Index: libcxx/trunk/include/__config
===================================================================
--- libcxx/trunk/include/__config
+++ libcxx/trunk/include/__config
@@ -258,6 +258,8 @@
 #  define _LIBCPP_NORETURN __attribute__ ((noreturn))
 #endif
 
+#define _LIBCPP_UNUSED __attribute__((__unused__))
+
 #if !(__has_feature(cxx_defaulted_functions))
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #endif  // !(__has_feature(cxx_defaulted_functions))
@@ -380,6 +382,8 @@
 
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 
+#define _LIBCPP_UNUSED __attribute__((__unused__))
+
 #if _GNUC_VER >= 407
 #define _LIBCPP_UNDERLYING_TYPE(T) __underlying_type(T)
 #define _LIBCPP_IS_LITERAL(T) __is_literal_type(T)
@@ -477,6 +481,7 @@
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 #define __alignof__ __alignof
 #define _LIBCPP_NORETURN __declspec(noreturn)
+#define _LIBCPP_UNUSED
 #define _ALIGNAS(x) __declspec(align(x))
 #define _LIBCPP_HAS_NO_VARIADICS
 
@@ -500,6 +505,7 @@
 #define _ALIGNAS_TYPE(x) __attribute__((__aligned__(__alignof(x))))
 #define _ATTRIBUTE(x) __attribute__((x))
 #define _LIBCPP_NORETURN __attribute__((noreturn))
+#define _LIBCPP_UNUSED
 
 #define _NOEXCEPT throw()
 #define _NOEXCEPT_(x)
Index: libcxx/trunk/include/stdexcept
===================================================================
--- libcxx/trunk/include/stdexcept
+++ libcxx/trunk/include/stdexcept
@@ -53,7 +53,7 @@
 #ifndef _LIBCPP___REFSTRING
 _LIBCPP_BEGIN_NAMESPACE_STD
 class _LIBCPP_HIDDEN __libcpp_refstring {
-    const char *__imp_;
+    const char *__imp_ _LIBCPP_UNUSED;
 };
 _LIBCPP_END_NAMESPACE_STD
 #endif