Restrict the units that can be added to PlainYearMonth (#671)

This implements the spec text from
tc39/proposal-temporal#3253

That change hasn't landed yet, but there are test262 tests relying on
this (tc39/test262#4883)

Author: Manishearth

src/builtins/core/plain_year_month.rs

@@ -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
@@ -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)
     }
 
@@ -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]"
+        );
+    }
 }