Separate warning/error thresholds for -Wfoo=<n>

[Alexander Monakov <amonakov@ispras.ru>]

Greetings, David, community,

I'd like to get your input on how GCC command line interface should support
making a "tiered" warning like -Warray-bounds={1,2} an error for "tier 1"
where fewer false positives are expected, and a plain warning otherwise.

There was a recent thread mentioning the current limitation [1]:

> This also shows nicely why I don't like warnings with levels, what if I want
> -Werror=array-bounds=2 + -Warray-bounds=1?

Also in PR 48088 [2] there was a request to make it work for stack size usage:

> Stumbled on this bug today. I tried to use it in more intricate way:
> 
>     -Wframe-larger-than=4096 -Werror=frame-larger-than=32768
> 
> which would only warn about any stack more than 4096+, but would fail on
> 32768+.
> 
> Does it make sense to implement desired behaviour?
> I guess it's not many '>=number' style options in gcc.

A problem with implementing DWIM semantics like above for -Wfoo=k -Werror=foo=n
combination is that technically it changes its current meaning.

If we don't want to risk that, an alternative is to introduce a new option
for selecting error threshold for a tiered warning, for example:

  -Warray-bounds=2 -Werror-level=array-bounds=1

Implementation-wise, we would need to extend common.opt language to annotate
which tier is more inclusive (generally smaller 'n' means fewer warnings, but
for -Wstack-usage and -Wstrict-aliasing it's the other way around).

Opinions? Does anybody envision problems with going the DWIM way?

Thanks.
Alexander

[1] https://inbox.sourceware.org/gcc-patches/2552ab22-916f-d0fe-2c78-d482f6ad8412@lauterbach.com/
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48088#c5