Editorial: Merge CalendarDateAddition into CalendarDateAdd

Having two separate operations for this was already confusing just because
of the names. Combine them into an ISO 8601 part and an implementation-
defined part, as in CalendarDateToISO.

See: #2948

Author: ptomato

polyfill/lib/calendar.mjs

@@ -174,9 +174,7 @@ impl['iso8601'] = {
     ({ year, month } = ES.BalanceISOYearMonth(year, month));
     ({ year, month, day } = ES.RegulateISODate(year, month, day, overflow));
     day += days + 7 * weeks;
-    ({ year, month, day } = ES.BalanceISODate(year, month, day));
-    ES.RejectDateRange(year, month, day);
-    return { year, month, day };
+    return ES.BalanceISODate(year, month, day);
   },
   dateUntil(one, two, largestUnit) {
     const sign = -ES.CompareISODate(one.year, one.month, one.day, two.year, two.month, two.day);

polyfill/lib/ecmascript.mjs

@@ -1852,7 +1852,9 @@ export function CalendarMergeFields(calendar, fields, additionalFields) {
 }
 
 export function CalendarDateAdd(calendar, isoDate, dateDuration, overflow) {
-  return GetIntrinsic('%calendarImpl%')(calendar).dateAdd(isoDate, dateDuration, overflow);
+  const result = GetIntrinsic('%calendarImpl%')(calendar).dateAdd(isoDate, dateDuration, overflow);
+  RejectDateRange(result.year, result.month, result.day);
+  return result;
 }
 
 export function CalendarDateUntil(calendar, isoDate, isoOtherDate, largestUnit) {

spec/calendar.html

@@ -418,7 +418,7 @@ <h1>
       </emu-alg>
     </emu-clause>
 
-    <emu-clause id="sec-temporal-calendardateadd" type="abstract operation">
+    <emu-clause id="sec-temporal-calendardateadd" type="implementation-defined abstract operation">
       <h1>
         CalendarDateAdd (
           _calendar_: a calendar type,
@@ -440,10 +440,15 @@ <h1>
           1. Set _intermediate_ to ? RegulateISODate(_intermediate_.[[Year]], _intermediate_.[[Month]], _isoDate_.[[Day]], _overflow_).
           1. Let _d_ be _intermediate_.[[Day]] + _duration_.[[Days]] + 7 × _duration_.[[Weeks]].
           1. Let _result_ be BalanceISODate(_intermediate_.[[Year]], _intermediate_.[[Month]], _d_).
-          1. If ISODateWithinLimits(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]) is *false*, throw a *RangeError* exception.
-          1. Return _result_.
-        1. Return ? CalendarDateAddition(_calendar_, _isoDate_, _duration_, _overflow_).
+        1. Else,
+          1. Let _result_ be an implementation-defined ISO Date Record, or throw a *RangeError* exception, as described below.
+        1. If ISODateWithinLimits(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]) is *false*, throw a *RangeError* exception.
+        1. Return _result_.
       </emu-alg>
+      <p>
+        When _calendar_ is not *"iso8601"*, the operation performs implementation-defined processing to add _duration_ to _date_ in the context of the calendar represented by _calendar_ and returns the corresponding day, month and year of the result in the ISO 8601 calendar values as an ISO Date Record.
+        It may throw a *RangeError* exception if _overflow_ is ~reject~ and the resulting month or day would need to be clamped in order to form a valid date in _calendar_.
+      </p>
     </emu-clause>
 
     <emu-clause id="sec-temporal-calendardateuntil" type="abstract operation">
@@ -1173,24 +1178,6 @@ <h1>
       </p>
     </emu-clause>
 
-    <emu-clause id="sec-temporal-calendardateaddition" type="implementation-defined abstract operation">
-      <h1>
-        CalendarDateAddition (
-          _calendar_: a calendar type that is not *"iso8601"*,
-          _date_: an ISO Date Record,
-          _duration_: a Date Duration Record,
-          _overflow_: ~constrain~ or ~reject~,
-        ): either a normal completion containing an ISO Date Record or a throw completion
-      </h1>
-      <dl class="header">
-        <dt>description</dt>
-        <dd>
-          It performs implementation-defined processing to add _duration_ to _date_ in the context of the calendar represented by _calendar_ and returns the corresponding day, month and year of the result in the ISO 8601 calendar values as an ISO Date Record.
-          It may throw a *RangeError* exception if _overflow_ is ~reject~ and the resulting month or day would need to be clamped in order to form a valid date in _calendar_, or if the date resulting from the addition is outside the range allowed by ISODateTimeWithinLimits.
-        </dd>
-      </dl>
-    </emu-clause>
-
     <emu-clause id="sec-temporal-calendardatedifference" type="implementation-defined abstract operation">
       <h1>
         CalendarDateDifference (