Question about _GLIBCXX_HAVE_BUILTIN_IS_SAME

[Ken Matsui <kmatsui@cs.washington.edu>]

Hi,

I found we are using _GLIBCXX_HAVE_BUILTIN_IS_SAME in type_traits, but I
think we can use _GLIBCXX_USE_BUILTIN_TRAIT(__is_same) instead.  I feel
this is a bit more readable and consistent with other traits.  With this
change, AFAIK, _GLIBCXX_HAVE_BUILTIN_IS_SAME is not used anywhere, but
can we completely remove it from gcc/libstdc++-v3/include/bits/c++config
or is there any reason to keep it?

   /// is_same
+#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_same)
   template<typename _Tp, typename _Up>
     struct is_same
-#ifdef _GLIBCXX_HAVE_BUILTIN_IS_SAME
     : public __bool_constant<__is_same(_Tp, _Up)>
+    { };
 #else
+  template<typename _Tp, typename _Up>
+    struct is_same
     : public false_type
-#endif
     { };
 
-#ifndef _GLIBCXX_HAVE_BUILTIN_IS_SAME
   template<typename _Tp>
     struct is_same<_Tp, _Tp>
     : public true_type
    { };
#endif

I am also wondering if we could replace other _GLIBCXX_HAVE_BUILTIN_*
macros with _GLIBCXX_USE_BUILTIN_TRAIT(*):

* _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP
* _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE
* _GLIBCXX_HAVE_BUILTIN_LAUNDER

-- 
Ken Matsui