Editorial: Second argument to CalendarExtraFields is only a List

The enum values are no longer passed anywhere in the spec text, so remove
them as a permissible type. Update the polyfill implementation to match,
which had a bug; it always assumed ~date~, which was incorrect.

Closes: #2999

polyfill/lib/calendar.mjs

@@ -1860,9 +1860,11 @@ const helperDangi = { ...helperChinese, id: 'dangi' };
  * ISO and non-ISO implementations vs. code that was very different.
  */
 const nonIsoGeneralImpl = {
-  extraFields(type = 'date') {
-    if (type === 'month-day') return [];
-    return ['era', 'eraYear'];
+  extraFields(fields) {
+    if (this.helper.hasEra && Call(ArrayPrototypeIncludes, fields, ['year'])) {
+      return ['era', 'eraYear'];
+    }
+    return [];
   },
   resolveFields(fields /* , type */) {
     if (this.helper.calendarType !== 'lunisolar') {

polyfill/lib/ecmascript.mjs

@@ -1075,7 +1075,7 @@ export function ISODateToFields(calendar, isoDate, type = 'date') {
 }
 
 export function PrepareCalendarFields(calendar, bag, calendarFieldNames, nonCalendarFieldNames, requiredFields) {
-  const extraFieldNames = calendarImplForID(calendar).extraFields();
+  const extraFieldNames = calendarImplForID(calendar).extraFields(calendarFieldNames);
   const fields = Call(ArrayPrototypeConcat, calendarFieldNames, [nonCalendarFieldNames, extraFieldNames]);
   const result = ObjectCreate(null);
   let any = false;

spec/calendar.html

@@ -862,12 +862,12 @@ <h1>
       <h1>
         CalendarExtraFields (
           _calendar_: a calendar type,
-          _type_: ~date~, ~year-month~, ~month-day~, or a List of values from the Enumeration Key column of <emu-xref href="#table-temporal-calendar-fields-record-fields"></emu-xref>,
+          _fields_: a List of values from the Enumeration Key column of <emu-xref href="#table-temporal-calendar-fields-record-fields"></emu-xref>,
         ): a List of values from the Enumeration Key column of <emu-xref href="#table-temporal-calendar-fields-record-fields"></emu-xref>
       </h1>
       <dl class="header">
         <dt>description</dt>
-        <dd>It characterizes calendar-specific fields that are relevant for values of the provided _type_ in the built-in calendar identified by _calendar_ (inferring the type when _type_ is a List by interpreting its elements as field names). For example, « ~era~, ~era-year~ » is returned when _calendar_ is *"gregory"* or *"japanese"* and _type_ is ~date~ or ~year-month~ or a List containing ~year~.</dd>
+        <dd>It characterizes calendar-specific fields that are relevant for the provided _fields_ in the built-in calendar identified by _calendar_. For example, « ~era~, ~era-year~ » is returned when _calendar_ is *"gregory"* or *"japanese"* and _fields_ is a List containing ~year~.</dd>
       </dl>
       <emu-alg>
         1. If _calendar_ is *"iso8601"*, return an empty List.