Index: cfe/trunk/include/clang/AST/Type.h
===================================================================
--- cfe/trunk/include/clang/AST/Type.h
+++ cfe/trunk/include/clang/AST/Type.h
@@ -1608,6 +1608,21 @@
     unsigned Keyword : 2;
   };
 
+  class SubstTemplateTypeParmPackTypeBitfields {
+    friend class SubstTemplateTypeParmPackType;
+
+    unsigned : NumTypeBits;
+
+    /// The number of template arguments in \c Arguments, which is
+    /// expected to be able to hold at least 1024 according to [implimits].
+    /// However as this limit is somewhat easy to hit with template
+    /// metaprogramming we'd prefer to keep it as large as possible.
+    /// At the moment it has been left as a non-bitfield since this type
+    /// safely fits in 64 bits as an unsigned, so there is no reason to
+    /// introduce the performance impact of a bitfield.
+    unsigned NumArgs;
+  };
+
   class TemplateSpecializationTypeBitfields {
     friend class TemplateSpecializationType;
 
@@ -1672,6 +1687,7 @@
     ReferenceTypeBitfields ReferenceTypeBits;
     TypeWithKeywordBitfields TypeWithKeywordBits;
     VectorTypeBitfields VectorTypeBits;
+    SubstTemplateTypeParmPackTypeBitfields SubstTemplateTypeParmPackTypeBits;
     TemplateSpecializationTypeBitfields TemplateSpecializationTypeBits;
     DependentTemplateSpecializationTypeBitfields
       DependentTemplateSpecializationTypeBits;
@@ -1697,6 +1713,9 @@
                   "TypeWithKeywordBitfields is larger than 8 bytes!");
     static_assert(sizeof(VectorTypeBitfields) <= 8,
                   "VectorTypeBitfields is larger than 8 bytes!");
+    static_assert(sizeof(SubstTemplateTypeParmPackTypeBitfields) <= 8,
+                  "SubstTemplateTypeParmPackTypeBitfields is larger"
+                  " than 8 bytes!");
     static_assert(sizeof(TemplateSpecializationTypeBitfields) <= 8,
                   "TemplateSpecializationTypeBitfields is larger"
                   " than 8 bytes!");
@@ -4551,9 +4570,6 @@
   /// parameter pack is instantiated with.
   const TemplateArgument *Arguments;
 
-  /// The number of template arguments in \c Arguments.
-  unsigned NumArguments;
-
   SubstTemplateTypeParmPackType(const TemplateTypeParmType *Param,
                                 QualType Canon,
                                 const TemplateArgument &ArgPack);
@@ -4566,6 +4582,10 @@
     return Replaced;
   }
 
+  unsigned getNumArgs() const {
+    return SubstTemplateTypeParmPackTypeBits.NumArgs;
+  }
+
   bool isSugared() const { return false; }
   QualType desugar() const { return QualType(this, 0); }
 
Index: cfe/trunk/lib/AST/Type.cpp
===================================================================
--- cfe/trunk/lib/AST/Type.cpp
+++ cfe/trunk/lib/AST/Type.cpp
@@ -3285,11 +3285,12 @@
                               QualType Canon,
                               const TemplateArgument &ArgPack)
     : Type(SubstTemplateTypeParmPack, Canon, true, true, false, true),
-      Replaced(Param),
-      Arguments(ArgPack.pack_begin()), NumArguments(ArgPack.pack_size()) {}
+      Replaced(Param), Arguments(ArgPack.pack_begin()) {
+  SubstTemplateTypeParmPackTypeBits.NumArgs = ArgPack.pack_size();
+}
 
 TemplateArgument SubstTemplateTypeParmPackType::getArgumentPack() const {
-  return TemplateArgument(llvm::makeArrayRef(Arguments, NumArguments));
+  return TemplateArgument(llvm::makeArrayRef(Arguments, getNumArgs()));
 }
 
 void SubstTemplateTypeParmPackType::Profile(llvm::FoldingSetNodeID &ID) {