use __mcall to make the return type consistent with the constraints and noexcept clause

Author: ericniebler

include/stdexec/__detail/__tuple.hpp

@@ -222,7 +222,7 @@ namespace STDEXEC {
         template <class... _Ts, class... _Us, __callable<_Us..., __mcall<_CvRef, _Ts>...> _Fn>
         auto operator()(_Fn&& __fn, __tuple_t<_Ts...>&& __tupl, _Us&&... __us) const
           noexcept(__nothrow_callable<_Fn, _Us..., __mcall<_CvRef, _Ts>...>)
-            -> __call_result_t<_Fn, _Us..., typename _CvRef::template __f<_Ts>...>;
+            -> __call_result_t<_Fn, _Us..., __mcall<_CvRef, _Ts>...>;
       };
 
       template <class _Tuple>