Polyfill: Implement era aliases

As of the current state of the Intl Era and Month Codes proposal, the
eras table in https://tc39.es/proposal-intl-era-monthcode/#table-eras has
a canonical name for each era, and a number of aliases which should also
be accepted. Legacy ICU names such as "era0" should not be accepted,
although they still need to be handled when using Intl.DateTimeFormat to
convert dates.

Closes: #2901

polyfill/lib/calendar.mjs

@@ -5,6 +5,8 @@ import * as ES from './ecmascript.mjs';
 import { DefineIntrinsic } from './intrinsicclass.mjs';
 
 const ArrayFrom = Array.from;
+const ArrayPrototypeFind = Array.prototype.find;
+const ArrayPrototypeIncludes = Array.prototype.includes;
 const ArrayPrototypeSort = Array.prototype.sort;
 const IntlDateTimeFormat = globalThis.Intl.DateTimeFormat;
 const MathAbs = Math.abs;
@@ -456,6 +458,11 @@ const nonIsoHelperBase = {
         `Intl.DateTimeFormat.formatToParts lacks relatedYear in ${this.id} calendar. Try Node 14+ or modern browsers.`
       );
     }
+    // Translate old ICU era codes "ERA0" etc. into canonical era names.
+    if (this.hasEra) {
+      const replacement = ES.Call(ArrayPrototypeFind, this.eras, [(e) => result.era === e.genericName]);
+      if (replacement) result.era = replacement.name;
+    }
     // Translate eras that may be handled differently by Temporal vs. by Intl
     // (e.g. Japanese pre-Meiji eras). See #526 for details.
     if (this.reviseIntlEra) {
@@ -1154,6 +1161,9 @@ const helperIndian = ObjectAssign({}, nonIsoHelperBase, {
  *   /** name of the era
  *   name: string;
  *
+ *   // Aliases, see https://tc39.es/proposal-intl-era-monthcode/#table-eras
+ *   aliases: string[];
+ *
  *   // alternate name of the era used in old versions of ICU data
  *   // format is `era{n}` where n is the zero-based index of the era
  *   // with the oldest era being 0.
@@ -1370,8 +1380,10 @@ const makeHelperGregorian = (id, originalEras) => {
         checkField('era', era);
         checkField('eraYear', eraYear);
       } else if (eraYear != null) {
-        const matchingEra =
-          era === undefined ? undefined : this.eras.find((e) => e.name === era || e.genericName === era);
+        if (era === undefined) throw new RangeError('era and eraYear must be provided together');
+        const matchingEra = ES.Call(ArrayPrototypeFind, this.eras, [
+          ({ name, aliases = [] }) => name === era || ES.Call(ArrayPrototypeIncludes, aliases, [era])
+        ]);
         if (!matchingEra) throw new RangeError(`Era ${era} (ISO year ${eraYear}) was not matched by any era`);
         if (eraYear < 1 && matchingEra.reverseOf) {
           throw new RangeError(`Years in ${era} era must be positive, not ${year}`);
@@ -1471,21 +1483,21 @@ const makeHelperOrthodox = (id, originalEras) => {
 //   zero era of ethioaa.
 const helperEthioaa = ObjectAssign({}, makeHelperOrthodox('ethioaa'), { isoEpoch: { year: -5492, month: 7, day: 17 } });
 const helperCoptic = makeHelperOrthodox('coptic', [
-  { name: 'era1', isoEpoch: { year: 284, month: 8, day: 29 } },
-  { name: 'era0', reverseOf: 'era1' }
+  { name: 'coptic', isoEpoch: { year: 284, month: 8, day: 29 } },
+  { name: 'coptic-inverse', reverseOf: 'coptic' }
 ]);
 // Anchor is currently the older era to match ethioaa, but should it be the newer era?
 // See https://github.com/tc39/ecma402/issues/534 for discussion.
 const helperEthiopic = makeHelperOrthodox('ethiopic', [
-  { name: 'era0', isoEpoch: { year: -5492, month: 7, day: 17 } },
-  { name: 'era1', isoEpoch: { year: 8, month: 8, day: 27 }, anchorEpoch: { year: 5501 } }
+  { name: 'ethioaa', aliases: ['ethiopic-amete-alem', 'mundi'], isoEpoch: { year: -5492, month: 7, day: 17 } },
+  { name: 'ethiopic', aliases: ['incar'], isoEpoch: { year: 8, month: 8, day: 27 }, anchorEpoch: { year: 5501 } }
 ]);
 
 const helperRoc = ObjectAssign(
   {},
   makeHelperSameMonthDayAsGregorian('roc', [
-    { name: 'minguo', isoEpoch: { year: 1912, month: 1, day: 1 } },
-    { name: 'before-roc', reverseOf: 'minguo' }
+    { name: 'roc', aliases: ['minguo'], isoEpoch: { year: 1912, month: 1, day: 1 } },
+    { name: 'roc-inverse', aliases: ['before-roc'], reverseOf: 'roc' }
   ])
 );
 
@@ -1496,8 +1508,8 @@ const helperBuddhist = ObjectAssign({}, makeHelperGregorianFixedEpoch('buddhist'
 const helperGregory = ObjectAssign(
   {},
   makeHelperSameMonthDayAsGregorian('gregory', [
-    { name: 'ce', isoEpoch: { year: 1, month: 1, day: 1 } },
-    { name: 'bce', reverseOf: 'ce' }
+    { name: 'gregory', aliases: ['ad', 'ce'], isoEpoch: { year: 1, month: 1, day: 1 } },
+    { name: 'gregory-inverse', aliases: ['bc', 'bce'], reverseOf: 'gregory' }
   ]),
   {
     reviseIntlEra(calendarDate /*, isoDate*/) {
@@ -1506,8 +1518,8 @@ const helperGregory = ObjectAssign(
       // option mistakenly returns the one-letter (narrow) format instead. The
       // code below handles either the correct or Firefox-buggy format. See
       // https://bugzilla.mozilla.org/show_bug.cgi?id=1752253
-      if (era === 'bc' || era === 'b') era = 'bce';
-      if (era === 'ad' || era === 'a') era = 'ce';
+      if (era === 'b') era = 'gregory-inverse';
+      if (era === 'a') era = 'gregory';
       return { era, eraYear };
     },
     getFirstDayOfWeek() {
@@ -1558,16 +1570,16 @@ const helperJapanese = ObjectAssign(
     { name: 'showa', isoEpoch: { year: 1926, month: 12, day: 25 }, anchorEpoch: { year: 1926, month: 12, day: 25 } },
     { name: 'taisho', isoEpoch: { year: 1912, month: 7, day: 30 }, anchorEpoch: { year: 1912, month: 7, day: 30 } },
     { name: 'meiji', isoEpoch: { year: 1868, month: 9, day: 8 }, anchorEpoch: { year: 1868, month: 9, day: 8 } },
-    { name: 'ce', isoEpoch: { year: 1, month: 1, day: 1 } },
-    { name: 'bce', reverseOf: 'ce' }
+    { name: 'japanese', aliases: ['gregory', 'ad', 'ce'], isoEpoch: { year: 1, month: 1, day: 1 } },
+    { name: 'japanese-inverse', aliases: ['gregory-inverse', 'bc', 'bce'], reverseOf: 'japanese' }
   ]),
   {
     erasBeginMidYear: true,
     reviseIntlEra(calendarDate, isoDate) {
       const { era, eraYear } = calendarDate;
       const { year: isoYear } = isoDate;
       if (this.eras.find((e) => e.name === era)) return { era, eraYear };
-      return isoYear < 1 ? { era: 'bce', eraYear: 1 - isoYear } : { era: 'ce', eraYear: isoYear };
+      return isoYear < 1 ? { era: 'japanese-inverse', eraYear: 1 - isoYear } : { era: 'japanese', eraYear: isoYear };
     }
   }
 );