Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -13076,7 +13076,8 @@
 
   StringRef LiteralName
     = FnDecl->getDeclName().getCXXLiteralIdentifier()->getName();
-  if (LiteralName[0] != '_') {
+  if (LiteralName[0] != '_' &&
+      !getSourceManager().isInSystemHeader(FnDecl->getLocation())) {
     // C++11 [usrlit.suffix]p1:
     //   Literal suffix identifiers that do not start with an underscore
     //   are reserved for future standardization.
Index: test/SemaCXX/no-warn-user-defined-literals-in-system-headers.h
===================================================================
--- test/SemaCXX/no-warn-user-defined-literals-in-system-headers.h
+++ test/SemaCXX/no-warn-user-defined-literals-in-system-headers.h
@@ -0,0 +1,2 @@
+// Header for no-warn-user-defined-literals-in-system-headers.cpp
+void operator "" foo (const char *);
Index: test/SemaCXX/no-warn-user-defined-literals-in-system-headers.cpp
===================================================================
--- test/SemaCXX/no-warn-user-defined-literals-in-system-headers.cpp
+++ test/SemaCXX/no-warn-user-defined-literals-in-system-headers.cpp
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wsystem-headers -isystem %S %s
+
+#include <no-warn-user-defined-literals-in-system-headers.h>
+
+void operator "" bar(long double); // expected-warning{{user-defined literal suffixes not starting with '_' are reserved}}