[Bug libstdc++/112942] [LWG2766] swap(variant&, variant&) is incorrectly marked as deleted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112942

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
            Summary|swap(variant&, variant&) is |[LWG2766] swap(variant&,
                   |incorrectly marked as       |variant&) is incorrectly
                   |deleted                     |marked as deleted
   Last reconfirmed|                            |2023-12-10
             Status|UNCONFIRMED                 |SUSPENDED

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> https://wg21.cmeerw.net/lwg/issue2749

LWG 2749 added the current constraint to [variant.specalg]. The libstdc++
behaviour is because we implement the proposed resolution of LWG 2766:
https://cplusplus.github.io/LWG/issue2766

That issue points out that swapping variant<A> when is_swappable_v<A> is false
makes no sense. The author of A explicitly disabled swapping of A. Falling back
to the generic std::swap that uses moves seems wrong. If variant's own swap
overload is disabled, then swapping should be disabled.

But LWG 2766 seems to have languished for years. I'll poke LWG about it.