another attempt at fixing single_sender

Author: dietmarkuehl

include/beman/execution26/detail/single_sender.hpp

@@ -11,15 +11,13 @@
 
 namespace beman::execution26::detail
 {
-#if 1
     template <typename Sender, typename Env>
     concept single_sender
         =  ::beman::execution26::sender_in<Sender, Env>
         && requires{
             typename ::beman::execution26::detail::single_sender_value_type<Sender, Env>;
         }
         ;
-#endif
 }
 
 // ----------------------------------------------------------------------------

include/beman/execution26/detail/single_sender_value_type.hpp

@@ -40,7 +40,7 @@ namespace beman::execution26::detail
                     ::beman::execution26::value_types_of_t<
                         Sender, Env, ::beman::execution26::detail::decayed_tuple, ::std::type_identity_t>>();
             else
-                return ::std::enable_if<false>();
+                return ::std::enable_if<false, Sender>();
         }())::type;
 }