Polyfill: Implement IsValidMonthCodeForCalendar as in spec

This makes the reference implementation slightly more like the spec
text, which should help make sure the spec text is correct.

The derivation of monthsPerYear from the table for non-lunisolar
calendars would ideally be written differently, but this is temporary
until we can refactor the implementation to be even more like the spec.

Author: ptomato

polyfill/lib/calendar.mjs

@@ -161,7 +161,7 @@ impl['iso8601'] = {
     if ((type === 'date' || type === 'month-day') && fields.day === undefined) {
       throw new TypeErrorCtor('day is required');
     }
-    ObjectAssign(fields, resolveNonLunisolarMonth(fields));
+    ObjectAssign(fields, resolveNonLunisolarMonth(fields, 'iso8601'));
   },
   dateToISO(fields, overflow) {
     return ES.RegulateISODate(fields.year, fields.month, fields.day, overflow);
@@ -287,34 +287,60 @@ impl['iso8601'] = {
   }
 };
 
+const monthCodeInfo = {
+  chinese: {
+    additionalMonths: ['M01L', 'M02L', 'M03L', 'M04L', 'M05L', 'M06L', 'M07L', 'M08L', 'M09L', 'M10L', 'M11L', 'M12L']
+  },
+  coptic: {
+    additionalMonths: ['M13']
+  },
+  dangi: {
+    additionalMonths: ['M01L', 'M02L', 'M03L', 'M04L', 'M05L', 'M06L', 'M07L', 'M08L', 'M09L', 'M10L', 'M11L', 'M12L']
+  },
+  ethioaa: {
+    additionalMonths: ['M13']
+  },
+  ethiopic: {
+    additionalMonths: ['M13']
+  },
+  hebrew: {
+    additionalMonths: ['M05L']
+  }
+};
+
+function IsValidMonthCodeForCalendar(calendar, monthCode) {
+  const { monthNumber, isLeapMonth } = ParseMonthCode(monthCode);
+  if (!isLeapMonth && monthNumber >= 1 && monthNumber <= 12) return true;
+  if (!ObjectHasOwn(monthCodeInfo, calendar)) return false;
+  return Call(ArrayPrototypeIncludes, monthCodeInfo[calendar].additionalMonths, [monthCode]);
+}
+
 /**
  * Safely merge a month, monthCode pair into an integer month.
  * If both are present, make sure they match.
  * This logic doesn't work for lunisolar calendars!
  * */
-function resolveNonLunisolarMonth(calendarDate, overflow = undefined, monthsPerYear = 12) {
+function resolveNonLunisolarMonth(calendarDate, calendar, overflow = undefined) {
   let { month, monthCode } = calendarDate;
   if (monthCode === undefined) {
     if (month === undefined) throw new TypeErrorCtor('Either month or monthCode are required');
     // The ISO calendar uses the default (undefined) value because it does
     // constrain/reject after this method returns. Non-ISO calendars, however,
     // rely on this function to constrain/reject out-of-range `month` values.
+    const monthsPerYear =
+      12 + (ObjectHasOwn(monthCodeInfo, calendar) ? monthCodeInfo[calendar].additionalMonths.length : 0);
     if (overflow === 'reject') ES.RejectToRange(month, 1, monthsPerYear);
     if (overflow === 'constrain') month = ES.ConstrainToRange(month, 1, monthsPerYear);
     monthCode = CreateMonthCode(month, false);
   } else {
-    const { monthNumber, isLeapMonth } = ParseMonthCode(monthCode);
-    if (isLeapMonth) {
-      throw new RangeErrorCtor(`Invalid monthCode: ${monthCode}. Leap months do not exist in this calendar`);
-    }
-    if (monthCode !== CreateMonthCode(monthNumber, false)) {
-      throw new RangeErrorCtor(`Invalid month code: ${monthCode}`);
+    if (!IsValidMonthCodeForCalendar(calendar, monthCode)) {
+      throw new RangeErrorCtor(`Invalid monthCode: ${monthCode} does not exist in calendar ${calendar}`);
     }
+    const { monthNumber } = ParseMonthCode(monthCode);
     if (month !== undefined && month !== monthNumber) {
       throw new RangeErrorCtor(`monthCode ${monthCode} and month ${month} must match if both are present`);
     }
     month = monthNumber;
-    if (month < 1 || month > monthsPerYear) throw new RangeErrorCtor(`Invalid monthCode: ${monthCode}`);
   }
   return { ...calendarDate, month, monthCode };
 }
@@ -752,9 +778,8 @@ const nonIsoHelperBase = {
   adjustCalendarDate(calendarDate, cache, overflow /*, fromLegacyDate = false */) {
     if (this.calendarType === 'lunisolar') throw new RangeErrorCtor('Override required for lunisolar calendars');
     this.validateCalendarDate(calendarDate);
-    const largestMonth = this.monthsInYear(calendarDate, cache);
     let { month, monthCode } = calendarDate;
-    ({ month, monthCode } = resolveNonLunisolarMonth(calendarDate, overflow, largestMonth));
+    ({ month, monthCode } = resolveNonLunisolarMonth(calendarDate, this.id, overflow));
     calendarDate = { ...calendarDate, month, monthCode };
     if (CalendarSupportsEra(this.id)) calendarDate = this.completeEraYear(calendarDate);
     return calendarDate;
@@ -1994,9 +2019,7 @@ const nonIsoGeneralImpl = {
   },
   resolveFields(fields /* , type */) {
     if (this.helper.calendarType !== 'lunisolar') {
-      const cache = new OneObjectCache();
-      const largestMonth = this.helper.monthsInYear(fields, cache);
-      resolveNonLunisolarMonth(fields, undefined, largestMonth);
+      resolveNonLunisolarMonth(fields, this.helper.id);
     }
   },
   dateToISO(fields, overflow) {