The standard says ([dcl.fct.def.coroutine]):


6 The unqualified-ids return_­void and return_­value are looked up in
the scope of the promise type. If both are found, the program is ill-formed.


However, there are good reasons to want both:


std::future<void> yet_another();


std::future<void> my_coroutine() {

     if (some_check) {

         co_return; // calls promise_type::return_void()

     }

     co_await something_else();

     co_return yet_another(); // calls
promise_type::return_value(std::future<void>)

}


The last statement calls another coroutine, but this is not legal under
the current wording (though gcc accepts it).


I could rewrite the coroutine as follows:


std::future<void> my_coroutine() {

     if (some_check) {

         co_return; // calls promise_type::return_void()

     }

     co_await something_else();

     co_await yet_another(); // typically suspends the coroutine here

     co_return; // optional

}


But this will cost something - typically the coroutine body has to be
suspended and awakened for co_await.


Remediation: allow return_value() and return_void() to coexist.