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
