Index: test/SemaCXX/libstdcxx_common_type_hack.cpp
===================================================================
--- test/SemaCXX/libstdcxx_common_type_hack.cpp	(revision 0)
+++ test/SemaCXX/libstdcxx_common_type_hack.cpp	(revision 166455)
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fsyntax-only %s -std=c++11 -verify
+
+// This is a test for an egregious hack in Clang that works around
+// an issue with GCC's <type_traits> implementation. std::common_type
+// relies on pre-standard rules for decltype(), in which it doesn't
+// produce reference types so frequently.
+
+#ifdef BE_THE_HEADER
+
+#pragma GCC system_header
+namespace std {
+  template<typename T> T &&declval();
+
+  template<typename...Ts> struct common_type {};
+  template<typename A, typename B> struct common_type<A, B> {
+    // Under the rules in the standard, this always produces a
+    // reference type.
+    typedef decltype(true ? declval<A>() : declval<B>()) type;
+  };
+}
+
+#else
+
+#define BE_THE_HEADER
+#include "libstdcxx_common_type_hack.cpp"
+
+using T = int;
+using T = std::common_type<int, int>::type;
+
+using U = int; // expected-note {{here}}
+using U = decltype(true ? std::declval<int>() : std::declval<int>()); // expected-error {{different types}}
+
+#endif
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp	(revision 166454)
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp	(revision 166455)
@@ -158,6 +158,22 @@
     SemaRef.MarkDeclarationsReferencedInType(D->getLocation(), DI->getType());
   }
 
+  // HACK: g++ has a bug where it gets the value kind of ?: wrong.
+  // libstdc++ relies upon this bug in its implementation of common_type.
+  // If we happen to be processing that implementation, fake up the g++ ?:
+  // semantics. See LWG issue 2141 for more information on the bug.
+  const DecltypeType *DT = DI->getType()->getAs<DecltypeType>();
+  CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(D->getDeclContext());
+  if (DT && RD && isa<ConditionalOperator>(DT->getUnderlyingExpr()) &&
+      DT->isReferenceType() &&
+      RD->getEnclosingNamespaceContext() == SemaRef.getStdNamespace() &&
+      RD->getIdentifier() && RD->getIdentifier()->isStr("common_type") &&
+      D->getIdentifier() && D->getIdentifier()->isStr("type") &&
+      SemaRef.getSourceManager().isInSystemHeader(D->getLocStart()))
+    // Fold it to the (non-reference) type which g++ would have produced.
+    DI = SemaRef.Context.getTrivialTypeSourceInfo(
+      DI->getType().getNonReferenceType());
+
   // Create the new typedef
   TypedefNameDecl *Typedef;
   if (IsTypeAlias)