[Bug libstdc++/63860] Ill-formed std::pair::swap implementation?

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63860

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 33965
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33965&action=edit
untested patch

I think this makes the code valid