exec: fork join trailing return type (#2083)

* exec: fork join trailing return type

To avoid issues with incomplete types.

* in `fork_join_t::__connect`, take receiver by value

---------

Signed-off-by: romintomasetti <romin.tomasetti@gmail.com>
Co-authored-by: Eric Niebler <eniebler@nvidia.com>

Author: romintomasetti

include/exec/fork_join.hpp

@@ -291,8 +291,11 @@ namespace experimental::execution::__fork_join
       }
     }
 
-    static constexpr auto __connect =
-      []<class _Receiver, class _Sender>(_Sender&& __sndr, _Receiver&& __rcvr) noexcept
+    static constexpr auto __connect = []<class _Receiver, class _Sender>(_Sender&& __sndr,
+                                                                         _Receiver __rcvr) noexcept
+      -> fork_join_impl_t::_opstate_t<STDEXEC::__child_of<_Sender>,
+                                      STDEXEC::__data_of<_Sender>,
+                                      _Receiver>
     {
       using _closures_t = STDEXEC::__data_of<_Sender>;
       using _sndr_t     = STDEXEC::__child_of<_Sender>;