[Bug libstdc++/81468] is_constructible gives the wrong answer for time_point construction

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81468

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
It seems that the implementation simply forgot to constrain overload
resolution, since this is the complete definition of the affected constructor:

template<typename _Dur2>
  constexpr time_point(const time_point<clock, _Dur2>& __t)
  : __d(__t.time_since_epoch())
  { }

The constraints were added by LWG 1177,

http://cplusplus.github.io/LWG/lwg-defects.html#1177