Re: [PATCH] libstdc++: Fix backward compatibility of P2325R3 backport [PR106320]

[Jonathan Wakely <jwakely@redhat.com>]

On Fri, 22 Jul 2022 at 16:52, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> The 11 and 10 partial backports of P2325R3, r11-9555-g92d612cccc1eec and
> r10-10808-g22b86cdc4d7fdd, unnecessarily preserved some changes from the
> paper that made certain views no longer default constructible, changes
> which aren't required to reap the overall benefits of the paper and
> which conflicted with the goal to maximize backwards compatibility with
> pre-P2325R3 code.
>
> This patch reverts the problematic changes, specifically it relaxes
> the constraints on various views' default constructors so that they
> reflect only the requirements that were already implicitly imposed by
> the NSDMIs of the view.  Thus for example this patch retains the
> default_initializable<_Vp> constraint on transform_view's default
> constructor since its '_Vp _M_base = _Vp()' NSDMI already reflects this
> constraint, and it removes the default_initializable<_Fp> constraint
> since the corresponding member '__detail::__box<_Fp> _M_fun' doesn't
> require default initializability (specializations of __box are always
> default constructible).
>
> After reverting these changes, all static_asserts from p2325.cc that
> verify lack of default constructibility now fail as expected, matching
> the pre-P2325R3 behavior.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for the 11 and 10
> branches?

OK for gcc-10 and gcc-11 (with the completed ChangeLog), thanks.