[Bug libstdc++/89194] [9 regression] build failure with libstdc++ with std=c++2a

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89194

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-02-04
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's the problem I mentioned in PR 87603 comment 5. I need to change the
definition of is_nothrow_convertible to workaround it.