[Bug c++/109323] No error when neither of return_value or return_void is defined

[enricomaria.dean6elis at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109323

--- Comment #4 from Enrico Maria De Angelis <enricomaria.dean6elis at gmail dot com> ---
The more I read the standard, specifically
http://eel.is/c++draft/dcl.fct.def.coroutine#6 and
http://eel.is/c++draft/stmt.return.coroutine, the more I'm convinced that it's
just fine that a coroutine's promise_type lacks both return_void and
return_value, as long as it has no co_return and the execution never flows off
the end of the coroutine itself.