I read on the date_time change history about a new function for calculating ISO 8601 week number.
I should note that this week number is rather useless without the corresponding year number. ISO 8601 week-based year is not always the same as the actual year. For example, 2nd January 1999 is week 53 of year 1998 (not 1999!). 30th December 1997 is week 01 of year 1998 (not 1997!). (examples taken from the C99 standard)
My experience is that people often forget about this important fact and thus introduce bugs that may be not be catched until such situation really happens in life. To force library users consider this thing, I think that gregorian::date::week_number should return a pair: week number and week-based year number.
Another option would be to just add a separate function for calculating week-based year number, however in my opinion it is less desirable because of reasons mentined above. In any case, providing week number without providing week-based year number is undesirable.
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