From: Douglas Gregor <dgregor@apple.com>
Date: Fri, 14 Jan 2011 17:04:44 +0000 (+0000)
Subject: Keep track of the number of expansions to be produced from a type pack
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=cded4f649cd4b7ba7d461c25c6482ef52b8d3a2a;p=clang

Keep track of the number of expansions to be produced from a type pack
expansion, when it is known due to the substitution of an out
parameter pack. This allows us to properly handle substitution into
pack expansions that involve multiple parameter packs at different
template parameter levels, even when this substitution happens one
level at a time (as with partial specializations of member class
templates and the signatures of member function templates).

Note that the diagnostic we provide when there is an arity mismatch
between an outer parameter pack and an inner parameter pack in this
case isn't as clear as the normal diagnostic for an arity
mismatch. However, this doesn't matter because these cases are very,
very rare and (even then) only typically occur in a SFINAE context.

The other kinds of pack expansions (expression, template, etc.) still
need to support optional tracking of the number of expansions, and we
need the moral equivalent of SubstTemplateTypeParmPackType for
substituted argument packs of template template and non-type template
parameters.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123448 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/include/clang/AST/ASTContext.h b/include/clang/AST/ASTContext.h
index 15bc301bd9..93b793eb78 100644
--- a/include/clang/AST/ASTContext.h
+++ b/include/clang/AST/ASTContext.h
@@ -712,7 +712,8 @@ public:
                                                   unsigned NumArgs,
                                             const TemplateArgument *Args) const;
 
-  QualType getPackExpansionType(QualType Pattern);
+  QualType getPackExpansionType(QualType Pattern,
+                                llvm::Optional<unsigned> NumExpansions);
 
   QualType getObjCInterfaceType(const ObjCInterfaceDecl *Decl) const;
 
diff --git a/include/clang/AST/TemplateBase.h b/include/clang/AST/TemplateBase.h
index 649c005020..93ec35bebe 100644
--- a/include/clang/AST/TemplateBase.h
+++ b/include/clang/AST/TemplateBase.h
@@ -465,7 +465,11 @@ public:
   /// the pattern of the pack expansion.
   ///
   /// \param Ellipsis Will be set to the location of the ellipsis.
+  ///
+  /// \param NumExpansions Will be set to the number of expansions that will
+  /// be generated from this pack expansion, if known a priori.
   TemplateArgumentLoc getPackExpansionPattern(SourceLocation &Ellipsis,
+                                        llvm::Optional<unsigned> &NumExpansions,
                                               ASTContext &Context) const;
 };
 
diff --git a/include/clang/AST/Type.h b/include/clang/AST/Type.h
index c081a161de..e32676a397 100644
--- a/include/clang/AST/Type.h
+++ b/include/clang/AST/Type.h
@@ -25,6 +25,7 @@
 #include "llvm/Support/type_traits.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/FoldingSet.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/PointerUnion.h"
 
@@ -3379,29 +3380,52 @@ class PackExpansionType : public Type, public llvm::FoldingSetNode {
   /// \brief The pattern of the pack expansion.
   QualType Pattern;
 
-  PackExpansionType(QualType Pattern, QualType Canon)
+  /// \brief The number of expansions that this pack expansion will
+  /// generate when substituted (+1), or indicates that 
+  ///
+  /// This field will only have a non-zero value when some of the parameter 
+  /// packs that occur within the pattern have been substituted but others have 
+  /// not.
+  unsigned NumExpansions;
+  
+  PackExpansionType(QualType Pattern, QualType Canon,
+                    llvm::Optional<unsigned> NumExpansions)
     : Type(PackExpansion, Canon, /*Dependent=*/true,
            /*VariableModified=*/Pattern->isVariablyModifiedType(),
            /*ContainsUnexpandedParameterPack=*/false),
-      Pattern(Pattern) { }
+      Pattern(Pattern), 
+      NumExpansions(NumExpansions? *NumExpansions + 1: 0) { }
 
   friend class ASTContext;  // ASTContext creates these
-
+  
 public:
   /// \brief Retrieve the pattern of this pack expansion, which is the
   /// type that will be repeatedly instantiated when instantiating the
   /// pack expansion itself.
   QualType getPattern() const { return Pattern; }
 
+  /// \brief Retrieve the number of expansions that this pack expansion will
+  /// generate, if known.
+  llvm::Optional<unsigned> getNumExpansions() const {
+    if (NumExpansions)
+      return NumExpansions - 1;
+    
+    return llvm::Optional<unsigned>();
+  }
+  
   bool isSugared() const { return false; }
   QualType desugar() const { return QualType(this, 0); }
 
   void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, getPattern());
+    Profile(ID, getPattern(), getNumExpansions());
   }
 
-  static void Profile(llvm::FoldingSetNodeID &ID, QualType Pattern) {
+  static void Profile(llvm::FoldingSetNodeID &ID, QualType Pattern,
+                      llvm::Optional<unsigned> NumExpansions) {
     ID.AddPointer(Pattern.getAsOpaquePtr());
+    ID.AddBoolean(NumExpansions);
+    if (NumExpansions)
+      ID.AddInteger(*NumExpansions);
   }
 
   static bool classof(const Type *T) {
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 8c3a57622b..a5a3d83833 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1868,6 +1868,9 @@ def err_pack_expansion_without_parameter_packs : Error<
 def err_pack_expansion_length_conflict : Error<
   "pack expansion contains parameter packs %0 and %1 that have different "
   "lengths (%2 vs. %3)">;
+def err_pack_expansion_length_conflict_multilevel : Error<
+  "pack expansion contains parameter pack %0 that has a different "
+  "length (%1 vs. %2) from outer parameter packs">;
 def err_pack_expansion_member_init : Error<
   "pack expansion for initialization of member %0">;
 
diff --git a/include/clang/Sema/Sema.h b/include/clang/Sema/Sema.h
index 1b22cd8ad7..ee286ddf1d 100644
--- a/include/clang/Sema/Sema.h
+++ b/include/clang/Sema/Sema.h
@@ -3330,13 +3330,15 @@ public:
   /// \brief Construct a pack expansion type from the pattern of the pack
   /// expansion.
   TypeSourceInfo *CheckPackExpansion(TypeSourceInfo *Pattern,
-                                     SourceLocation EllipsisLoc);
+                                     SourceLocation EllipsisLoc,
+                                     llvm::Optional<unsigned> NumExpansions);
 
   /// \brief Construct a pack expansion type from the pattern of the pack
   /// expansion.
   QualType CheckPackExpansion(QualType Pattern,
                               SourceRange PatternRange,
-                              SourceLocation EllipsisLoc);
+                              SourceLocation EllipsisLoc,
+                              llvm::Optional<unsigned> NumExpansions);
 
   /// \brief Invoked when parsing an expression followed by an ellipsis, which
   /// creates a pack expansion.
@@ -3373,8 +3375,12 @@ public:
   /// C++0x [temp.arg.explicit]p9.
   ///
   /// \param NumExpansions The number of separate arguments that will be in
-  /// the expanded form of the corresponding pack expansion. Must be set when
-  /// \c ShouldExpand is \c true.
+  /// the expanded form of the corresponding pack expansion. This is both an
+  /// input and an output parameter, which can be set by the caller if the
+  /// number of expansions is known a priori (e.g., due to a prior substitution)
+  /// and will be set by the callee when the number of expansions is known.
+  /// The callee must set this value when \c ShouldExpand is \c true; it may
+  /// set this value in other cases.
   ///
   /// \returns true if an error occurred (e.g., because the parameter packs 
   /// are to be instantiated with arguments of different lengths), false 
@@ -3387,7 +3393,7 @@ public:
                              const MultiLevelTemplateArgumentList &TemplateArgs,
                                        bool &ShouldExpand,
                                        bool &RetainExpansion,
-                                       unsigned &NumExpansions);
+                                       llvm::Optional<unsigned> &NumExpansions);
 
   /// \brief Determine the number of arguments in the given pack expansion
   /// type.
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index 35d4b7a097..a98e0636d4 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -2246,9 +2246,10 @@ ASTContext::getDependentTemplateSpecializationType(
   return QualType(T, 0);
 }
 
-QualType ASTContext::getPackExpansionType(QualType Pattern) {
+QualType ASTContext::getPackExpansionType(QualType Pattern,
+                                      llvm::Optional<unsigned> NumExpansions) {
   llvm::FoldingSetNodeID ID;
-  PackExpansionType::Profile(ID, Pattern);
+  PackExpansionType::Profile(ID, Pattern, NumExpansions);
 
   assert(Pattern->containsUnexpandedParameterPack() &&
          "Pack expansions must expand one or more parameter packs");
@@ -2260,13 +2261,13 @@ QualType ASTContext::getPackExpansionType(QualType Pattern) {
 
   QualType Canon;
   if (!Pattern.isCanonical()) {
-    Canon = getPackExpansionType(getCanonicalType(Pattern));
+    Canon = getPackExpansionType(getCanonicalType(Pattern), NumExpansions);
 
     // Find the insert position again.
     PackExpansionTypes.FindNodeOrInsertPos(ID, InsertPos);
   }
 
-  T = new (*this) PackExpansionType(Pattern, Canon);
+  T = new (*this) PackExpansionType(Pattern, Canon, NumExpansions);
   Types.push_back(T);
   PackExpansionTypes.InsertNode(T, InsertPos);
   return QualType(T, 0);  
diff --git a/lib/AST/DeclTemplate.cpp b/lib/AST/DeclTemplate.cpp
index 6ad62eab30..bd91facd08 100644
--- a/lib/AST/DeclTemplate.cpp
+++ b/lib/AST/DeclTemplate.cpp
@@ -319,7 +319,8 @@ ClassTemplateDecl::getInjectedClassNameSpecialization() {
     if (TemplateTypeParmDecl *TTP = dyn_cast<TemplateTypeParmDecl>(*Param)) {
       QualType ArgType = Context.getTypeDeclType(TTP);
       if (TTP->isParameterPack())
-        ArgType = Context.getPackExpansionType(ArgType);
+        ArgType = Context.getPackExpansionType(ArgType, 
+                                               llvm::Optional<unsigned>());
       
       Arg = TemplateArgument(ArgType);
     } else if (NonTypeTemplateParmDecl *NTTP =
diff --git a/lib/AST/TemplateBase.cpp b/lib/AST/TemplateBase.cpp
index 42a936c500..f7c4ac832f 100644
--- a/lib/AST/TemplateBase.cpp
+++ b/lib/AST/TemplateBase.cpp
@@ -346,6 +346,7 @@ SourceRange TemplateArgumentLoc::getSourceRange() const {
 
 TemplateArgumentLoc 
 TemplateArgumentLoc::getPackExpansionPattern(SourceLocation &Ellipsis,
+                                       llvm::Optional<unsigned> &NumExpansions,
                                              ASTContext &Context) const {
   assert(Argument.isPackExpansion());
   
@@ -363,6 +364,7 @@ TemplateArgumentLoc::getPackExpansionPattern(SourceLocation &Ellipsis,
     Ellipsis = Expansion.getEllipsisLoc();
     
     TypeLoc Pattern = Expansion.getPatternLoc();
+    NumExpansions = Expansion.getTypePtr()->getNumExpansions();
     
     // FIXME: This is horrible. We know where the source location data is for
     // the pattern, and we have the pattern's type, but we are forced to copy
@@ -382,10 +384,12 @@ TemplateArgumentLoc::getPackExpansionPattern(SourceLocation &Ellipsis,
       = cast<PackExpansionExpr>(Argument.getAsExpr());
     Expr *Pattern = Expansion->getPattern();
     Ellipsis = Expansion->getEllipsisLoc();
+    // FIXME: Variadic templates num expansions
     return TemplateArgumentLoc(Pattern, Pattern);
   }
 
   case TemplateArgument::TemplateExpansion:
+    // FIXME: Variadic templates num expansions
     Ellipsis = getTemplateEllipsisLoc();
     return TemplateArgumentLoc(Argument.getPackExpansionPattern(),
                                getTemplateQualifierRange(),
diff --git a/lib/Sema/SemaTemplateInstantiate.cpp b/lib/Sema/SemaTemplateInstantiate.cpp
index 629dc58958..c181a568b5 100644
--- a/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/lib/Sema/SemaTemplateInstantiate.cpp
@@ -622,7 +622,7 @@ namespace {
                                  unsigned NumUnexpanded,
                                  bool &ShouldExpand,
                                  bool &RetainExpansion,
-                                 unsigned &NumExpansions) {
+                                 llvm::Optional<unsigned> &NumExpansions) {
       return getSema().CheckParameterPacksForExpansion(EllipsisLoc, 
                                                        PatternRange, Unexpanded,
                                                        NumUnexpanded, 
@@ -1260,7 +1260,9 @@ ParmVarDecl *Sema::SubstParmVarDecl(ParmVarDecl *OldParm,
       // We still have unexpanded parameter packs, which means that
       // our function parameter is still a function parameter pack.
       // Therefore, make its type a pack expansion type.
-      NewDI = CheckPackExpansion(NewDI, ExpansionTL.getEllipsisLoc());
+      // FIXME: Variadic templates num expansions.
+      NewDI = CheckPackExpansion(NewDI, ExpansionTL.getEllipsisLoc(),
+                                 llvm::Optional<unsigned>());
     }
   } else {
     NewDI = SubstType(OldDI, TemplateArgs, OldParm->getLocation(), 
@@ -1360,7 +1362,7 @@ Sema::SubstBaseSpecifiers(CXXRecordDecl *Instantiation,
                                       Unexpanded);
       bool ShouldExpand = false;
       bool RetainExpansion = false;
-      unsigned NumExpansions = 0;
+      llvm::Optional<unsigned> NumExpansions;
       if (CheckParameterPacksForExpansion(Base->getEllipsisLoc(), 
                                           Base->getSourceRange(),
                                           Unexpanded.data(), Unexpanded.size(),
@@ -1373,7 +1375,7 @@ Sema::SubstBaseSpecifiers(CXXRecordDecl *Instantiation,
       
       // If we should expand this pack expansion now, do so.
       if (ShouldExpand) {
-        for (unsigned I = 0; I != NumExpansions; ++I) {
+        for (unsigned I = 0; I != *NumExpansions; ++I) {
             Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this, I);
           
           TypeSourceInfo *BaseTypeLoc = SubstType(Base->getTypeSourceInfo(),
diff --git a/lib/Sema/SemaTemplateInstantiateDecl.cpp b/lib/Sema/SemaTemplateInstantiateDecl.cpp
index a18bfbafc0..6b5713a2fa 100644
--- a/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1977,7 +1977,8 @@ TemplateDeclInstantiator::InitFunctionInstantiation(FunctionDecl *New,
         
         bool Expand = false;
         bool RetainExpansion = false;
-        unsigned NumExpansions = 0;
+        llvm::Optional<unsigned> NumExpansions
+                                          = PackExpansion->getNumExpansions();
         if (SemaRef.CheckParameterPacksForExpansion(New->getLocation(), 
                                                     SourceRange(),
                                                     Unexpanded.data(), 
@@ -1990,7 +1991,8 @@ TemplateDeclInstantiator::InitFunctionInstantiation(FunctionDecl *New,
                       
         if (!Expand) {
           // We can't expand this pack expansion into separate arguments yet;
-          // just substitute into the argument pack.
+          // just substitute into the pattern and create a new pack expansion 
+          // type.
           Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, -1);
           QualType T = SemaRef.SubstType(PackExpansion->getPattern(), 
                                          TemplateArgs,
@@ -1998,13 +2000,14 @@ TemplateDeclInstantiator::InitFunctionInstantiation(FunctionDecl *New,
           if (T.isNull())
             break;
           
+          T = SemaRef.Context.getPackExpansionType(T, NumExpansions);
           Exceptions.push_back(T);
           continue;
         }
         
         // Substitute into the pack expansion pattern for each template
         bool Invalid = false;
-        for (unsigned ArgIdx = 0; ArgIdx != NumExpansions; ++ArgIdx) {
+        for (unsigned ArgIdx = 0; ArgIdx != *NumExpansions; ++ArgIdx) {
           Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(SemaRef, ArgIdx);
           
           QualType T = SemaRef.SubstType(PackExpansion->getPattern(), 
@@ -2384,7 +2387,7 @@ Sema::InstantiateMemInitializers(CXXConstructorDecl *New,
       collectUnexpandedParameterPacks(BaseTL, Unexpanded);
       bool ShouldExpand = false;
       bool RetainExpansion = false;
-      unsigned NumExpansions = 0;
+      llvm::Optional<unsigned> NumExpansions;
       if (CheckParameterPacksForExpansion(Init->getEllipsisLoc(), 
                                           BaseTL.getSourceRange(),
                                           Unexpanded.data(), 
@@ -2399,7 +2402,7 @@ Sema::InstantiateMemInitializers(CXXConstructorDecl *New,
       assert(ShouldExpand && "Partial instantiation of base initializer?");
       
       // Loop over all of the arguments in the argument pack(s), 
-      for (unsigned I = 0; I != NumExpansions; ++I) {
+      for (unsigned I = 0; I != *NumExpansions; ++I) {
         Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(*this, I);
 
         // Instantiate the initializer.
diff --git a/lib/Sema/SemaTemplateVariadic.cpp b/lib/Sema/SemaTemplateVariadic.cpp
index dc5013c25d..899b58e558 100644
--- a/lib/Sema/SemaTemplateVariadic.cpp
+++ b/lib/Sema/SemaTemplateVariadic.cpp
@@ -368,7 +368,8 @@ TypeResult Sema::ActOnPackExpansion(ParsedType Type,
   if (!TSInfo)
     return true;
 
-  TypeSourceInfo *TSResult = CheckPackExpansion(TSInfo, EllipsisLoc);
+  TypeSourceInfo *TSResult = CheckPackExpansion(TSInfo, EllipsisLoc,
+                                                llvm::Optional<unsigned>());
   if (!TSResult)
     return true;
   
@@ -376,11 +377,12 @@ TypeResult Sema::ActOnPackExpansion(ParsedType Type,
 }
 
 TypeSourceInfo *Sema::CheckPackExpansion(TypeSourceInfo *Pattern,
-                                         SourceLocation EllipsisLoc) {
+                                         SourceLocation EllipsisLoc,
+                                       llvm::Optional<unsigned> NumExpansions) {
   // Create the pack expansion type and source-location information.
   QualType Result = CheckPackExpansion(Pattern->getType(), 
                                        Pattern->getTypeLoc().getSourceRange(),
-                                       EllipsisLoc);
+                                       EllipsisLoc, NumExpansions);
   if (Result.isNull())
     return 0;
   
@@ -397,7 +399,8 @@ TypeSourceInfo *Sema::CheckPackExpansion(TypeSourceInfo *Pattern,
 
 QualType Sema::CheckPackExpansion(QualType Pattern,
                                   SourceRange PatternRange,
-                                  SourceLocation EllipsisLoc) {
+                                  SourceLocation EllipsisLoc,
+                                  llvm::Optional<unsigned> NumExpansions) {
   // 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
@@ -408,7 +411,7 @@ QualType Sema::CheckPackExpansion(QualType Pattern,
     return QualType();
   }
 
-  return Context.getPackExpansionType(Pattern);
+  return Context.getPackExpansionType(Pattern, NumExpansions);
 }
 
 ExprResult Sema::ActOnPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc) {
@@ -450,7 +453,7 @@ bool Sema::CheckParameterPacksForExpansion(SourceLocation EllipsisLoc,
                              const MultiLevelTemplateArgumentList &TemplateArgs,
                                            bool &ShouldExpand,
                                            bool &RetainExpansion,
-                                           unsigned &NumExpansions) {                                        
+                                     llvm::Optional<unsigned> &NumExpansions) {                                        
   ShouldExpand = true;
   RetainExpansion = false;
   std::pair<IdentifierInfo *, SourceLocation> FirstPack;
@@ -527,7 +530,7 @@ bool Sema::CheckParameterPacksForExpansion(SourceLocation EllipsisLoc,
         RetainExpansion = true;
     }
 
-    if (!HaveFirstPack) {
+    if (!NumExpansions) {
       // The is the first pack we've seen for which we have an argument. 
       // Record it.
       NumExpansions = NewPackSize;
@@ -537,13 +540,18 @@ bool Sema::CheckParameterPacksForExpansion(SourceLocation EllipsisLoc,
       continue;
     }
     
-    if (NewPackSize != NumExpansions) {
+    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);
+      if (HaveFirstPack)
+        Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict)
+          << FirstPack.first << Name << *NumExpansions << NewPackSize
+          << SourceRange(FirstPack.second) << SourceRange(Unexpanded[I].second);
+      else
+        Diag(EllipsisLoc, diag::err_pack_expansion_length_conflict_multilevel)
+          << Name << *NumExpansions << NewPackSize
+          << SourceRange(Unexpanded[I].second);
       return true;
     }
   }
diff --git a/lib/Sema/SemaType.cpp b/lib/Sema/SemaType.cpp
index ada8a1d8a2..e843ca5fca 100644
--- a/lib/Sema/SemaType.cpp
+++ b/lib/Sema/SemaType.cpp
@@ -1927,7 +1927,7 @@ TypeSourceInfo *Sema::GetTypeForDeclarator(Declarator &D, Scope *S,
           << T <<  D.getSourceRange();
         D.setEllipsisLoc(SourceLocation());
       } else {
-        T = Context.getPackExpansionType(T);
+        T = Context.getPackExpansionType(T, llvm::Optional<unsigned>());
       }
       break;
         
@@ -1941,7 +1941,7 @@ TypeSourceInfo *Sema::GetTypeForDeclarator(Declarator &D, Scope *S,
       // parameter packs in the type of the non-type template parameter, then
       // it expands those parameter packs.
       if (T->containsUnexpandedParameterPack())
-        T = Context.getPackExpansionType(T);
+        T = Context.getPackExpansionType(T, llvm::Optional<unsigned>());
       else if (!getLangOptions().CPlusPlus0x)
         Diag(D.getEllipsisLoc(), diag::err_variadic_templates);
       break;
diff --git a/lib/Sema/TreeTransform.h b/lib/Sema/TreeTransform.h
index 4127d5045a..f2496c2f1a 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -227,8 +227,12 @@ public:
   /// C++0x [temp.arg.explicit]p9.
   ///
   /// \param NumExpansions The number of separate arguments that will be in
-  /// the expanded form of the corresponding pack expansion. Must be set when
-  /// \c ShouldExpand is \c true.
+  /// the expanded form of the corresponding pack expansion. This is both an
+  /// input and an output parameter, which can be set by the caller if the
+  /// number of expansions is known a priori (e.g., due to a prior substitution)
+  /// and will be set by the callee when the number of expansions is known.
+  /// The callee must set this value when \c ShouldExpand is \c true; it may
+  /// set this value in other cases.
   ///
   /// \returns true if an error occurred (e.g., because the parameter packs 
   /// are to be instantiated with arguments of different lengths), false 
@@ -240,7 +244,7 @@ public:
                                unsigned NumUnexpanded,
                                bool &ShouldExpand,
                                bool &RetainExpansion,
-                               unsigned &NumExpansions) {
+                               llvm::Optional<unsigned> &NumExpansions) {
     ShouldExpand = false;
     return false;
   }
@@ -814,8 +818,10 @@ public:
   /// Subclasses may override this routine to provide different behavior.
   QualType RebuildPackExpansionType(QualType Pattern, 
                                     SourceRange PatternRange,
-                                    SourceLocation EllipsisLoc) {
-    return getSema().CheckPackExpansion(Pattern, PatternRange, EllipsisLoc);
+                                    SourceLocation EllipsisLoc,
+                                    llvm::Optional<unsigned> NumExpansions) {
+    return getSema().CheckPackExpansion(Pattern, PatternRange, EllipsisLoc,
+                                        NumExpansions);
   }
 
   /// \brief Build a new nested-name-specifier given the prefix and an
@@ -2165,7 +2171,8 @@ public:
   /// for a template argument. Subclasses may override this routine to provide 
   /// different behavior.
   TemplateArgumentLoc RebuildPackExpansion(TemplateArgumentLoc Pattern,
-                                           SourceLocation EllipsisLoc) {
+                                           SourceLocation EllipsisLoc,
+                                       llvm::Optional<unsigned> NumExpansions) {
     switch (Pattern.getArgument().getKind()) {
     case TemplateArgument::Expression: {
       ExprResult Result
@@ -2195,7 +2202,8 @@ public:
     case TemplateArgument::Type:
       if (TypeSourceInfo *Expansion 
             = getSema().CheckPackExpansion(Pattern.getTypeSourceInfo(),
-                                           EllipsisLoc))
+                                           EllipsisLoc,
+                                           NumExpansions))
         return TemplateArgumentLoc(TemplateArgument(Expansion->getType()),
                                    Expansion);
       break;
@@ -2300,7 +2308,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr **Inputs,
       // be expanded.
       bool Expand = true;
       bool RetainExpansion = false;
-      unsigned NumExpansions = 0;
+      llvm::Optional<unsigned> NumExpansions;
       if (getDerived().TryExpandParameterPacks(Expansion->getEllipsisLoc(),
                                                Pattern->getSourceRange(),
                                                Unexpanded.data(),
@@ -2318,6 +2326,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr **Inputs,
         if (OutPattern.isInvalid())
           return true;
         
+        // FIXME: Variadic templates NumExpansions
         ExprResult Out = getDerived().RebuildPackExpansion(OutPattern.get(), 
                                                 Expansion->getEllipsisLoc());
         if (Out.isInvalid())
@@ -2331,7 +2340,7 @@ bool TreeTransform<Derived>::TransformExprs(Expr **Inputs,
       
       // The transform has determined that we should perform an elementwise
       // expansion of the pattern. Do so.
-      for (unsigned I = 0; I != NumExpansions; ++I) {
+      for (unsigned I = 0; I != *NumExpansions; ++I) {
         Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
         ExprResult Out = getDerived().TransformExpr(Pattern);
         if (Out.isInvalid())
@@ -2817,8 +2826,10 @@ bool TreeTransform<Derived>::TransformTemplateArguments(InputIterator First,
       // We have a pack expansion, for which we will be substituting into
       // the pattern.
       SourceLocation Ellipsis;
+      llvm::Optional<unsigned> OrigNumExpansions;
       TemplateArgumentLoc Pattern
-        = In.getPackExpansionPattern(Ellipsis, getSema().Context);
+        = In.getPackExpansionPattern(Ellipsis, OrigNumExpansions, 
+                                     getSema().Context);
       
       llvm::SmallVector<UnexpandedParameterPack, 2> Unexpanded;
       getSema().collectUnexpandedParameterPacks(Pattern, Unexpanded);
@@ -2828,7 +2839,7 @@ bool TreeTransform<Derived>::TransformTemplateArguments(InputIterator First,
       // be expanded.
       bool Expand = true;
       bool RetainExpansion = false;
-      unsigned NumExpansions = 0;
+      llvm::Optional<unsigned> NumExpansions = OrigNumExpansions;
       if (getDerived().TryExpandParameterPacks(Ellipsis,
                                                Pattern.getSourceRange(),
                                                Unexpanded.data(),
@@ -2847,7 +2858,8 @@ bool TreeTransform<Derived>::TransformTemplateArguments(InputIterator First,
         if (getDerived().TransformTemplateArgument(Pattern, OutPattern))
           return true;
                 
-        Out = getDerived().RebuildPackExpansion(OutPattern, Ellipsis);
+        Out = getDerived().RebuildPackExpansion(OutPattern, Ellipsis,
+                                                NumExpansions);
         if (Out.getArgument().isNull())
           return true;
         
@@ -2857,14 +2869,15 @@ bool TreeTransform<Derived>::TransformTemplateArguments(InputIterator First,
       
       // The transform has determined that we should perform an elementwise
       // expansion of the pattern. Do so.
-      for (unsigned I = 0; I != NumExpansions; ++I) {
+      for (unsigned I = 0; I != *NumExpansions; ++I) {
         Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
 
         if (getDerived().TransformTemplateArgument(Pattern, Out))
           return true;
         
         if (Out.getArgument().containsUnexpandedParameterPack()) {
-          Out = getDerived().RebuildPackExpansion(Out, Ellipsis);
+          Out = getDerived().RebuildPackExpansion(Out, Ellipsis,
+                                                  OrigNumExpansions);
           if (Out.getArgument().isNull())
             return true;
         }
@@ -2880,7 +2893,8 @@ bool TreeTransform<Derived>::TransformTemplateArguments(InputIterator First,
         if (getDerived().TransformTemplateArgument(Pattern, Out))
           return true;
         
-        Out = getDerived().RebuildPackExpansion(Out, Ellipsis);
+        Out = getDerived().RebuildPackExpansion(Out, Ellipsis,
+                                                OrigNumExpansions);
         if (Out.getArgument().isNull())
           return true;
         
@@ -3497,7 +3511,7 @@ bool TreeTransform<Derived>::
         // Determine whether we should expand the parameter packs.
         bool ShouldExpand = false;
         bool RetainExpansion = false;
-        unsigned NumExpansions = 0;
+        llvm::Optional<unsigned> NumExpansions;
         if (getDerived().TryExpandParameterPacks(ExpansionTL.getEllipsisLoc(),
                                                  Pattern.getSourceRange(),
                                                  Unexpanded.data(), 
@@ -3512,7 +3526,7 @@ bool TreeTransform<Derived>::
           // Expand the function parameter pack into multiple, separate
           // parameters.
           getDerived().ExpandingFunctionParameterPack(OldParm);
-          for (unsigned I = 0; I != NumExpansions; ++I) {
+          for (unsigned I = 0; I != *NumExpansions; ++I) {
             Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
             ParmVarDecl *NewParm 
               = getDerived().TransformFunctionTypeParam(OldParm);
@@ -3546,6 +3560,7 @@ bool TreeTransform<Derived>::
         // expansion.
       }
       
+      // FIXME: Variadic templates num expansions
       Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
       ParmVarDecl *NewParm = getDerived().TransformFunctionTypeParam(OldParm);
       if (!NewParm)
@@ -3561,6 +3576,7 @@ bool TreeTransform<Derived>::
     // declaration for this parameter.
     QualType OldType = ParamTypes[i];
     bool IsPackExpansion = false;
+    llvm::Optional<unsigned> NumExpansions;
     if (const PackExpansionType *Expansion 
                                        = dyn_cast<PackExpansionType>(OldType)) {
       // We have a function parameter pack that may need to be expanded.
@@ -3571,7 +3587,6 @@ bool TreeTransform<Derived>::
       // Determine whether we should expand the parameter packs.
       bool ShouldExpand = false;
       bool RetainExpansion = false;
-      unsigned NumExpansions = 0;
       if (getDerived().TryExpandParameterPacks(Loc, SourceRange(),
                                                Unexpanded.data(), 
                                                Unexpanded.size(),
@@ -3584,7 +3599,7 @@ bool TreeTransform<Derived>::
       if (ShouldExpand) {
         // Expand the function parameter pack into multiple, separate 
         // parameters.
-        for (unsigned I = 0; I != NumExpansions; ++I) {
+        for (unsigned I = 0; I != *NumExpansions; ++I) {
           Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
           QualType NewType = getDerived().TransformType(Pattern);
           if (NewType.isNull())
@@ -3624,7 +3639,8 @@ bool TreeTransform<Derived>::
       return true;
 
     if (IsPackExpansion)
-      NewType = getSema().Context.getPackExpansionType(NewType);
+      NewType = getSema().Context.getPackExpansionType(NewType,
+                                                       NumExpansions);
       
     OutParamTypes.push_back(NewType);
     if (PVars)
@@ -4259,7 +4275,8 @@ QualType TreeTransform<Derived>::TransformPackExpansionType(TypeLocBuilder &TLB,
       Pattern != TL.getPatternLoc().getType()) {
     Result = getDerived().RebuildPackExpansionType(Pattern, 
                                            TL.getPatternLoc().getSourceRange(),
-                                                   TL.getEllipsisLoc());
+                                                   TL.getEllipsisLoc(),
+                                           TL.getTypePtr()->getNumExpansions());
     if (Result.isNull())
       return QualType();
   }
@@ -6818,7 +6835,7 @@ TreeTransform<Derived>::TransformSizeOfPackExpr(SizeOfPackExpr *E) {
   UnexpandedParameterPack Unexpanded(E->getPack(), E->getPackLoc());
   bool ShouldExpand = false;
   bool RetainExpansion = false;
-  unsigned NumExpansions = 0;
+  llvm::Optional<unsigned> NumExpansions;
   if (getDerived().TryExpandParameterPacks(E->getOperatorLoc(), E->getPackLoc(), 
                                            &Unexpanded, 1, 
                                            ShouldExpand, RetainExpansion,
@@ -6832,7 +6849,7 @@ TreeTransform<Derived>::TransformSizeOfPackExpr(SizeOfPackExpr *E) {
   // that stores that length.
   return getDerived().RebuildSizeOfPackExpr(E->getOperatorLoc(), E->getPack(), 
                                             E->getPackLoc(), E->getRParenLoc(), 
-                                            NumExpansions);
+                                            *NumExpansions);
 }
 
 template<typename Derived>
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index edb79ee808..e7a3b0aebb 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -2954,8 +2954,10 @@ QualType ASTReader::ReadTypeRecord(unsigned Index) {
     QualType Pattern = GetType(Record[0]);
     if (Pattern.isNull())
       return QualType();
-
-    return Context->getPackExpansionType(Pattern);
+    llvm::Optional<unsigned> NumExpansions;
+    if (Record[1])
+      NumExpansions = Record[1] - 1;
+    return Context->getPackExpansionType(Pattern, NumExpansions);
   }
 
   case TYPE_ELABORATED: {
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index 52cf3829dc..7582d1a450 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -316,6 +316,10 @@ ASTTypeWriter::VisitDependentTemplateSpecializationType(
 
 void ASTTypeWriter::VisitPackExpansionType(const PackExpansionType *T) {
   Writer.AddTypeRef(T->getPattern(), Record);
+  if (llvm::Optional<unsigned> NumExpansions = T->getNumExpansions())
+    Record.push_back(*NumExpansions + 1);
+  else
+    Record.push_back(0);
   Code = TYPE_PACK_EXPANSION;
 }
 
diff --git a/test/CXX/temp/temp.decls/temp.variadic/multi-level-substitution.cpp b/test/CXX/temp/temp.decls/temp.variadic/multi-level-substitution.cpp
index ff2134dc33..a01ffc338f 100644
--- a/test/CXX/temp/temp.decls/temp.variadic/multi-level-substitution.cpp
+++ b/test/CXX/temp/temp.decls/temp.variadic/multi-level-substitution.cpp
@@ -18,16 +18,23 @@ namespace PacksAtDifferentLevels {
 
   template<typename ...Types>
   struct X {
-    template<typename> struct Inner;
+    template<typename> struct Inner {
+      static const unsigned value = 1;
+    };
 
     template<typename ...YTypes>
     struct Inner<tuple<pair<Types, YTypes>...> > {
-      static const unsigned zero = sizeof...(Types) - sizeof...(YTypes);
+      static const unsigned value = sizeof...(Types) - sizeof...(YTypes);
     };
   };
 
   int check0[X<short, int, long>::Inner<tuple<pair<short, unsigned short>,
                                              pair<int, unsigned int>,
                                              pair<long, unsigned long>>
-                                       >::zero == 0? 1 : -1];
+                                       >::value == 0? 1 : -1];
+
+  int check1[X<short, int>::Inner<tuple<pair<short, unsigned short>,
+                                        pair<int, unsigned int>,
+                                        pair<long, unsigned long>>
+                                       >::value == 1? 1 : -1]; 
 }