CWG2934 [dcl.fct.def.coroutine] Unclear semantics of exception escaping from `unhandled_exception`

[t3nsor]

Full name of submitter: Brian Bi

Reference (section label): [dcl.fct.def.coroutine]

Issue description: [dcl.fct.def.coroutine]/14 says:

If the evaluation of the expression promise.unhandled_exception() exits via an exception, the coroutine is considered suspended at the final suspend point and the exception propagates to the caller or resumer.

All implementations first destroy the promise object followed by the parameter copies, then deallocate the coroutine state, and finally return control to the caller: https://godbolt.org/z/e7edh8d9o

The behaviour of the implementations is expected based on the code in [dcl.fct.def.coroutine]/5, but the wording in p14 suggests that suspension occurs immediately when unhandled_exception() exits, prior to any stack unwinding in the coroutine. The intent should be clarified.

[t3nsor]

Prior discussion can be found here and here.

[t3nsor]

Bump

[jensmaurer]

Any wording suggestion?

[jensmaurer]

CWG2934

[t3nsor]

I think the reason why I didn't write a wording suggestion for this one is that it depends on what we decide we want for [CWG2935]. The wording is cleaner if we assume that an exception propagating out of the coroutine always destroys the coroutine state.