namespace std::chrono {
class weekday_last {
chrono::weekday wd_; // exposition only
public:
explicit constexpr weekday_last(const chrono::weekday& wd) noexcept;
constexpr chrono::weekday weekday() const noexcept;
constexpr bool ok() const noexcept;
};
}
constexpr auto wdl = Sunday[last]; // wdl is the last Sunday of an as yet unspecified month
static_assert(wdl.weekday() == Sunday);
— end example