[Bug c++/95616] New: [coroutines] coroutines with potentially-throwing 'co_await promise.final_suspend()' expressions should be ill-formed

["lewissbaker.opensource at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95616

            Bug ID: 95616
           Summary: [coroutines] coroutines with potentially-throwing
                    'co_await promise.final_suspend()' expressions should
                    be ill-formed
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lewissbaker.opensource at gmail dot com
  Target Milestone: ---

The wording of [dcl.fct.def.coroutine]/15 states:
> The expression co_­await promise.final_­suspend() shall not be potentially-throwing ([except.spec]).

See http://eel.is/c++draft/dcl.fct.def.coroutine#15
and http://eel.is/c++draft/except.spec#6

I believe this means that the compiler should reject with a diagnostic any
coroutine with a promise_type where `co_await promise.final_suspend()` is not
noexcept. ie. all of the following must be declared noexcept (if they form part
of the await-expression):
- promise_type::final_suspend()
- finalSuspendObj.operator co_await()
- finalSuspendAwaiter.await_ready()
- finalSuspendAwaiter.await_suspend()
- finalSuspendAwaiter.await_resume()
- finalSuspedObj destructor
- finalSuspendAwaiter destructor

GCC (and Clang) currently accepts code that that does not declare
final_suspend() as
noexcept.

e.g. see https://godbolt.org/z/Et_5eo