lib/Sema/SemaTemplateVariadic.cpp

//===------- SemaTemplateVariadic.cpp - C++ Variadic Templates ------------===/
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//===----------------------------------------------------------------------===/
//
//  This file implements semantic analysis for C++0x variadic templates.
//===----------------------------------------------------------------------===/

#include "clang/Sema/Sema.h"
#include "clang/Sema/Lookup.h"
#include "clang/Sema/ParsedTemplate.h"
#include "clang/Sema/SemaInternal.h"
#include "clang/Sema/Template.h"
#include "clang/AST/Expr.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/AST/TypeLoc.h"

using namespace clang;

//----------------------------------------------------------------------------
// Visitor that collects unexpanded parameter packs
//----------------------------------------------------------------------------

namespace {
  /// \brief A class that collects unexpanded parameter packs.
  class CollectUnexpandedParameterPacksVisitor :
    public RecursiveASTVisitor<CollectUnexpandedParameterPacksVisitor> 
  {
    typedef RecursiveASTVisitor<CollectUnexpandedParameterPacksVisitor>
      inherited;

    llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded;

  public:
    explicit CollectUnexpandedParameterPacksVisitor(
                  llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded)
      : Unexpanded(Unexpanded) { }

    bool shouldWalkTypesOfTypeLocs() const { return false; }
    
    //------------------------------------------------------------------------
    // Recording occurrences of (unexpanded) parameter packs.
    //------------------------------------------------------------------------

    /// \brief Record occurrences of template type parameter packs.
    bool VisitTemplateTypeParmTypeLoc(TemplateTypeParmTypeLoc TL) {
      if (TL.getTypePtr()->isParameterPack())
        Unexpanded.push_back(std::make_pair(TL.getTypePtr(), TL.getNameLoc()));
      return true;
    }

    /// \brief Record occurrences of template type parameter packs
    /// when we don't have proper source-location information for
    /// them.
    ///
    /// Ideally, this routine would never be used.
    bool VisitTemplateTypeParmType(TemplateTypeParmType *T) {
      if (T->isParameterPack())
        Unexpanded.push_back(std::make_pair(T, SourceLocation()));

      return true;
    }

    /// \brief Record occurrences of (FIXME: function and) non-type template
    /// parameter packs in an expression.
    bool VisitDeclRefExpr(DeclRefExpr *E) {
      if (E->getDecl()->isParameterPack())
        Unexpanded.push_back(std::make_pair(E->getDecl(), E->getLocation()));
      
      return true;
    }
    
    /// \brief Record occurrences of template template parameter packs.
    bool TraverseTemplateName(TemplateName Template) {
      if (TemplateTemplateParmDecl *TTP 
            = dyn_cast_or_null<TemplateTemplateParmDecl>(
                                                  Template.getAsTemplateDecl()))
        if (TTP->isParameterPack())
          Unexpanded.push_back(std::make_pair(TTP, SourceLocation()));
      
      return inherited::TraverseTemplateName(Template);
    }

    //------------------------------------------------------------------------
    // Pruning the search for unexpanded parameter packs.
    //------------------------------------------------------------------------

    /// \brief Suppress traversal into statements and expressions that
    /// do not contain unexpanded parameter packs.
    bool TraverseStmt(Stmt *S) { 
      if (Expr *E = dyn_cast_or_null<Expr>(S))
        if (E->containsUnexpandedParameterPack())
          return inherited::TraverseStmt(E);

      return true; 
    }

    /// \brief Suppress traversal into types that do not contain
    /// unexpanded parameter packs.
    bool TraverseType(QualType T) {
      if (!T.isNull() && T->containsUnexpandedParameterPack())
        return inherited::TraverseType(T);

      return true;
    }

    /// \brief Suppress traversel into types with location information
    /// that do not contain unexpanded parameter packs.
    bool TraverseTypeLoc(TypeLoc TL) {
      if (!TL.getType().isNull() && 
          TL.getType()->containsUnexpandedParameterPack())
        return inherited::TraverseTypeLoc(TL);

      return true;
    }

    /// \brief Suppress traversal of non-parameter declarations, since
    /// they cannot contain unexpanded parameter packs.
    bool TraverseDecl(Decl *D) { 
      if (D && isa<ParmVarDecl>(D))
        return inherited::TraverseDecl(D);

      return true; 
    }

    /// \brief Suppress traversal of template argument pack expansions.
    bool TraverseTemplateArgument(const TemplateArgument &Arg) {
      if (Arg.isPackExpansion())
        return true;

      return inherited::TraverseTemplateArgument(Arg);
    }

    /// \brief Suppress traversal of template argument pack expansions.
    bool TraverseTemplateArgumentLoc(const TemplateArgumentLoc &ArgLoc) {
      if (ArgLoc.getArgument().isPackExpansion())
        return true;
      
      return inherited::TraverseTemplateArgumentLoc(ArgLoc);
    }
  };
}

/// \brief Diagnose all of the unexpanded parameter packs in the given
/// vector.
static void 
DiagnoseUnexpandedParameterPacks(Sema &S, SourceLocation Loc,
                                 Sema::UnexpandedParameterPackContext UPPC,
             const llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
  llvm::SmallVector<SourceLocation, 4> Locations;
  llvm::SmallVector<IdentifierInfo *, 4> Names;
  llvm::SmallPtrSet<IdentifierInfo *, 4> NamesKnown;

  for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
    IdentifierInfo *Name = 0;
    if (const TemplateTypeParmType *TTP
          = Unexpanded[I].first.dyn_cast<const TemplateTypeParmType *>())
      Name = TTP->getName();
    else
      Name = Unexpanded[I].first.get<NamedDecl *>()->getIdentifier();

    if (Name && NamesKnown.insert(Name))
      Names.push_back(Name);

    if (Unexpanded[I].second.isValid())
      Locations.push_back(Unexpanded[I].second);
  }

  DiagnosticBuilder DB
    = Names.size() == 0? S.Diag(Loc, diag::err_unexpanded_parameter_pack_0)
                           << (int)UPPC
    : Names.size() == 1? S.Diag(Loc, diag::err_unexpanded_parameter_pack_1)
                           << (int)UPPC << Names[0]
    : Names.size() == 2? S.Diag(Loc, diag::err_unexpanded_parameter_pack_2)
                           << (int)UPPC << Names[0] << Names[1]
    : S.Diag(Loc, diag::err_unexpanded_parameter_pack_3_or_more)
        << (int)UPPC << Names[0] << Names[1];

  for (unsigned I = 0, N = Locations.size(); I != N; ++I)
    DB << SourceRange(Locations[I]);
}

bool Sema::DiagnoseUnexpandedParameterPack(SourceLocation Loc, 
                                           TypeSourceInfo *T,
                                         UnexpandedParameterPackContext UPPC) {
  // C++0x [temp.variadic]p5:
  //   An appearance of a name of a parameter pack that is not expanded is 
  //   ill-formed.
  if (!T->getType()->containsUnexpandedParameterPack())
    return false;

  llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseTypeLoc(
                                                              T->getTypeLoc());
  assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
  DiagnoseUnexpandedParameterPacks(*this, Loc, UPPC, Unexpanded);
  return true;
}

bool Sema::DiagnoseUnexpandedParameterPack(Expr *E,
                                        UnexpandedParameterPackContext UPPC) {
  // C++0x [temp.variadic]p5:
  //   An appearance of a name of a parameter pack that is not expanded is 
  //   ill-formed.
  if (!E->containsUnexpandedParameterPack())
    return false;

  llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseStmt(E);
  assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
  DiagnoseUnexpandedParameterPacks(*this, E->getLocStart(), UPPC, Unexpanded);
  return true;
}

bool Sema::DiagnoseUnexpandedParameterPack(const CXXScopeSpec &SS,
                                        UnexpandedParameterPackContext UPPC) {
  // C++0x [temp.variadic]p5:
  //   An appearance of a name of a parameter pack that is not expanded is 
  //   ill-formed.
  if (!SS.getScopeRep() || 
      !SS.getScopeRep()->containsUnexpandedParameterPack())
    return false;

  llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  CollectUnexpandedParameterPacksVisitor(Unexpanded)
    .TraverseNestedNameSpecifier(SS.getScopeRep());
  assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
  DiagnoseUnexpandedParameterPacks(*this, SS.getRange().getBegin(), 
                                   UPPC, Unexpanded);
  return true;
}

bool Sema::DiagnoseUnexpandedParameterPack(const DeclarationNameInfo &NameInfo,
                                         UnexpandedParameterPackContext UPPC) {
  // C++0x [temp.variadic]p5:
  //   An appearance of a name of a parameter pack that is not expanded is 
  //   ill-formed.
  switch (NameInfo.getName().getNameKind()) {
  case DeclarationName::Identifier:
  case DeclarationName::ObjCZeroArgSelector:
  case DeclarationName::ObjCOneArgSelector:
  case DeclarationName::ObjCMultiArgSelector:
  case DeclarationName::CXXOperatorName:
  case DeclarationName::CXXLiteralOperatorName:
  case DeclarationName::CXXUsingDirective:
    return false;

  case DeclarationName::CXXConstructorName:
  case DeclarationName::CXXDestructorName:
  case DeclarationName::CXXConversionFunctionName:
    // FIXME: We shouldn't need this null check!
    if (TypeSourceInfo *TSInfo = NameInfo.getNamedTypeInfo())
      return DiagnoseUnexpandedParameterPack(NameInfo.getLoc(), TSInfo, UPPC);

    if (!NameInfo.getName().getCXXNameType()->containsUnexpandedParameterPack())
      return false;

    break;
  }

  llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  CollectUnexpandedParameterPacksVisitor(Unexpanded)
    .TraverseType(NameInfo.getName().getCXXNameType());
  assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
  DiagnoseUnexpandedParameterPacks(*this, NameInfo.getLoc(), UPPC, Unexpanded);
  return true;
}

bool Sema::DiagnoseUnexpandedParameterPack(SourceLocation Loc,
                                           TemplateName Template,
                                       UnexpandedParameterPackContext UPPC) {
  
  if (Template.isNull() || !Template.containsUnexpandedParameterPack())
    return false;

  llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  CollectUnexpandedParameterPacksVisitor(Unexpanded)
    .TraverseTemplateName(Template);
  assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
  DiagnoseUnexpandedParameterPacks(*this, Loc, UPPC, Unexpanded);
  return true;
}

bool Sema::DiagnoseUnexpandedParameterPack(TemplateArgumentLoc Arg,
                                         UnexpandedParameterPackContext UPPC) {
  if (Arg.getArgument().isNull() || 
      !Arg.getArgument().containsUnexpandedParameterPack())
    return false;
  
  llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
  CollectUnexpandedParameterPacksVisitor(Unexpanded)
    .TraverseTemplateArgumentLoc(Arg);
  assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
  DiagnoseUnexpandedParameterPacks(*this, Arg.getLocation(), UPPC, Unexpanded);
  return true;  
}

void Sema::collectUnexpandedParameterPacks(TemplateArgument Arg,
                   llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
  CollectUnexpandedParameterPacksVisitor(Unexpanded)
    .TraverseTemplateArgument(Arg);
}

void Sema::collectUnexpandedParameterPacks(TemplateArgumentLoc Arg,
                   llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
  CollectUnexpandedParameterPacksVisitor(Unexpanded)
    .TraverseTemplateArgumentLoc(Arg);
}

void Sema::collectUnexpandedParameterPacks(QualType T,
                   llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
  CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseType(T);  
}  

void Sema::collectUnexpandedParameterPacks(TypeLoc TL,
                   llvm::SmallVectorImpl<UnexpandedParameterPack> &Unexpanded) {
  CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseTypeLoc(TL);  
}  

ParsedTemplateArgument 
Sema::ActOnPackExpansion(const ParsedTemplateArgument &Arg,
                         SourceLocation EllipsisLoc) {
  if (Arg.isInvalid())
    return Arg;

  switch (Arg.getKind()) {
  case ParsedTemplateArgument::Type: {
    TypeResult Result = ActOnPackExpansion(Arg.getAsType(), EllipsisLoc);
    if (Result.isInvalid())
      return ParsedTemplateArgument();

    return ParsedTemplateArgument(Arg.getKind(), Result.get().getAsOpaquePtr(), 
                                  Arg.getLocation());
  }

  case ParsedTemplateArgument::NonType: {
    ExprResult Result = ActOnPackExpansion(Arg.getAsExpr(), EllipsisLoc);
    if (Result.isInvalid())
      return ParsedTemplateArgument();
    
    return ParsedTemplateArgument(Arg.getKind(), Result.get(), 
                                  Arg.getLocation());
  }
    
  case ParsedTemplateArgument::Template:
    if (!Arg.getAsTemplate().get().containsUnexpandedParameterPack()) {
      SourceRange R(Arg.getLocation());
      if (Arg.getScopeSpec().isValid())
        R.setBegin(Arg.getScopeSpec().getBeginLoc());
      Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
        << R;
      return ParsedTemplateArgument();
    }
      
    return Arg.getTemplatePackExpansion(EllipsisLoc);
  }
  llvm_unreachable("Unhandled template argument kind?");
  return ParsedTemplateArgument();
}

TypeResult Sema::ActOnPackExpansion(ParsedType Type, 
                                    SourceLocation EllipsisLoc) {
  TypeSourceInfo *TSInfo;
  GetTypeFromParser(Type, &TSInfo);
  if (!TSInfo)
    return true;

  TypeSourceInfo *TSResult = CheckPackExpansion(TSInfo, EllipsisLoc);
  if (!TSResult)
    return true;
  
  return CreateParsedType(TSResult->getType(), TSResult);
}

TypeSourceInfo *Sema::CheckPackExpansion(TypeSourceInfo *Pattern,
                                         SourceLocation EllipsisLoc) {
  // C++0x [temp.variadic]p5:
  //   The pattern of a pack expansion shall name one or more
  //   parameter packs that are not expanded by a nested pack
  //   expansion.
  if (!Pattern->getType()->containsUnexpandedParameterPack()) {
    Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
      << Pattern->getTypeLoc().getSourceRange();
    return 0;
  }
  
  // Create the pack expansion type and source-location information.
  QualType Result = Context.getPackExpansionType(Pattern->getType());
  TypeSourceInfo *TSResult = Context.CreateTypeSourceInfo(Result);
  PackExpansionTypeLoc TL = cast<PackExpansionTypeLoc>(TSResult->getTypeLoc());
  TL.setEllipsisLoc(EllipsisLoc);
  
  // Copy over the source-location information from the type.
  memcpy(TL.getNextTypeLoc().getOpaqueData(),
         Pattern->getTypeLoc().getOpaqueData(),
         Pattern->getTypeLoc().getFullDataSize());
  return TSResult;
}

ExprResult Sema::ActOnPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc) {
  if (!Pattern)
    return ExprError();
  
  // C++0x [temp.variadic]p5:
  //   The pattern of a pack expansion shall name one or more
  //   parameter packs that are not expanded by a nested pack
  //   expansion.
  if (!Pattern->containsUnexpandedParameterPack()) {
    Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
    << Pattern->getSourceRange();
    return ExprError();
  }
  
  // Create the pack expansion expression and source-location information.
  return Owned(new (Context) PackExpansionExpr(Context.DependentTy, Pattern,
                                               EllipsisLoc));
}

/// \brief Retrieve the depth and index of a parameter pack.
static std::pair<unsigned, unsigned> 
getDepthAndIndex(NamedDecl *ND) {
  if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(ND))
    return std::make_pair(TTP->getDepth(), TTP->getIndex());
  
  if (NonTypeTemplateParmDecl *NTTP = dyn_cast<NonTypeTemplateParmDecl>(ND))
    return std::make_pair(NTTP->getDepth(), NTTP->getIndex());
  
  TemplateTemplateParmDecl *TTP = cast<TemplateTemplateParmDecl>(ND);
  return std::make_pair(TTP->getDepth(), TTP->getIndex());
}

bool Sema::CheckParameterPacksForExpansion(SourceLocation EllipsisLoc,
                                           SourceRange PatternRange,
                                     const UnexpandedParameterPack *Unexpanded,
                                           unsigned NumUnexpanded,
                             const MultiLevelTemplateArgumentList &TemplateArgs,
                                           bool &ShouldExpand,
                                           bool &RetainExpansion,
                                           unsigned &NumExpansions) {                                        
  ShouldExpand = true;
  RetainExpansion = false;
  std::pair<IdentifierInfo *, SourceLocation> FirstPack;
  bool HaveFirstPack = false;
  
  for (unsigned I = 0; I != NumUnexpanded; ++I) {
    // Compute the depth and index for this parameter pack.
    unsigned Depth;
    unsigned Index;
    IdentifierInfo *Name;
    bool IsFunctionParameterPack = false;
    
    if (const TemplateTypeParmType *TTP
        = Unexpanded[I].first.dyn_cast<const TemplateTypeParmType *>()) {
      Depth = TTP->getDepth();
      Index = TTP->getIndex();
      Name = TTP->getName();
    } else {
      NamedDecl *ND = Unexpanded[I].first.get<NamedDecl *>();
      if (isa<ParmVarDecl>(ND))
        IsFunctionParameterPack = true;
      else
        llvm::tie(Depth, Index) = getDepthAndIndex(ND);        
      
      Name = ND->getIdentifier();
    }
    
    // Determine the size of this argument pack.
    unsigned NewPackSize;    
    if (IsFunctionParameterPack) {
      // Figure out whether we're instantiating to an argument pack or not.
      typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
      
      llvm::PointerUnion<Decl *, DeclArgumentPack *> *Instantiation
        = CurrentInstantiationScope->findInstantiationOf(
                                        Unexpanded[I].first.get<NamedDecl *>());
      if (Instantiation &&
          Instantiation->is<DeclArgumentPack *>()) {
        // We could expand this function parameter pack.
        NewPackSize = Instantiation->get<DeclArgumentPack *>()->size();
      } else {
        // We can't expand this function parameter pack, so we can't expand
        // the pack expansion.
        ShouldExpand = false;
        continue;
      }
    } else {
      // If we don't have a template argument at this depth/index, then we 
      // cannot expand the pack expansion. Make a note of this, but we still 
      // want to check any parameter packs we *do* have arguments for.
      if (Depth >= TemplateArgs.getNumLevels() ||
          !TemplateArgs.hasTemplateArgument(Depth, Index)) {
        ShouldExpand = false;
        continue;
      }
      
      // Determine the size of the argument pack.
      NewPackSize = TemplateArgs(Depth, Index).pack_size();
    }
    
    // C++0x [temp.arg.explicit]p9:
    //   Template argument deduction can extend the sequence of template 
    //   arguments corresponding to a template parameter pack, even when the
    //   sequence contains explicitly specified template arguments.
    if (NamedDecl *PartialPack 
                  = CurrentInstantiationScope->getPartiallySubstitutedPack()) {
      unsigned PartialDepth, PartialIndex;
      llvm::tie(PartialDepth, PartialIndex) = getDepthAndIndex(PartialPack);
      if (PartialDepth == Depth && PartialIndex == Index)
        RetainExpansion = true;
    }

    if (!HaveFirstPack) {
      // The is the first pack we've seen for which we have an argument. 
      // Record it.
      NumExpansions = NewPackSize;
      FirstPack.first = Name;
      FirstPack.second = Unexpanded[I].second;
      HaveFirstPack = true;
      continue;
    }
    
    if (NewPackSize != NumExpansions) {
      // C++0x [temp.variadic]p5:
      //   All of the parameter packs expanded by a pack expansion shall have 
      //   the same number of arguments specified.
      Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict)
        << FirstPack.first << Name << NumExpansions << NewPackSize
        << SourceRange(FirstPack.second) << SourceRange(Unexpanded[I].second);
      return true;
    }
  }
  
  return false;
}

bool Sema::containsUnexpandedParameterPacks(Declarator &D) {
  const DeclSpec &DS = D.getDeclSpec();
  switch (DS.getTypeSpecType()) {
  case TST_typename:
  case TST_typeofType: {
    QualType T = DS.getRepAsType().get();
    if (!T.isNull() && T->containsUnexpandedParameterPack())
      return true;
    break;
  }
      
  case TST_typeofExpr:
  case TST_decltype:
    if (DS.getRepAsExpr() && 
        DS.getRepAsExpr()->containsUnexpandedParameterPack())
      return true;
    break;
      
  case TST_unspecified:
  case TST_void:
  case TST_char:
  case TST_wchar:
  case TST_char16:
  case TST_char32:
  case TST_int:
  case TST_float:
  case TST_double:
  case TST_bool:
  case TST_decimal32:
  case TST_decimal64:
  case TST_decimal128:
  case TST_enum:
  case TST_union:
  case TST_struct:
  case TST_class:
  case TST_auto:
  case TST_error:
    break;
  }
  
  for (unsigned I = 0, N = D.getNumTypeObjects(); I != N; ++I) {
    const DeclaratorChunk &Chunk = D.getTypeObject(I);
    switch (Chunk.Kind) {
    case DeclaratorChunk::Pointer:
    case DeclaratorChunk::Reference:
    case DeclaratorChunk::Paren:
      // These declarator chunks cannot contain any parameter packs.
      break;
        
    case DeclaratorChunk::Array:
    case DeclaratorChunk::Function:
    case DeclaratorChunk::BlockPointer:
      // Syntactically, these kinds of declarator chunks all come after the
      // declarator-id (conceptually), so the parser should not invoke this
      // routine at this time.
      llvm_unreachable("Could not have seen this kind of declarator chunk");
      break;
        
    case DeclaratorChunk::MemberPointer:
      if (Chunk.Mem.Scope().getScopeRep() &&
          Chunk.Mem.Scope().getScopeRep()->containsUnexpandedParameterPack())
        return true;
      break;
    }
  }
  
  return false;
}

/// \brief Called when an expression computing the size of a parameter pack
/// is parsed.
///
/// \code
/// template<typename ...Types> struct count {
///   static const unsigned value = sizeof...(Types);
/// };
/// \endcode
///
//
/// \param OpLoc The location of the "sizeof" keyword.
/// \param Name The name of the parameter pack whose size will be determined.
/// \param NameLoc The source location of the name of the parameter pack.
/// \param RParenLoc The location of the closing parentheses.
ExprResult Sema::ActOnSizeofParameterPackExpr(Scope *S,
                                              SourceLocation OpLoc,
                                              IdentifierInfo &Name,
                                              SourceLocation NameLoc,
                                              SourceLocation RParenLoc) {
  // C++0x [expr.sizeof]p5:
  //   The identifier in a sizeof... expression shall name a parameter pack.
  LookupResult R(*this, &Name, NameLoc, LookupOrdinaryName);
  LookupName(R, S);
  
  NamedDecl *ParameterPack = 0;
  switch (R.getResultKind()) {
  case LookupResult::Found:
    ParameterPack = R.getFoundDecl();
    break;
    
  case LookupResult::NotFound:
  case LookupResult::NotFoundInCurrentInstantiation:
    if (DeclarationName CorrectedName = CorrectTypo(R, S, 0, 0, false, 
                                                    CTC_NoKeywords)) {
      if (NamedDecl *CorrectedResult = R.getAsSingle<NamedDecl>())
        if (CorrectedResult->isParameterPack()) {
          ParameterPack = CorrectedResult;
          Diag(NameLoc, diag::err_sizeof_pack_no_pack_name_suggest)
            << &Name << CorrectedName
            << FixItHint::CreateReplacement(NameLoc, 
                                            CorrectedName.getAsString());
          Diag(ParameterPack->getLocation(), diag::note_parameter_pack_here)
            << CorrectedName;
        }
    }
      
  case LookupResult::FoundOverloaded:
  case LookupResult::FoundUnresolvedValue:
    break;
    
  case LookupResult::Ambiguous:
    DiagnoseAmbiguousLookup(R);
    return ExprError();
  }
  
  if (!ParameterPack || !ParameterPack->isParameterPack()) {
    Diag(NameLoc, diag::err_sizeof_pack_no_pack_name)
      << &Name;
    return ExprError();
  }

  return new (Context) SizeOfPackExpr(Context.getSizeType(), OpLoc, 
                                      ParameterPack, NameLoc, RParenLoc);
}