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

[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]

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

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

[iains at gcc dot gnu.org]

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

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-06-09
           Keywords|                            |accepts-invalid
   Target Milestone|---                         |10.2
           Assignee|unassigned at gcc dot gnu.org      |iains at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Iain Sandoe <iains at gcc dot gnu.org> ---
thanks for the report

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

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.2                        |10.3

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10.2 is released, adjusting target milestone.

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

[iains at gcc dot gnu.org]

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

--- Comment #3 from Iain Sandoe <iains at gcc dot gnu.org> ---
Created attachment 50197
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50197&action=edit
Patch being tested

this fixes the top-level check, but doesn't yet look at (a) operator co_await
(b) the await_* functions.

needs some tidying up.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Iain D Sandoe <iains@gcc.gnu.org>:

https://gcc.gnu.org/g:9ee91079fd5879cba046e452ab5593372166b2ab

commit r11-7528-g9ee91079fd5879cba046e452ab5593372166b2ab
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Mon Feb 15 17:11:31 2021 +0000

    coroutines : Do not accept throwing final await expressions [PR95616].

    From the PR:

    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

    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

    This implements the checks for these cases and rejects such code with
    a diagnostic if exceptions are enabled.

    gcc/cp/ChangeLog:

            PR c++/95616
            * coroutines.cc (coro_diagnose_throwing_fn): New helper.
            (coro_diagnose_throwing_final_aw_expr): New helper.
            (build_co_await): Diagnose throwing final await expression
            components.
            (build_init_or_final_await): Diagnose a throwing promise
            final_suspend() call.

    gcc/testsuite/ChangeLog:

            PR c++/95616
            * g++.dg/coroutines/pr95616-0-no-exceptions.C: New test.
            * g++.dg/coroutines/pr95616-0.C: New test.
            * g++.dg/coroutines/pr95616-1-no-exceptions.C: New test.
            * g++.dg/coroutines/pr95616-1.C: New test.
            * g++.dg/coroutines/pr95616-2.C: New test.
            * g++.dg/coroutines/pr95616-3-no-exceptions.C: New test.
            * g++.dg/coroutines/pr95616-3.C: New test.
            * g++.dg/coroutines/pr95616-4.C: New test.
            * g++.dg/coroutines/pr95616-5.C: New test.
            * g++.dg/coroutines/pr95616-6.C: New test.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Iain D Sandoe
<iains@gcc.gnu.org>:

https://gcc.gnu.org/g:00f703000fea2e8384f643413b53baa66ec4ddc2

commit r10-9513-g00f703000fea2e8384f643413b53baa66ec4ddc2
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Mon Feb 15 17:11:31 2021 +0000

    coroutines : Do not accept throwing final await expressions [PR95616].

    From the PR:

    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

    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

    This implements the checks for these cases and rejects such code with
    a diagnostic if exceptions are enabled.

            Backported from 9ee91079fd5879cba046e452ab5593372166b2ab and
            4e252e23d34932f13f39cc6544bf5c9379fa2a87

    gcc/cp/ChangeLog:

            PR c++/95616
            * coroutines.cc (coro_diagnose_throwing_fn): New helper.
            (coro_diagnose_throwing_final_aw_expr): New helper.
            (build_co_await): Diagnose throwing final await expression
            components. Look through NOP_EXPRs inbuild_special_member_call
            return value to find the CALL_EXPR. Simplify.
            (build_init_or_final_await): Diagnose a throwing promise
            final_suspend() call.

    gcc/testsuite/ChangeLog:

            PR c++/95616
            * g++.dg/coroutines/pr95616-0-no-exceptions.C: New test.
            * g++.dg/coroutines/pr95616-0.C: New test.
            * g++.dg/coroutines/pr95616-1-no-exceptions.C: New test.
            * g++.dg/coroutines/pr95616-1.C: New test.
            * g++.dg/coroutines/pr95616-2.C: New test.
            * g++.dg/coroutines/pr95616-3-no-exceptions.C: New test.
            * g++.dg/coroutines/pr95616-3.C: New test.
            * g++.dg/coroutines/pr95616-4.C: New test.
            * g++.dg/coroutines/pr95616-5.C: New test.
            * g++.dg/coroutines/pr95616-6.C: New test.

    Co-authored-by: Jakub Jelinek  <jakub@redhat.com>

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

[iains at gcc dot gnu.org]

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

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #6 from Iain Sandoe <iains at gcc dot gnu.org> ---
fixed for master and 10.3.