Seems like the following specialization of impl<> works:

 

 

   template <typename r>

   struct impl<true, r(void)>

   {

     static const bool value =

       sizeof(

            return_value_check<type, r>::deduce( (

             (((derived_type*)0)->operator()(),  details::void_exp_result<type>())

                         ) )

            ) == sizeof(yes);

 

   };