Re: [PATCH v2] c, c++: -Wswitch warning on [[maybe_unused]] enumerator [PR105497]

[Marek Polacek <polacek@redhat.com>]

On Tue, May 10, 2022 at 08:58:46AM -0400, Jason Merrill wrote:
> On 5/7/22 18:26, Marek Polacek wrote:
> > Corrected version that avoids an uninitialized warning:
> > 
> > This PR complains that we emit the "enumeration value not handled in
> > switch" warning even though the enumerator was marked with the
> > [[maybe_unused]] attribute.
> > 
> > The first snag was that I couldn't just check TREE_USED, because
> > the enumerator could have been used earlier in the function, which
> > doesn't play well with the c_do_switch_warnings warning.  Instead,
> > I had to check the attributes on the CONST_DECL directly, which led
> > to the second, and worse, snag: in C we don't have direct access to
> > the CONST_DECL for the enumerator.
> 
> I wonder if you want to change that instead of working around it?

I wouldn't mind looking into that; I've hit this discrepancy numerous
times throughout the years and it'd be good to unify it so that the
c-common code doesn't need to hack around it.
 
Let's see how far I'll get...

> > +      const bool unused_p = (lookup_attribute ("unused", attrs)
> > +			     || lookup_attribute ("maybe_unused", attrs));
> 
> Why is this calculation...
> 
> >         node = splay_tree_lookup (cases, (splay_tree_key) value);
> >         if (node)
> >   	{
> > @@ -1769,6 +1784,10 @@ c_do_switch_warnings (splay_tree cases, location_t switch_location,
> >         /* We've now determined that this enumerated literal isn't
> >   	 handled by the case labels of the switch statement.  */
> > +      /* Don't warn if the enumerator was marked as unused.  */
> > +      if (unused_p)
> > +	continue;
> 
> ...separate from this test?

Ah, that must be a remnant from a previous version of the patch.  No reason
for the separation anymore.

Thanks,
Marek