[Bug c++/108974] New: std::barrier except completion function which is not manifestly noexcept

[Bug c++/108974] New: std::barrier except completion function which is not manifestly noexcept

[oliver.rosten at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

            Bug ID: 108974
           Summary: std::barrier except completion function which is not
                    manifestly noexcept
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: oliver.rosten at googlemail dot com
  Target Milestone: ---

std::barrier<std::function<void()>> b{1, [](){}};

is accepted by gcc (and clang).

However, it fails on MSVC - as I think it should - since std::function's call
operator isn't manifestly noexcept.

https://godbolt.org/z/7vfoa1qhn

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 54558
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54558&action=edit
Full testcase

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-02-28
                 CC|                            |rodgertq at gcc dot gnu.org

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
"is_nothrow_invocable_v<CompletionFunction&> shall be true."

Failing to meet that requirement is undefined, so we're not required to reject
it as ill-formed.

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #3 from Jiang An <de34 at live dot cn> ---
> is_nothrow_invocable_v<CompletionFunction&> shall be true.

If implementation divergence is not intendedly permitted, I don't think it
makes much sense to introduce UB in this way.

I guess we should either turn it into a mandating requirement:
> Instantiation of barrier<CompletionFunction> is ill-formed
> if is_nothrow_invocable_v<CompletionFunction&> is not true.

Or relax the preconditions:
> If any invocation to the completion function throws an exception,
> the behavior is undefined.

I've mailed to LWG Chair for this...

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[rodgertq at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

--- Comment #4 from Thomas Rodgers <rodgertq at gcc dot gnu.org> ---
(In reply to Jiang An from comment #3)
> > is_nothrow_invocable_v<CompletionFunction&> shall be true.
> 
> If implementation divergence is not intendedly permitted, I don't think it
> makes much sense to introduce UB in this way.
> 
> I guess we should either turn it into a mandating requirement:
> > Instantiation of barrier<CompletionFunction> is ill-formed
> > if is_nothrow_invocable_v<CompletionFunction&> is not true.
> 
> Or relax the preconditions:
> > If any invocation to the completion function throws an exception,
> > the behavior is undefined.
> 
> I've mailed to LWG Chair for this...

LWG Chair has already weighed in on this thread.

libstdc++ and libc++ have essentially the same implementation of std::barrier
(libstdc++'s implementation is derived from libc++'s). So that they behave
consistently and MSVC is different is not surprising.

I don't have a strong opinion on whether this should be UB ore mandated.
Raising an LWG issue seems reasonable.

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[rodgertq at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

Thomas Rodgers <rodgertq at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rodgertq at gcc dot gnu.org

[Bug libstdc++/108974] std::barrier except completion function which is not manifestly noexcept

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108974

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jiang An from comment #3)
> I've mailed to LWG Chair for this...

Thanks, the new issue will be opened soon.