Index: test/Sema/128bitfloat.cc
===================================================================
--- test/Sema/128bitfloat.cc	(revision 0)
+++ test/Sema/128bitfloat.cc	(revision 0)
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=gnu++11 %s -DHAVE
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+
+#ifdef HAVE
+__float128 f;  // expected-error{{variable has incomplete type '__float128'}}
+// But this should work:
+template<typename> struct __is_floating_point_helper {};
+template<> struct __is_floating_point_helper<__float128> {};
+#else
+__float128 f;  // expected-error {{unknown type name '__float128'}}
+template<typename> struct __is_floating_point_helper {};
+template<> struct __is_floating_point_helper<__float128> {};  // expected-error {{use of undeclared identifier '__float128'}}
+#endif
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h	(revision 184377)
+++ include/clang/Sema/Sema.h	(working copy)
@@ -7600,6 +7600,7 @@
   Scope *CurScope;
 
   mutable IdentifierInfo *Ident_super;
+  mutable IdentifierInfo *Ident___float128;
 
 protected:
   friend class Parser;
@@ -7619,6 +7620,7 @@
   Scope *getCurScope() const { return CurScope; }
 
   IdentifierInfo *getSuperIdentifier() const;
+  IdentifierInfo *getFloat128Identifier() const;
 
   Decl *getObjCDeclContext() const;
 
Index: include/clang/AST/ASTContext.h
===================================================================
--- include/clang/AST/ASTContext.h	(revision 184377)
+++ include/clang/AST/ASTContext.h	(working copy)
@@ -196,6 +196,9 @@
 
   /// \brief The typedef for the __uint128_t type.
   mutable TypedefDecl *UInt128Decl;
+
+  /// \brief The typedef for the __float128 stub type.
+  mutable NamedDecl *Float128StubDecl;
   
   /// \brief The typedef for the target specific predefined
   /// __builtin_va_list type.
@@ -808,6 +811,9 @@
 
   /// \brief Retrieve the declaration for the 128-bit unsigned integer type.
   TypedefDecl *getUInt128Decl() const;
+
+  /// \brief Retrieve the declaration for a 128-bit float stub type.
+  NamedDecl *getFloat128StubType() const;
   
   //===--------------------------------------------------------------------===//
   //                           Type Constructors
Index: lib/Sema/SemaLookup.cpp
===================================================================
--- lib/Sema/SemaLookup.cpp	(revision 184377)
+++ lib/Sema/SemaLookup.cpp	(working copy)
@@ -511,6 +511,14 @@
       NameKind == Sema::LookupRedeclarationWithLinkage) {
     IdentifierInfo *II = R.getLookupName().getAsIdentifierInfo();
     if (II) {
+      if (S.getLangOpts().CPlusPlus11 && S.getLangOpts().GNUMode &&
+          II == S.getFloat128Identifier()) {
+        // libstdc++4.7's type_traits expects type __float128 to exist, so
+        // insert a dummy type to make that header build in gnu++11 mode.
+        R.addDecl(S.getASTContext().getFloat128StubType());
+        return true;
+      }
+
       // If this is a builtin on this (or all) targets, create the decl.
       if (unsigned BuiltinID = II->getBuiltinID()) {
         // In C++, we don't have any predefined library functions like
Index: lib/Sema/Sema.cpp
===================================================================
--- lib/Sema/Sema.cpp	(revision 184377)
+++ lib/Sema/Sema.cpp	(working copy)
@@ -90,7 +90,7 @@
     AccessCheckingSFINAE(false), InNonInstantiationSFINAEContext(false),
     NonInstantiationEntries(0), ArgumentPackSubstitutionIndex(-1),
     CurrentInstantiationScope(0), TyposCorrected(0),
-    AnalysisWarnings(*this), CurScope(0), Ident_super(0)
+    AnalysisWarnings(*this), CurScope(0), Ident_super(0), Ident___float128(0)
 {
   TUScope = 0;
 
@@ -1320,6 +1320,12 @@
   return Ident_super;
 }
 
+IdentifierInfo *Sema::getFloat128Identifier() const {
+  if (!Ident___float128)
+    Ident___float128 = &Context.Idents.get("__float128");
+  return Ident___float128;
+}
+
 void Sema::PushCapturedRegionScope(Scope *S, CapturedDecl *CD, RecordDecl *RD,
                                    CapturedRegionKind K) {
   CapturingScopeInfo *CSI = new CapturedRegionScopeInfo(getDiagnostics(), S, CD, RD,
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp	(revision 184377)
+++ lib/AST/ASTContext.cpp	(working copy)
@@ -698,7 +698,7 @@
     DependentTemplateSpecializationTypes(this_()),
     SubstTemplateTemplateParmPacks(this_()),
     GlobalNestedNameSpecifier(0), 
-    Int128Decl(0), UInt128Decl(0),
+    Int128Decl(0), UInt128Decl(0), Float128StubDecl(0),
     BuiltinVaListDecl(0),
     ObjCIdDecl(0), ObjCSelDecl(0), ObjCClassDecl(0), ObjCProtocolClassDecl(0),
     BOOLDecl(0),
@@ -857,6 +857,19 @@
   return UInt128Decl;
 }
 
+NamedDecl *ASTContext::getFloat128StubType() const {
+  if (!Float128StubDecl) {
+    Float128StubDecl = RecordDecl::Create(const_cast<ASTContext &>(*this), 
+                                          TTK_Struct,
+                                          getTranslationUnitDecl(),
+                                          SourceLocation(),
+                                          SourceLocation(),
+                                          &Idents.get("__float128"));
+  }
+  
+  return Float128StubDecl;
+}
+
 void ASTContext::InitBuiltinType(CanQualType &R, BuiltinType::Kind K) {
   BuiltinType *Ty = new (*this, TypeAlignment) BuiltinType(K);
   R = CanQualType::CreateUnsafe(QualType(Ty, 0));