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4357. connect-awaitable should use is_void_v to check for result-type of co_await expression instead of same_as<void>

Section: 33.9.10 [exec.connect] Status: New Submitter: Lewis Baker Opened: 2025-08-27 Last modified: 2025-10-23

Priority: 4

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Discussion:

The wording in 33.9.10 [exec.connect] p5 defines the connect-awaitable() function as follows:

operation-state-task connect-awaitable(DS sndr, DR rcvr) requires receiver_of<DR, Sigs> {
  exception_ptr ep;
  try {
    if constexpr (same_as<V, void>) {
      co_await std::move(sndr);
      co_await suspend-complete(set_value, std::move(rcvr));
    } else {
      co_await suspend-complete(set_value, std::move(rcvr), co_await std::move(sndr));
    }
  } catch(...) {
    ep = current_exception();
  }
  co_await suspend-complete(set_error, std::move(rcvr), std::move(ep));
}

The use of same_as<V, void> in the if-constexpr condition does not cover the case where the result of the co_await expression has type cv void. It should use is_void_v<V> instead to allow it to match all void types, not just unqualified void.

[2025-10-23; Reflector poll.]

Set priority to 4 after reflector poll.

"NAD, V is await-result-type<...> which is decltype(...) and you never get cv void from decltype (7.2.2 [expr.type] p2)."

"But using is_void_v more clearly expresses the intent."

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.9.10 [exec.connect] as indicated:

   -5- Let V name the type await-result-type<DS, connect-awaitable-promise>, let Sigs name the type

   completion_signatures<
     SET-VALUE-SIG(V), // see 33.9.3 [exec.snd.concepts]
     set_error_t(exception_ptr),
     set_stopped_t()>
   

   and let connect-awaitable be an exposition-only coroutine defined as follows:

   namespace std::execution {
     […]
     operation-state-task connect-awaitable(DS sndr, DR rcvr) requires receiver_of<DR, Sigs> {
       exception_ptr ep;
       try {
         if constexpr (same_as<V, void>is_void_v<V>) {
           co_await std::move(sndr);
           co_await suspend-complete(set_value, std::move(rcvr));
         } else {
           co_await suspend-complete(set_value, std::move(rcvr), co_await std::move(sndr));
         }
       } catch(...) {
         ep = current_exception();
       }
       co_await suspend-complete(set_error, std::move(rcvr), std::move(ep));
     }
   }