represents a day of the week in the civil calendar.
It normally holds values in the range 0
corresponding to Sunday through Saturday, but
it may hold non-negative values outside this range.
It can be constructed with any unsigned
which will be subsequently truncated to fit into weekday
's unspecified internal storage.
meets the Cpp17EqualityComparable
because there is no universal consensus on which day is the first day of the week.
's arithmetic operations treat the days of the week as a circular range,
with no beginning and no end.
— end note