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tomaszkam / date

A date and time library based on the C++11/14/17 <chrono> header
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[LWG3231] year_month_day_last::day specification does not cover !ok() values #6

Closed tomaszkam closed 5 years ago

tomaszkam commented 5 years ago

Original comment:

Fix year_month_day_last::day() when !ok().

E-mail conversation: [isocpp-lib] year_month_day_last::day when it stores an invalid year or month

tomaszkam commented 5 years ago

Given other functions I think that we expect to return unspecified values, as it is done for other classes, like year_month_day or year_month_weekday. Also, I think we need to appy similiar change sys_days conversion.

Example spec: http://eel.is/c++draft/time.cal.ymwdlast.members#15

tomaszkam commented 5 years ago

Current implementation produced undefined result for !month().ok(), as it indexes table out of range. See: https://github.com/HowardHinnant/date/blob/master/include/date/date.h#L2514-L2525

tomaszkam commented 5 years ago

Created pull request that makes behavior unspecified: https://github.com/HowardHinnant/date/pull/456

tomaszkam commented 5 years ago

Discussion: The current specification of year_month_day_last::day method does not cover the behaviour in situation when year_month_day_last value is not ok(). To illustrate the sentence from http://eel.is/c++draft/time.cal#ymdlast.members-13:

A day representing the last day of the (year, month) pair represented by *this.

is unclear in situation when month member has !ok value, e.g. what is last day of 14th month of 2019.

Proposed resolution makes the value of ymdl.day() (and by consequence conversion to sys_days/local_days) unspecifed if ymdl.ok is false. This make is consistent with rest of the library, that produces unspecified values in similiar situation, e.g.: http://eel.is/c++draft/time.cal#ymd.members-18, http://eel.is/c++draft/time.cal#ymwd.members-19, http://eel.is/c++draft/time.cal#ymwdlast.members-14.

Proposed wording: Apply the following changes to the specification of year_month_day_last::day method in [time.cal.ymdlast.members]:

constexpr chrono::day day() const noexcept;

Returns: If ok() == true, returns aA day representing the last day of the (year, month) pair represented by *this. Otherwise the returned value is unspecified. [ Note: This value may be computed on demand. — end note  ]