Normative: Treat daysInMonth as count of all days rather than number of the last day

Updates PlainYearMonth arithmetic accordingly and clarifies language
around non-ISO calendar operations.

Fixes #1315

Co-authored-by: Guillaume Emont <[email protected]>

UPSTREAM_COMMIT=3aa97fa6fec9bb83272e7f62c497d163f83255ae

Author: cjtenny

lib/ecmascript.ts

@@ -5664,16 +5664,27 @@ export function AddDurationToOrSubtractDurationFromPlainYearMonth(
   const calendar = GetSlot(yearMonth, CALENDAR);
   const fieldNames = CalendarFields(calendar, ['monthCode', 'year'] as const);
   const fields = PrepareTemporalFields(yearMonth, fieldNames, []);
-  const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
-  fields.day = sign < 0 ? CalendarDaysInMonth(calendar, yearMonth) : 1;
+  const fieldsCopy = ObjectCreate(null);
+  CopyDataProperties(fieldsCopy, fields, []);
+  fields.day = 1;
   // PrepareTemporalFields returns a type where 'day' is potentially undefined,
-  // and TS doesn't narrow the type as a result of the assignment above, so we
-  // cast the fields input to the new type.
-  const startDate = CalendarDateFromFields(calendar, fields as typeof fields & { day: number });
+  // but TS doesn't narrow the type as a result of the assignment above.
+  uncheckedAssertNarrowedType<typeof fields & { day: number }>(fields, '`day` is guaranteed to be non-undefined');
+  let startDate = CalendarDateFromFields(calendar, fields);
+  const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
+  const dateAdd = GetMethod(calendar, 'dateAdd');
   const Duration = GetIntrinsic('%Temporal.Duration%');
+  if (sign < 0) {
+    const oneMonthDuration = new Duration(0, 1, 0, 0, 0, 0, 0, 0, 0, 0);
+    const nextMonth = CalendarDateAdd(calendar, startDate, oneMonthDuration, undefined, dateAdd);
+    const minusDayDuration = new Duration(0, 0, 0, -1, 0, 0, 0, 0, 0, 0);
+    const endOfMonth = CalendarDateAdd(calendar, nextMonth, minusDayDuration, undefined, dateAdd);
+    fieldsCopy.day = CalendarDay(calendar, endOfMonth);
+    startDate = CalendarDateFromFields(calendar, fieldsCopy);
+  }
   const durationToAdd = new Duration(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
   const optionsCopy = CopyOptions(options);
-  const addedDate = CalendarDateAdd(calendar, startDate, durationToAdd, options);
+  const addedDate = CalendarDateAdd(calendar, startDate, durationToAdd, options, dateAdd);
   const addedDateFields = PrepareTemporalFields(addedDate, fieldNames, []);
 
   return CalendarYearMonthFromFields(calendar, addedDateFields, optionsCopy);