CWG2556 [stmt.return.coroutine] Unusable promise::return_void

[opensdh]

Reference (section label)

[stmt.return.coroutine]

Link to reflector thread (if any)

https://lists.isocpp.org/core/2020/09/9958.php

Issue description

The note in [dcl.fct.def.coroutine]/6 suggests that the existence of promise::return_void causes flowing off the end of a coroutine to be well-defined. [stmt.return.coroutine]/3, however, actually checks whether calling it with no arguments is a valid expression (presumably from the coroutine). As such, a return_void that is inaccessible, deleted, or requires arguments will produce undefined behavior even though the implementation knows that return_void was "intended" (since no return_value can coexist with it). This seems unnecessarily user-hostile.

Suggested resolution

Say in [stmt.return.coroutine]/3 that p.return_void() is used if name lookup succeeded even if it's ill-formed.

[jensmaurer]

CWG2556