Normative: Don't observably iterate array in built-in calendar's fiel…

…ds()

When calling CalendarFields with a built-in calendar, previously we would
create a JS Array from the passed List, and pass it to %Temporal.Calendar.
prototype.fields%, which would iterate it observably. So, instead we call
CalendarDateFields when the calendar is a built-in calendar, which doesn't
do anything observable at all.

See: #2289

polyfill/lib/calendar.mjs

@@ -2160,3 +2160,10 @@ impl['indian'] = ObjectAssign({}, nonIsoGeneralImpl, { helper: helperIndian });
 impl['buddhist'] = ObjectAssign({}, nonIsoGeneralImpl, { helper: helperBuddhist });
 impl['japanese'] = ObjectAssign({}, nonIsoGeneralImpl, { helper: helperJapanese });
 impl['gregory'] = ObjectAssign({}, nonIsoGeneralImpl, { helper: helperGregory });
+
+function calendarFieldsImpl(calendar, fieldNames) {
+  return impl[calendar].fields(fieldNames);
+}
+// Probably not what the intrinsics mechanism was intended for, but view this as
+// an export of calendarFieldsImpl while avoiding circular dependencies
+DefineIntrinsic('calendarFieldsImpl', calendarFieldsImpl);

polyfill/lib/ecmascript.mjs

@@ -1803,9 +1803,8 @@ export function CreateTemporalZonedDateTime(epochNanoseconds, timeZone, calendar
 
 export function CalendarFields(calendar, fieldNames) {
   if (typeof calendar === 'string') {
-    const TemporalCalendar = GetIntrinsic('%Temporal.Calendar%');
-    calendar = new TemporalCalendar(calendar);
-    return Call(GetIntrinsic('%Temporal.Calendar.prototype.fields%'), calendar, [fieldNames]);
+    if (calendar === 'iso8601') return fieldNames;
+    return GetIntrinsic('%calendarFieldsImpl%')(calendar, fieldNames);
   }
   const fields = GetMethod(calendar, 'fields');
   fieldNames = Call(fields, calendar, [fieldNames]);

spec/calendar.html

@@ -99,9 +99,12 @@ <h1>
       </dl>
       <emu-alg>
         1. If _calendar_ is a String, then
-          1. Set _calendar_ to ! CreateTemporalCalendar(_calendar_).
-          1. Let _fieldsArray_ be ? Call(%Temporal.Calendar.prototype.fields%, _calendar_, « CreateArrayFromList(_fieldNames_) »).
-          1. Return ! CreateListFromArrayLike(_fieldsArray_, « String »).
+          1. Assert: _fieldNames_ contains no String other than *"year"*, *"month"*, *"monthCode"*, or *"day"*.
+          1. If _calendar_ is *"iso8601"*, return _fieldNames_.
+          1. Let _extraFieldDescriptors_ be CalendarFieldDescriptors(_calendar_, _fieldNames_).
+          1. For each Calendar Field Descriptor Record _desc_ of _extraFieldDescriptors_, do
+            1. Append _desc_.[[Property]] to _fieldNames_.
+          1. Return _fieldNames_.
         1. Let _fieldsArray_ be ? Invoke(_calendar_, *"fields"*, « CreateArrayFromList(_fieldNames_) »).
         1. Let _iteratorRecord_ be ? GetIterator(_fieldsArray_, ~sync~).
         1. Return ? IteratorToListOfType(_iteratorRecord_, « String »).