[libsigc++2] C++11: deduce_result_type: Simpifying, removing use of std::is_same().

[Murray Cumming <murrayc src gnome org>]

commit 960f48920ea86d9dab2c84344992c19b55895f20
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Sep 17 14:54:55 2015 +0200

    C++11: deduce_result_type: Simpifying, removing use of std::is_same().
    
    See https://bugzilla.gnome.org/show_bug.cgi?id=753612#c9

 sigc++/adaptors/macros/deduce_result_type.h.m4 |   17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)
---
diff --git a/sigc++/adaptors/macros/deduce_result_type.h.m4 b/sigc++/adaptors/macros/deduce_result_type.h.m4
index f83347f..c7c8fca 100644
--- a/sigc++/adaptors/macros/deduce_result_type.h.m4
+++ b/sigc++/adaptors/macros/deduce_result_type.h.m4
@@ -65,13 +65,20 @@ struct adaptor_base : public functor_base {};
 template<class T_functor, class... T_args>
 struct deduce_result_type
 {
-  template<class U, typename=typename std::is_same<typename std::is_base_of<adaptor_base, T_functor>::type, 
std::true_type>::type>
-  static typename U::template deduce_result_type<T_args...> test (int);
+  //The compiler will choose this method overload if T_functor derives from adaptor_base,
+  //and if it has its own deduce_result_type member (which has its own ::type member).
+  template<class U_functor, typename = typename std::is_base_of<adaptor_base, T_functor>::type>
+  static
+  typename U_functor::template deduce_result_type<T_args...>::type
+  test();
 
-  template<class U>
-  static typename functor_trait<T_functor>::result_type test (...);
+  //Otherwise, the compiler will choose this fallback method.
+  template<class U_functor>
+  static
+  typename functor_trait<T_functor>::result_type
+  test();
 
-  using type = decltype (test<T_functor> (0));
+  using type = decltype (test<T_functor> ());
 };
 
 template<typename T_functor, typename... T_args>