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[dcl.fct.def.coroutine] Unclear semantics of exception escaping from `unhandled_exception` #574

Open t3nsor opened 1 month ago

t3nsor commented 1 month ago

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 commented 1 month ago

Prior discussion can be found here and here.