[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

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
   Last reconfirmed|                            |2016-09-21
         Resolution|FIXED                       |---
     Ever confirmed|0                           |1

--- Comment #6 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Reopen for tracking the backport.