[llvm-bugs] [Bug 46256] New: [coroutines] coroutines with potentially-throwing 'co_await promise.final_suspend()' expressions should be ill-formed

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46256

            Bug ID: 46256
           Summary: [coroutines] coroutines with potentially-throwing
                    'co_await promise.final_suspend()' expressions should
                    be ill-formed
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lewissbaker at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

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

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

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

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