[Bug c++/98704] New: coroutine_handle::done() == false after promise's unhandled_exception() has thrown

["m.krause@tu-harburg.de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98704

            Bug ID: 98704
           Summary: coroutine_handle::done() == false after promise's
                    unhandled_exception() has thrown
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.krause@tu-harburg.de
  Target Milestone: ---

Created attachment 49981
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49981&action=edit
Reproducer

[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". So, after this exception has
propagated to the caller, I expect that the coroutine_handle's done() function
returns true (because of [coroutine.handle.observers]/3 "Returns: true if the
coroutine is suspended at its final suspend point, otherwise false").

However, the compiler for which I am reporting this makes done() return false
in this situation. See the attached example code (my expectation is 1 instead
of 0 in the final two lines of output).

By the way, gcc 10.2, gcc trunk, clang 10.0.0, clang trunk, msvc 14.28 all
behave in this way. See the short discussion on Slack Cpplang #coroutines
starting on 02-Jan-2021. A fix for msvc is already underway:
https://developercommunity.visualstudio.com/content/problem/1305540/coroutine-handledone-false-after-promises-unhandle.html