template< class Rep1, class Period1, class Rep2, class Period2 >
typename std::common_type<duration<Rep1,Period1>, duration<Rep2,Period2>>::type
constexpr operator+( const duration<Rep1,Period1>& lhs,
const duration<Rep2,Period2>& rhs ); (1) template< class Rep1, class Period1, class Rep2, class Period2 >
typename std::common_type<duration<Rep1,Period1>, duration<Rep2,Period2>>::type
cons