%Y-%W-%w parsing for 2019

[Bidek56]

let date = NaiveDate::parse_from_str("2020-01-0", "%Y-%W-%w")?;
Returns: 2020-01-12
But:
let date = NaiveDate::parse_from_str("2019-01-0", "%Y-%W-%w")?;
Returns: Error: ParseError(Impossible)

Any reasons? Thx

[djc]

Okay, there seems to be a bug in the logic for verify_ordinal that you've hit. My guess is the logic for the week_from_sun verification was copied to week_from_mon, but the latter version is incorrect in this case (potentially only for the particular case of years starting on a Tuesday?) with an off-by-one error.

I can't quite work out the correct logic right now, please submit a PR if you can (and add the failing case as a test case).

[esheppa]

I think the fix is the same as from https://github.com/chronotope/chrono/pull/921/files - I'll push a commit to that PR

[FObersteiner]

a related question: why is %w needed here? The docs don't say so explicitly and for example %Y-%m also works without needing a day, it just defaults to the first day of the month.

I could make a separate issue if this is too much off-topic here ;-)

[djc]

%W identifies a week, but not a date, so you can't parse a date out of it.

[FObersteiner]

@djc that was my first thought as well. But if you take %m for example, similarly that identifies a month and not a date. Nevertheless, you can parse 2019-01 with %Y-%m to a full date. I don't see why this shouldn't work technically with %W. Just default to the first day of the week. Or, if there is a clear reason why not to do so, be more explicit in the docs.

[djc]

I guess if someone wants to contribute code to allow leaving off %w I'd be open to merging that.

[pitdicker]

Fixed in #966.

Allowing default/missing values when parsing is a duplicate of #528 and a couple of other issues.