diff --git a/lib/Sema/SemaTemplateInstantiate.cpp b/lib/Sema/SemaTemplateInstantiate.cpp
index 629dc58..c181a56 100644
--- a/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/lib/Sema/SemaTemplateInstantiate.cpp
@@ -622,7 +622,7 @@
                                  unsigned NumUnexpanded,
                                  bool &ShouldExpand,
                                  bool &RetainExpansion,
-                                 unsigned &NumExpansions) {
+                                 llvm::Optional<unsigned> &NumExpansions) {
       return getSema().CheckParameterPacksForExpansion(EllipsisLoc, 
                                                        PatternRange, Unexpanded,
                                                        NumUnexpanded, 
@@ -1260,7 +1260,9 @@
       // 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 @@
                                       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 @@
       
       // 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 a18bfba..6b5713a 100644
--- a/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1977,7 +1977,8 @@
         
         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 @@
                       
         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 @@
           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 @@
       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 @@
       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 dc5013c..899b58e 100644
--- a/lib/Sema/SemaTemplateVariadic.cpp
+++ b/lib/Sema/SemaTemplateVariadic.cpp
@@ -368,7 +368,8 @@
   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 @@
 }
 
 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 @@
 
 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 @@
     return QualType();
   }
 
-  return Context.getPackExpansionType(Pattern);
+  return Context.getPackExpansionType(Pattern, NumExpansions);
 }
 
 ExprResult Sema::ActOnPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc) {
@@ -450,7 +453,7 @@
                              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 @@
         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 @@
       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 ada8a1d..e843ca5 100644
--- a/lib/Sema/SemaType.cpp
+++ b/lib/Sema/SemaType.cpp
@@ -1927,7 +1927,7 @@
           << T <<  D.getSourceRange();
         D.setEllipsisLoc(SourceLocation());
       } else {
-        T = Context.getPackExpansionType(T);
+        T = Context.getPackExpansionType(T, llvm::Optional<unsigned>());
       }
       break;
         
@@ -1941,7 +1941,7 @@
       // 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 4127d50..f2496c2 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -227,8 +227,12 @@
   /// 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 @@
                                unsigned NumUnexpanded,
                                bool &ShouldExpand,
                                bool &RetainExpansion,
-                               unsigned &NumExpansions) {
+                               llvm::Optional<unsigned> &NumExpansions) {
     ShouldExpand = false;
     return false;
   }
@@ -814,8 +818,10 @@
   /// 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 @@
   /// 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 @@
     case TemplateArgument::Type:
       if (TypeSourceInfo *Expansion 
             = getSema().CheckPackExpansion(Pattern.getTypeSourceInfo(),
-                                           EllipsisLoc))
+                                           EllipsisLoc,
+                                           NumExpansions))
         return TemplateArgumentLoc(TemplateArgument(Expansion->getType()),
                                    Expansion);
       break;
@@ -2300,7 +2308,7 @@
       // 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 @@
         if (OutPattern.isInvalid())
           return true;
         
+        // FIXME: Variadic templates NumExpansions
         ExprResult Out = getDerived().RebuildPackExpansion(OutPattern.get(), 
                                                 Expansion->getEllipsisLoc());
         if (Out.isInvalid())
@@ -2331,7 +2340,7 @@
       
       // 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 @@
       // 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 @@
       // 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 @@
         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 @@
       
       // 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 @@
         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 @@
         // 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 @@
           // 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 @@
         // expansion.
       }
       
+      // FIXME: Variadic templates num expansions
       Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
       ParmVarDecl *NewParm = getDerived().TransformFunctionTypeParam(OldParm);
       if (!NewParm)
@@ -3561,6 +3576,7 @@
     // 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 @@
       // 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 @@
       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 @@
       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 @@
       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 @@
   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 @@
   // that stores that length.
   return getDerived().RebuildSizeOfPackExpr(E->getOperatorLoc(), E->getPack(), 
                                             E->getPackLoc(), E->getRParenLoc(), 
-                                            NumExpansions);
+                                            *NumExpansions);
 }
 
 template<typename Derived>