[Bug libstdc++/77537] pair constructors do not properly SFINAE

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77537

--- Comment #5 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
There's a fairly decent chance, sure. :) I'll cook up a patch.