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

[Ville Voutilainen <ville dot voutilainen at gmail dot com>]

On 19 April 2017 at 20:07, Jonathan Wakely <jwakely@redhat.com> wrote:
> I know it's a bit late, but I'd like to propose deprecating the
> libstdc++ extension that allows arithmetic on std::atomic<void*>.
> Currently we make it behave like arithmetic on void*, which is also a
> GNU extension (https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html).
> We also allow arithmetic on types such as std::atomic<void(*)()> which
> is probably not useful (PR 69769).
>
> C++11 and C++14 were silent on the matter, but C++17 makes it
> explicitly ill-formed (see https://wg21.link/p0558r1 page 2). I'm
> implementing that for C++17 mode but would like it to be ill-formed
> for all modes in a future release.
>
> So I'd like to apply this patch to the gcc-7/changes.html page:
>
> --- htdocs/gcc-7/changes.html   17 Apr 2017 22:12:35 -0000      1.77
> +++ htdocs/gcc-7/changes.html   19 Apr 2017 17:05:27 -0000
> @@ -38,6 +38,10 @@
>   </li>
>
>   <li>The Cilk+ extensions to the C and C++ languages have been
> deprecated.</li>
> +
> +  <li>The extension allowing arithmetic on
> <code>std::atomic&lt;void*&gt;</code>
> +      and types like <code>std::atomic&lt;R(*)()&gt;</code>
> +      has been deprecated.</li>
> </ul>
>
> <!-- .................................................................. -->
>
>
> Does this seem reasonable, or do people think we need to keep this
> feature, to match the core language extensions?
>

I think it's reasonable. If users report massive concerns, we can
certainly un-deprecate it.
At the moment all that deprecation is doing for non-C++17 modes is
giving users a fair warning
of things to be aware of, and we can get feedback on that.