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

[Jonathan Wakely <jwakely at redhat dot com>]

On 20/04/17 11:12 +0100, Jonathan Wakely wrote:
> On 20/04/17 12:07 +0200, Jakub Jelinek wrote:
> > On Thu, Apr 20, 2017 at 11:03:38AM +0100, Jonathan Wakely wrote:
> > > 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".
> >
> > Can #pragma GCC warning "-Wno-system-headers" do that?
>
> I don't think so. It didn't work last time I tried to use that to fix
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58876
>
> I don't remember the details, but I think there was some weirdness
> like the "pop" didn't restore the old value, or some other class of
> warning disappeaed.


I might be misremembering. I thought I'd found a problem with that
approach, but I can't find any evidence of what the problem was. I
have an old draft email from October 2013 with the text below. Maybe I
just never committed it due to moving jobs at the time.

===
I'm inclined to commit this patch to locally enable -Wsystem-headers
around default_delete and the _Sp_counter_ptr manager used by
shared_ptr.

This would ensure we don't suppress "deleting object of abstract type
without virtual destructor will cause undefined behaviour" errors when
giving ownership of such a pointer to shared_ptr or unique_ptr.

The downside is that users have to use -Wno-delete-non-virtual-dtor to
suppress the warning again if they really don't want it, but I think
that's good. This is a real error in user code, the only reason we
don't already warn is that the error happens in a template defined in
a system header.

We could similar thing in other places too, e.g. disable
-Wpointer-arith around __atomic_base which relies on this extension
for __atomic_base<cv void*>
http://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html
===