      DI->getType().getNonReferenceType());
 
  // Create the new typedef
  TypedefNameDecl *Typedef;
  if (IsTypeAlias)
    Typedef = TypeAliasDecl::Create(SemaRef.Context, Owner, D->getBeginLoc(),
                                    D->getLocation(), D->getIdentifier(), DI);
  else
    Typedef = TypedefDecl::Create(SemaRef.Context, Owner, D->getBeginLoc(),
                                  D->getLocation(), D->getIdentifier(), DI);
  if (Invalid)
    Typedef->setInvalidDecl();
 
  // If the old typedef was the name for linkage purposes of an anonymous
  // tag decl, re-establish that relationship for the new typedef.
  if (const TagType *oldTagType = D->getUnderlyingType()->getAs<TagType>()) {
    TagDecl *oldTag = oldTagType->getDecl();
    if (oldTag->getTypedefNameForAnonDecl() == D && !Invalid) {
      TagDecl *newTag = DI->getType()->castAs<TagType>()->getDecl();
      assert(!newTag->hasNameForLinkage());
      newTag->setTypedefNameForAnonDecl(Typedef);
    }
  }
 
  if (TypedefNameDecl *Prev = getPreviousDeclForInstantiation(D)) {
    NamedDecl *InstPrev = SemaRef.FindInstantiatedDecl(D->getLocation(), Prev,
                                                       TemplateArgs);
    if (!InstPrev)
      return nullptr;
 
    TypedefNameDecl *InstPrevTypedef = cast<TypedefNameDecl>(InstPrev);
 
    // If the typedef types are not identical, reject them.
    SemaRef.isIncompatibleTypedef(InstPrevTypedef, Typedef);
 
    Typedef->setPreviousDecl(InstPrevTypedef);
  }
 
  SemaRef.InstantiateAttrs(TemplateArgs, D, Typedef);
 
  if (D->getUnderlyingType()->getAs<DependentNameType>())
    SemaRef.inferGslPointerAttribute(Typedef);
 
  Typedef->setAccess(D->getAccess());
  Typedef->setReferenced(D->isReferenced());
 
  return Typedef;
}
 
Decl *TemplateDeclInstantiator::VisitTypedefDecl(TypedefDecl *D) {
  Decl *Typedef = InstantiateTypedefNameDecl(D, /*IsTypeAlias=*/false);
  if (Typedef)
    Owner->addDecl(Typedef);
  return Typedef;
}
 
Decl *TemplateDeclInstantiator::VisitTypeAliasDecl(TypeAliasDecl *D) {
  Decl *Typedef = InstantiateTypedefNameDecl(D, /*IsTypeAlias=*/true);
  if (Typedef)
    Owner->addDecl(Typedef);
  return Typedef;
}
 
Decl *
TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
  // Create a local instantiation scope for this type alias template, which
  // will contain the instantiations of the template parameters.
  LocalInstantiationScope Scope(SemaRef);
 
  TemplateParameterList *TempParams = D->getTemplateParameters();
  TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
  if (!InstParams)
    return nullptr;
 
  TypeAliasDecl *Pattern = D->getTemplatedDecl();
 
  TypeAliasTemplateDecl *PrevAliasTemplate = nullptr;
  if (getPreviousDeclForInstantiation<TypedefNameDecl>(Pattern)) {
    DeclContext::lookup_result Found = Owner->lookup(Pattern->getDeclName());
    if (!Found.empty()) {
      PrevAliasTemplate = dyn_cast<TypeAliasTemplateDecl>(Found.front());
    }
  }
 
  TypeAliasDecl *AliasInst = cast_or_null<TypeAliasDecl>(
    InstantiateTypedefNameDecl(Pattern, /*IsTypeAlias=*/true));
  if (!AliasInst)
    return nullptr;
 
  TypeAliasTemplateDecl *Inst
    = TypeAliasTemplateDecl::Create(SemaRef.Context, Owner, D->getLocation(),
                                    D->getDeclName(), InstParams, AliasInst);
  AliasInst->setDescribedAliasTemplate(Inst);
  if (PrevAliasTemplate)
    Inst->setPreviousDecl(PrevAliasTemplate);
 
  Inst->setAccess(D->getAccess());
 
  if (!PrevAliasTemplate)
    Inst->setInstantiatedFromMemberTemplate(D);
 
  Owner->addDecl(Inst);
 
  return Inst;
}
 
Decl *TemplateDeclInstantiator::VisitBindingDecl(BindingDecl *D) {
  auto *NewBD = BindingDecl::Create(SemaRef.Context, Owner, D->getLocation(),
                                    D->getIdentifier());
  NewBD->setReferenced(D->isReferenced());
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewBD);
  return NewBD;
}
 
Decl *TemplateDeclInstantiator::VisitDecompositionDecl(DecompositionDecl *D) {
  // Transform the bindings first.
  SmallVector<BindingDecl*, 16> NewBindings;
  for (auto *OldBD : D->bindings())
    NewBindings.push_back(cast<BindingDecl>(VisitBindingDecl(OldBD)));
  ArrayRef<BindingDecl*> NewBindingArray = NewBindings;
 
  auto *NewDD = cast_or_null<DecompositionDecl>(
      VisitVarDecl(D, /*InstantiatingVarTemplate=*/false, &NewBindingArray));
 
  if (!NewDD || NewDD->isInvalidDecl())
    for (auto *NewBD : NewBindings)
      NewBD->setInvalidDecl();
 
  return NewDD;
}
 
Decl *TemplateDeclInstantiator::VisitVarDecl(VarDecl *D) {
  return VisitVarDecl(D, /*InstantiatingVarTemplate=*/false);
}
 
Decl *TemplateDeclInstantiator::VisitVarDecl(VarDecl *D,
                                             bool InstantiatingVarTemplate,
                                             ArrayRef<BindingDecl*> *Bindings) {
 
  // Do substitution on the type of the declaration
  TypeSourceInfo *DI = SemaRef.SubstType(
      D->getTypeSourceInfo(), TemplateArgs, D->getTypeSpecStartLoc(),
      D->getDeclName(), /*AllowDeducedTST*/true);
  if (!DI)
    return nullptr;
 
  if (DI->getType()->isFunctionType()) {
    SemaRef.Diag(D->getLocation(), diag::err_variable_instantiates_to_function)
      << D->isStaticDataMember() << DI->getType();
    return nullptr;
  }
 
  DeclContext *DC = Owner;
  if (D->isLocalExternDecl())
    SemaRef.adjustContextForLocalExternDecl(DC);
 
  // Build the instantiated declaration.
  VarDecl *Var;
  if (Bindings)
    Var = DecompositionDecl::Create(SemaRef.Context, DC, D->getInnerLocStart(),
                                    D->getLocation(), DI->getType(), DI,
                                    D->getStorageClass(), *Bindings);
  else
    Var = VarDecl::Create(SemaRef.Context, DC, D->getInnerLocStart(),
                          D->getLocation(), D->getIdentifier(), DI->getType(),
                          DI, D->getStorageClass());
 
  // In ARC, infer 'retaining' for variables of retainable type.
  if (SemaRef.getLangOpts().ObjCAutoRefCount &&
      SemaRef.inferObjCARCLifetime(Var))
    Var->setInvalidDecl();
 
  if (SemaRef.getLangOpts().OpenCL)
    SemaRef.deduceOpenCLAddressSpace(Var);
 
  // Substitute the nested name specifier, if any.
  if (SubstQualifier(D, Var))
    return nullptr;
 
  SemaRef.BuildVariableInstantiation(Var, D, TemplateArgs, LateAttrs, Owner,
                                     StartingScope, InstantiatingVarTemplate);
  if (D->isNRVOVariable() && !Var->isInvalidDecl()) {
    QualType RT;
    if (auto *F = dyn_cast<FunctionDecl>(DC))
      RT = F->getReturnType();
    else if (isa<BlockDecl>(DC))
      RT = cast<FunctionType>(SemaRef.getCurBlock()->FunctionType)
               ->getReturnType();
    else
      llvm_unreachable("Unknown context type");
 
    // This is the last chance we have of checking copy elision eligibility
    // for functions in dependent contexts. The sema actions for building
    // the return statement during template instantiation will have no effect
    // regarding copy elision, since NRVO propagation runs on the scope exit
    // actions, and these are not run on instantiation.
    // This might run through some VarDecls which were returned from non-taken
    // 'if constexpr' branches, and these will end up being constructed on the
    // return slot even if they will never be returned, as a sort of accidental
    // 'optimization'. Notably, functions with 'auto' return types won't have it
    // deduced by this point. Coupled with the limitation described
    // previously, this makes it very hard to support copy elision for these.
    Sema::NamedReturnInfo Info = SemaRef.getNamedReturnInfo(Var);
    bool NRVO = SemaRef.getCopyElisionCandidate(Info, RT) != nullptr;
    Var->setNRVOVariable(NRVO);
  }
 
  Var->setImplicit(D->isImplicit());
 
  if (Var->isStaticLocal())
    SemaRef.CheckStaticLocalForDllExport(Var);
 
  if (Var->getTLSKind())
    SemaRef.CheckThreadLocalForLargeAlignment(Var);
 
  return Var;
}
 
Decl *TemplateDeclInstantiator::VisitAccessSpecDecl(AccessSpecDecl *D) {
  AccessSpecDecl* AD
    = AccessSpecDecl::Create(SemaRef.Context, D->getAccess(), Owner,
                             D->getAccessSpecifierLoc(), D->getColonLoc());
  Owner->addHiddenDecl(AD);
  return AD;
}
 
Decl *TemplateDeclInstantiator::VisitFieldDecl(FieldDecl *D) {
  bool Invalid = false;
  TypeSourceInfo *DI = D->getTypeSourceInfo();
  if (DI->getType()->isInstantiationDependentType() ||
      DI->getType()->isVariablyModifiedType())  {
    DI = SemaRef.SubstType(DI, TemplateArgs,
                           D->getLocation(), D->getDeclName());
    if (!DI) {
      DI = D->getTypeSourceInfo();
      Invalid = true;
    } else if (DI->getType()->isFunctionType()) {
      // C++ [temp.arg.type]p3:
      //   If a declaration acquires a function type through a type
      //   dependent on a template-parameter and this causes a
      //   declaration that does not use the syntactic form of a
      //   function declarator to have function type, the program is
      //   ill-formed.
      SemaRef.Diag(D->getLocation(), diag::err_field_instantiates_to_function)
        << DI->getType();
      Invalid = true;
    }
  } else {
    SemaRef.MarkDeclarationsReferencedInType(D->getLocation(), DI->getType());
  }
 
  Expr *BitWidth = D->getBitWidth();
  if (Invalid)
    BitWidth = nullptr;
  else if (BitWidth) {
    // The bit-width expression is a constant expression.
    EnterExpressionEvaluationContext Unevaluated(
        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
    ExprResult InstantiatedBitWidth
      = SemaRef.SubstExpr(BitWidth, TemplateArgs);
    if (InstantiatedBitWidth.isInvalid()) {
      Invalid = true;
      BitWidth = nullptr;
    } else
      BitWidth = InstantiatedBitWidth.getAs<Expr>();
  }
 
  FieldDecl *Field = SemaRef.CheckFieldDecl(D->getDeclName(),
                                            DI->getType(), DI,
                                            cast<RecordDecl>(Owner),
                                            D->getLocation(),
                                            D->isMutable(),
                                            BitWidth,
                                            D->getInClassInitStyle(),
                                            D->getInnerLocStart(),
                                            D->getAccess(),
                                            nullptr);
  if (!Field) {
    cast<Decl>(Owner)->setInvalidDecl();
    return nullptr;
  }
 
  SemaRef.InstantiateAttrs(TemplateArgs, D, Field, LateAttrs, StartingScope);
 
  if (Field->hasAttrs())
    SemaRef.CheckAlignasUnderalignment(Field);
 
  if (Invalid)
    Field->setInvalidDecl();
 
  if (!Field->getDeclName()) {
    // Keep track of where this decl came from.
    SemaRef.Context.setInstantiatedFromUnnamedFieldDecl(Field, D);
  }
  if (CXXRecordDecl *Parent= dyn_cast<CXXRecordDecl>(Field->getDeclContext())) {
    if (Parent->isAnonymousStructOrUnion() &&
        Parent->getRedeclContext()->isFunctionOrMethod())
      SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Field);
  }
 
  Field->setImplicit(D->isImplicit());
  Field->setAccess(D->getAccess());
  Owner->addDecl(Field);
 
  return Field;
}
 
Decl *TemplateDeclInstantiator::VisitMSPropertyDecl(MSPropertyDecl *D) {
  bool Invalid = false;
  TypeSourceInfo *DI = D->getTypeSourceInfo();
 
  if (DI->getType()->isVariablyModifiedType()) {
    SemaRef.Diag(D->getLocation(), diag::err_property_is_variably_modified)
      << D;
    Invalid = true;
  } else if (DI->getType()->isInstantiationDependentType())  {
    DI = SemaRef.SubstType(DI, TemplateArgs,
                           D->getLocation(), D->getDeclName());
    if (!DI) {
      DI = D->getTypeSourceInfo();
      Invalid = true;
    } else if (DI->getType()->isFunctionType()) {
      // C++ [temp.arg.type]p3:
      //   If a declaration acquires a function type through a type
      //   dependent on a template-parameter and this causes a
      //   declaration that does not use the syntactic form of a
      //   function declarator to have function type, the program is
      //   ill-formed.
      SemaRef.Diag(D->getLocation(), diag::err_field_instantiates_to_function)
      << DI->getType();
      Invalid = true;
    }
  } else {
    SemaRef.MarkDeclarationsReferencedInType(D->getLocation(), DI->getType());
  }
 
  MSPropertyDecl *Property = MSPropertyDecl::Create(
      SemaRef.Context, Owner, D->getLocation(), D->getDeclName(), DI->getType(),
      DI, D->getBeginLoc(), D->getGetterId(), D->getSetterId());
 
  SemaRef.InstantiateAttrs(TemplateArgs, D, Property, LateAttrs,
                           StartingScope);
 
  if (Invalid)
    Property->setInvalidDecl();
 
  Property->setAccess(D->getAccess());
  Owner->addDecl(Property);
 
  return Property;
}
 
Decl *TemplateDeclInstantiator::VisitIndirectFieldDecl(IndirectFieldDecl *D) {
  NamedDecl **NamedChain =
    new (SemaRef.Context)NamedDecl*[D->getChainingSize()];
 
  int i = 0;
  for (auto *PI : D->chain()) {
    NamedDecl *Next = SemaRef.FindInstantiatedDecl(D->getLocation(), PI,
                                              TemplateArgs);
    if (!Next)
      return nullptr;
 
    NamedChain[i++] = Next;
  }
 
  QualType T = cast<FieldDecl>(NamedChain[i-1])->getType();
  IndirectFieldDecl *IndirectField = IndirectFieldDecl::Create(
      SemaRef.Context, Owner, D->getLocation(), D->getIdentifier(), T,
      {NamedChain, D->getChainingSize()});
 
  for (const auto *Attr : D->attrs())
    IndirectField->addAttr(Attr->clone(SemaRef.Context));
 
  IndirectField->setImplicit(D->isImplicit());
  IndirectField->setAccess(D->getAccess());
  Owner->addDecl(IndirectField);
  return IndirectField;
}
 
Decl *TemplateDeclInstantiator::VisitFriendDecl(FriendDecl *D) {
  // Handle friend type expressions by simply substituting template
  // parameters into the pattern type and checking the result.
  if (TypeSourceInfo *Ty = D->getFriendType()) {
    TypeSourceInfo *InstTy;
    // If this is an unsupported friend, don't bother substituting template
    // arguments into it. The actual type referred to won't be used by any
    // parts of Clang, and may not be valid for instantiating. Just use the
    // same info for the instantiated friend.
    if (D->isUnsupportedFriend()) {
      InstTy = Ty;
    } else {
      InstTy = SemaRef.SubstType(Ty, TemplateArgs,
                                 D->getLocation(), DeclarationName());
    }
    if (!InstTy)
      return nullptr;
 
    FriendDecl *FD = SemaRef.CheckFriendTypeDecl(D->getBeginLoc(),
                                                 D->getFriendLoc(), InstTy);
    if (!FD)
      return nullptr;
 
    FD->setAccess(AS_public);
    FD->setUnsupportedFriend(D->isUnsupportedFriend());
    Owner->addDecl(FD);
    return FD;
  }
 
  NamedDecl *ND = D->getFriendDecl();
  assert(ND && "friend decl must be a decl or a type!");
 
  // All of the Visit implementations for the various potential friend
  // declarations have to be carefully written to work for friend
  // objects, with the most important detail being that the target
  // decl should almost certainly not be placed in Owner.
  Decl *NewND = Visit(ND);
  if (!NewND) return nullptr;
 
  FriendDecl *FD =
    FriendDecl::Create(SemaRef.Context, Owner, D->getLocation(),
                       cast<NamedDecl>(NewND), D->getFriendLoc());
  FD->setAccess(AS_public);
  FD->setUnsupportedFriend(D->isUnsupportedFriend());
  Owner->addDecl(FD);
  return FD;
}
 
Decl *TemplateDeclInstantiator::VisitStaticAssertDecl(StaticAssertDecl *D) {
  Expr *AssertExpr = D->getAssertExpr();
 
  // The expression in a static assertion is a constant expression.
  EnterExpressionEvaluationContext Unevaluated(
      SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
  ExprResult InstantiatedAssertExpr
    = SemaRef.SubstExpr(AssertExpr, TemplateArgs);
  if (InstantiatedAssertExpr.isInvalid())
    return nullptr;
 
  return SemaRef.BuildStaticAssertDeclaration(D->getLocation(),
                                              InstantiatedAssertExpr.get(),
                                              D->getMessage(),
                                              D->getRParenLoc(),
                                              D->isFailed());
}
 
Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
  EnumDecl *PrevDecl = nullptr;
  if (EnumDecl *PatternPrev = getPreviousDeclForInstantiation(D)) {
    NamedDecl *Prev = SemaRef.FindInstantiatedDecl(D->getLocation(),
                                                   PatternPrev,
                                                   TemplateArgs);
    if (!Prev) return nullptr;
    PrevDecl = cast<EnumDecl>(Prev);
  }
 
  EnumDecl *Enum =
      EnumDecl::Create(SemaRef.Context, Owner, D->getBeginLoc(),
                       D->getLocation(), D->getIdentifier(), PrevDecl,
                       D->isScoped(), D->isScopedUsingClassTag(), D->isFixed());
  if (D->isFixed()) {
    if (TypeSourceInfo *TI = D->getIntegerTypeSourceInfo()) {
      // If we have type source information for the underlying type, it means it
      // has been explicitly set by the user. Perform substitution on it before
      // moving on.
      SourceLocation UnderlyingLoc = TI->getTypeLoc().getBeginLoc();
      TypeSourceInfo *NewTI = SemaRef.SubstType(TI, TemplateArgs, UnderlyingLoc,
                                                DeclarationName());
      if (!NewTI || SemaRef.CheckEnumUnderlyingType(NewTI))
        Enum->setIntegerType(SemaRef.Context.IntTy);
      else
        Enum->setIntegerTypeSourceInfo(NewTI);
    } else {
      assert(!D->getIntegerType()->isDependentType()
             && "Dependent type without type source info");
      Enum->setIntegerType(D->getIntegerType());
    }
  }
 
  SemaRef.InstantiateAttrs(TemplateArgs, D, Enum);
 
  Enum->setInstantiationOfMemberEnum(D, TSK_ImplicitInstantiation);
  Enum->setAccess(D->getAccess());
  // Forward the mangling number from the template to the instantiated decl.
  SemaRef.Context.setManglingNumber(Enum, SemaRef.Context.getManglingNumber(D));
  // See if the old tag was defined along with a declarator.
  // If it did, mark the new tag as being associated with that declarator.
  if (DeclaratorDecl *DD = SemaRef.Context.getDeclaratorForUnnamedTagDecl(D))
    SemaRef.Context.addDeclaratorForUnnamedTagDecl(Enum, DD);
  // See if the old tag was defined along with a typedef.
  // If it did, mark the new tag as being associated with that typedef.
  if (TypedefNameDecl *TND = SemaRef.Context.getTypedefNameForUnnamedTagDecl(D))
    SemaRef.Context.addTypedefNameForUnnamedTagDecl(Enum, TND);
  if (SubstQualifier(D, Enum)) return nullptr;
  Owner->addDecl(Enum);
 
  EnumDecl *Def = D->getDefinition();
  if (Def && Def != D) {
    // If this is an out-of-line definition of an enum member template, check
    // that the underlying types match in the instantiation of both
    // declarations.
    if (TypeSourceInfo *TI = Def->getIntegerTypeSourceInfo()) {
      SourceLocation UnderlyingLoc = TI->getTypeLoc().getBeginLoc();
      QualType DefnUnderlying =
        SemaRef.SubstType(TI->getType(), TemplateArgs,
                          UnderlyingLoc, DeclarationName());
      SemaRef.CheckEnumRedeclaration(Def->getLocation(), Def->isScoped(),
                                     DefnUnderlying, /*IsFixed=*/true, Enum);
    }
  }
 
  // C++11 [temp.inst]p1: The implicit instantiation of a class template
  // specialization causes the implicit instantiation of the declarations, but
  // not the definitions of scoped member enumerations.
  //
  // DR1484 clarifies that enumeration definitions inside of a template
  // declaration aren't considered entities that can be separately instantiated
  // from the rest of the entity they are declared inside of.
  if (isDeclWithinFunction(D) ? D == Def : Def && !Enum->isScoped()) {
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Enum);
    InstantiateEnumDefinition(Enum, Def);
  }
 
  return Enum;
}
 
void TemplateDeclInstantiator::InstantiateEnumDefinition(
    EnumDecl *Enum, EnumDecl *Pattern) {
  Enum->startDefinition();
 
  // Update the location to refer to the definition.
  Enum->setLocation(Pattern->getLocation());
 
  SmallVector<Decl*, 4> Enumerators;
 
  EnumConstantDecl *LastEnumConst = nullptr;
  for (auto *EC : Pattern->enumerators()) {
    // The specified value for the enumerator.
    ExprResult Value((Expr *)nullptr);
    if (Expr *UninstValue = EC->getInitExpr()) {
      // The enumerator's value expression is a constant expression.
      EnterExpressionEvaluationContext Unevaluated(
          SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
 
      Value = SemaRef.SubstExpr(UninstValue, TemplateArgs);
    }
 
    // Drop the initial value and continue.
    bool isInvalid = false;
    if (Value.isInvalid()) {
      Value = nullptr;
      isInvalid = true;
    }
 
    EnumConstantDecl *EnumConst
      = SemaRef.CheckEnumConstant(Enum, LastEnumConst,
                                  EC->getLocation(), EC->getIdentifier(),
                                  Value.get());
 
    if (isInvalid) {
      if (EnumConst)
        EnumConst->setInvalidDecl();
      Enum->setInvalidDecl();
    }
 
    if (EnumConst) {
      SemaRef.InstantiateAttrs(TemplateArgs, EC, EnumConst);
 
      EnumConst->setAccess(Enum->getAccess());
      Enum->addDecl(EnumConst);
      Enumerators.push_back(EnumConst);
      LastEnumConst = EnumConst;
 
      if (Pattern->getDeclContext()->isFunctionOrMethod() &&
          !Enum->isScoped()) {
        // If the enumeration is within a function or method, record the enum
        // constant as a local.
        SemaRef.CurrentInstantiationScope->InstantiatedLocal(EC, EnumConst);
      }
    }
  }
 
  SemaRef.ActOnEnumBody(Enum->getLocation(), Enum->getBraceRange(), Enum,
                        Enumerators, nullptr, ParsedAttributesView());
}
 
Decl *TemplateDeclInstantiator::VisitEnumConstantDecl(EnumConstantDecl *D) {
  llvm_unreachable("EnumConstantDecls can only occur within EnumDecls.");
}
 
Decl *
TemplateDeclInstantiator::VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D) {
  llvm_unreachable("BuiltinTemplateDecls cannot be instantiated.");
}
 
Decl *TemplateDeclInstantiator::VisitClassTemplateDecl(ClassTemplateDecl *D) {
  bool isFriend = (D->getFriendObjectKind() != Decl::FOK_None);
 
  // Create a local instantiation scope for this class template, which
  // will contain the instantiations of the template parameters.
  LocalInstantiationScope Scope(SemaRef);
  TemplateParameterList *TempParams = D->getTemplateParameters();
  TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
  if (!InstParams)
    return nullptr;
 
  CXXRecordDecl *Pattern = D->getTemplatedDecl();
 
  // Instantiate the qualifier.  We have to do this first in case
  // we're a friend declaration, because if we are then we need to put
  // the new declaration in the appropriate context.
  NestedNameSpecifierLoc QualifierLoc = Pattern->getQualifierLoc();
  if (QualifierLoc) {
    QualifierLoc = SemaRef.SubstNestedNameSpecifierLoc(QualifierLoc,
                                                       TemplateArgs);
    if (!QualifierLoc)
      return nullptr;
  }
 
  CXXRecordDecl *PrevDecl = nullptr;
  ClassTemplateDecl *PrevClassTemplate = nullptr;
 
  if (!isFriend && getPreviousDeclForInstantiation(Pattern)) {
    DeclContext::lookup_result Found = Owner->lookup(Pattern->getDeclName());
    if (!Found.empty()) {
      PrevClassTemplate = dyn_cast<ClassTemplateDecl>(Found.front());
      if (PrevClassTemplate)
        PrevDecl = PrevClassTemplate->getTemplatedDecl();
    }
  }
 
  // If this isn't a friend, then it's a member template, in which
  // case we just want to build the instantiation in the
  // specialization.  If it is a friend, we want to build it in
  // the appropriate context.
  DeclContext *DC = Owner;
  if (isFriend) {
    if (QualifierLoc) {
      CXXScopeSpec SS;
      SS.Adopt(QualifierLoc);
      DC = SemaRef.computeDeclContext(SS);
      if (!DC) return nullptr;
    } else {
      DC = SemaRef.FindInstantiatedContext(Pattern->getLocation(),
                                           Pattern->getDeclContext(),
                                           TemplateArgs);
    }
 
    // Look for a previous declaration of the template in the owning
    // context.
    LookupResult R(SemaRef, Pattern->getDeclName(), Pattern->getLocation(),
                   Sema::LookupOrdinaryName,
                   SemaRef.forRedeclarationInCurContext());
    SemaRef.LookupQualifiedName(R, DC);
 
    if (R.isSingleResult()) {
      PrevClassTemplate = R.getAsSingle<ClassTemplateDecl>();
      if (PrevClassTemplate)
        PrevDecl = PrevClassTemplate->getTemplatedDecl();
    }
 
    if (!PrevClassTemplate && QualifierLoc) {
      SemaRef.Diag(Pattern->getLocation(), diag::err_not_tag_in_scope)
        << D->getTemplatedDecl()->getTagKind() << Pattern->getDeclName() << DC
        << QualifierLoc.getSourceRange();
      return nullptr;
    }
 
    if (PrevClassTemplate) {
      const ClassTemplateDecl *MostRecentPrevCT =
          PrevClassTemplate->getMostRecentDecl();
      TemplateParameterList *PrevParams =
          MostRecentPrevCT->getTemplateParameters();
 
      // Make sure the parameter lists match.
      if (!SemaRef.TemplateParameterListsAreEqual(
              D->getTemplatedDecl(), InstParams,
              MostRecentPrevCT->getTemplatedDecl(), PrevParams, true,
              Sema::TPL_TemplateMatch))
        return nullptr;
 
      // Do some additional validation, then merge default arguments
      // from the existing declarations.
      if (SemaRef.CheckTemplateParameterList(InstParams, PrevParams,
                                             Sema::TPC_ClassTemplate))
        return nullptr;
    }
  }
 
  CXXRecordDecl *RecordInst = CXXRecordDecl::Create(
      SemaRef.Context, Pattern->getTagKind(), DC, Pattern->getBeginLoc(),
      Pattern->getLocation(), Pattern->getIdentifier(), PrevDecl,
      /*DelayTypeCreation=*/true);
 
  if (QualifierLoc)
    RecordInst->setQualifierInfo(QualifierLoc);
 
  SemaRef.InstantiateAttrsForDecl(TemplateArgs, Pattern, RecordInst, LateAttrs,
                                                              StartingScope);
 
  ClassTemplateDecl *Inst
    = ClassTemplateDecl::Create(SemaRef.Context, DC, D->getLocation(),
                                D->getIdentifier(), InstParams, RecordInst);
  assert(!(isFriend && Owner->isDependentContext()));
  Inst->setPreviousDecl(PrevClassTemplate);
 
  RecordInst->setDescribedClassTemplate(Inst);
 
  if (isFriend) {
    if (PrevClassTemplate)
      Inst->setAccess(PrevClassTemplate->getAccess());
    else
      Inst->setAccess(D->getAccess());
 
    Inst->setObjectOfFriendDecl();
    // TODO: do we want to track the instantiation progeny of this
    // friend target decl?
  } else {
    Inst->setAccess(D->getAccess());
    if (!PrevClassTemplate)
      Inst->setInstantiatedFromMemberTemplate(D);
  }
 
  // Trigger creation of the type for the instantiation.
  SemaRef.Context.getInjectedClassNameType(RecordInst,
                                    Inst->getInjectedClassNameSpecialization());
 
  // Finish handling of friends.
  if (isFriend) {
    DC->makeDeclVisibleInContext(Inst);
    Inst->setLexicalDeclContext(Owner);
    RecordInst->setLexicalDeclContext(Owner);
    return Inst;
  }
 
  if (D->isOutOfLine()) {
    Inst->setLexicalDeclContext(D->getLexicalDeclContext());
    RecordInst->setLexicalDeclContext(D->getLexicalDeclContext());
  }
 
  Owner->addDecl(Inst);
 
  if (!PrevClassTemplate) {
    // Queue up any out-of-line partial specializations of this member
    // class template; the client will force their instantiation once
    // the enclosing class has been instantiated.
    SmallVector<ClassTemplatePartialSpecializationDecl *, 4> PartialSpecs;
    D->getPartialSpecializations(PartialSpecs);
    for (unsigned I = 0, N = PartialSpecs.size(); I != N; ++I)
      if (PartialSpecs[I]->getFirstDecl()->isOutOfLine())
        OutOfLinePartialSpecs.push_back(std::make_pair(Inst, PartialSpecs[I]));
  }
 
  return Inst;
}
 
Decl *
TemplateDeclInstantiator::VisitClassTemplatePartialSpecializationDecl(
                                   ClassTemplatePartialSpecializationDecl *D) {
  ClassTemplateDecl *ClassTemplate = D->getSpecializedTemplate();
 
  // Lookup the already-instantiated declaration in the instantiation
  // of the class template and return that.
  DeclContext::lookup_result Found
    = Owner->lookup(ClassTemplate->getDeclName());
  if (Found.empty())
    return nullptr;
 
  ClassTemplateDecl *InstClassTemplate
    = dyn_cast<ClassTemplateDecl>(Found.front());
  if (!InstClassTemplate)
    return nullptr;
 
  if (ClassTemplatePartialSpecializationDecl *Result
        = InstClassTemplate->findPartialSpecInstantiatedFromMember(D))
    return Result;
 
  return InstantiateClassTemplatePartialSpecialization(InstClassTemplate, D);
}
 
Decl *TemplateDeclInstantiator::VisitVarTemplateDecl(VarTemplateDecl *D) {
  assert(D->getTemplatedDecl()->isStaticDataMember() &&
         "Only static data member templates are allowed.");
 
  // Create a local instantiation scope for this variable template, which
  // will contain the instantiations of the template parameters.
  LocalInstantiationScope Scope(SemaRef);
  TemplateParameterList *TempParams = D->getTemplateParameters();
  TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
  if (!InstParams)
    return nullptr;
 
  VarDecl *Pattern = D->getTemplatedDecl();
  VarTemplateDecl *PrevVarTemplate = nullptr;
 
  if (getPreviousDeclForInstantiation(Pattern)) {
    DeclContext::lookup_result Found = Owner->lookup(Pattern->getDeclName());
    if (!Found.empty())
      PrevVarTemplate = dyn_cast<VarTemplateDecl>(Found.front());
  }
 
  VarDecl *VarInst =
      cast_or_null<VarDecl>(VisitVarDecl(Pattern,
                                         /*InstantiatingVarTemplate=*/true));
  if (!VarInst) return nullptr;
 
  DeclContext *DC = Owner;
 
  VarTemplateDecl *Inst = VarTemplateDecl::Create(
      SemaRef.Context, DC, D->getLocation(), D->getIdentifier(), InstParams,
      VarInst);
  VarInst->setDescribedVarTemplate(Inst);
  Inst->setPreviousDecl(PrevVarTemplate);
 
  Inst->setAccess(D->getAccess());
  if (!PrevVarTemplate)
    Inst->setInstantiatedFromMemberTemplate(D);
 
  if (D->isOutOfLine()) {
    Inst->setLexicalDeclContext(D->getLexicalDeclContext());
    VarInst->setLexicalDeclContext(D->getLexicalDeclContext());
  }
 
  Owner->addDecl(Inst);
 
  if (!PrevVarTemplate) {
    // Queue up any out-of-line partial specializations of this member
    // variable template; the client will force their instantiation once
    // the enclosing class has been instantiated.
    SmallVector<VarTemplatePartialSpecializationDecl *, 4> PartialSpecs;
    D->getPartialSpecializations(PartialSpecs);
    for (unsigned I = 0, N = PartialSpecs.size(); I != N; ++I)
      if (PartialSpecs[I]->getFirstDecl()->isOutOfLine())
        OutOfLineVarPartialSpecs.push_back(
            std::make_pair(Inst, PartialSpecs[I]));
  }
 
  return Inst;
}
 
Decl *TemplateDeclInstantiator::VisitVarTemplatePartialSpecializationDecl(
    VarTemplatePartialSpecializationDecl *D) {
  assert(D->isStaticDataMember() &&
         "Only static data member templates are allowed.");
 
  VarTemplateDecl *VarTemplate = D->getSpecializedTemplate();
 
  // Lookup the already-instantiated declaration and return that.
  DeclContext::lookup_result Found = Owner->lookup(VarTemplate->getDeclName());
  assert(!Found.empty() && "Instantiation found nothing?");
 
  VarTemplateDecl *InstVarTemplate = dyn_cast<VarTemplateDecl>(Found.front());
  assert(InstVarTemplate && "Instantiation did not find a variable template?");
 
  if (VarTemplatePartialSpecializationDecl *Result =
          InstVarTemplate->findPartialSpecInstantiatedFromMember(D))
    return Result;
 
  return InstantiateVarTemplatePartialSpecialization(InstVarTemplate, D);
}
 
Decl *
TemplateDeclInstantiator::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
  // Create a local instantiation scope for this function template, which
  // will contain the instantiations of the template parameters and then get
  // merged with the local instantiation scope for the function template
  // itself.
  LocalInstantiationScope Scope(SemaRef);
  Sema::ConstraintEvalRAII<TemplateDeclInstantiator> RAII(*this);
 
  TemplateParameterList *TempParams = D->getTemplateParameters();
  TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
  if (!InstParams)
    return nullptr;
 
  FunctionDecl *Instantiated = nullptr;
  if (CXXMethodDecl *DMethod = dyn_cast<CXXMethodDecl>(D->getTemplatedDecl()))
    Instantiated = cast_or_null<FunctionDecl>(VisitCXXMethodDecl(DMethod,
                                                                 InstParams));
  else
    Instantiated = cast_or_null<FunctionDecl>(VisitFunctionDecl(
                                                          D->getTemplatedDecl(),
                                                                InstParams));
 
  if (!Instantiated)
    return nullptr;
 
  // Link the instantiated function template declaration to the function
  // template from which it was instantiated.
  FunctionTemplateDecl *InstTemplate
    = Instantiated->getDescribedFunctionTemplate();
  InstTemplate->setAccess(D->getAccess());
  assert(InstTemplate &&
         "VisitFunctionDecl/CXXMethodDecl didn't create a template!");
 
  bool isFriend = (InstTemplate->getFriendObjectKind() != Decl::FOK_None);
 
  // Link the instantiation back to the pattern *unless* this is a
  // non-definition friend declaration.
  if (!InstTemplate->getInstantiatedFromMemberTemplate() &&
      !(isFriend && !D->getTemplatedDecl()->isThisDeclarationADefinition()))
    InstTemplate->setInstantiatedFromMemberTemplate(D);
 
  // Make declarations visible in the appropriate context.
  if (!isFriend) {
    Owner->addDecl(InstTemplate);
  } else if (InstTemplate->getDeclContext()->isRecord() &&
             !getPreviousDeclForInstantiation(D)) {
    SemaRef.CheckFriendAccess(InstTemplate);
  }
 
  return InstTemplate;
}
 
Decl *TemplateDeclInstantiator::VisitCXXRecordDecl(CXXRecordDecl *D) {
  CXXRecordDecl *PrevDecl = nullptr;
  if (CXXRecordDecl *PatternPrev = getPreviousDeclForInstantiation(D)) {
    NamedDecl *Prev = SemaRef.FindInstantiatedDecl(D->getLocation(),
                                                   PatternPrev,
                                                   TemplateArgs);
    if (!Prev) return nullptr;
    PrevDecl = cast<CXXRecordDecl>(Prev);
  }
 
  CXXRecordDecl *Record = nullptr;
  bool IsInjectedClassName = D->isInjectedClassName();
  if (D->isLambda())
    Record = CXXRecordDecl::CreateLambda(
        SemaRef.Context, Owner, D->getLambdaTypeInfo(), D->getLocation(),
        D->getLambdaDependencyKind(), D->isGenericLambda(),
        D->getLambdaCaptureDefault());
  else
    Record = CXXRecordDecl::Create(SemaRef.Context, D->getTagKind(), Owner,
                                   D->getBeginLoc(), D->getLocation(),
                                   D->getIdentifier(), PrevDecl,
                                   /*DelayTypeCreation=*/IsInjectedClassName);
  // Link the type of the injected-class-name to that of the outer class.
  if (IsInjectedClassName)
    (void)SemaRef.Context.getTypeDeclType(Record, cast<CXXRecordDecl>(Owner));
 
  // Substitute the nested name specifier, if any.
  if (SubstQualifier(D, Record))
    return nullptr;
 
  SemaRef.InstantiateAttrsForDecl(TemplateArgs, D, Record, LateAttrs,
                                                              StartingScope);
 
  Record->setImplicit(D->isImplicit());
  // FIXME: Check against AS_none is an ugly hack to work around the issue that
  // the tag decls introduced by friend class declarations don't have an access
  // specifier. Remove once this area of the code gets sorted out.
  if (D->getAccess() != AS_none)
    Record->setAccess(D->getAccess());
  if (!IsInjectedClassName)
    Record->setInstantiationOfMemberClass(D, TSK_ImplicitInstantiation);
 
  // If the original function was part of a friend declaration,
  // inherit its namespace state.
  if (D->getFriendObjectKind())
    Record->setObjectOfFriendDecl();
 
  // Make sure that anonymous structs and unions are recorded.
  if (D->isAnonymousStructOrUnion())
    Record->setAnonymousStructOrUnion(true);
 
  if (D->isLocalClass())
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Record);
 
  // Forward the mangling number from the template to the instantiated decl.
  SemaRef.Context.setManglingNumber(Record,
                                    SemaRef.Context.getManglingNumber(D));
 
  // See if the old tag was defined along with a declarator.
  // If it did, mark the new tag as being associated with that declarator.
  if (DeclaratorDecl *DD = SemaRef.Context.getDeclaratorForUnnamedTagDecl(D))
    SemaRef.Context.addDeclaratorForUnnamedTagDecl(Record, DD);
 
  // See if the old tag was defined along with a typedef.
  // If it did, mark the new tag as being associated with that typedef.
  if (TypedefNameDecl *TND = SemaRef.Context.getTypedefNameForUnnamedTagDecl(D))
    SemaRef.Context.addTypedefNameForUnnamedTagDecl(Record, TND);
 
  Owner->addDecl(Record);
 
  // DR1484 clarifies that the members of a local class are instantiated as part
  // of the instantiation of their enclosing entity.
  if (D->isCompleteDefinition() && D->isLocalClass()) {
    Sema::LocalEagerInstantiationScope LocalInstantiations(SemaRef);
 
    SemaRef.InstantiateClass(D->getLocation(), Record, D, TemplateArgs,
                             TSK_ImplicitInstantiation,
                             /*Complain=*/true);
 
    // For nested local classes, we will instantiate the members when we
    // reach the end of the outermost (non-nested) local class.
    if (!D->isCXXClassMember())
      SemaRef.InstantiateClassMembers(D->getLocation(), Record, TemplateArgs,
                                      TSK_ImplicitInstantiation);
 
    // This class may have local implicit instantiations that need to be
    // performed within this scope.
    LocalInstantiations.perform();
  }
 
  SemaRef.DiagnoseUnusedNestedTypedefs(Record);
 
  if (IsInjectedClassName)
    assert(Record->isInjectedClassName() && "Broken injected-class-name");
 
  return Record;
}
 
/// Adjust the given function type for an instantiation of the
/// given declaration, to cope with modifications to the function's type that
/// aren't reflected in the type-source information.
///
/// \param D The declaration we're instantiating.
/// \param TInfo The already-instantiated type.
static QualType adjustFunctionTypeForInstantiation(ASTContext &Context,
                                                   FunctionDecl *D,
                                                   TypeSourceInfo *TInfo) {
  const FunctionProtoType *OrigFunc
    = D->getType()->castAs<FunctionProtoType>();
  const FunctionProtoType *NewFunc
    = TInfo->getType()->castAs<FunctionProtoType>();
  if (OrigFunc->getExtInfo() == NewFunc->getExtInfo())
    return TInfo->getType();
 
  FunctionProtoType::ExtProtoInfo NewEPI = NewFunc->getExtProtoInfo();
  NewEPI.ExtInfo = OrigFunc->getExtInfo();
  return Context.getFunctionType(NewFunc->getReturnType(),
                                 NewFunc->getParamTypes(), NewEPI);
}
 
/// Normal class members are of more specific types and therefore
/// don't make it here.  This function serves three purposes:
///   1) instantiating function templates
///   2) substituting friend and local function declarations
///   3) substituting deduction guide declarations for nested class templates
Decl *TemplateDeclInstantiator::VisitFunctionDecl(
    FunctionDecl *D, TemplateParameterList *TemplateParams,
    RewriteKind FunctionRewriteKind) {
  // Check whether there is already a function template specialization for
  // this declaration.
  FunctionTemplateDecl *FunctionTemplate = D->getDescribedFunctionTemplate();
  if (FunctionTemplate && !TemplateParams) {
    ArrayRef<TemplateArgument> Innermost = TemplateArgs.getInnermost();
 
    void *InsertPos = nullptr;
    FunctionDecl *SpecFunc
      = FunctionTemplate->findSpecialization(Innermost, InsertPos);
 
    // If we already have a function template specialization, return it.
    if (SpecFunc)
      return SpecFunc;
  }
 
  bool isFriend;
  if (FunctionTemplate)
    isFriend = (FunctionTemplate->getFriendObjectKind() != Decl::FOK_None);
  else
    isFriend = (D->getFriendObjectKind() != Decl::FOK_None);
 
  bool MergeWithParentScope = (TemplateParams != nullptr) ||
    Owner->isFunctionOrMethod() ||
    !(isa<Decl>(Owner) &&
      cast<Decl>(Owner)->isDefinedOutsideFunctionOrMethod());
  LocalInstantiationScope Scope(SemaRef, MergeWithParentScope);
 
  ExplicitSpecifier InstantiatedExplicitSpecifier;
  if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
    InstantiatedExplicitSpecifier = instantiateExplicitSpecifier(
        SemaRef, TemplateArgs, DGuide->getExplicitSpecifier(), DGuide);
    if (InstantiatedExplicitSpecifier.isInvalid())
      return nullptr;
  }
 
  SmallVector<ParmVarDecl *, 4> Params;
  TypeSourceInfo *TInfo = SubstFunctionType(D, Params);
  if (!TInfo)
    return nullptr;
  QualType T = adjustFunctionTypeForInstantiation(SemaRef.Context, D, TInfo);
 
  if (TemplateParams && TemplateParams->size()) {
    auto *LastParam =
        dyn_cast<TemplateTypeParmDecl>(TemplateParams->asArray().back());
    if (LastParam && LastParam->isImplicit() &&
        LastParam->hasTypeConstraint()) {
      // In abbreviated templates, the type-constraints of invented template
      // type parameters are instantiated with the function type, invalidating
      // the TemplateParameterList which relied on the template type parameter
      // not having a type constraint. Recreate the TemplateParameterList with
      // the updated parameter list.
      TemplateParams = TemplateParameterList::Create(
          SemaRef.Context, TemplateParams->getTemplateLoc(),
          TemplateParams->getLAngleLoc(), TemplateParams->asArray(),
          TemplateParams->getRAngleLoc(), TemplateParams->getRequiresClause());
    }
  }
 
  NestedNameSpecifierLoc QualifierLoc = D->getQualifierLoc();
  if (QualifierLoc) {
    QualifierLoc = SemaRef.SubstNestedNameSpecifierLoc(QualifierLoc,
                                                       TemplateArgs);
    if (!QualifierLoc)
      return nullptr;
  }
 
  Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
 
  // If we're instantiating a local function declaration, put the result
  // in the enclosing namespace; otherwise we need to find the instantiated
  // context.
  DeclContext *DC;
  if (D->isLocalExternDecl()) {
    DC = Owner;
    SemaRef.adjustContextForLocalExternDecl(DC);
  } else if (isFriend && QualifierLoc) {
    CXXScopeSpec SS;
    SS.Adopt(QualifierLoc);
    DC = SemaRef.computeDeclContext(SS);
    if (!DC) return nullptr;
  } else {
    DC = SemaRef.FindInstantiatedContext(D->getLocation(), D->getDeclContext(),
                                         TemplateArgs);
  }
 
  DeclarationNameInfo NameInfo
    = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
 
  if (FunctionRewriteKind != RewriteKind::None)
    adjustForRewrite(FunctionRewriteKind, D, T, TInfo, NameInfo);
 
  FunctionDecl *Function;
  if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
    Function = CXXDeductionGuideDecl::Create(
        SemaRef.Context, DC, D->getInnerLocStart(),
        InstantiatedExplicitSpecifier, NameInfo, T, TInfo,
        D->getSourceRange().getEnd());
    if (DGuide->isCopyDeductionCandidate())
      cast<CXXDeductionGuideDecl>(Function)->setIsCopyDeductionCandidate();
    Function->setAccess(D->getAccess());
  } else {
    Function = FunctionDecl::Create(
        SemaRef.Context, DC, D->getInnerLocStart(), NameInfo, T, TInfo,
        D->getCanonicalDecl()->getStorageClass(), D->UsesFPIntrin(),
        D->isInlineSpecified(), D->hasWrittenPrototype(), D->getConstexprKind(),
        TrailingRequiresClause);
    Function->setFriendConstraintRefersToEnclosingTemplate(
        D->FriendConstraintRefersToEnclosingTemplate());
    Function->setRangeEnd(D->getSourceRange().getEnd());
  }
 
  if (D->isInlined())
    Function->setImplicitlyInline();
 
  if (QualifierLoc)
    Function->setQualifierInfo(QualifierLoc);
 
  if (D->isLocalExternDecl())
    Function->setLocalExternDecl();
 
  DeclContext *LexicalDC = Owner;
  if (!isFriend && D->isOutOfLine() && !D->isLocalExternDecl()) {
    assert(D->getDeclContext()->isFileContext());
    LexicalDC = D->getDeclContext();
  }
  else if (D->isLocalExternDecl()) {
    LexicalDC = SemaRef.CurContext;
  }
 
  Function->setLexicalDeclContext(LexicalDC);
 
  // Attach the parameters
  for (unsigned P = 0; P < Params.size(); ++P)
    if (Params[P])
      Params[P]->setOwningFunction(Function);
  Function->setParams(Params);
 
  if (TrailingRequiresClause)
    Function->setTrailingRequiresClause(TrailingRequiresClause);
 
  if (TemplateParams) {
    // Our resulting instantiation is actually a function template, since we
    // are substituting only the outer template parameters. For example, given
    //
    //   template<typename T>
    //   struct X {
    //     template<typename U> friend void f(T, U);
    //   };
    //
    //   X<int> x;
    //
    // We are instantiating the friend function template "f" within X<int>,
    // which means substituting int for T, but leaving "f" as a friend function
    // template.
    // Build the function template itself.
    FunctionTemplate = FunctionTemplateDecl::Create(SemaRef.Context, DC,
                                                    Function->getLocation(),
                                                    Function->getDeclName(),
                                                    TemplateParams, Function);
    Function->setDescribedFunctionTemplate(FunctionTemplate);
 
    FunctionTemplate->setLexicalDeclContext(LexicalDC);
 
    if (isFriend && D->isThisDeclarationADefinition()) {
      FunctionTemplate->setInstantiatedFromMemberTemplate(
                                           D->getDescribedFunctionTemplate());
    }
  } else if (FunctionTemplate) {
    // Record this function template specialization.
    ArrayRef<TemplateArgument> Innermost = TemplateArgs.getInnermost();
    Function->setFunctionTemplateSpecialization(FunctionTemplate,
                            TemplateArgumentList::CreateCopy(SemaRef.Context,
                                                             Innermost),
                                                /*InsertPos=*/nullptr);
  } else if (isFriend && D->isThisDeclarationADefinition()) {
    // Do not connect the friend to the template unless it's actually a
    // definition. We don't want non-template functions to be marked as being
    // template instantiations.
    Function->setInstantiationOfMemberFunction(D, TSK_ImplicitInstantiation);
  } else if (!isFriend) {
    // If this is not a function template, and this is not a friend (that is,
    // this is a locally declared function), save the instantiation relationship
    // for the purposes of constraint instantiation.
    Function->setInstantiatedFromDecl(D);
  }
 
  if (isFriend) {
    Function->setObjectOfFriendDecl();
    if (FunctionTemplateDecl *FT = Function->getDescribedFunctionTemplate())
      FT->setObjectOfFriendDecl();
  }
 
  if (InitFunctionInstantiation(Function, D))
    Function->setInvalidDecl();
 
  bool IsExplicitSpecialization = false;
 
  LookupResult Previous(
      SemaRef, Function->getDeclName(), SourceLocation(),
      D->isLocalExternDecl() ? Sema::LookupRedeclarationWithLinkage
                             : Sema::LookupOrdinaryName,
      D->isLocalExternDecl() ? Sema::ForExternalRedeclaration
                             : SemaRef.forRedeclarationInCurContext());
 
  if (DependentFunctionTemplateSpecializationInfo *Info
        = D->getDependentSpecializationInfo()) {
    assert(isFriend && "non-friend has dependent specialization info?");
 
    // Instantiate the explicit template arguments.
    TemplateArgumentListInfo ExplicitArgs(Info->getLAngleLoc(),
                                          Info->getRAngleLoc());
    if (SemaRef.SubstTemplateArguments(Info->arguments(), TemplateArgs,
                                       ExplicitArgs))
      return nullptr;
 
    // Map the candidate templates to their instantiations.
    for (unsigned I = 0, E = Info->getNumTemplates(); I != E; ++I) {
      Decl *Temp = SemaRef.FindInstantiatedDecl(D->getLocation(),
                                                Info->getTemplate(I),
                                                TemplateArgs);
      if (!Temp) return nullptr;
 
      Previous.addDecl(cast<FunctionTemplateDecl>(Temp));
    }
 
    if (SemaRef.CheckFunctionTemplateSpecialization(Function,
                                                    &ExplicitArgs,
                                                    Previous))
      Function->setInvalidDecl();
 
    IsExplicitSpecialization = true;
  } else if (const ASTTemplateArgumentListInfo *Info =
                 D->getTemplateSpecializationArgsAsWritten()) {
    // The name of this function was written as a template-id.
    SemaRef.LookupQualifiedName(Previous, DC);
 
    // Instantiate the explicit template arguments.
    TemplateArgumentListInfo ExplicitArgs(Info->getLAngleLoc(),
                                          Info->getRAngleLoc());
    if (SemaRef.SubstTemplateArguments(Info->arguments(), TemplateArgs,
                                       ExplicitArgs))
      return nullptr;
 
    if (SemaRef.CheckFunctionTemplateSpecialization(Function,
                                                    &ExplicitArgs,
                                                    Previous))
      Function->setInvalidDecl();
 
    IsExplicitSpecialization = true;
  } else if (TemplateParams || !FunctionTemplate) {
    // Look only into the namespace where the friend would be declared to
    // find a previous declaration. This is the innermost enclosing namespace,
    // as described in ActOnFriendFunctionDecl.
    SemaRef.LookupQualifiedName(Previous, DC->getRedeclContext());
 
    // In C++, the previous declaration we find might be a tag type
    // (class or enum). In this case, the new declaration will hide the
    // tag type. Note that this does not apply if we're declaring a
    // typedef (C++ [dcl.typedef]p4).
    if (Previous.isSingleTagDecl())
      Previous.clear();
 
    // Filter out previous declarations that don't match the scope. The only
    // effect this has is to remove declarations found in inline namespaces
    // for friend declarations with unqualified names.
    if (isFriend && !QualifierLoc && !FunctionTemplate) {
      SemaRef.FilterLookupForScope(Previous, DC, /*Scope=*/ nullptr,
                                   /*ConsiderLinkage=*/ true,
                                   QualifierLoc.hasQualifier());
    }
  }
 
  // Per [temp.inst], default arguments in function declarations at local scope
  // are instantiated along with the enclosing declaration. For example:
  //
  //   template<typename T>
  //   void ft() {
  //     void f(int = []{ return T::value; }());
  //   }
  //   template void ft<int>(); // error: type 'int' cannot be used prior
  //                                      to '::' because it has no members
  //
  // The error is issued during instantiation of ft<int>() because substitution
  // into the default argument fails; the default argument is instantiated even
  // though it is never used.
  if (Function->isLocalExternDecl()) {
    for (ParmVarDecl *PVD : Function->parameters()) {
      if (!PVD->hasDefaultArg())
        continue;
      if (SemaRef.SubstDefaultArgument(D->getInnerLocStart(), PVD, TemplateArgs)) {
        // If substitution fails, the default argument is set to a
        // RecoveryExpr that wraps the uninstantiated default argument so
        // that downstream diagnostics are omitted.
        Expr *UninstExpr = PVD->getUninstantiatedDefaultArg();
        ExprResult ErrorResult = SemaRef.CreateRecoveryExpr(
            UninstExpr->getBeginLoc(), UninstExpr->getEndLoc(),
            { UninstExpr }, UninstExpr->getType());
        if (ErrorResult.isUsable())
          PVD->setDefaultArg(ErrorResult.get());
      }
    }
  }
 
  SemaRef.CheckFunctionDeclaration(/*Scope*/ nullptr, Function, Previous,
                                   IsExplicitSpecialization,
                                   Function->isThisDeclarationADefinition());
 
  // Check the template parameter list against the previous declaration. The
  // goal here is to pick up default arguments added since the friend was
  // declared; we know the template parameter lists match, since otherwise
  // we would not have picked this template as the previous declaration.
  if (isFriend && TemplateParams && FunctionTemplate->getPreviousDecl()) {
    SemaRef.CheckTemplateParameterList(
        TemplateParams,
        FunctionTemplate->getPreviousDecl()->getTemplateParameters(),
        Function->isThisDeclarationADefinition()
            ? Sema::TPC_FriendFunctionTemplateDefinition
            : Sema::TPC_FriendFunctionTemplate);
  }
 
  // If we're introducing a friend definition after the first use, trigger
  // instantiation.
  // FIXME: If this is a friend function template definition, we should check
  // to see if any specializations have been used.
  if (isFriend && D->isThisDeclarationADefinition() && Function->isUsed(false)) {
    if (MemberSpecializationInfo *MSInfo =
            Function->getMemberSpecializationInfo()) {
      if (MSInfo->getPointOfInstantiation().isInvalid()) {
        SourceLocation Loc = D->getLocation(); // FIXME
        MSInfo->setPointOfInstantiation(Loc);
        SemaRef.PendingLocalImplicitInstantiations.push_back(
            std::make_pair(Function, Loc));
      }
    }
  }
 
  if (D->isExplicitlyDefaulted()) {
    if (SubstDefaultedFunction(Function, D))
      return nullptr;
  }
  if (D->isDeleted())
    SemaRef.SetDeclDeleted(Function, D->getLocation());
 
  NamedDecl *PrincipalDecl =
      (TemplateParams ? cast<NamedDecl>(FunctionTemplate) : Function);
 
  // If this declaration lives in a different context from its lexical context,
  // add it to the corresponding lookup table.
  if (isFriend ||
      (Function->isLocalExternDecl() && !Function->getPreviousDecl()))
    DC->makeDeclVisibleInContext(PrincipalDecl);
 
  if (Function->isOverloadedOperator() && !DC->isRecord() &&
      PrincipalDecl->isInIdentifierNamespace(Decl::IDNS_Ordinary))
    PrincipalDecl->setNonMemberOperator();
 
  return Function;
}
 
Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
    CXXMethodDecl *D, TemplateParameterList *TemplateParams,
    std::optional<const ASTTemplateArgumentListInfo *>
        ClassScopeSpecializationArgs,
    RewriteKind FunctionRewriteKind) {
  FunctionTemplateDecl *FunctionTemplate = D->getDescribedFunctionTemplate();
  if (FunctionTemplate && !TemplateParams) {
    // We are creating a function template specialization from a function
    // template. Check whether there is already a function template
    // specialization for this particular set of template arguments.
    ArrayRef<TemplateArgument> Innermost = TemplateArgs.getInnermost();
 
    void *InsertPos = nullptr;
    FunctionDecl *SpecFunc
      = FunctionTemplate->findSpecialization(Innermost, InsertPos);
 
    // If we already have a function template specialization, return it.
    if (SpecFunc)
      return SpecFunc;
  }
 
  bool isFriend;
  if (FunctionTemplate)
    isFriend = (FunctionTemplate->getFriendObjectKind() != Decl::FOK_None);
  else
    isFriend = (D->getFriendObjectKind() != Decl::FOK_None);
 
  bool MergeWithParentScope = (TemplateParams != nullptr) ||
    !(isa<Decl>(Owner) &&
      cast<Decl>(Owner)->isDefinedOutsideFunctionOrMethod());
  LocalInstantiationScope Scope(SemaRef, MergeWithParentScope);
 
  // Instantiate enclosing template arguments for friends.
  SmallVector<TemplateParameterList *, 4> TempParamLists;
  unsigned NumTempParamLists = 0;
  if (isFriend && (NumTempParamLists = D->getNumTemplateParameterLists())) {
    TempParamLists.resize(NumTempParamLists);
    for (unsigned I = 0; I != NumTempParamLists; ++I) {
      TemplateParameterList *TempParams = D->getTemplateParameterList(I);
      TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
      if (!InstParams)
        return nullptr;
      TempParamLists[I] = InstParams;
    }
  }
 
  ExplicitSpecifier InstantiatedExplicitSpecifier =
      instantiateExplicitSpecifier(SemaRef, TemplateArgs,
                                   ExplicitSpecifier::getFromDecl(D), D);
  if (InstantiatedExplicitSpecifier.isInvalid())
    return nullptr;
 
  // Implicit destructors/constructors created for local classes in
  // DeclareImplicit* (see SemaDeclCXX.cpp) might not have an associated TSI.
  // Unfortunately there isn't enough context in those functions to
  // conditionally populate the TSI without breaking non-template related use
  // cases. Populate TSIs prior to calling SubstFunctionType to make sure we get
  // a proper transformation.
  if (cast<CXXRecordDecl>(D->getParent())->isLambda() &&
      !D->getTypeSourceInfo() &&
      isa<CXXConstructorDecl, CXXDestructorDecl>(D)) {
    TypeSourceInfo *TSI =
        SemaRef.Context.getTrivialTypeSourceInfo(D->getType());
    D->setTypeSourceInfo(TSI);
  }
 
  SmallVector<ParmVarDecl *, 4> Params;
  TypeSourceInfo *TInfo = SubstFunctionType(D, Params);
  if (!TInfo)
    return nullptr;
  QualType T = adjustFunctionTypeForInstantiation(SemaRef.Context, D, TInfo);
 
  if (TemplateParams && TemplateParams->size()) {
    auto *LastParam =
        dyn_cast<TemplateTypeParmDecl>(TemplateParams->asArray().back());
    if (LastParam && LastParam->isImplicit() &&
        LastParam->hasTypeConstraint()) {
      // In abbreviated templates, the type-constraints of invented template
      // type parameters are instantiated with the function type, invalidating
      // the TemplateParameterList which relied on the template type parameter
      // not having a type constraint. Recreate the TemplateParameterList with
      // the updated parameter list.
      TemplateParams = TemplateParameterList::Create(
          SemaRef.Context, TemplateParams->getTemplateLoc(),
          TemplateParams->getLAngleLoc(), TemplateParams->asArray(),
          TemplateParams->getRAngleLoc(), TemplateParams->getRequiresClause());
    }
  }
 
  NestedNameSpecifierLoc QualifierLoc = D->getQualifierLoc();
  if (QualifierLoc) {
    QualifierLoc = SemaRef.SubstNestedNameSpecifierLoc(QualifierLoc,
                                                 TemplateArgs);
    if (!QualifierLoc)
      return nullptr;
  }
 
  DeclContext *DC = Owner;
  if (isFriend) {
    if (QualifierLoc) {
      CXXScopeSpec SS;
      SS.Adopt(QualifierLoc);
      DC = SemaRef.computeDeclContext(SS);
 
      if (DC && SemaRef.RequireCompleteDeclContext(SS, DC))
        return nullptr;
    } else {
      DC = SemaRef.FindInstantiatedContext(D->getLocation(),
                                           D->getDeclContext(),
                                           TemplateArgs);
    }
    if (!DC) return nullptr;
  }
 
  CXXRecordDecl *Record = cast<CXXRecordDecl>(DC);
  Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
 
  DeclarationNameInfo NameInfo
    = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
 
  if (FunctionRewriteKind != RewriteKind::None)
    adjustForRewrite(FunctionRewriteKind, D, T, TInfo, NameInfo);
 
  // Build the instantiated method declaration.
  CXXMethodDecl *Method = nullptr;
 
  SourceLocation StartLoc = D->getInnerLocStart();
  if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(D)) {
    Method = CXXConstructorDecl::Create(
        SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
        InstantiatedExplicitSpecifier, Constructor->UsesFPIntrin(),
        Constructor->isInlineSpecified(), false,
        Constructor->getConstexprKind(), InheritedConstructor(),
        TrailingRequiresClause);
    Method->setRangeEnd(Constructor->getEndLoc());
    if (Constructor->isDefaultConstructor() ||
        Constructor->isCopyOrMoveConstructor())
      Method->setIneligibleOrNotSelected(true);
  } else if (CXXDestructorDecl *Destructor = dyn_cast<CXXDestructorDecl>(D)) {
    Method = CXXDestructorDecl::Create(
        SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
        Destructor->UsesFPIntrin(), Destructor->isInlineSpecified(), false,
        Destructor->getConstexprKind(), TrailingRequiresClause);
    Method->setIneligibleOrNotSelected(true);
    Method->setRangeEnd(Destructor->getEndLoc());
    Method->setDeclName(SemaRef.Context.DeclarationNames.getCXXDestructorName(
        SemaRef.Context.getCanonicalType(
            SemaRef.Context.getTypeDeclType(Record))));
  } else if (CXXConversionDecl *Conversion = dyn_cast<CXXConversionDecl>(D)) {
    Method = CXXConversionDecl::Create(
        SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo,
        Conversion->UsesFPIntrin(), Conversion->isInlineSpecified(),
        InstantiatedExplicitSpecifier, Conversion->getConstexprKind(),
        Conversion->getEndLoc(), TrailingRequiresClause);
  } else {
    StorageClass SC = D->isStatic() ? SC_Static : SC_None;
    Method = CXXMethodDecl::Create(
        SemaRef.Context, Record, StartLoc, NameInfo, T, TInfo, SC,
        D->UsesFPIntrin(), D->isInlineSpecified(), D->getConstexprKind(),
        D->getEndLoc(), TrailingRequiresClause);
    if (D->isMoveAssignmentOperator() || D->isCopyAssignmentOperator())
      Method->setIneligibleOrNotSelected(true);
  }
 
  if (D->isInlined())
    Method->setImplicitlyInline();
 
  if (QualifierLoc)
    Method->setQualifierInfo(QualifierLoc);
 
  if (TemplateParams) {
    // Our resulting instantiation is actually a function template, since we
    // are substituting only the outer template parameters. For example, given
    //
    //   template<typename T>
    //   struct X {
    //     template<typename U> void f(T, U);
    //   };
    //
    //   X<int> x;
    //
    // We are instantiating the member template "f" within X<int>, which means
    // substituting int for T, but leaving "f" as a member function template.
    // Build the function template itself.
    FunctionTemplate = FunctionTemplateDecl::Create(SemaRef.Context, Record,
                                                    Method->getLocation(),
                                                    Method->getDeclName(),
                                                    TemplateParams, Method);
    if (isFriend) {
      FunctionTemplate->setLexicalDeclContext(Owner);
      FunctionTemplate->setObjectOfFriendDecl();
    } else if (D->isOutOfLine())
      FunctionTemplate->setLexicalDeclContext(D->getLexicalDeclContext());
    Method->setDescribedFunctionTemplate(FunctionTemplate);
  } else if (FunctionTemplate) {
    // Record this function template specialization.
    ArrayRef<TemplateArgument> Innermost = TemplateArgs.getInnermost();
    Method->setFunctionTemplateSpecialization(FunctionTemplate,
                         TemplateArgumentList::CreateCopy(SemaRef.Context,
                                                          Innermost),
                                              /*InsertPos=*/nullptr);
  } else if (!isFriend) {
    // Record that this is an instantiation of a member function.
    Method->setInstantiationOfMemberFunction(D, TSK_ImplicitInstantiation);
  }
 
  // If we are instantiating a member function defined
  // out-of-line, the instantiation will have the same lexical
  // context (which will be a namespace scope) as the template.
  if (isFriend) {
    if (NumTempParamLists)
      Method->setTemplateParameterListsInfo(
          SemaRef.Context,
          llvm::ArrayRef(TempParamLists.data(), NumTempParamLists));
 
    Method->setLexicalDeclContext(Owner);
    Method->setObjectOfFriendDecl();
  } else if (D->isOutOfLine())
    Method->setLexicalDeclContext(D->getLexicalDeclContext());
 
  // Attach the parameters
  for (unsigned P = 0; P < Params.size(); ++P)
    Params[P]->setOwningFunction(Method);
  Method->setParams(Params);
 
  if (InitMethodInstantiation(Method, D))
    Method->setInvalidDecl();
 
  LookupResult Previous(SemaRef, NameInfo, Sema::LookupOrdinaryName,
                        Sema::ForExternalRedeclaration);
 
  bool IsExplicitSpecialization = false;
 
  // If the name of this function was written as a template-id, instantiate
  // the explicit template arguments.
  if (DependentFunctionTemplateSpecializationInfo *Info
        = D->getDependentSpecializationInfo()) {
    assert(isFriend && "non-friend has dependent specialization info?");
 
    // Instantiate the explicit template arguments.
    TemplateArgumentListInfo ExplicitArgs(Info->getLAngleLoc(),
                                          Info->getRAngleLoc());
    if (SemaRef.SubstTemplateArguments(Info->arguments(), TemplateArgs,
                                       ExplicitArgs))
      return nullptr;
 
    // Map the candidate templates to their instantiations.
    for (unsigned I = 0, E = Info->getNumTemplates(); I != E; ++I) {
      Decl *Temp = SemaRef.FindInstantiatedDecl(D->getLocation(),
                                                Info->getTemplate(I),
                                                TemplateArgs);
      if (!Temp) return nullptr;
 
      Previous.addDecl(cast<FunctionTemplateDecl>(Temp));
    }
 
    if (SemaRef.CheckFunctionTemplateSpecialization(Method,
                                                    &ExplicitArgs,
                                                    Previous))
      Method->setInvalidDecl();
 
    IsExplicitSpecialization = true;
  } else if (const ASTTemplateArgumentListInfo *Info =
                 ClassScopeSpecializationArgs.value_or(
                     D->getTemplateSpecializationArgsAsWritten())) {
    SemaRef.LookupQualifiedName(Previous, DC);
 
    TemplateArgumentListInfo ExplicitArgs(Info->getLAngleLoc(),
                                          Info->getRAngleLoc());
    if (SemaRef.SubstTemplateArguments(Info->arguments(), TemplateArgs,
                                       ExplicitArgs))
      return nullptr;
 
    if (SemaRef.CheckFunctionTemplateSpecialization(Method,
                                                    &ExplicitArgs,
                                                    Previous))
      Method->setInvalidDecl();
 
    IsExplicitSpecialization = true;
  } else if (ClassScopeSpecializationArgs) {
    // Class-scope explicit specialization written without explicit template
    // arguments.
    SemaRef.LookupQualifiedName(Previous, DC);
    if (SemaRef.CheckFunctionTemplateSpecialization(Method, nullptr, Previous))
      Method->setInvalidDecl();
 
    IsExplicitSpecialization = true;
  } else if (!FunctionTemplate || TemplateParams || isFriend) {
    SemaRef.LookupQualifiedName(Previous, Record);
 
    // In C++, the previous declaration we find might be a tag type
    // (class or enum). In this case, the new declaration will hide the
    // tag type. Note that this does not apply if we're declaring a
    // typedef (C++ [dcl.typedef]p4).
    if (Previous.isSingleTagDecl())
      Previous.clear();
  }
 
  // Per [temp.inst], default arguments in member functions of local classes
  // are instantiated along with the member function declaration. For example:
  //
  //   template<typename T>
  //   void ft() {
  //     struct lc {
  //       int operator()(int p = []{ return T::value; }());
  //     };
  //   }
  //   template void ft<int>(); // error: type 'int' cannot be used prior
  //                                      to '::'because it has no members
  //
  // The error is issued during instantiation of ft<int>()::lc::operator()
  // because substitution into the default argument fails; the default argument
  // is instantiated even though it is never used.
  if (D->isInLocalScopeForInstantiation()) {
    for (unsigned P = 0; P < Params.size(); ++P) {
      if (!Params[P]->hasDefaultArg())
        continue;
      if (SemaRef.SubstDefaultArgument(StartLoc, Params[P], TemplateArgs)) {
        // If substitution fails, the default argument is set to a
        // RecoveryExpr that wraps the uninstantiated default argument so
        // that downstream diagnostics are omitted.
        Expr *UninstExpr = Params[P]->getUninstantiatedDefaultArg();
        ExprResult ErrorResult = SemaRef.CreateRecoveryExpr(
            UninstExpr->getBeginLoc(), UninstExpr->getEndLoc(),
            { UninstExpr }, UninstExpr->getType());
        if (ErrorResult.isUsable())
          Params[P]->setDefaultArg(ErrorResult.get());
      }
    }
  }
 
  SemaRef.CheckFunctionDeclaration(nullptr, Method, Previous,
                                   IsExplicitSpecialization,
                                   Method->isThisDeclarationADefinition());
 
  if (D->isPure())
    SemaRef.CheckPureMethod(Method, SourceRange());
 
  // Propagate access.  For a non-friend declaration, the access is
  // whatever we're propagating from.  For a friend, it should be the
  // previous declaration we just found.
  if (isFriend && Method->getPreviousDecl())
    Method->setAccess(Method->getPreviousDecl()->getAccess());
  else
    Method->setAccess(D->getAccess());
  if (FunctionTemplate)
    FunctionTemplate->setAccess(Method->getAccess());
 
  SemaRef.CheckOverrideControl(Method);
 
  // If a function is defined as defaulted or deleted, mark it as such now.
  if (D->isExplicitlyDefaulted()) {
    if (SubstDefaultedFunction(Method, D))
      return nullptr;
  }
  if (D->isDeletedAsWritten())
    SemaRef.SetDeclDeleted(Method, Method->getLocation());
 
  // If this is an explicit specialization, mark the implicitly-instantiated
  // template specialization as being an explicit specialization too.
  // FIXME: Is this necessary?
  if (IsExplicitSpecialization && !isFriend)
    SemaRef.CompleteMemberSpecialization(Method, Previous);
 
  // If there's a function template, let our caller handle it.
  if (FunctionTemplate) {
    // do nothing
 
  // Don't hide a (potentially) valid declaration with an invalid one.
  } else if (Method->isInvalidDecl() && !Previous.empty()) {
    // do nothing
 
  // Otherwise, check access to friends and make them visible.
  } else if (isFriend) {
    // We only need to re-check access for methods which we didn't
    // manage to match during parsing.
    if (!D->getPreviousDecl())
      SemaRef.CheckFriendAccess(Method);
 
    Record->makeDeclVisibleInContext(Method);
 
  // Otherwise, add the declaration.  We don't need to do this for
  // class-scope specializations because we'll have matched them with
  // the appropriate template.
  } else {
    Owner->addDecl(Method);
  }
 
  // PR17480: Honor the used attribute to instantiate member function
  // definitions
  if (Method->hasAttr<UsedAttr>()) {
    if (const auto *A = dyn_cast<CXXRecordDecl>(Owner)) {
      SourceLocation Loc;
      if (const MemberSpecializationInfo *MSInfo =
              A->getMemberSpecializationInfo())
        Loc = MSInfo->getPointOfInstantiation();
      else if (const auto *Spec = dyn_cast<ClassTemplateSpecializationDecl>(A))
        Loc = Spec->getPointOfInstantiation();
      SemaRef.MarkFunctionReferenced(Loc, Method);
    }
  }
 
  return Method;
}
 
Decl *TemplateDeclInstantiator::VisitCXXConstructorDecl(CXXConstructorDecl *D) {
  return VisitCXXMethodDecl(D);
}
 
Decl *TemplateDeclInstantiator::VisitCXXDestructorDecl(CXXDestructorDecl *D) {
  return VisitCXXMethodDecl(D);
}
 
Decl *TemplateDeclInstantiator::VisitCXXConversionDecl(CXXConversionDecl *D) {
  return VisitCXXMethodDecl(D);
}
 
Decl *TemplateDeclInstantiator::VisitParmVarDecl(ParmVarDecl *D) {
  return SemaRef.SubstParmVarDecl(D, TemplateArgs, /*indexAdjustment*/ 0,
                                  std::nullopt,
                                  /*ExpectParameterPack=*/false);
}
 
Decl *TemplateDeclInstantiator::VisitTemplateTypeParmDecl(
                                                    TemplateTypeParmDecl *D) {
  assert(D->getTypeForDecl()->isTemplateTypeParmType());
 
  std::optional<unsigned> NumExpanded;
 
  if (const TypeConstraint *TC = D->getTypeConstraint()) {
    if (D->isPackExpansion() && !D->isExpandedParameterPack()) {
      assert(TC->getTemplateArgsAsWritten() &&
             "type parameter can only be an expansion when explicit arguments "
             "are specified");
      // The template type parameter pack's type is a pack expansion of types.
      // Determine whether we need to expand this parameter pack into separate
      // types.
      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
      for (auto &ArgLoc : TC->getTemplateArgsAsWritten()->arguments())
        SemaRef.collectUnexpandedParameterPacks(ArgLoc, Unexpanded);
 
      // Determine whether the set of unexpanded parameter packs can and should
      // be expanded.
      bool Expand = true;
      bool RetainExpansion = false;
      if (SemaRef.CheckParameterPacksForExpansion(
              cast<CXXFoldExpr>(TC->getImmediatelyDeclaredConstraint())
                  ->getEllipsisLoc(),
              SourceRange(TC->getConceptNameLoc(),
                          TC->hasExplicitTemplateArgs() ?
                          TC->getTemplateArgsAsWritten()->getRAngleLoc() :
                          TC->getConceptNameInfo().getEndLoc()),
              Unexpanded, TemplateArgs, Expand, RetainExpansion, NumExpanded))
        return nullptr;
    }
  }
 
  TemplateTypeParmDecl *Inst = TemplateTypeParmDecl::Create(
      SemaRef.Context, Owner, D->getBeginLoc(), D->getLocation(),
      D->getDepth() - TemplateArgs.getNumSubstitutedLevels(), D->getIndex(),
      D->getIdentifier(), D->wasDeclaredWithTypename(), D->isParameterPack(),
      D->hasTypeConstraint(), NumExpanded);
 
  Inst->setAccess(AS_public);
  Inst->setImplicit(D->isImplicit());
  if (auto *TC = D->getTypeConstraint()) {
    if (!D->isImplicit()) {
      // Invented template parameter type constraints will be instantiated
      // with the corresponding auto-typed parameter as it might reference
      // other parameters.
      if (SemaRef.SubstTypeConstraint(Inst, TC, TemplateArgs,
                                      EvaluateConstraints))
        return nullptr;
    }
  }
  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
    TypeSourceInfo *InstantiatedDefaultArg =
        SemaRef.SubstType(D->getDefaultArgumentInfo(), TemplateArgs,
                          D->getDefaultArgumentLoc(), D->getDeclName());
    if (InstantiatedDefaultArg)
      Inst->setDefaultArgument(InstantiatedDefaultArg);
  }
 
  // Introduce this template parameter's instantiation into the instantiation
  // scope.
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Inst);
 
  return Inst;
}
 
Decl *TemplateDeclInstantiator::VisitNonTypeTemplateParmDecl(
                                                 NonTypeTemplateParmDecl *D) {
  // Substitute into the type of the non-type template parameter.
  TypeLoc TL = D->getTypeSourceInfo()->getTypeLoc();
  SmallVector<TypeSourceInfo *, 4> ExpandedParameterPackTypesAsWritten;
  SmallVector<QualType, 4> ExpandedParameterPackTypes;
  bool IsExpandedParameterPack = false;
  TypeSourceInfo *DI;
  QualType T;
  bool Invalid = false;
 
  if (D->isExpandedParameterPack()) {
    // The non-type template parameter pack is an already-expanded pack
    // expansion of types. Substitute into each of the expanded types.
    ExpandedParameterPackTypes.reserve(D->getNumExpansionTypes());
    ExpandedParameterPackTypesAsWritten.reserve(D->getNumExpansionTypes());
    for (unsigned I = 0, N = D->getNumExpansionTypes(); I != N; ++I) {
      TypeSourceInfo *NewDI =
          SemaRef.SubstType(D->getExpansionTypeSourceInfo(I), TemplateArgs,
                            D->getLocation(), D->getDeclName());
      if (!NewDI)
        return nullptr;
 
      QualType NewT =
          SemaRef.CheckNonTypeTemplateParameterType(NewDI, D->getLocation());
      if (NewT.isNull())
        return nullptr;
 
      ExpandedParameterPackTypesAsWritten.push_back(NewDI);
      ExpandedParameterPackTypes.push_back(NewT);
    }
 
    IsExpandedParameterPack = true;
    DI = D->getTypeSourceInfo();
    T = DI->getType();
  } else if (D->isPackExpansion()) {
    // The non-type template parameter pack's type is a pack expansion of types.
    // Determine whether we need to expand this parameter pack into separate
    // types.
    PackExpansionTypeLoc Expansion = TL.castAs<PackExpansionTypeLoc>();
    TypeLoc Pattern = Expansion.getPatternLoc();
    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
    SemaRef.collectUnexpandedParameterPacks(Pattern, Unexpanded);
 
    // Determine whether the set of unexpanded parameter packs can and should
    // be expanded.
    bool Expand = true;
    bool RetainExpansion = false;
    std::optional<unsigned> OrigNumExpansions =
        Expansion.getTypePtr()->getNumExpansions();
    std::optional<unsigned> NumExpansions = OrigNumExpansions;
    if (SemaRef.CheckParameterPacksForExpansion(Expansion.getEllipsisLoc(),
                                                Pattern.getSourceRange(),
                                                Unexpanded,
                                                TemplateArgs,
                                                Expand, RetainExpansion,
                                                NumExpansions))
      return nullptr;
 
    if (Expand) {
      for (unsigned I = 0; I != *NumExpansions; ++I) {
        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, I);
        TypeSourceInfo *NewDI = SemaRef.SubstType(Pattern, TemplateArgs,
                                                  D->getLocation(),
                                                  D->getDeclName());
        if (!NewDI)
          return nullptr;
 
        QualType NewT =
            SemaRef.CheckNonTypeTemplateParameterType(NewDI, D->getLocation());
        if (NewT.isNull())
          return nullptr;
 
        ExpandedParameterPackTypesAsWritten.push_back(NewDI);
        ExpandedParameterPackTypes.push_back(NewT);
      }
 
      // Note that we have an expanded parameter pack. The "type" of this
      // expanded parameter pack is the original expansion type, but callers
      // will end up using the expanded parameter pack types for type-checking.
      IsExpandedParameterPack = true;
      DI = D->getTypeSourceInfo();
      T = DI->getType();
    } else {
      // We cannot fully expand the pack expansion now, so substitute into the
      // pattern and create a new pack expansion type.
      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, -1);
      TypeSourceInfo *NewPattern = SemaRef.SubstType(Pattern, TemplateArgs,
                                                     D->getLocation(),
                                                     D->getDeclName());
      if (!NewPattern)
        return nullptr;
 
      SemaRef.CheckNonTypeTemplateParameterType(NewPattern, D->getLocation());
      DI = SemaRef.CheckPackExpansion(NewPattern, Expansion.getEllipsisLoc(),
                                      NumExpansions);
      if (!DI)
        return nullptr;
 
      T = DI->getType();
    }
  } else {
    // Simple case: substitution into a parameter that is not a parameter pack.
    DI = SemaRef.SubstType(D->getTypeSourceInfo(), TemplateArgs,
                           D->getLocation(), D->getDeclName());
    if (!DI)
      return nullptr;
 
    // Check that this type is acceptable for a non-type template parameter.
    T = SemaRef.CheckNonTypeTemplateParameterType(DI, D->getLocation());
    if (T.isNull()) {
      T = SemaRef.Context.IntTy;
      Invalid = true;
    }
  }
 
  NonTypeTemplateParmDecl *Param;
  if (IsExpandedParameterPack)
    Param = NonTypeTemplateParmDecl::Create(
        SemaRef.Context, Owner, D->getInnerLocStart(), D->getLocation(),
        D->getDepth() - TemplateArgs.getNumSubstitutedLevels(),
        D->getPosition(), D->getIdentifier(), T, DI, ExpandedParameterPackTypes,
        ExpandedParameterPackTypesAsWritten);
  else
    Param = NonTypeTemplateParmDecl::Create(
        SemaRef.Context, Owner, D->getInnerLocStart(), D->getLocation(),
        D->getDepth() - TemplateArgs.getNumSubstitutedLevels(),
        D->getPosition(), D->getIdentifier(), T, D->isParameterPack(), DI);
 
  if (AutoTypeLoc AutoLoc = DI->getTypeLoc().getContainedAutoTypeLoc())
    if (AutoLoc.isConstrained())
      // Note: We attach the uninstantiated constriant here, so that it can be
      // instantiated relative to the top level, like all our other constraints.
      if (SemaRef.AttachTypeConstraint(
              AutoLoc, Param, D,
              IsExpandedParameterPack
                ? DI->getTypeLoc().getAs<PackExpansionTypeLoc>()
                    .getEllipsisLoc()
                : SourceLocation()))
        Invalid = true;
 
  Param->setAccess(AS_public);
  Param->setImplicit(D->isImplicit());
  if (Invalid)
    Param->setInvalidDecl();
 
  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
    EnterExpressionEvaluationContext ConstantEvaluated(
        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
    ExprResult Value = SemaRef.SubstExpr(D->getDefaultArgument(), TemplateArgs);
    if (!Value.isInvalid())
      Param->setDefaultArgument(Value.get());
  }
 
  // Introduce this template parameter's instantiation into the instantiation
  // scope.
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Param);
  return Param;
}
 
static void collectUnexpandedParameterPacks(
    Sema &S,
    TemplateParameterList *Params,
    SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
  for (const auto &P : *Params) {
    if (P->isTemplateParameterPack())
      continue;
    if (NonTypeTemplateParmDecl *NTTP = dyn_cast<NonTypeTemplateParmDecl>(P))
      S.collectUnexpandedParameterPacks(NTTP->getTypeSourceInfo()->getTypeLoc(),
                                        Unexpanded);
    if (TemplateTemplateParmDecl *TTP = dyn_cast<TemplateTemplateParmDecl>(P))
      collectUnexpandedParameterPacks(S, TTP->getTemplateParameters(),
                                      Unexpanded);
  }
}
 
Decl *
TemplateDeclInstantiator::VisitTemplateTemplateParmDecl(
                                                  TemplateTemplateParmDecl *D) {
  // Instantiate the template parameter list of the template template parameter.
  TemplateParameterList *TempParams = D->getTemplateParameters();
  TemplateParameterList *InstParams;
  SmallVector<TemplateParameterList*, 8> ExpandedParams;
 
  bool IsExpandedParameterPack = false;
 
  if (D->isExpandedParameterPack()) {
    // The template template parameter pack is an already-expanded pack
    // expansion of template parameters. Substitute into each of the expanded
    // parameters.
    ExpandedParams.reserve(D->getNumExpansionTemplateParameters());
    for (unsigned I = 0, N = D->getNumExpansionTemplateParameters();
         I != N; ++I) {
      LocalInstantiationScope Scope(SemaRef);
      TemplateParameterList *Expansion =
        SubstTemplateParams(D->getExpansionTemplateParameters(I));
      if (!Expansion)
        return nullptr;
      ExpandedParams.push_back(Expansion);
    }
 
    IsExpandedParameterPack = true;
    InstParams = TempParams;
  } else if (D->isPackExpansion()) {
    // The template template parameter pack expands to a pack of template
    // template parameters. Determine whether we need to expand this parameter
    // pack into separate parameters.
    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
    collectUnexpandedParameterPacks(SemaRef, D->getTemplateParameters(),
                                    Unexpanded);
 
    // Determine whether the set of unexpanded parameter packs can and should
    // be expanded.
    bool Expand = true;
    bool RetainExpansion = false;
    std::optional<unsigned> NumExpansions;
    if (SemaRef.CheckParameterPacksForExpansion(D->getLocation(),
                                                TempParams->getSourceRange(),
                                                Unexpanded,
                                                TemplateArgs,
                                                Expand, RetainExpansion,
                                                NumExpansions))
      return nullptr;
 
    if (Expand) {
      for (unsigned I = 0; I != *NumExpansions; ++I) {
        Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, I);
        LocalInstantiationScope Scope(SemaRef);
        TemplateParameterList *Expansion = SubstTemplateParams(TempParams);
        if (!Expansion)
          return nullptr;
        ExpandedParams.push_back(Expansion);
      }
 
      // Note that we have an expanded parameter pack. The "type" of this
      // expanded parameter pack is the original expansion type, but callers
      // will end up using the expanded parameter pack types for type-checking.
      IsExpandedParameterPack = true;
      InstParams = TempParams;
    } else {
      // We cannot fully expand the pack expansion now, so just substitute
      // into the pattern.
      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, -1);
 
      LocalInstantiationScope Scope(SemaRef);
      InstParams = SubstTemplateParams(TempParams);
      if (!InstParams)
        return nullptr;
    }
  } else {
    // Perform the actual substitution of template parameters within a new,
    // local instantiation scope.
    LocalInstantiationScope Scope(SemaRef);
    InstParams = SubstTemplateParams(TempParams);
    if (!InstParams)
      return nullptr;
  }
 
  // Build the template template parameter.
  TemplateTemplateParmDecl *Param;
  if (IsExpandedParameterPack)
    Param = TemplateTemplateParmDecl::Create(
        SemaRef.Context, Owner, D->getLocation(),
        D->getDepth() - TemplateArgs.getNumSubstitutedLevels(),
        D->getPosition(), D->getIdentifier(), InstParams, ExpandedParams);
  else
    Param = TemplateTemplateParmDecl::Create(
        SemaRef.Context, Owner, D->getLocation(),
        D->getDepth() - TemplateArgs.getNumSubstitutedLevels(),
        D->getPosition(), D->isParameterPack(), D->getIdentifier(), InstParams);
  if (D->hasDefaultArgument() && !D->defaultArgumentWasInherited()) {
    NestedNameSpecifierLoc QualifierLoc =
        D->getDefaultArgument().getTemplateQualifierLoc();
    QualifierLoc =
        SemaRef.SubstNestedNameSpecifierLoc(QualifierLoc, TemplateArgs);
    TemplateName TName = SemaRef.SubstTemplateName(
        QualifierLoc, D->getDefaultArgument().getArgument().getAsTemplate(),
        D->getDefaultArgument().getTemplateNameLoc(), TemplateArgs);
    if (!TName.isNull())
      Param->setDefaultArgument(
          SemaRef.Context,
          TemplateArgumentLoc(SemaRef.Context, TemplateArgument(TName),
                              D->getDefaultArgument().getTemplateQualifierLoc(),
                              D->getDefaultArgument().getTemplateNameLoc()));
  }
  Param->setAccess(AS_public);
  Param->setImplicit(D->isImplicit());
 
  // Introduce this template parameter's instantiation into the instantiation
  // scope.
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, Param);
 
  return Param;
}
 
Decl *TemplateDeclInstantiator::VisitUsingDirectiveDecl(UsingDirectiveDecl *D) {
  // Using directives are never dependent (and never contain any types or
  // expressions), so they require no explicit instantiation work.
 
  UsingDirectiveDecl *Inst
    = UsingDirectiveDecl::Create(SemaRef.Context, Owner, D->getLocation(),
                                 D->getNamespaceKeyLocation(),
                                 D->getQualifierLoc(),
                                 D->getIdentLocation(),
                                 D->getNominatedNamespace(),
                                 D->getCommonAncestor());
 
  // Add the using directive to its declaration context
  // only if this is not a function or method.
  if (!Owner->isFunctionOrMethod())
    Owner->addDecl(Inst);
 
  return Inst;
}
 
Decl *TemplateDeclInstantiator::VisitBaseUsingDecls(BaseUsingDecl *D,
                                                    BaseUsingDecl *Inst,
                                                    LookupResult *Lookup) {
 
  bool isFunctionScope = Owner->isFunctionOrMethod();
 
  for (auto *Shadow : D->shadows()) {
    // FIXME: UsingShadowDecl doesn't preserve its immediate target, so
    // reconstruct it in the case where it matters. Hm, can we extract it from
    // the DeclSpec when parsing and save it in the UsingDecl itself?
    NamedDecl *OldTarget = Shadow->getTargetDecl();
    if (auto *CUSD = dyn_cast<ConstructorUsingShadowDecl>(Shadow))
      if (auto *BaseShadow = CUSD->getNominatedBaseClassShadowDecl())
        OldTarget = BaseShadow;
 
    NamedDecl *InstTarget = nullptr;
    if (auto *EmptyD =
            dyn_cast<UnresolvedUsingIfExistsDecl>(Shadow->getTargetDecl())) {
      InstTarget = UnresolvedUsingIfExistsDecl::Create(
          SemaRef.Context, Owner, EmptyD->getLocation(), EmptyD->getDeclName());
    } else {
      InstTarget = cast_or_null<NamedDecl>(SemaRef.FindInstantiatedDecl(
          Shadow->getLocation(), OldTarget, TemplateArgs));
    }
    if (!InstTarget)
      return nullptr;
 
    UsingShadowDecl *PrevDecl = nullptr;
    if (Lookup &&
        SemaRef.CheckUsingShadowDecl(Inst, InstTarget, *Lookup, PrevDecl))
      continue;
 
    if (UsingShadowDecl *OldPrev = getPreviousDeclForInstantiation(Shadow))
      PrevDecl = cast_or_null<UsingShadowDecl>(SemaRef.FindInstantiatedDecl(
          Shadow->getLocation(), OldPrev, TemplateArgs));
 
    UsingShadowDecl *InstShadow = SemaRef.BuildUsingShadowDecl(
        /*Scope*/ nullptr, Inst, InstTarget, PrevDecl);
    SemaRef.Context.setInstantiatedFromUsingShadowDecl(InstShadow, Shadow);
 
    if (isFunctionScope)
      SemaRef.CurrentInstantiationScope->InstantiatedLocal(Shadow, InstShadow);
  }
 
  return Inst;
}
 
Decl *TemplateDeclInstantiator::VisitUsingDecl(UsingDecl *D) {
 
  // The nested name specifier may be dependent, for example
  //     template <typename T> struct t {
  //       struct s1 { T f1(); };
  //       struct s2 : s1 { using s1::f1; };
  //     };
  //     template struct t<int>;
  // Here, in using s1::f1, s1 refers to t<T>::s1;
  // we need to substitute for t<int>::s1.
  NestedNameSpecifierLoc QualifierLoc
    = SemaRef.SubstNestedNameSpecifierLoc(D->getQualifierLoc(),
                                          TemplateArgs);
  if (!QualifierLoc)
    return nullptr;
 
  // For an inheriting constructor declaration, the name of the using
  // declaration is the name of a constructor in this class, not in the
  // base class.
  DeclarationNameInfo NameInfo = D->getNameInfo();
  if (NameInfo.getName().getNameKind() == DeclarationName::CXXConstructorName)
    if (auto *RD = dyn_cast<CXXRecordDecl>(SemaRef.CurContext))
      NameInfo.setName(SemaRef.Context.DeclarationNames.getCXXConstructorName(
          SemaRef.Context.getCanonicalType(SemaRef.Context.getRecordType(RD))));
 
  // We only need to do redeclaration lookups if we're in a class scope (in
  // fact, it's not really even possible in non-class scopes).
  bool CheckRedeclaration = Owner->isRecord();
  LookupResult Prev(SemaRef, NameInfo, Sema::LookupUsingDeclName,
                    Sema::ForVisibleRedeclaration);
 
  UsingDecl *NewUD = UsingDecl::Create(SemaRef.Context, Owner,
                                       D->getUsingLoc(),
                                       QualifierLoc,
                                       NameInfo,
                                       D->hasTypename());
 
  CXXScopeSpec SS;
  SS.Adopt(QualifierLoc);
  if (CheckRedeclaration) {
    Prev.setHideTags(false);
    SemaRef.LookupQualifiedName(Prev, Owner);
 
    // Check for invalid redeclarations.
    if (SemaRef.CheckUsingDeclRedeclaration(D->getUsingLoc(),
                                            D->hasTypename(), SS,
                                            D->getLocation(), Prev))
      NewUD->setInvalidDecl();
  }
 
  if (!NewUD->isInvalidDecl() &&
      SemaRef.CheckUsingDeclQualifier(D->getUsingLoc(), D->hasTypename(), SS,
                                      NameInfo, D->getLocation(), nullptr, D))
    NewUD->setInvalidDecl();
 
  SemaRef.Context.setInstantiatedFromUsingDecl(NewUD, D);
  NewUD->setAccess(D->getAccess());
  Owner->addDecl(NewUD);
 
  // Don't process the shadow decls for an invalid decl.
  if (NewUD->isInvalidDecl())
    return NewUD;
 
  // If the using scope was dependent, or we had dependent bases, we need to
  // recheck the inheritance
  if (NameInfo.getName().getNameKind() == DeclarationName::CXXConstructorName)
    SemaRef.CheckInheritingConstructorUsingDecl(NewUD);
 
  return VisitBaseUsingDecls(D, NewUD, CheckRedeclaration ? &Prev : nullptr);
}
 
Decl *TemplateDeclInstantiator::VisitUsingEnumDecl(UsingEnumDecl *D) {
  // Cannot be a dependent type, but still could be an instantiation
  EnumDecl *EnumD = cast_or_null<EnumDecl>(SemaRef.FindInstantiatedDecl(
      D->getLocation(), D->getEnumDecl(), TemplateArgs));
 
  if (SemaRef.RequireCompleteEnumDecl(EnumD, EnumD->getLocation()))
    return nullptr;
 
  TypeSourceInfo *TSI = SemaRef.SubstType(D->getEnumType(), TemplateArgs,
                                          D->getLocation(), D->getDeclName());
  UsingEnumDecl *NewUD =
      UsingEnumDecl::Create(SemaRef.Context, Owner, D->getUsingLoc(),
                            D->getEnumLoc(), D->getLocation(), TSI);
 
  SemaRef.Context.setInstantiatedFromUsingEnumDecl(NewUD, D);
  NewUD->setAccess(D->getAccess());
  Owner->addDecl(NewUD);
 
  // Don't process the shadow decls for an invalid decl.
  if (NewUD->isInvalidDecl())
    return NewUD;
 
  // We don't have to recheck for duplication of the UsingEnumDecl itself, as it
  // cannot be dependent, and will therefore have been checked during template
  // definition.
 
  return VisitBaseUsingDecls(D, NewUD, nullptr);
}
 
Decl *TemplateDeclInstantiator::VisitUsingShadowDecl(UsingShadowDecl *D) {
  // Ignore these;  we handle them in bulk when processing the UsingDecl.
  return nullptr;
}
 
Decl *TemplateDeclInstantiator::VisitConstructorUsingShadowDecl(
    ConstructorUsingShadowDecl *D) {
  // Ignore these;  we handle them in bulk when processing the UsingDecl.
  return nullptr;
}
 
template <typename T>
Decl *TemplateDeclInstantiator::instantiateUnresolvedUsingDecl(
    T *D, bool InstantiatingPackElement) {
  // If this is a pack expansion, expand it now.
  if (D->isPackExpansion() && !InstantiatingPackElement) {
    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
    SemaRef.collectUnexpandedParameterPacks(D->getQualifierLoc(), Unexpanded);
    SemaRef.collectUnexpandedParameterPacks(D->getNameInfo(), Unexpanded);
 
    // Determine whether the set of unexpanded parameter packs can and should
    // be expanded.
    bool Expand = true;
    bool RetainExpansion = false;
    std::optional<unsigned> NumExpansions;
    if (SemaRef.CheckParameterPacksForExpansion(
          D->getEllipsisLoc(), D->getSourceRange(), Unexpanded, TemplateArgs,
            Expand, RetainExpansion, NumExpansions))
      return nullptr;
 
    // This declaration cannot appear within a function template signature,
    // so we can't have a partial argument list for a parameter pack.
    assert(!RetainExpansion &&
           "should never need to retain an expansion for UsingPackDecl");
 
    if (!Expand) {
      // We cannot fully expand the pack expansion now, so substitute into the
      // pattern and create a new pack expansion.
      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, -1);
      return instantiateUnresolvedUsingDecl(D, true);
    }
 
    // Within a function, we don't have any normal way to check for conflicts
    // between shadow declarations from different using declarations in the
    // same pack expansion, but this is always ill-formed because all expansions
    // must produce (conflicting) enumerators.
    //
    // Sadly we can't just reject this in the template definition because it
    // could be valid if the pack is empty or has exactly one expansion.
    if (D->getDeclContext()->isFunctionOrMethod() && *NumExpansions > 1) {
      SemaRef.Diag(D->getEllipsisLoc(),
                   diag::err_using_decl_redeclaration_expansion);
      return nullptr;
    }
 
    // Instantiate the slices of this pack and build a UsingPackDecl.
    SmallVector<NamedDecl*, 8> Expansions;
    for (unsigned I = 0; I != *NumExpansions; ++I) {
      Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, I);
      Decl *Slice = instantiateUnresolvedUsingDecl(D, true);
      if (!Slice)
        return nullptr;
      // Note that we can still get unresolved using declarations here, if we
      // had arguments for all packs but the pattern also contained other
      // template arguments (this only happens during partial substitution, eg
      // into the body of a generic lambda in a function template).
      Expansions.push_back(cast<NamedDecl>(Slice));
    }
 
    auto *NewD = SemaRef.BuildUsingPackDecl(D, Expansions);
    if (isDeclWithinFunction(D))
      SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewD);
    return NewD;
  }
 
  UnresolvedUsingTypenameDecl *TD = dyn_cast<UnresolvedUsingTypenameDecl>(D);
  SourceLocation TypenameLoc = TD ? TD->getTypenameLoc() : SourceLocation();
 
  NestedNameSpecifierLoc QualifierLoc
    = SemaRef.SubstNestedNameSpecifierLoc(D->getQualifierLoc(),
                                          TemplateArgs);
  if (!QualifierLoc)
    return nullptr;
 
  CXXScopeSpec SS;
  SS.Adopt(QualifierLoc);
 
  DeclarationNameInfo NameInfo
    = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);
 
  // Produce a pack expansion only if we're not instantiating a particular
  // slice of a pack expansion.
  bool InstantiatingSlice = D->getEllipsisLoc().isValid() &&
                            SemaRef.ArgumentPackSubstitutionIndex != -1;
  SourceLocation EllipsisLoc =
      InstantiatingSlice ? SourceLocation() : D->getEllipsisLoc();
 
  bool IsUsingIfExists = D->template hasAttr<UsingIfExistsAttr>();
  NamedDecl *UD = SemaRef.BuildUsingDeclaration(
      /*Scope*/ nullptr, D->getAccess(), D->getUsingLoc(),
      /*HasTypename*/ TD, TypenameLoc, SS, NameInfo, EllipsisLoc,
      ParsedAttributesView(),
      /*IsInstantiation*/ true, IsUsingIfExists);
  if (UD) {
    SemaRef.InstantiateAttrs(TemplateArgs, D, UD);
    SemaRef.Context.setInstantiatedFromUsingDecl(UD, D);
  }
 
  return UD;
}
 
Decl *TemplateDeclInstantiator::VisitUnresolvedUsingTypenameDecl(
    UnresolvedUsingTypenameDecl *D) {
  return instantiateUnresolvedUsingDecl(D);
}
 
Decl *TemplateDeclInstantiator::VisitUnresolvedUsingValueDecl(
    UnresolvedUsingValueDecl *D) {
  return instantiateUnresolvedUsingDecl(D);
}
 
Decl *TemplateDeclInstantiator::VisitUnresolvedUsingIfExistsDecl(
    UnresolvedUsingIfExistsDecl *D) {
  llvm_unreachable("referring to unresolved decl out of UsingShadowDecl");
}
 
Decl *TemplateDeclInstantiator::VisitUsingPackDecl(UsingPackDecl *D) {
  SmallVector<NamedDecl*, 8> Expansions;
  for (auto *UD : D->expansions()) {
    if (NamedDecl *NewUD =
            SemaRef.FindInstantiatedDecl(D->getLocation(), UD, TemplateArgs))
      Expansions.push_back(NewUD);
    else
      return nullptr;
  }
 
  auto *NewD = SemaRef.BuildUsingPackDecl(D, Expansions);
  if (isDeclWithinFunction(D))
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewD);
  return NewD;
}
 
Decl *TemplateDeclInstantiator::VisitClassScopeFunctionSpecializationDecl(
    ClassScopeFunctionSpecializationDecl *Decl) {
  CXXMethodDecl *OldFD = Decl->getSpecialization();
  return cast_or_null<CXXMethodDecl>(
      VisitCXXMethodDecl(OldFD, nullptr, Decl->getTemplateArgsAsWritten()));
}
 
Decl *TemplateDeclInstantiator::VisitOMPThreadPrivateDecl(
                                     OMPThreadPrivateDecl *D) {
  SmallVector<Expr *, 5> Vars;
  for (auto *I : D->varlists()) {
    Expr *Var = SemaRef.SubstExpr(I, TemplateArgs).get();
    assert(isa<DeclRefExpr>(Var) && "threadprivate arg is not a DeclRefExpr");
    Vars.push_back(Var);
  }
 
  OMPThreadPrivateDecl *TD =
    SemaRef.CheckOMPThreadPrivateDecl(D->getLocation(), Vars);
 
  TD->setAccess(AS_public);
  Owner->addDecl(TD);
 
  return TD;
}
 
Decl *TemplateDeclInstantiator::VisitOMPAllocateDecl(OMPAllocateDecl *D) {
  SmallVector<Expr *, 5> Vars;
  for (auto *I : D->varlists()) {
    Expr *Var = SemaRef.SubstExpr(I, TemplateArgs).get();
    assert(isa<DeclRefExpr>(Var) && "allocate arg is not a DeclRefExpr");
    Vars.push_back(Var);
  }
  SmallVector<OMPClause *, 4> Clauses;
  // Copy map clauses from the original mapper.
  for (OMPClause *C : D->clauselists()) {
    OMPClause *IC = nullptr;
    if (auto *AC = dyn_cast<OMPAllocatorClause>(C)) {
      ExprResult NewE = SemaRef.SubstExpr(AC->getAllocator(), TemplateArgs);
      if (!NewE.isUsable())
        continue;
      IC = SemaRef.ActOnOpenMPAllocatorClause(
          NewE.get(), AC->getBeginLoc(), AC->getLParenLoc(), AC->getEndLoc());
    } else if (auto *AC = dyn_cast<OMPAlignClause>(C)) {
      ExprResult NewE = SemaRef.SubstExpr(AC->getAlignment(), TemplateArgs);
      if (!NewE.isUsable())
        continue;
      IC = SemaRef.ActOnOpenMPAlignClause(NewE.get(), AC->getBeginLoc(),
                                          AC->getLParenLoc(), AC->getEndLoc());
      // If align clause value ends up being invalid, this can end up null.
      if (!IC)
        continue;
    }
    Clauses.push_back(IC);
  }
 
  Sema::DeclGroupPtrTy Res = SemaRef.ActOnOpenMPAllocateDirective(
      D->getLocation(), Vars, Clauses, Owner);
  if (Res.get().isNull())
    return nullptr;
  return Res.get().getSingleDecl();
}
 
Decl *TemplateDeclInstantiator::VisitOMPRequiresDecl(OMPRequiresDecl *D) {
  llvm_unreachable(
      "Requires directive cannot be instantiated within a dependent context");
}
 
Decl *TemplateDeclInstantiator::VisitOMPDeclareReductionDecl(
    OMPDeclareReductionDecl *D) {
  // Instantiate type and check if it is allowed.
  const bool RequiresInstantiation =
      D->getType()->isDependentType() ||
      D->getType()->isInstantiationDependentType() ||
      D->getType()->containsUnexpandedParameterPack();
  QualType SubstReductionType;
  if (RequiresInstantiation) {
    SubstReductionType = SemaRef.ActOnOpenMPDeclareReductionType(
        D->getLocation(),
        ParsedType::make(SemaRef.SubstType(
            D->getType(), TemplateArgs, D->getLocation(), DeclarationName())));
  } else {
    SubstReductionType = D->getType();
  }
  if (SubstReductionType.isNull())
    return nullptr;
  Expr *Combiner = D->getCombiner();
  Expr *Init = D->getInitializer();
  bool IsCorrect = true;
  // Create instantiated copy.
  std::pair<QualType, SourceLocation> ReductionTypes[] = {
      std::make_pair(SubstReductionType, D->getLocation())};
  auto *PrevDeclInScope = D->getPrevDeclInScope();
  if (PrevDeclInScope && !PrevDeclInScope->isInvalidDecl()) {
    PrevDeclInScope = cast<OMPDeclareReductionDecl>(
        SemaRef.CurrentInstantiationScope->findInstantiationOf(PrevDeclInScope)
            ->get<Decl *>());
  }
  auto DRD = SemaRef.ActOnOpenMPDeclareReductionDirectiveStart(
      /*S=*/nullptr, Owner, D->getDeclName(), ReductionTypes, D->getAccess(),
      PrevDeclInScope);
  auto *NewDRD = cast<OMPDeclareReductionDecl>(DRD.get().getSingleDecl());
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewDRD);
  Expr *SubstCombiner = nullptr;
  Expr *SubstInitializer = nullptr;
  // Combiners instantiation sequence.
  if (Combiner) {
    SemaRef.ActOnOpenMPDeclareReductionCombinerStart(
        /*S=*/nullptr, NewDRD);
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(
        cast<DeclRefExpr>(D->getCombinerIn())->getDecl(),
        cast<DeclRefExpr>(NewDRD->getCombinerIn())->getDecl());
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(
        cast<DeclRefExpr>(D->getCombinerOut())->getDecl(),
        cast<DeclRefExpr>(NewDRD->getCombinerOut())->getDecl());
    auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(Owner);
    Sema::CXXThisScopeRAII ThisScope(SemaRef, ThisContext, Qualifiers(),
                                     ThisContext);
    SubstCombiner = SemaRef.SubstExpr(Combiner, TemplateArgs).get();
    SemaRef.ActOnOpenMPDeclareReductionCombinerEnd(NewDRD, SubstCombiner);
  }
  // Initializers instantiation sequence.
  if (Init) {
    VarDecl *OmpPrivParm = SemaRef.ActOnOpenMPDeclareReductionInitializerStart(
        /*S=*/nullptr, NewDRD);
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(
        cast<DeclRefExpr>(D->getInitOrig())->getDecl(),
        cast<DeclRefExpr>(NewDRD->getInitOrig())->getDecl());
    SemaRef.CurrentInstantiationScope->InstantiatedLocal(
        cast<DeclRefExpr>(D->getInitPriv())->getDecl(),
        cast<DeclRefExpr>(NewDRD->getInitPriv())->getDecl());
    if (D->getInitializerKind() == OMPDeclareReductionDecl::CallInit) {
      SubstInitializer = SemaRef.SubstExpr(Init, TemplateArgs).get();
    } else {
      auto *OldPrivParm =
          cast<VarDecl>(cast<DeclRefExpr>(D->getInitPriv())->getDecl());
      IsCorrect = IsCorrect && OldPrivParm->hasInit();
      if (IsCorrect)
        SemaRef.InstantiateVariableInitializer(OmpPrivParm, OldPrivParm,
                                               TemplateArgs);
    }
    SemaRef.ActOnOpenMPDeclareReductionInitializerEnd(NewDRD, SubstInitializer,
                                                      OmpPrivParm);
  }
  IsCorrect = IsCorrect && SubstCombiner &&
              (!Init ||
               (D->getInitializerKind() == OMPDeclareReductionDecl::CallInit &&
                SubstInitializer) ||
               (D->getInitializerKind() != OMPDeclareReductionDecl::CallInit &&
                !SubstInitializer));
 
  (void)SemaRef.ActOnOpenMPDeclareReductionDirectiveEnd(
      /*S=*/nullptr, DRD, IsCorrect && !D->isInvalidDecl());
 
  return NewDRD;
}
 
Decl *
TemplateDeclInstantiator::VisitOMPDeclareMapperDecl(OMPDeclareMapperDecl *D) {
  // Instantiate type and check if it is allowed.
  const bool RequiresInstantiation =
      D->getType()->isDependentType() ||
      D->getType()->isInstantiationDependentType() ||
      D->getType()->containsUnexpandedParameterPack();
  QualType SubstMapperTy;
  DeclarationName VN = D->getVarName();
  if (RequiresInstantiation) {
    SubstMapperTy = SemaRef.ActOnOpenMPDeclareMapperType(
        D->getLocation(),
        ParsedType::make(SemaRef.SubstType(D->getType(), TemplateArgs,
                                           D->getLocation(), VN)));
  } else {
    SubstMapperTy = D->getType();
  }
  if (SubstMapperTy.isNull())
    return nullptr;
  // Create an instantiated copy of mapper.
  auto *PrevDeclInScope = D->getPrevDeclInScope();
  if (PrevDeclInScope && !PrevDeclInScope->isInvalidDecl()) {
    PrevDeclInScope = cast<OMPDeclareMapperDecl>(
        SemaRef.CurrentInstantiationScope->findInstantiationOf(PrevDeclInScope)
            ->get<Decl *>());
  }
  bool IsCorrect = true;
  SmallVector<OMPClause *, 6> Clauses;
  // Instantiate the mapper variable.
  DeclarationNameInfo DirName;
  SemaRef.StartOpenMPDSABlock(llvm::omp::OMPD_declare_mapper, DirName,
                              /*S=*/nullptr,
                              (*D->clauselist_begin())->getBeginLoc());
  ExprResult MapperVarRef = SemaRef.ActOnOpenMPDeclareMapperDirectiveVarDecl(
      /*S=*/nullptr, SubstMapperTy, D->getLocation(), VN);
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(
      cast<DeclRefExpr>(D->getMapperVarRef())->getDecl(),
      cast<DeclRefExpr>(MapperVarRef.get())->getDecl());
  auto *ThisContext = dyn_cast_or_null<CXXRecordDecl>(Owner);
  Sema::CXXThisScopeRAII ThisScope(SemaRef, ThisContext, Qualifiers(),
                                   ThisContext);
  // Instantiate map clauses.
  for (OMPClause *C : D->clauselists()) {
    auto *OldC = cast<OMPMapClause>(C);
    SmallVector<Expr *, 4> NewVars;
    for (Expr *OE : OldC->varlists()) {
      Expr *NE = SemaRef.SubstExpr(OE, TemplateArgs).get();
      if (!NE) {
        IsCorrect = false;
        break;
      }
      NewVars.push_back(NE);
    }
    if (!IsCorrect)
      break;
    NestedNameSpecifierLoc NewQualifierLoc =
        SemaRef.SubstNestedNameSpecifierLoc(OldC->getMapperQualifierLoc(),
                                            TemplateArgs);
    CXXScopeSpec SS;
    SS.Adopt(NewQualifierLoc);
    DeclarationNameInfo NewNameInfo =
        SemaRef.SubstDeclarationNameInfo(OldC->getMapperIdInfo(), TemplateArgs);
    OMPVarListLocTy Locs(OldC->getBeginLoc(), OldC->getLParenLoc(),
                         OldC->getEndLoc());
    OMPClause *NewC = SemaRef.ActOnOpenMPMapClause(
        OldC->getIteratorModifier(), OldC->getMapTypeModifiers(),
        OldC->getMapTypeModifiersLoc(), SS, NewNameInfo, OldC->getMapType(),
        OldC->isImplicitMapType(), OldC->getMapLoc(), OldC->getColonLoc(),
        NewVars, Locs);
    Clauses.push_back(NewC);
  }
  SemaRef.EndOpenMPDSABlock(nullptr);
  if (!IsCorrect)
    return nullptr;
  Sema::DeclGroupPtrTy DG = SemaRef.ActOnOpenMPDeclareMapperDirective(
      /*S=*/nullptr, Owner, D->getDeclName(), SubstMapperTy, D->getLocation(),
      VN, D->getAccess(), MapperVarRef.get(), Clauses, PrevDeclInScope);
  Decl *NewDMD = DG.get().getSingleDecl();
  SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewDMD);
  return NewDMD;
}
 
Decl *TemplateDeclInstantiator::VisitOMPCapturedExprDecl(
    OMPCapturedExprDecl * /*D*/) {
  llvm_unreachable("Should not be met in templates");
}
 
Decl *TemplateDeclInstantiator::VisitFunctionDecl(FunctionDecl *D) {
  return VisitFunctionDecl(D, nullptr);
}
 
Decl *
TemplateDeclInstantiator::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
  Decl *Inst = VisitFunctionDecl(D, nullptr);
  if (Inst && !D->getDescribedFunctionTemplate())
    Owner->addDecl(Inst);
  return Inst;
}
 
Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(CXXMethodDecl *D) {
  return VisitCXXMethodDecl(D, nullptr);
}
 
Decl *TemplateDeclInstantiator::VisitRecordDecl(RecordDecl *D) {
  llvm_unreachable("There are only CXXRecordDecls in C++");
}
 
Decl *
TemplateDeclInstantiator::VisitClassTemplateSpecializationDecl(
    ClassTemplateSpecializationDecl *D) {
  // As a MS extension, we permit class-scope explicit specialization
  // of member class templates.
  ClassTemplateDecl *ClassTemplate = D->getSpecializedTemplate();
  assert(ClassTemplate->getDeclContext()->isRecord() &&
         D->getTemplateSpecializationKind() == TSK_ExplicitSpecialization &&
         "can only instantiate an explicit specialization "
         "for a member class template");
 
  // Lookup the already-instantiated declaration in the instantiation
  // of the class template.
  ClassTemplateDecl *InstClassTemplate =
      cast_or_null<ClassTemplateDecl>(SemaRef.FindInstantiatedDecl(
          D->getLocation(), ClassTemplate, TemplateArgs));
  if (!InstClassTemplate)
    return nullptr;
 
  // Substitute into the template arguments of the class template explicit
  // specialization.
  TemplateSpecializationTypeLoc Loc = D->getTypeAsWritten()->getTypeLoc().
                                        castAs<TemplateSpecializationTypeLoc>();
  TemplateArgumentListInfo InstTemplateArgs(Loc.getLAngleLoc(),
                                            Loc.getRAngleLoc());
  SmallVector<TemplateArgumentLoc, 4> ArgLocs;
  for (unsigned I = 0; I != Loc.getNumArgs(); ++I)
    ArgLocs.push_back(Loc.getArgLoc(I));
  if (SemaRef.SubstTemplateArguments(ArgLocs, TemplateArgs, InstTemplateArgs))
    return nullptr;
 
  // Check that the template argument list is well-formed for this
  // class template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (SemaRef.CheckTemplateArgumentList(InstClassTemplate, D->getLocation(),
                                        InstTemplateArgs, false,
                                        SugaredConverted, CanonicalConverted,
                                        /*UpdateArgsWithConversions=*/true))
    return nullptr;
 
  // Figure out where to insert this class template explicit specialization
  // in the member template's set of class template explicit specializations.
  void *InsertPos = nullptr;
  ClassTemplateSpecializationDecl *PrevDecl =
      InstClassTemplate->findSpecialization(CanonicalConverted, InsertPos);
 
  // Check whether we've already seen a conflicting instantiation of this
  // declaration (for instance, if there was a prior implicit instantiation).
  bool Ignored;
  if (PrevDecl &&
      SemaRef.CheckSpecializationInstantiationRedecl(D->getLocation(),
                                                     D->getSpecializationKind(),
                                                     PrevDecl,
                                                     PrevDecl->getSpecializationKind(),
                                                     PrevDecl->getPointOfInstantiation(),
                                                     Ignored))
    return nullptr;
 
  // If PrevDecl was a definition and D is also a definition, diagnose.
  // This happens in cases like:
  //
  //   template<typename T, typename U>
  //   struct Outer {
  //     template<typename X> struct Inner;
  //     template<> struct Inner<T> {};
  //     template<> struct Inner<U> {};
  //   };
  //
  //   Outer<int, int> outer; // error: the explicit specializations of Inner
  //                          // have the same signature.
  if (PrevDecl && PrevDecl->getDefinition() &&
      D->isThisDeclarationADefinition()) {
    SemaRef.Diag(D->getLocation(), diag::err_redefinition) << PrevDecl;
    SemaRef.Diag(PrevDecl->getDefinition()->getLocation(),
                 diag::note_previous_definition);
    return nullptr;
  }
 
  // Create the class template partial specialization declaration.
  ClassTemplateSpecializationDecl *InstD =
      ClassTemplateSpecializationDecl::Create(
          SemaRef.Context, D->getTagKind(), Owner, D->getBeginLoc(),
          D->getLocation(), InstClassTemplate, CanonicalConverted, PrevDecl);
 
  // Add this partial specialization to the set of class template partial
  // specializations.
  if (!PrevDecl)
    InstClassTemplate->AddSpecialization(InstD, InsertPos);
 
  // Substitute the nested name specifier, if any.
  if (SubstQualifier(D, InstD))
    return nullptr;
 
  // Build the canonical type that describes the converted template
  // arguments of the class template explicit specialization.
  QualType CanonType = SemaRef.Context.getTemplateSpecializationType(
      TemplateName(InstClassTemplate), CanonicalConverted,
      SemaRef.Context.getRecordType(InstD));
 
  // Build the fully-sugared type for this class template
  // specialization as the user wrote in the specialization
  // itself. This means that we'll pretty-print the type retrieved
  // from the specialization's declaration the way that the user
  // actually wrote the specialization, rather than formatting the
  // name based on the "canonical" representation used to store the
  // template arguments in the specialization.
  TypeSourceInfo *WrittenTy = SemaRef.Context.getTemplateSpecializationTypeInfo(
      TemplateName(InstClassTemplate), D->getLocation(), InstTemplateArgs,
      CanonType);
 
  InstD->setAccess(D->getAccess());
  InstD->setInstantiationOfMemberClass(D, TSK_ImplicitInstantiation);
  InstD->setSpecializationKind(D->getSpecializationKind());
  InstD->setTypeAsWritten(WrittenTy);
  InstD->setExternLoc(D->getExternLoc());
  InstD->setTemplateKeywordLoc(D->getTemplateKeywordLoc());
 
  Owner->addDecl(InstD);
 
  // Instantiate the members of the class-scope explicit specialization eagerly.
  // We don't have support for lazy instantiation of an explicit specialization
  // yet, and MSVC eagerly instantiates in this case.
  // FIXME: This is wrong in standard C++.
  if (D->isThisDeclarationADefinition() &&
      SemaRef.InstantiateClass(D->getLocation(), InstD, D, TemplateArgs,
                               TSK_ImplicitInstantiation,
                               /*Complain=*/true))
    return nullptr;
 
  return InstD;
}
 
Decl *TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl(
    VarTemplateSpecializationDecl *D) {
 
  TemplateArgumentListInfo VarTemplateArgsInfo;
  VarTemplateDecl *VarTemplate = D->getSpecializedTemplate();
  assert(VarTemplate &&
         "A template specialization without specialized template?");
 
  VarTemplateDecl *InstVarTemplate =
      cast_or_null<VarTemplateDecl>(SemaRef.FindInstantiatedDecl(
          D->getLocation(), VarTemplate, TemplateArgs));
  if (!InstVarTemplate)
    return nullptr;
 
  // Substitute the current template arguments.
  if (const ASTTemplateArgumentListInfo *TemplateArgsInfo =
          D->getTemplateArgsInfo()) {
    VarTemplateArgsInfo.setLAngleLoc(TemplateArgsInfo->getLAngleLoc());
    VarTemplateArgsInfo.setRAngleLoc(TemplateArgsInfo->getRAngleLoc());
 
    if (SemaRef.SubstTemplateArguments(TemplateArgsInfo->arguments(),
                                       TemplateArgs, VarTemplateArgsInfo))
      return nullptr;
  }
 
  // Check that the template argument list is well-formed for this template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (SemaRef.CheckTemplateArgumentList(InstVarTemplate, D->getLocation(),
                                        VarTemplateArgsInfo, false,
                                        SugaredConverted, CanonicalConverted,
                                        /*UpdateArgsWithConversions=*/true))
    return nullptr;
 
  // Check whether we've already seen a declaration of this specialization.
  void *InsertPos = nullptr;
  VarTemplateSpecializationDecl *PrevDecl =
      InstVarTemplate->findSpecialization(CanonicalConverted, InsertPos);
 
  // Check whether we've already seen a conflicting instantiation of this
  // declaration (for instance, if there was a prior implicit instantiation).
  bool Ignored;
  if (PrevDecl && SemaRef.CheckSpecializationInstantiationRedecl(
                      D->getLocation(), D->getSpecializationKind(), PrevDecl,
                      PrevDecl->getSpecializationKind(),
                      PrevDecl->getPointOfInstantiation(), Ignored))
    return nullptr;
 
  return VisitVarTemplateSpecializationDecl(
      InstVarTemplate, D, VarTemplateArgsInfo, CanonicalConverted, PrevDecl);
}
 
Decl *TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl(
    VarTemplateDecl *VarTemplate, VarDecl *D,
    const TemplateArgumentListInfo &TemplateArgsInfo,
    ArrayRef<TemplateArgument> Converted,
    VarTemplateSpecializationDecl *PrevDecl) {
 
  // Do substitution on the type of the declaration
  TypeSourceInfo *DI =
      SemaRef.SubstType(D->getTypeSourceInfo(), TemplateArgs,
                        D->getTypeSpecStartLoc(), D->getDeclName());
  if (!DI)
    return nullptr;
 
  if (DI->getType()->isFunctionType()) {
    SemaRef.Diag(D->getLocation(), diag::err_variable_instantiates_to_function)
        << D->isStaticDataMember() << DI->getType();
    return nullptr;
  }
 
  // Build the instantiated declaration
  VarTemplateSpecializationDecl *Var = VarTemplateSpecializationDecl::Create(
      SemaRef.Context, Owner, D->getInnerLocStart(), D->getLocation(),
      VarTemplate, DI->getType(), DI, D->getStorageClass(), Converted);
  Var->setTemplateArgsInfo(TemplateArgsInfo);
  if (!PrevDecl) {
    void *InsertPos = nullptr;
    VarTemplate->findSpecialization(Converted, InsertPos);
    VarTemplate->AddSpecialization(Var, InsertPos);
  }
 
  if (SemaRef.getLangOpts().OpenCL)
    SemaRef.deduceOpenCLAddressSpace(Var);
 
  // Substitute the nested name specifier, if any.
  if (SubstQualifier(D, Var))
    return nullptr;
 
  SemaRef.BuildVariableInstantiation(Var, D, TemplateArgs, LateAttrs, Owner,
                                     StartingScope, false, PrevDecl);
 
  return Var;
}
 
Decl *TemplateDeclInstantiator::VisitObjCAtDefsFieldDecl(ObjCAtDefsFieldDecl *D) {
  llvm_unreachable("@defs is not supported in Objective-C++");
}
 
Decl *TemplateDeclInstantiator::VisitFriendTemplateDecl(FriendTemplateDecl *D) {
  // FIXME: We need to be able to instantiate FriendTemplateDecls.
  unsigned DiagID = SemaRef.getDiagnostics().getCustomDiagID(
                                               DiagnosticsEngine::Error,
                                               "cannot instantiate %0 yet");
  SemaRef.Diag(D->getLocation(), DiagID)
    << D->getDeclKindName();
 
  return nullptr;
}
 
Decl *TemplateDeclInstantiator::VisitConceptDecl(ConceptDecl *D) {
  llvm_unreachable("Concept definitions cannot reside inside a template");
}
 
Decl *TemplateDeclInstantiator::VisitImplicitConceptSpecializationDecl(
    ImplicitConceptSpecializationDecl *D) {
  llvm_unreachable("Concept specializations cannot reside inside a template");
}
 
Decl *
TemplateDeclInstantiator::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
  return RequiresExprBodyDecl::Create(SemaRef.Context, D->getDeclContext(),
                                      D->getBeginLoc());
}
 
Decl *TemplateDeclInstantiator::VisitDecl(Decl *D) {
  llvm_unreachable("Unexpected decl");
}
 
Decl *Sema::SubstDecl(Decl *D, DeclContext *Owner,
                      const MultiLevelTemplateArgumentList &TemplateArgs) {
  TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs);
  if (D->isInvalidDecl())
    return nullptr;
 
  Decl *SubstD;
  runWithSufficientStackSpace(D->getLocation(), [&] {
    SubstD = Instantiator.Visit(D);
  });
  return SubstD;
}
 
void TemplateDeclInstantiator::adjustForRewrite(RewriteKind RK,
                                                FunctionDecl *Orig, QualType &T,
                                                TypeSourceInfo *&TInfo,
                                                DeclarationNameInfo &NameInfo) {
  assert(RK == RewriteKind::RewriteSpaceshipAsEqualEqual);
 
  // C++2a [class.compare.default]p3:
  //   the return type is replaced with bool
  auto *FPT = T->castAs<FunctionProtoType>();
  T = SemaRef.Context.getFunctionType(
      SemaRef.Context.BoolTy, FPT->getParamTypes(), FPT->getExtProtoInfo());
 
  // Update the return type in the source info too. The most straightforward
  // way is to create new TypeSourceInfo for the new type. Use the location of
  // the '= default' as the location of the new type.
  //
  // FIXME: Set the correct return type when we initially transform the type,
  // rather than delaying it to now.
  TypeSourceInfo *NewTInfo =
      SemaRef.Context.getTrivialTypeSourceInfo(T, Orig->getEndLoc());
  auto OldLoc = TInfo->getTypeLoc().getAsAdjusted<FunctionProtoTypeLoc>();
  assert(OldLoc && "type of function is not a function type?");
  auto NewLoc = NewTInfo->getTypeLoc().castAs<FunctionProtoTypeLoc>();
  for (unsigned I = 0, N = OldLoc.getNumParams(); I != N; ++I)
    NewLoc.setParam(I, OldLoc.getParam(I));
  TInfo = NewTInfo;
 
  //   and the declarator-id is replaced with operator==
  NameInfo.setName(
      SemaRef.Context.DeclarationNames.getCXXOperatorName(OO_EqualEqual));
}
 
FunctionDecl *Sema::SubstSpaceshipAsEqualEqual(CXXRecordDecl *RD,
                                               FunctionDecl *Spaceship) {
  if (Spaceship->isInvalidDecl())
    return nullptr;
 
  // C++2a [class.compare.default]p3:
  //   an == operator function is declared implicitly [...] with the same
  //   access and function-definition and in the same class scope as the
  //   three-way comparison operator function
  MultiLevelTemplateArgumentList NoTemplateArgs;
  NoTemplateArgs.setKind(TemplateSubstitutionKind::Rewrite);
  NoTemplateArgs.addOuterRetainedLevels(RD->getTemplateDepth());
  TemplateDeclInstantiator Instantiator(*this, RD, NoTemplateArgs);
  Decl *R;
  if (auto *MD = dyn_cast<CXXMethodDecl>(Spaceship)) {
    R = Instantiator.VisitCXXMethodDecl(
        MD, nullptr, std::nullopt,
        TemplateDeclInstantiator::RewriteKind::RewriteSpaceshipAsEqualEqual);
  } else {
    assert(Spaceship->getFriendObjectKind() &&
           "defaulted spaceship is neither a member nor a friend");
 
    R = Instantiator.VisitFunctionDecl(
        Spaceship, nullptr,
        TemplateDeclInstantiator::RewriteKind::RewriteSpaceshipAsEqualEqual);
    if (!R)
      return nullptr;
 
    FriendDecl *FD =
        FriendDecl::Create(Context, RD, Spaceship->getLocation(),
                           cast<NamedDecl>(R), Spaceship->getBeginLoc());
    FD->setAccess(AS_public);
    RD->addDecl(FD);
  }
  return cast_or_null<FunctionDecl>(R);
}
 
/// Instantiates a nested template parameter list in the current
/// instantiation context.
///
/// \param L The parameter list to instantiate
///
/// \returns NULL if there was an error
TemplateParameterList *
TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
  // Get errors for all the parameters before bailing out.
  bool Invalid = false;
 
  unsigned N = L->size();
  typedef SmallVector<NamedDecl *, 8> ParamVector;
  ParamVector Params;
  Params.reserve(N);
  for (auto &P : *L) {
    NamedDecl *D = cast_or_null<NamedDecl>(Visit(P));
    Params.push_back(D);
    Invalid = Invalid || !D || D->isInvalidDecl();
  }
 
  // Clean up if we had an error.
  if (Invalid)
    return nullptr;
 
  Expr *InstRequiresClause = L->getRequiresClause();
 
  TemplateParameterList *InstL
    = TemplateParameterList::Create(SemaRef.Context, L->getTemplateLoc(),
                                    L->getLAngleLoc(), Params,
                                    L->getRAngleLoc(), InstRequiresClause);
  return InstL;
}
 
TemplateParameterList *
Sema::SubstTemplateParams(TemplateParameterList *Params, DeclContext *Owner,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
                          bool EvaluateConstraints) {
  TemplateDeclInstantiator Instantiator(*this, Owner, TemplateArgs);
  Instantiator.setEvaluateConstraints(EvaluateConstraints);
  return Instantiator.SubstTemplateParams(Params);
}
 
/// Instantiate the declaration of a class template partial
/// specialization.
///
/// \param ClassTemplate the (instantiated) class template that is partially
// specialized by the instantiation of \p PartialSpec.
///
/// \param PartialSpec the (uninstantiated) class template partial
/// specialization that we are instantiating.
///
/// \returns The instantiated partial specialization, if successful; otherwise,
/// NULL to indicate an error.
ClassTemplatePartialSpecializationDecl *
TemplateDeclInstantiator::InstantiateClassTemplatePartialSpecialization(
                                            ClassTemplateDecl *ClassTemplate,
                          ClassTemplatePartialSpecializationDecl *PartialSpec) {
  // Create a local instantiation scope for this class template partial
  // specialization, which will contain the instantiations of the template
  // parameters.
  LocalInstantiationScope Scope(SemaRef);
 
  // Substitute into the template parameters of the class template partial
  // specialization.
  TemplateParameterList *TempParams = PartialSpec->getTemplateParameters();
  TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
  if (!InstParams)
    return nullptr;
 
  // Substitute into the template arguments of the class template partial
  // specialization.
  const ASTTemplateArgumentListInfo *TemplArgInfo
    = PartialSpec->getTemplateArgsAsWritten();
  TemplateArgumentListInfo InstTemplateArgs(TemplArgInfo->LAngleLoc,
                                            TemplArgInfo->RAngleLoc);
  if (SemaRef.SubstTemplateArguments(TemplArgInfo->arguments(), TemplateArgs,
                                     InstTemplateArgs))
    return nullptr;
 
  // Check that the template argument list is well-formed for this
  // class template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (SemaRef.CheckTemplateArgumentList(
          ClassTemplate, PartialSpec->getLocation(), InstTemplateArgs,
          /*PartialTemplateArgs=*/false, SugaredConverted, CanonicalConverted))
    return nullptr;
 
  // Check these arguments are valid for a template partial specialization.
  if (SemaRef.CheckTemplatePartialSpecializationArgs(
          PartialSpec->getLocation(), ClassTemplate, InstTemplateArgs.size(),
          CanonicalConverted))
    return nullptr;
 
  // Figure out where to insert this class template partial specialization
  // in the member template's set of class template partial specializations.
  void *InsertPos = nullptr;
  ClassTemplateSpecializationDecl *PrevDecl =
      ClassTemplate->findPartialSpecialization(CanonicalConverted, InstParams,
                                               InsertPos);
 
  // Build the canonical type that describes the converted template
  // arguments of the class template partial specialization.
  QualType CanonType = SemaRef.Context.getTemplateSpecializationType(
      TemplateName(ClassTemplate), CanonicalConverted);
 
  // Build the fully-sugared type for this class template
  // specialization as the user wrote in the specialization
  // itself. This means that we'll pretty-print the type retrieved
  // from the specialization's declaration the way that the user
  // actually wrote the specialization, rather than formatting the
  // name based on the "canonical" representation used to store the
  // template arguments in the specialization.
  TypeSourceInfo *WrittenTy
    = SemaRef.Context.getTemplateSpecializationTypeInfo(
                                                    TemplateName(ClassTemplate),
                                                    PartialSpec->getLocation(),
                                                    InstTemplateArgs,
                                                    CanonType);
 
  if (PrevDecl) {
    // We've already seen a partial specialization with the same template
    // parameters and template arguments. This can happen, for example, when
    // substituting the outer template arguments ends up causing two
    // class template partial specializations of a member class template
    // to have identical forms, e.g.,
    //
    //   template<typename T, typename U>
    //   struct Outer {
    //     template<typename X, typename Y> struct Inner;
    //     template<typename Y> struct Inner<T, Y>;
    //     template<typename Y> struct Inner<U, Y>;
    //   };
    //
    //   Outer<int, int> outer; // error: the partial specializations of Inner
    //                          // have the same signature.
    SemaRef.Diag(PartialSpec->getLocation(), diag::err_partial_spec_redeclared)
      << WrittenTy->getType();
    SemaRef.Diag(PrevDecl->getLocation(), diag::note_prev_partial_spec_here)
      << SemaRef.Context.getTypeDeclType(PrevDecl);
    return nullptr;
  }
 
 
  // Create the class template partial specialization declaration.
  ClassTemplatePartialSpecializationDecl *InstPartialSpec =
      ClassTemplatePartialSpecializationDecl::Create(
          SemaRef.Context, PartialSpec->getTagKind(), Owner,
          PartialSpec->getBeginLoc(), PartialSpec->getLocation(), InstParams,
          ClassTemplate, CanonicalConverted, InstTemplateArgs, CanonType,
          nullptr);
  // Substitute the nested name specifier, if any.
  if (SubstQualifier(PartialSpec, InstPartialSpec))
    return nullptr;
 
  InstPartialSpec->setInstantiatedFromMember(PartialSpec);
  InstPartialSpec->setTypeAsWritten(WrittenTy);
 
  // Check the completed partial specialization.
  SemaRef.CheckTemplatePartialSpecialization(InstPartialSpec);
 
  // Add this partial specialization to the set of class template partial
  // specializations.
  ClassTemplate->AddPartialSpecialization(InstPartialSpec,
                                          /*InsertPos=*/nullptr);
  return InstPartialSpec;
}
 
/// Instantiate the declaration of a variable template partial
/// specialization.
///
/// \param VarTemplate the (instantiated) variable template that is partially
/// specialized by the instantiation of \p PartialSpec.
///
/// \param PartialSpec the (uninstantiated) variable template partial
/// specialization that we are instantiating.
///
/// \returns The instantiated partial specialization, if successful; otherwise,
/// NULL to indicate an error.
VarTemplatePartialSpecializationDecl *
TemplateDeclInstantiator::InstantiateVarTemplatePartialSpecialization(
    VarTemplateDecl *VarTemplate,
    VarTemplatePartialSpecializationDecl *PartialSpec) {
  // Create a local instantiation scope for this variable template partial
  // specialization, which will contain the instantiations of the template
  // parameters.
  LocalInstantiationScope Scope(SemaRef);
 
  // Substitute into the template parameters of the variable template partial
  // specialization.
  TemplateParameterList *TempParams = PartialSpec->getTemplateParameters();
  TemplateParameterList *InstParams = SubstTemplateParams(TempParams);
  if (!InstParams)
    return nullptr;
 
  // Substitute into the template arguments of the variable template partial
  // specialization.
  const ASTTemplateArgumentListInfo *TemplArgInfo
    = PartialSpec->getTemplateArgsAsWritten();
  TemplateArgumentListInfo InstTemplateArgs(TemplArgInfo->LAngleLoc,
                                            TemplArgInfo->RAngleLoc);
  if (SemaRef.SubstTemplateArguments(TemplArgInfo->arguments(), TemplateArgs,
                                     InstTemplateArgs))
    return nullptr;
 
  // Check that the template argument list is well-formed for this
  // class template.
  SmallVector<TemplateArgument, 4> SugaredConverted, CanonicalConverted;
  if (SemaRef.CheckTemplateArgumentList(
          VarTemplate, PartialSpec->getLocation(), InstTemplateArgs,
          /*PartialTemplateArgs=*/false, SugaredConverted, CanonicalConverted))
    return nullptr;
 
  // Check these arguments are valid for a template partial specialization.
  if (SemaRef.CheckTemplatePartialSpecializationArgs(
          PartialSpec->getLocation(), VarTemplate, InstTemplateArgs.size(),
          CanonicalConverted))
    return nullptr;
 
  // Figure out where to insert this variable template partial specialization
  // in the member template's set of variable template partial specializations.
  void *InsertPos = nullptr;
  VarTemplateSpecializationDecl *PrevDecl =
      VarTemplate->findPartialSpecialization(CanonicalConverted, InstParams,
                                             InsertPos);
 
  // Build the canonical type that describes the converted template
  // arguments of the variable template partial specialization.
  QualType CanonType = SemaRef.Context.getTemplateSpecializationType(
      TemplateName(VarTemplate), CanonicalConverted);
 
  // Build the fully-sugared type for this variable template
  // specialization as the user wrote in the specialization
  // itself. This means that we'll pretty-print the type retrieved
  // from the specialization's declaration the way that the user
  // actually wrote the specialization, rather than formatting the
  // name based on the "canonical" representation used to store the
  // template arguments in the specialization.
  TypeSourceInfo *WrittenTy = SemaRef.Context.getTemplateSpecializationTypeInfo(
      TemplateName(VarTemplate), PartialSpec->getLocation(), InstTemplateArgs,
      CanonType);
 
  if (PrevDecl) {
    // We've already seen a partial specialization with the same template
    // parameters and template arguments. This can happen, for example, when
    // substituting the outer template arguments ends up causing two
    // variable template partial specializations of a member variable template
    // to have identical forms, e.g.,
    //
    //   template<typename T, typename U>
    //   struct Outer {
    //     template<typename X, typename Y> pair<X,Y> p;
    //     template<typename Y> pair<T, Y> p;
    //     template<typename Y> pair<U, Y> p;
    //   };
    //
    //   Outer<int, int> outer; // error: the partial specializations of Inner
    //                          // have the same signature.
    SemaRef.Diag(PartialSpec->getLocation(),
                 diag::err_var_partial_spec_redeclared)
        << WrittenTy->getType();
    SemaRef.Diag(PrevDecl->getLocation(),
                 diag::note_var_prev_partial_spec_here);
    return nullptr;
  }
 
  // Do substitution on the type of the declaration
  TypeSourceInfo *DI = SemaRef.SubstType(
      PartialSpec->getTypeSourceInfo(), TemplateArgs,
      PartialSpec->getTypeSpecStartLoc(), PartialSpec->getDeclName());
  if (!DI)
    return nullptr;
 
  if (DI->getType()->isFunctionType()) {
    SemaRef.Diag(PartialSpec->getLocation(),
                 diag::err_variable_instantiates_to_function)
        << PartialSpec->isStaticDataMember() << DI->getType();
    return nullptr;
  }
 
  // Create the variable template partial specialization declaration.
  VarTemplatePartialSpecializationDecl *InstPartialSpec =
      VarTemplatePartialSpecializationDecl::Create(
          SemaRef.Context, Owner, PartialSpec->getInnerLocStart(),
          PartialSpec->getLocation(), InstParams, VarTemplate, DI->getType(),
          DI, PartialSpec->getStorageClass(), CanonicalConverted,
          InstTemplateArgs);
 
  // Substitute the nested name specifier, if any.
  if (SubstQualifier(PartialSpec, InstPartialSpec))
    return nullptr;
 
  InstPartialSpec->setInstantiatedFromMember(PartialSpec);
  InstPartialSpec->setTypeAsWritten(WrittenTy);
 
  // Check the completed partial specialization.
  SemaRef.CheckTemplatePartialSpecialization(InstPartialSpec);
 
  // Add this partial specialization to the set of variable template partial
  // specializations. The instantiation of the initializer is not necessary.
  VarTemplate->AddPartialSpecialization(InstPartialSpec, /*InsertPos=*/nullptr);
 
  SemaRef.BuildVariableInstantiation(InstPartialSpec, PartialSpec, TemplateArgs,
                                     LateAttrs, Owner, StartingScope);
 
  return InstPartialSpec;
}
 
TypeSourceInfo*
TemplateDeclInstantiator::SubstFunctionType(FunctionDecl *D,
                              SmallVectorImpl<ParmVarDecl *> &Params) {
  TypeSourceInfo *OldTInfo = D->getTypeSourceInfo();
  assert(OldTInfo && "substituting function without type source info");
  assert(Params.empty() && "parameter vector is non-empty at start");
 
  CXXRecordDecl *ThisContext = nullptr;
  Qualifiers ThisTypeQuals;
  if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(D)) {
    ThisContext = cast<CXXRecordDecl>(Owner);
    ThisTypeQuals = Method->getMethodQualifiers();
  }
 
  TypeSourceInfo *NewTInfo = SemaRef.SubstFunctionDeclType(
      OldTInfo, TemplateArgs, D->getTypeSpecStartLoc(), D->getDeclName(),
      ThisContext, ThisTypeQuals, EvaluateConstraints);
  if (!NewTInfo)
    return nullptr;
 
  TypeLoc OldTL = OldTInfo->getTypeLoc().IgnoreParens();
  if (FunctionProtoTypeLoc OldProtoLoc = OldTL.getAs<FunctionProtoTypeLoc>()) {
    if (NewTInfo != OldTInfo) {
      // Get parameters from the new type info.
      TypeLoc NewTL = NewTInfo->getTypeLoc().IgnoreParens();
      FunctionProtoTypeLoc NewProtoLoc = NewTL.castAs<FunctionProtoTypeLoc>();
      unsigned NewIdx = 0;
      for (unsigned OldIdx = 0, NumOldParams = OldProtoLoc.getNumParams();
           OldIdx != NumOldParams; ++OldIdx) {
        ParmVarDecl *OldParam = OldProtoLoc.getParam(OldIdx);
        if (!OldParam)
          return nullptr;
 
        LocalInstantiationScope *Scope = SemaRef.CurrentInstantiationScope;
 
        std::optional<unsigned> NumArgumentsInExpansion;
        if (OldParam->isParameterPack())
          NumArgumentsInExpansion =
              SemaRef.getNumArgumentsInExpansion(OldParam->getType(),
                                                 TemplateArgs);
        if (!NumArgumentsInExpansion) {
          // Simple case: normal parameter, or a parameter pack that's
          // instantiated to a (still-dependent) parameter pack.
          ParmVarDecl *NewParam = NewProtoLoc.getParam(NewIdx++);
          Params.push_back(NewParam);
          Scope->InstantiatedLocal(OldParam, NewParam);
        } else {
          // Parameter pack expansion: make the instantiation an argument pack.
          Scope->MakeInstantiatedLocalArgPack(OldParam);
          for (unsigned I = 0; I != *NumArgumentsInExpansion; ++I) {
            ParmVarDecl *NewParam = NewProtoLoc.getParam(NewIdx++);
            Params.push_back(NewParam);
            Scope->InstantiatedLocalPackArg(OldParam, NewParam);
          }
        }
      }
    } else {
      // The function type itself was not dependent and therefore no
      // substitution occurred. However, we still need to instantiate
      // the function parameters themselves.
      const FunctionProtoType *OldProto =
          cast<FunctionProtoType>(OldProtoLoc.getType());
      for (unsigned i = 0, i_end = OldProtoLoc.getNumParams(); i != i_end;
           ++i) {
        ParmVarDecl *OldParam = OldProtoLoc.getParam(i);
        if (!OldParam) {
          Params.push_back(SemaRef.BuildParmVarDeclForTypedef(
              D, D->getLocation(), OldProto->getParamType(i)));
          continue;
        }
 
        ParmVarDecl *Parm =
            cast_or_null<ParmVarDecl>(VisitParmVarDecl(OldParam));
        if (!Parm)
          return nullptr;
        Params.push_back(Parm);
      }
    }
  } else {
    // If the type of this function, after ignoring parentheses, is not
    // *directly* a function type, then we're instantiating a function that
    // was declared via a typedef or with attributes, e.g.,
    //
    //   typedef int functype(int, int);
    //   functype func;
    //   int __cdecl meth(int, int);
    //
    // In this case, we'll just go instantiate the ParmVarDecls that we
    // synthesized in the method declaration.
    SmallVector<QualType, 4> ParamTypes;
    Sema::ExtParameterInfoBuilder ExtParamInfos;
    if (SemaRef.SubstParmTypes(D->getLocation(), D->parameters(), nullptr,
                               TemplateArgs, ParamTypes, &Params,
                               ExtParamInfos))
      return nullptr;
  }
 
  return NewTInfo;
}
 
/// Introduce the instantiated function parameters into the local
/// instantiation scope, and set the parameter names to those used
/// in the template.
bool Sema::addInstantiatedParametersToScope(
    FunctionDecl *Function, const FunctionDecl *PatternDecl,
    LocalInstantiationScope &Scope,
    const MultiLevelTemplateArgumentList &TemplateArgs) {
  unsigned FParamIdx = 0;
  for (unsigned I = 0, N = PatternDecl->getNumParams(); I != N; ++I) {
    const ParmVarDecl *PatternParam = PatternDecl->getParamDecl(I);
    if (!PatternParam->isParameterPack()) {
      // Simple case: not a parameter pack.
      assert(FParamIdx < Function->getNumParams());
      ParmVarDecl *FunctionParam = Function->getParamDecl(FParamIdx);
      FunctionParam->setDeclName(PatternParam->getDeclName());
      // If the parameter's type is not dependent, update it to match the type
      // in the pattern. They can differ in top-level cv-qualifiers, and we want
      // the pattern's type here. If the type is dependent, they can't differ,
      // per core issue 1668. Substitute into the type from the pattern, in case
      // it's instantiation-dependent.
      // FIXME: Updating the type to work around this is at best fragile.
      if (!PatternDecl->getType()->isDependentType()) {
        QualType T = SubstType(PatternParam->getType(), TemplateArgs,
                               FunctionParam->getLocation(),
                               FunctionParam->getDeclName());
        if (T.isNull())
          return true;
        FunctionParam->setType(T);
      }
 
      Scope.InstantiatedLocal(PatternParam, FunctionParam);
      ++FParamIdx;
      continue;
    }
 
    // Expand the parameter pack.
    Scope.MakeInstantiatedLocalArgPack(PatternParam);
    std::optional<unsigned> NumArgumentsInExpansion =
        getNumArgumentsInExpansion(PatternParam->getType(), TemplateArgs);
    if (NumArgumentsInExpansion) {
      QualType PatternType =
          PatternParam->getType()->castAs<PackExpansionType>()->getPattern();
      for (unsigned Arg = 0; Arg < *NumArgumentsInExpansion; ++Arg) {
        ParmVarDecl *FunctionParam = Function->getParamDecl(FParamIdx);
        FunctionParam->setDeclName(PatternParam->getDeclName());
        if (!PatternDecl->getType()->isDependentType()) {
          Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this, Arg);
          QualType T =
              SubstType(PatternType, TemplateArgs, FunctionParam->getLocation(),
                        FunctionParam->getDeclName());
          if (T.isNull())
            return true;
          FunctionParam->setType(T);
        }
 
        Scope.InstantiatedLocalPackArg(PatternParam, FunctionParam);
        ++FParamIdx;
      }
    }
  }
 
  return false;
}
 
bool Sema::InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
                                      ParmVarDecl *Param) {
  assert(Param->hasUninstantiatedDefaultArg());
 
  // Instantiate the expression.
  //
  // FIXME: Pass in a correct Pattern argument, otherwise
  // getTemplateInstantiationArgs uses the lexical context of FD, e.g.
  //
  // template<typename T>
  // struct A {
  //   static int FooImpl();
  //
  //   template<typename Tp>
  //   // bug: default argument A<T>::FooImpl() is evaluated with 2-level
  //   // template argument list [[T], [Tp]], should be [[Tp]].
  //   friend A<Tp> Foo(int a);
  // };
  //
  // template<typename T>
  // A<T> Foo(int a = A<T>::FooImpl());
  MultiLevelTemplateArgumentList TemplateArgs = getTemplateInstantiationArgs(
      FD, /*Final=*/false, nullptr, /*RelativeToPrimary=*/true);
 
  if (SubstDefaultArgument(CallLoc, Param, TemplateArgs, /*ForCallExpr*/ true))
    return true;
 
  if (ASTMutationListener *L = getASTMutationListener())
    L->DefaultArgumentInstantiated(Param);
 
  return false;
}
 
void Sema::InstantiateExceptionSpec(SourceLocation PointOfInstantiation,
                                    FunctionDecl *Decl) {
  const FunctionProtoType *Proto = Decl->getType()->castAs<FunctionProtoType>();
  if (Proto->getExceptionSpecType() != EST_Uninstantiated)
    return;
 
  InstantiatingTemplate Inst(*this, PointOfInstantiation, Decl,
                             InstantiatingTemplate::ExceptionSpecification());
  if (Inst.isInvalid()) {
    // We hit the instantiation depth limit. Clear the exception specification
    // so that our callers don't have to cope with EST_Uninstantiated.
    UpdateExceptionSpec(Decl, EST_None);
    return;
  }
  if (Inst.isAlreadyInstantiating()) {
    // This exception specification indirectly depends on itself. Reject.
    // FIXME: Corresponding rule in the standard?
    Diag(PointOfInstantiation, diag::err_exception_spec_cycle) << Decl;
    UpdateExceptionSpec(Decl, EST_None);
    return;
  }
 
  // Enter the scope of this instantiation. We don't use
  // PushDeclContext because we don't have a scope.
  Sema::ContextRAII savedContext(*this, Decl);
  LocalInstantiationScope Scope(*this);
 
  MultiLevelTemplateArgumentList TemplateArgs = getTemplateInstantiationArgs(
      Decl, /*Final=*/false, nullptr, /*RelativeToPrimary*/ true);
 
  // FIXME: We can't use getTemplateInstantiationPattern(false) in general
  // here, because for a non-defining friend declaration in a class template,
  // we don't store enough information to map back to the friend declaration in
  // the template.
  FunctionDecl *Template = Proto->getExceptionSpecTemplate();
  if (addInstantiatedParametersToScope(Decl, Template, Scope, TemplateArgs)) {
    UpdateExceptionSpec(Decl, EST_None);
    return;
  }
 
  SubstExceptionSpec(Decl, Template->getType()->castAs<FunctionProtoType>(),
                     TemplateArgs);
}
 
/// Initializes the common fields of an instantiation function
/// declaration (New) from the corresponding fields of its template (Tmpl).
///
/// \returns true if there was an error
bool
TemplateDeclInstantiator::InitFunctionInstantiation(FunctionDecl *New,
                                                    FunctionDecl *Tmpl) {
  New->setImplicit(Tmpl->isImplicit());
 
  // Forward the mangling number from the template to the instantiated decl.
  SemaRef.Context.setManglingNumber(New,
                                    SemaRef.Context.getManglingNumber(Tmpl));
 
  // If we are performing substituting explicitly-specified template arguments
  // or deduced template arguments into a function template and we reach this
  // point, we are now past the point where SFINAE applies and have committed
  // to keeping the new function template specialization. We therefore
  // convert the active template instantiation for the function template
  // into a template instantiation for this specific function template
  // specialization, which is not a SFINAE context, so that we diagnose any
  // further errors in the declaration itself.
  //
  // FIXME: This is a hack.
  typedef Sema::CodeSynthesisContext ActiveInstType;
  ActiveInstType &ActiveInst = SemaRef.CodeSynthesisContexts.back();
  if (ActiveInst.Kind == ActiveInstType::ExplicitTemplateArgumentSubstitution ||
      ActiveInst.Kind == ActiveInstType::DeducedTemplateArgumentSubstitution) {
    if (FunctionTemplateDecl *FunTmpl
          = dyn_cast<FunctionTemplateDecl>(ActiveInst.Entity)) {
      assert(FunTmpl->getTemplatedDecl() == Tmpl &&
             "Deduction from the wrong function template?");
      (void) FunTmpl;
      SemaRef.InstantiatingSpecializations.erase(
          {ActiveInst.Entity->getCanonicalDecl(), ActiveInst.Kind});
      atTemplateEnd(SemaRef.TemplateInstCallbacks, SemaRef, ActiveInst);
      ActiveInst.Kind = ActiveInstType::TemplateInstantiation;
      ActiveInst.Entity = New;
      atTemplateBegin(SemaRef.TemplateInstCallbacks, SemaRef, ActiveInst);
    }
  }
 
  const FunctionProtoType *Proto = Tmpl->getType()->getAs<FunctionProtoType>();
  assert(Proto && "Function template without prototype?");
 
  if (Proto->hasExceptionSpec() || Proto->getNoReturnAttr()) {
    FunctionProtoType::ExtProtoInfo EPI = Proto->getExtProtoInfo();
 
    // DR1330: In C++11, defer instantiation of a non-trivial
    // exception specification.
    // DR1484: Local classes and their members are instantiated along with the
    // containing function.
    if (SemaRef.getLangOpts().CPlusPlus11 &&
        EPI.ExceptionSpec.Type != EST_None &&
        EPI.ExceptionSpec.Type != EST_DynamicNone &&
        EPI.ExceptionSpec.Type != EST_BasicNoexcept &&
        !Tmpl->isInLocalScopeForInstantiation()) {
      FunctionDecl *ExceptionSpecTemplate = Tmpl;
      if (EPI.ExceptionSpec.Type == EST_Uninstantiated)
        ExceptionSpecTemplate = EPI.ExceptionSpec.SourceTemplate;
      ExceptionSpecificationType NewEST = EST_Uninstantiated;
      if (EPI.ExceptionSpec.Type == EST_Unevaluated)
        NewEST = EST_Unevaluated;
 
      // Mark the function has having an uninstantiated exception specification.
      const FunctionProtoType *NewProto
        = New->getType()->getAs<FunctionProtoType>();
      assert(NewProto && "Template instantiation without function prototype?");
      EPI = NewProto->getExtProtoInfo();
      EPI.ExceptionSpec.Type = NewEST;
      EPI.ExceptionSpec.SourceDecl = New;
      EPI.ExceptionSpec.SourceTemplate = ExceptionSpecTemplate;
      New->setType(SemaRef.Context.getFunctionType(
          NewProto->getReturnType(), NewProto->getParamTypes(), EPI));
    } else {
      Sema::ContextRAII SwitchContext(SemaRef, New);
      SemaRef.SubstExceptionSpec(New, Proto, TemplateArgs);
    }
  }
 
  // Get the definition. Leaves the variable unchanged if undefined.
  const FunctionDecl *Definition = Tmpl;
  Tmpl->isDefined(Definition);
 
  SemaRef.InstantiateAttrs(TemplateArgs, Definition, New,
                           LateAttrs, StartingScope);
 
  return false;
}
 
/// Initializes common fields of an instantiated method
/// declaration (New) from the corresponding fields of its template
/// (Tmpl).
///
/// \returns true if there was an error
bool
TemplateDeclInstantiator::InitMethodInstantiation(CXXMethodDecl *New,
                                                  CXXMethodDecl *Tmpl) {
  if (InitFunctionInstantiation(New, Tmpl))
    return true;
 
  if (isa<CXXDestructorDecl>(New) && SemaRef.getLangOpts().CPlusPlus11)
    SemaRef.AdjustDestructorExceptionSpec(cast<CXXDestructorDecl>(New));
 
  New->setAccess(Tmpl->getAccess());
  if (Tmpl->isVirtualAsWritten())
    New->setVirtualAsWritten(true);
 
  // FIXME: New needs a pointer to Tmpl
  return false;
}
 
bool TemplateDeclInstantiator::SubstDefaultedFunction(FunctionDecl *New,
                                                      FunctionDecl *Tmpl) {
  // Transfer across any unqualified lookups.
  if (auto *DFI = Tmpl->getDefaultedFunctionInfo()) {
    SmallVector<DeclAccessPair, 32> Lookups;
    Lookups.reserve(DFI->getUnqualifiedLookups().size());
    bool AnyChanged = false;
    for (DeclAccessPair DA : DFI->getUnqualifiedLookups()) {
      NamedDecl *D = SemaRef.FindInstantiatedDecl(New->getLocation(),
                                                  DA.getDecl(), TemplateArgs);
      if (!D)
        return true;
      AnyChanged |= (D != DA.getDecl());
      Lookups.push_back(DeclAccessPair::make(D, DA.getAccess()));
    }
 
    // It's unlikely that substitution will change any declarations. Don't
    // store an unnecessary copy in that case.
    New->setDefaultedFunctionInfo(
        AnyChanged ? FunctionDecl::DefaultedFunctionInfo::Create(
                         SemaRef.Context, Lookups)
                   : DFI);
  }
 
  SemaRef.SetDeclDefaulted(New, Tmpl->getLocation());
  return false;
}
 
/// Instantiate (or find existing instantiation of) a function template with a
/// given set of template arguments.
///
/// Usually this should not be used, and template argument deduction should be
/// used in its place.
FunctionDecl *
Sema::InstantiateFunctionDeclaration(FunctionTemplateDecl *FTD,
                                     const TemplateArgumentList *Args,
                                     SourceLocation Loc) {
  FunctionDecl *FD = FTD->getTemplatedDecl();
 
  sema::TemplateDeductionInfo Info(Loc);
  InstantiatingTemplate Inst(
      *this, Loc, FTD, Args->asArray(),
      CodeSynthesisContext::ExplicitTemplateArgumentSubstitution, Info);
  if (Inst.isInvalid())
    return nullptr;
 
  ContextRAII SavedContext(*this, FD);
  MultiLevelTemplateArgumentList MArgs(FTD, Args->asArray(),
                                       /*Final=*/false);
 
  return cast_or_null<FunctionDecl>(SubstDecl(FD, FD->getParent(), MArgs));
}
 
/// Instantiate the definition of the given function from its
/// template.
///
/// \param PointOfInstantiation the point at which the instantiation was
/// required. Note that this is not precisely a "point of instantiation"
/// for the function, but it's close.
///
/// \param Function the already-instantiated declaration of a
/// function template specialization or member function of a class template
/// specialization.
///
/// \param Recursive if true, recursively instantiates any functions that
/// are required by this instantiation.
///
/// \param DefinitionRequired if true, then we are performing an explicit
/// instantiation where the body of the function is required. Complain if
/// there is no such body.
void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
                                         FunctionDecl *Function,
                                         bool Recursive,
                                         bool DefinitionRequired,
                                         bool AtEndOfTU) {
  if (Function->isInvalidDecl() || isa<CXXDeductionGuideDecl>(Function))
    return;
 
  // Never instantiate an explicit specialization except if it is a class scope
  // explicit specialization.
  TemplateSpecializationKind TSK =
      Function->getTemplateSpecializationKindForInstantiation();
  if (TSK == TSK_ExplicitSpecialization)
    return;
 
  // Never implicitly instantiate a builtin; we don't actually need a function
  // body.
  if (Function->getBuiltinID() && TSK == TSK_ImplicitInstantiation &&
      !DefinitionRequired)
    return;
 
  // Don't instantiate a definition if we already have one.
  const FunctionDecl *ExistingDefn = nullptr;
  if (Function->isDefined(ExistingDefn,
                          /*CheckForPendingFriendDefinition=*/true)) {
    if (ExistingDefn->isThisDeclarationADefinition())
      return;
 
    // If we're asked to instantiate a function whose body comes from an
    // instantiated friend declaration, attach the instantiated body to the
    // corresponding declaration of the function.
    assert(ExistingDefn->isThisDeclarationInstantiatedFromAFriendDefinition());
    Function = const_cast<FunctionDecl*>(ExistingDefn);
  }
 
  // Find the function body that we'll be substituting.
  const FunctionDecl *PatternDecl = Function->getTemplateInstantiationPattern();
  assert(PatternDecl && "instantiating a non-template");
 
  const FunctionDecl *PatternDef = PatternDecl->getDefinition();
  Stmt *Pattern = nullptr;
  if (PatternDef) {
    Pattern = PatternDef->getBody(PatternDef);
    PatternDecl = PatternDef;
    if (PatternDef->willHaveBody())
      PatternDef = nullptr;
  }
 
  // FIXME: We need to track the instantiation stack in order to know which
  // definitions should be visible within this instantiation.
  if (DiagnoseUninstantiableTemplate(PointOfInstantiation, Function,
                                Function->getInstantiatedFromMemberFunction(),
                                     PatternDecl, PatternDef, TSK,
                                     /*Complain*/DefinitionRequired)) {
    if (DefinitionRequired)
      Function->setInvalidDecl();
    else if (TSK == TSK_ExplicitInstantiationDefinition ||
             (Function->isConstexpr() && !Recursive)) {
      // Try again at the end of the translation unit (at which point a
      // definition will be required).
      assert(!Recursive);
      Function->setInstantiationIsPending(true);
      PendingInstantiations.push_back(
        std::make_pair(Function, PointOfInstantiation));
    } else if (TSK == TSK_ImplicitInstantiation) {
      if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
          !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {
        Diag(PointOfInstantiation, diag::warn_func_template_missing)
          << Function;
        Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
        if (getLangOpts().CPlusPlus11)
          Diag(PointOfInstantiation, diag::note_inst_declaration_hint)
              << Function;
      }
    }
 
    return;
  }
 
  // Postpone late parsed template instantiations.
  if (PatternDecl->isLateTemplateParsed() &&
      !LateTemplateParser) {
    Function->setInstantiationIsPending(true);
    LateParsedInstantiations.push_back(
        std::make_pair(Function, PointOfInstantiation));
    return;
  }
 
  llvm::TimeTraceScope TimeScope("InstantiateFunction", [&]() {
    std::string Name;
    llvm::raw_string_ostream OS(Name);
    Function->getNameForDiagnostic(OS, getPrintingPolicy(),
                                   /*Qualified=*/true);
    return Name;
  });
 
  // If we're performing recursive template instantiation, create our own
  // queue of pending implicit instantiations that we will instantiate later,
  // while we're still within our own instantiation context.
  // This has to happen before LateTemplateParser below is called, so that
  // it marks vtables used in late parsed templates as used.
  GlobalEagerInstantiationScope GlobalInstantiations(*this,
                                                     /*Enabled=*/Recursive);
  LocalEagerInstantiationScope LocalInstantiations(*this);
 
  // Call the LateTemplateParser callback if there is a need to late parse
  // a templated function definition.
  if (!Pattern && PatternDecl->isLateTemplateParsed() &&
      LateTemplateParser) {
    // FIXME: Optimize to allow individual templates to be deserialized.
    if (PatternDecl->isFromASTFile())
      ExternalSource->ReadLateParsedTemplates(LateParsedTemplateMap);
 
    auto LPTIter = LateParsedTemplateMap.find(PatternDecl);
    assert(LPTIter != LateParsedTemplateMap.end() &&
           "missing LateParsedTemplate");
    LateTemplateParser(OpaqueParser, *LPTIter->second);
    Pattern = PatternDecl->getBody(PatternDecl);
    updateAttrsForLateParsedTemplate(PatternDecl, Function);
  }
 
  // Note, we should never try to instantiate a deleted function template.
  assert((Pattern || PatternDecl->isDefaulted() ||
          PatternDecl->hasSkippedBody()) &&
         "unexpected kind of function template definition");
 
  // C++1y [temp.explicit]p10:
  //   Except for inline functions, declarations with types deduced from their
  //   initializer or return value, and class template specializations, other
  //   explicit instantiation declarations have the effect of suppressing the
  //   implicit instantiation of the entity to which they refer.
  if (TSK == TSK_ExplicitInstantiationDeclaration &&
      !PatternDecl->isInlined() &&
      !PatternDecl->getReturnType()->getContainedAutoType())
    return;
 
  if (PatternDecl->isInlined()) {
    // Function, and all later redeclarations of it (from imported modules,
    // for instance), are now implicitly inline.
    for (auto *D = Function->getMostRecentDecl(); /**/;
         D = D->getPreviousDecl()) {
      D->setImplicitlyInline();
      if (D == Function)
        break;
    }
  }
 
  InstantiatingTemplate Inst(*this, PointOfInstantiation, Function);
  if (Inst.isInvalid() || Inst.isAlreadyInstantiating())
    return;
  PrettyDeclStackTraceEntry CrashInfo(Context, Function, SourceLocation(),
                                      "instantiating function definition");
 
  // The instantiation is visible here, even if it was first declared in an
  // unimported module.
  Function->setVisibleDespiteOwningModule();
 
  // Copy the inner loc start from the pattern.
  Function->setInnerLocStart(PatternDecl->getInnerLocStart());
 
  EnterExpressionEvaluationContext EvalContext(
      *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
 
  // Introduce a new scope where local variable instantiations will be
  // recorded, unless we're actually a member function within a local
  // class, in which case we need to merge our results with the parent
  // scope (of the enclosing function). The exception is instantiating
  // a function template specialization, since the template to be
  // instantiated already has references to locals properly substituted.
  bool MergeWithParentScope = false;
  if (CXXRecordDecl *Rec = dyn_cast<CXXRecordDecl>(Function->getDeclContext()))
    MergeWithParentScope =
        Rec->isLocalClass() && !Function->isFunctionTemplateSpecialization();
 
  LocalInstantiationScope Scope(*this, MergeWithParentScope);
  auto RebuildTypeSourceInfoForDefaultSpecialMembers = [&]() {
    // Special members might get their TypeSourceInfo set up w.r.t the
    // PatternDecl context, in which case parameters could still be pointing
    // back to the original class, make sure arguments are bound to the
    // instantiated record instead.
    assert(PatternDecl->isDefaulted() &&
           "Special member needs to be defaulted");
    auto PatternSM = getDefaultedFunctionKind(PatternDecl).asSpecialMember();
    if (!(PatternSM == Sema::CXXCopyConstructor ||
          PatternSM == Sema::CXXCopyAssignment ||
          PatternSM == Sema::CXXMoveConstructor ||
          PatternSM == Sema::CXXMoveAssignment))
      return;
 
    auto *NewRec = dyn_cast<CXXRecordDecl>(Function->getDeclContext());
    const auto *PatternRec =
        dyn_cast<CXXRecordDecl>(PatternDecl->getDeclContext());