Normative: Disallow adding weeks, days, and time units to PlainYearMonth

docs/plainyearmonth.md

@@ -336,9 +336,7 @@ This method adds `duration` to `yearMonth`, returning a month that is in the fut
 
 The `duration` argument is an object with properties denoting a duration, such as `{ months: 5 }`, or a string such as `P5M`, or a `Temporal.Duration` object.
 If `duration` is not a `Temporal.Duration` object, then it will be converted to one as if it were passed to `Temporal.Duration.from()`.
-
-If `duration` has any units smaller than `months`, they will be treated as if they are being added to the first moment of the month given by `yearMonth`.
-Effectively, this means that adding things like `{ days: 1 }` will be ignored.
+There must not be any units smaller than `months` in `duration`.
 
 If the result is earlier or later than the range of dates that `Temporal.PlainYearMonth` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), then this method will throw a `RangeError` regardless of `overflow`.
 
@@ -372,9 +370,7 @@ This method subtracts `duration` from `yearMonth`, returning a month that is in
 
 The `duration` argument is an object with properties denoting a duration, such as `{ months: 5 }`, or a string such as `P5M`, or a `Temporal.Duration` object.
 If `duration` is not a `Temporal.Duration` object, then it will be converted to one as if it were passed to `Temporal.Duration.from()`.
-
-If `duration` has any units smaller than `months`, they will be treated as if they are being subtracted from the last moment of the month given by `yearMonth`.
-Effectively, this means that subtracting things like `{ days: 1 }` will be ignored.
+There must not be any units smaller than `months` in `duration`.
 
 If the result is earlier or later than the range of dates that `Temporal.PlainYearMonth` can represent (approximately half a million years centered on the [Unix epoch](https://en.wikipedia.org/wiki/Unix_time)), then this method will throw a `RangeError` regardless of `overflow`.
 

polyfill/lib/ecmascript.mjs

@@ -4128,21 +4128,21 @@ export function AddDurationToTime(operation, temporalTime, durationLike) {
 export function AddDurationToYearMonth(operation, yearMonth, durationLike, options) {
   let duration = ToTemporalDuration(durationLike);
   if (operation === 'subtract') duration = CreateNegatedTemporalDuration(duration);
+  const internalDuration = ToInternalDurationRecord(duration);
+
   const resolvedOptions = GetOptionsObject(options);
   const overflow = GetTemporalOverflowOption(resolvedOptions);
-  const sign = DurationSign(duration);
+
+  const durationToAdd = internalDuration.date;
+  if (durationToAdd.weeks !== 0 || durationToAdd.days !== 0 || !internalDuration.time.isZero()) {
+    throw new RangeErrorCtor('only years and months can be added to Temporal.PlainYearMonth');
+  }
 
   const calendar = GetSlot(yearMonth, CALENDAR);
   const fields = ISODateToFields(calendar, GetSlot(yearMonth, ISO_DATE), 'year-month');
   fields.day = 1;
-  let startDate = CalendarDateFromFields(calendar, fields, 'constrain');
-  if (sign < 0) {
-    const nextMonth = CalendarDateAdd(calendar, startDate, { months: 1 }, 'constrain');
-    startDate = AddDaysToISODate(nextMonth, -1);
-  }
-  const durationToAdd = ToDateDurationRecordWithoutTime(duration);
-  RejectDateRange(startDate);
-  const addedDate = CalendarDateAdd(calendar, startDate, durationToAdd, overflow);
+  const date = CalendarDateFromFields(calendar, fields, 'constrain');
+  const addedDate = CalendarDateAdd(calendar, date, durationToAdd, overflow);
   const addedDateFields = ISODateToFields(calendar, addedDate, 'year-month');
 
   const isoDate = CalendarYearMonthFromFields(calendar, addedDateFields, overflow);

polyfill/test/thorough/yearmonthaddition.mjs

@@ -8,7 +8,11 @@ import {
 } from './support.mjs';
 
 const interestingYearMonths = makeYearMonthCases();
-const interestingDurations = makeDurationCases();
+const interestingDurations = makeDurationCases().filter(
+  // We don't need to test over and over that units lower than months are not
+  // allowed, but years/months mixed with lower units are still interesting
+  ([duration]) => duration.blank || duration.years !== 0 || duration.months !== 0
+);
 const total = interestingYearMonths.length * interestingDurations.length;
 
 await time(async (start) => {