Re: [PATCH] c++: Implement -Wdangling-reference [PR106393]

[Marek Polacek <polacek@redhat.com>]

On Tue, Oct 25, 2022 at 12:34:50PM +0100, Jonathan Wakely wrote:
> On Mon, 24 Oct 2022 at 18:30, Jason Merrill wrote:
> >
> > On 10/21/22 19:28, Marek Polacek wrote:
> > > When testing a previous version of the patch, there were many FAILs in
> > > libstdc++'s 22_locale/; all of them because the warning triggered on
> > >
> > >    const test_type& obj = std::use_facet<test_type>(std::locale());
> > >
> > > but this code looks valid -- std::use_facet doesn't return a reference
> > > to its parameter.  Therefore I added code to suppress the warning when
> > > the call is std::use_facet.  Now 22_locale/* pass even with the warning
> > > on.  We could exclude more std:: functions like this if desirable.
> >
> > Instead of adding special cases in the compiler, let's disable the
> > warning around the definition of use_facet (and adjust the compiler as
> > needed so that avoids the warning).
> 
> I assume you mean using #pragma here. If we disable it around the
> definition of use_facet, will that disable it for callers of
> use_facet, or only within the definition of use_facet itself?

Right, a #pragma will not help, it would only disable the warning
within the definition of use_facet itself.

Another way would be to use some kind of attribute on use_facet but
I didn't find any that would be relevant in this scenario (so I guess
we'd have to add one).

Marek