[Bug c/86134] earlier diagnostic causes followup diagnostic about unknown -Wno-* options

["manu at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86134

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |manu at gcc dot gnu.org

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #4)
> fails the compile but with the warning not emitted the error isn't
> emitted.  Also we do know the warning is guarded by -Wreturn-type
> so I see no point in warning that it wasn't silenced by
> -Wno-unknown-warning-option?

I see what you mean: The warning that could have been silenced did not prevent
compilation but the warning about the unknown option did. However, 

1) Given -Wno-foo, there is no way to know if you actually meant -Wno-foooo or,
-Wfoo is an alias for -Wreturn-type in some other version of GCC. Maybe you
really meant to write -Wno-return-type but you mistyped it as -Wno-foo. The
current behavior means  "There is at least one diagnostic that could have been
silenced by -Wno-foo, but this GCC does not recognize -Wfoo, maybe you gave the
wrong option?" 

2) The warning behaves like any other warning: It gets converted to an error
with -Werror. Imagine an example where GCC gives both a warning [-Wfooo] and a
warning converted to an error [-Werror=foooo], what should we report for
-Wno-fooooo ? 

We can alleviate (2) by giving it a name (clang uses -Wunknown-warning-option)
so that you can do:

gcc -S -x c  -Wall -Werror -Wno-error=return-type
-Wno-error=unknown-warning-option -Wno-foo


Another example:

gcc -S -x c -Wall -Werror -Wno-misleading-identation -Wno-future

If there is no warning emitted, there is no harm in the above -Wno- options.

If there is a warning emitted, it will get converted to an error, and GCC does
not know if you actually meant -Wno-misleading-indentation. And maybe you
really want to use -Wno-future for a future -Wfuture enabled by -Wall, and you
don't care but you want the same command-line to work on previous GCC versions
so giving an extra warning (or error)  or may be you really meant  the warning
used to be called -Wmisleading-identation, but it got renamed.

The long history can be found here: https://gcc.gnu.org/wiki/FAQ#wnowarning