The defaulted copy constructor of duration shall be a
constexpr function if and only if the required initialization
of the member rep_ for copy and move, respectively, would
be constexpr-suitable ([dcl.constexpr]).
[Example 1: duration<long, ratio<60>> d0; // holds a count of minutes using a long
duration<longlong, milli> d1; // holds a count of milliseconds using a long long
duration<double, ratio<1, 30>> d2; // holds a count with a tick period of 130 of a second// (30 Hz) using a double — end example]