[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.