Re: Deprecating arithmetic on std::atomic<void*>

[Jonathan Wakely <jwakely at redhat dot com>]

On 20/04/17 11:57 +0200, Florian Weimer wrote:
> On 04/20/2017 11:52 AM, Jonathan Wakely wrote:
> > On 20/04/17 11:43 +0200, Florian Weimer wrote:
> > > On 04/20/2017 11:25 AM, Jonathan Wakely wrote:
> > >
> > > > > I mean, with -pedantic-errors we already error on void * arighmetics
> > > > > or function pointer arithmetics.  If std::atomic<void*> would use
> > > > > the void * arithmetics, it would also reject it.  Or does it 
> > > > > use integer
> > > > > arithmetics instead?
> > > >
> > > > No, it does it on void*, but the __atomic built-ins still perform that
> > > > arithmetic even with -pedantic-errors.
> > >
> > > sizeof is not defined for the relevant types, either, and already 
> > > triggers a warning by default:
> > >
> > > t.cc:25:58: warning: invalid application of ‘sizeof’ to a void 
> > > type [-Wpointer-arith]
> > >  _M_type_size(ptrdiff_t __d) const { return __d * sizeof(_PTp); }
> >
> > That could be easily avoided if we care about supporting arithmetic on
> > std::atomic<void*>.
> >
> > > I think the actual problem here is the suppression of warnings 
> > > from system headers.
> >
> > The atomic built-ins still don't warn even with -Wsystem-headers
>
> Sorry, this is what I meant: -pedantic-errors does not catch this case 
> due to the system header warning suppression.  Otherwise, with the 
> current std::atomic implementation, -pedantic-errors would already 
> disable the extension on std::atomic alongside with the extension on 
> built-in pointer types, due to the use of sizeof in the definition of 
> _M_type_size.

Ah yes, you're right.

Yet another case where warning suppression in system headers hurts the
library's ability to give diagnostics. We can't warn about using
incomplete types in std::unique_ptr because the -Wdelete-incomplete
warning gets suppressed, and the interaction of #pragma GCC diagnostic
with -Wsystem-headers is ... interesting. We need a way to enable
warnings for specific blocks of code, to say "do not suppress warnings
here, even though it's in a system header".