[Bug c++/94745] New: No error emitted for unknown -Wno-meow argument

[Bug c++/94745] New: No error emitted for unknown -Wno-meow argument

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94745

            Bug ID: 94745
           Summary: No error emitted for unknown -Wno-meow argument
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldionne.2 at gmail dot com
  Target Milestone: ---

GCC does not diagnose unsupported -Wno-meow command line flags unless another
error happens during compilation.

For example, the following compiles just fine even though -Wno-foo doesn't
exist:

    $ echo | g++ -xc++ - -Werror -fsyntax-only -Wno-foo

    <nothing>

If, however, another error happens during compilation, the unsupported
command-line flag is reported:

    $ echo '#error' | g++ -xc++ - -Werror -fsyntax-only -Wno-foo

    <stdin>:1:2: error: #error
    cc1plus: error: unrecognized command line option '-Wno-foo' [-Werror]
    cc1plus: all warnings being treated as errors


This makes detecting whether GCC supports a given warning flag very challenging
in build systems. This issue popped up while trying to support GCC when running
the libc++ Standard Library conformance test suite.

[Bug c++/94745] No error emitted for unknown -Wno-meow argument

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94745

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is by design. It means that makefiles written for different versions of
GCC do not trigger spurious diagnostics.

If somebody says "I don't want warnings about cat noises" and their version of
GCC doesn't support -Wmeow then their request is granted. Warning about it
would cause valid compilations to fail.

If other diagnostics are being emitted anyway, then adding one to point out the
unrecognized option isn't going to make a build go from passing to failing.

[Bug c++/94745] No error emitted for unknown -Wno-meow argument

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94745

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Louis Dionne from comment #0)
> This makes detecting whether GCC supports a given warning flag very
> challenging in build systems.

You're doing it wrong then.

Try 'g++ -Q --help=warnings' instead of probing individual options.

[Bug c++/94745] No error emitted for unknown -Wno-meow argument

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94745

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
If you want to detect whether a warning is supported use -Wfoo, not -Wno-foo.

[Bug c++/94745] No error emitted for unknown -Wno-meow argument

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94745

--- Comment #4 from Louis Dionne <ldionne.2 at gmail dot com> ---
Thanks for your replies, all. We resolved the problem on our side by not trying
to workaround the lack of error, which means that we might end up passing
`-Wno-foo` to GCC when it's not supported. I think that follows the design
intent explained by Jonathan.

Thanks again!