Editorial: Recommend never ignoring a specified year in CalendarMonth…

…DayToISOReferenceDate

(and update the polyfill accordingly)

Fixes tc39#2863

polyfill/lib/calendar.mjs

@@ -898,14 +898,16 @@ const nonIsoHelperBase = {
   // Override if calendar uses eras
   hasEra: false,
   monthDayFromFields(fields, overflow, cache) {
-    let { monthCode, day } = fields;
+    let { year, era, eraYear, monthCode, day } = fields;
+    let validateDate = monthCode === undefined || (year !== undefined || era !== undefined || eraYear !== undefined);
     if (monthCode === undefined) {
-      let { year, era, eraYear } = fields;
       if (year === undefined && (era === undefined || eraYear === undefined)) {
         throw new TypeErrorCtor(
           'when `monthCode` is omitted, `year` (or `era` and `eraYear`) and `month` are required'
         );
       }
+    }
+    if (validateDate) {
       // Apply overflow behaviour to year/month/day, to get correct monthCode/day
       ({ monthCode, day } = this.isoToCalendarDate(this.calendarToIsoDate(fields, overflow, cache), cache));
     }

spec/calendar.html

@@ -1209,6 +1209,10 @@ <h1>
         For example, when _calendar_ is *"gregory"* and _overflow_ is *"reject"*, _fields_ values of { [[MonthCode]]: *"M01"*, [[Day]]: 32 } and { [[Year]]: 2001, [[Month]]: 2, [[Day]]: 29 } would both cause a *RangeError* to be thrown.
         In the latter case, even though February 29 is a date in leap years of the Gregorian calendar, 2001 was not a leap year and a month code cannot be determined from the nonexistent date 2001-02-29 with the specified month index.
       </p>
+      <p>
+        Also like RegulateISODate, if _overflow_ is *"constrain"* and the date or month and day described by _fields_ does not exist (or does not exist within the year described by _fields_ when there is such a year), the values of those fields are clamped to their respective valid range. As in CalendarDateToISO, such clamping is calendar-specific.
+        For example, when _calendar_ is *"gregory"* and _overflow_ is *"constrain"*, a _fields_ value of { [[MonthCode]]: *"M02"*, [[Day]]: 30 } is clamped to { [[Year]]: 1972, [[Month]]: 2, [[Day]]: 29 } (because 29 is the maximum valid day for February) while _fields_ values of { [[Year]]: 2001, [[MonthCode]]: *"M02"*, [[Day]]: 30 } and { [[Era]]: *"ce"*, [[EraYear]]: 2001, [[Month]]: 2, [[Day]]: 30 } (or an equivalent with a supported value for [[Era]] representing the Common Era beginning in ISO 8601 year 1) are clamped to { [[Year]]: 1972, [[Month]]: 2, [[Day]]: 28 } (because 28 is the maximum valid day for February 2001, which did not include a leap day).
+      </p>
     </emu-clause>
 
     <emu-clause id="sec-temporal-calendardateaddition" type="implementation-defined abstract operation">