[OB PATCH] Build two gdb.cp testcases with -Wno-unused-comparison

[Gary Benson]

Pedro Alves wrote:
> On 5/29/20 7:08 PM, Tom Tromey wrote:
> >>>>>> "Gary" == Gary Benson via Gdb-patches <gdb-patches@sourceware.org> writes:
> > 
> > Gary> +if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
> > Gary> +      {debug c++ additional_flags=-Wno-unused-comparison}] } {
> > 
> > Won't this cause build (and therefore test) failures if the compiler
> > does not accept this option?
> 
> I think that by design, while GCC errors out about unknown -Wfoo warnings,
> it ignores -Wno-foo, so that you can disable newer warnings without worrying
> about older compilers:
> 
>  $ gcc main.c -o main -g3 -O0 -Wfoo
>  gcc: error: unrecognized command line option ‘-Wfoo’
>  $ gcc main.c -o main -g3 -O0 -Wno-foo

Nice.

> However, Clang seems to output a warning in either case:
> 
>  $ clang main.c -o main -g3 -O0 -Wfoo
>  warning: unknown warning option '-Wfoo' [-Wunknown-warning-option]
>  1 warning generated.
>  $ clang main.c -o main -g3 -O0 -Wno-foo
>  warning: unknown warning option '-Wno-foo' [-Wunknown-warning-option]
>  1 warning generated.
> 
> (This was clang 5.0.2)
> 
> So that new option can break tests with older Clangs that don't
> know about the option.

Ugh.

clang 9.0.1 behaves the same too:

 $ clang -Werror -Wno-step-on-grass test.c
 error: unknown warning option '-Wno-step-on-grass' [-Werror,-Wunknown-warning-option]

> I guess to prevent this sort of thing going forward, we could make
> gcc_compile always add -Wno-unknown-warning-option to the build
> flags.  I don't know when was that option added to Clang, but maybe
> it's ancient enough.

Does this buy us much?  Any test with a warning I disable for clang is
a test that didn't compile with clang anyway.

Cheers,
Gary