Index: include/tr1_impl/complex
===================================================================
--- include/tr1_impl/complex	(revision 156970)
+++ include/tr1_impl/complex	(working copy)
@@ -311,8 +311,12 @@
     arg(_Tp __x)
     {
       typedef typename __gnu_cxx::__promote<_Tp>::__type __type;
+#if (_GLIBCXX_USE_C99_MATH && !_GLIBCXX_USE_C99_FP_MACROS_DYNAMIC)
       return std::signbit(__x) ? __type(3.1415926535897932384626433832795029L)
 	                       : __type();
+#else
+      return std::arg(std::complex<__type>(__x));
+#endif
     }
 
   template<typename _Tp>