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Restrict the units that can be added to PlainYearMonth #671

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2bdca83
Restrict the units that can be added to PlainYearMonth
Manishearth Jan 28, 2026
53f0834
order of operations
Manishearth Jan 29, 2026
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96 changes: 44 additions & 52 deletions src/builtins/core/plain_year_month.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -16,14 +16,11 @@ use crate::{
parsed_intermediates::ParsedDate,
parsers::{FormattableCalendar, FormattableDate, FormattableYearMonth},
provider::{NeverProvider, TimeZoneProvider},
temporal_assert,
unix_time::EpochNanoseconds,
Calendar, MonthCode, TemporalError, TemporalResult, TemporalUnwrap, TimeZone,
Calendar, MonthCode, TemporalError, TemporalResult, TimeZone,
};

use super::{
duration::normalized::InternalDurationRecord, DateDuration, Duration, PlainDate, PlainDateTime,
};
use super::{duration::normalized::InternalDurationRecord, Duration, PlainDate, PlainDateTime};
use writeable::Writeable;

/// A partial PlainYearMonth record
Expand Down Expand Up @@ -191,70 +188,51 @@ impl PlainYearMonth {
// NOTE: The following operation has been moved to the caller.
// MOVE: 2. If operation is subtract, set duration to CreateNegatedTemporalDuration(duration).

// 3. Let internalDuration be ToInternalDurationRecord(duration).
let internal_duration = duration.to_internal_duration_record();

// 4. Let durationToAdd be internalDuration.[[Date]].
let duration_to_add = internal_duration.date();

// NOTE: The following are engine specific:
// SKIP: 3. Let resolvedOptions be ? GetOptionsObject(options).
// SKIP: 4. Let overflow be ? GetTemporalOverflowOption(resolvedOptions).
// SKIP: 5. Let resolvedOptions be ? GetOptionsObject(options).
// SKIP: 6. Let overflow be ? GetTemporalOverflowOption(resolvedOptions).

// 5. Let sign be DurationSign(duration).
let sign = duration.sign();
// 7. If durationToAdd.[[Weeks]] ≠ 0, or durationToAdd.[[Days]] ≠ 0, or internalDuration.[[Time]] ≠ 0,
// throw a RangeError exception.

if duration_to_add.weeks != 0
|| duration_to_add.days != 0
|| internal_duration.normalized_time_duration().0 != 0
{
return Err(TemporalError::range()
.with_message("Can only add years or months to PlainYearMonth."));
}

// 6. Let calendar be yearMonth.[[Calendar]].
// 8. Let calendar be yearMonth.[[Calendar]].
let calendar = self.calendar();

// 7. Let fields be ISODateToFields(calendar, yearMonth.[[ISODate]], year-month).
// 9. Let fields be ISODateToFields(calendar, yearMonth.[[ISODate]], year-month).
let fields = CalendarFields::from(YearMonthCalendarFields::try_from_year_month(self)?);

// 8. Set fields.[[Day]] to 1.
// 10. Set fields.[[Day]] to 1.
let fields = fields.with_day(1);

// 9. Let intermediateDate be ? CalendarDateFromFields(calendar, fields, constrain).
let intermediate_date = calendar.date_from_fields(fields, overflow)?;

// 10. If sign < 0, then
let date = if sign.as_sign_multiplier() < 0 {
// a. Let oneMonthDuration be ! CreateDateDurationRecord(0, 1, 0, 0).
let one_month_duration = DateDuration::new_unchecked(0, 1, 0, 0);

// b. Let nextMonth be ? CalendarDateAdd(calendar, intermediateDate, oneMonthDuration, constrain).
let next_month = calendar.date_add(
&intermediate_date.iso,
&one_month_duration,
Overflow::Constrain,
)?;
let next_month = next_month.iso;

// c. Let date be BalanceISODate(nextMonth.[[Year]], nextMonth.[[Month]], nextMonth.[[Day]] - 1).
let date = IsoDate::balance(
next_month.year,
i32::from(next_month.month),
i32::from(next_month.day).checked_sub(1).temporal_unwrap()?,
);

// d. Assert: ISODateWithinLimits(date) is true.
temporal_assert!(date.is_valid());

date
} else {
// 11. Else,
// a. Let date be intermediateDate.
intermediate_date.iso
};

// 12. Let durationToAdd be ToDateDurationRecordWithoutTime(duration).
let duration_to_add = duration.to_date_duration_record_without_time()?;
// 11. Let date be ? CalendarDateFromFields(calendar, fields, constrain).
let date = calendar.date_from_fields(fields, Overflow::Constrain)?;

// 13. Let addedDate be ? CalendarDateAdd(calendar, date, durationToAdd, overflow).
let added_date = calendar.date_add(&date, &duration_to_add, overflow)?;
// 12. Let addedDate be ? CalendarDateAdd(calendar, date, durationToAdd, overflow).
let added_date = calendar.date_add(&date.iso, &duration_to_add, overflow)?;

// 14. Let addedDateFields be ISODateToFields(calendar, addedDate, year-month).
// 13. Let addedDateFields be ISODateToFields(calendar, addedDate, year-month).
let added_date_fields = YearMonthCalendarFields::new()
.with_month_code(added_date.month_code())
.with_year(added_date.year());

// 15. Let isoDate be ? CalendarYearMonthFromFields(calendar, addedDateFields, overflow).
// 14. Let isoDate be ? CalendarYearMonthFromFields(calendar, addedDateFields, overflow).
let iso_date = calendar.year_month_from_fields(added_date_fields, overflow)?;

// 16. Return ! CreateTemporalYearMonth(isoDate, calendar).
// 15. Return ! CreateTemporalYearMonth(isoDate, calendar).
Ok(iso_date)
}

Expand Down Expand Up @@ -1172,4 +1150,18 @@ mod tests {
"2001-12-15[u-ca=chinese]"
);
}

#[test]
/// Should be able to subtract years from a leap month
///
/// Regression test for bugs fixed by <https://github.com/tc39/proposal-temporal/pull/3253/>
fn test_subtract_years_from_leap_month() {
let dangi_m03l_2012 = PlainYearMonth::from_str("2012-04-21[u-ca=dangi]").unwrap();
let minus_19y = Duration::from_str("-P19Y").unwrap();
let prev = dangi_m03l_2012.add(&minus_19y, Overflow::Reject).unwrap();
assert_eq!(
prev.to_ixdtf_string(Default::default()),
"1993-04-22[u-ca=dangi]"
);
}
}