[Bug c++/93805] [8/9/10 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[Bug c++/93805] [8/9/10 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

Patrick Palka <ppalka at gcc dot gnu.org> changed:

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

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Looking into it.

[Bug c++/93805] [8/9/10 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Reduced test case:

$ cat 93805.C
struct B
{
  B() {}
};

struct C
{
  B b = B();
};

C c;
$ g++ -Wnoexcept 93805.C
93805.C:11:3: warning: noexcept-expression evaluates to ‘false’ because of a
call to ‘B::B()’ [-Wnoexcept]
   11 | C c;                                   
      |   ^                                    
93805.C:3:3: note: but ‘B::B()’ does not throw; perhaps it should be declared
‘noexcept’
    3 |   B() {}                               
      |   ^                                    



The commit in question adds a complain parameter to get_defaulted_eh_spec and
passes it down to synthesized_method_walk, and we eventually call
expr_noexcept_p in walk_field_subops with complain=tf_warning_or_error instead
of with complain=tf_none originally, which is the immediate cause of the
warning.

We're calling expr_noexcept_p from there to determine whether the NSDMI of C::b
is noexcept (so that we can determine the exception specification of C's
synthesized default constructor), and we're warning because the NSDMI of C::b
is not noexcept, but it could be if the non-throwing ctor B::B were marked
noexcept.

ISTM we can either disable the -Wnoexcept warning here, or we can keep the
warning but improve the warning message in this case.

[Bug c++/93805] [8/9/10 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:1f6c1c82eb5001a844b5ac535f6aba4a3257031f

commit r10-7345-g1f6c1c82eb5001a844b5ac535f6aba4a3257031f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Mar 13 14:30:39 2020 -0400

    c++: Avoid a suspicious -Wnoexcept warning [PR93805]

    In this PR we're emitting -Wnoexcept warnings about potentially-throwing
NSDMIs
    when computing the noexcept specification of a class's defaulted default
    constructor.  Although these warnings are in some sense valid, this patch
takes
    the route of suppressing them, because:

      1. the warning message is confusing in its current form;
      2. warning for 'struct C { B b = B(); };' but not for 'struct C { B b;
};'
         is inconsistent; and
      3. emitting a warning here arguably doesn't fall under the umbrella of
         -Wnoexcept, whose documentation says it warns only when a
         noexcept-expression evaluates to false, but there are no
         noexcept-expressions here.

    gcc/cp/ChangeLog:

            PR c++/93805
            * except.c (maybe_noexcept_warning): Add TODO comment.
            * method.c (walk_field_subobs): Pass tf_none to expr_noexcept_p.

    gcc/testsuite/ChangeLog:

            PR c++/93805
            * g++.dg/warn/Wnoexcept2.C: New test.

[Bug c++/93805] [8/9 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[8/9/10 Regression] A       |[8/9 Regression] A
                   |suspicious -Werror=noexcept |suspicious -Werror=noexcept
                   |warning since               |warning since
                   |r8-2461-g9fb82e652cee118b   |r8-2461-g9fb82e652cee118b

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed on trunk.

[Bug c++/93805] [9 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|8.5                         |9.4

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 8 branch is being closed.

[Bug c++/93805] [9 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |9.5

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9.4 is being released, retargeting bugs to GCC 9.5.

[Bug c++/93805] [9 Regression] A suspicious -Werror=noexcept warning since r8-2461-g9fb82e652cee118b

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93805

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|9.5                         |10.0

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed for GCC 10.