[RFC] Deprecate non-standard constructors in std::pair

[Ville Voutilainen]

On Wed, 7 Apr 2021 at 15:31, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
> I propose that we deprecate the constructors for C++11/14/17/20 in
> stage 1, and do not support them at all in C++23 mode once P1951 is
> supported. I have a patch which I'll send in stage 1 (it also uses
> C++20 concepts to simplify std::pair and fix PR 97930).
>
> After a period of deprecation we could remove them, and support P1951
> for -std=gnu++11/14/17/20 too so that {} continues to work.

The proposal sounds good to me.