Allow ISO strings in Temporal.Calendar.from()

This brings the behaviour in line with Temporal.TimeZone. If a calendar
annotation is not given in the ISO string, then the ISO calendar is
assumed.

Closes: #862

polyfill/lib/calendar.mjs

@@ -119,7 +119,15 @@ export class Calendar {
   static from(item) {
     if (ES.Type(item) === 'Object') return item;
     const stringIdent = ES.ToString(item);
-    return GetBuiltinCalendar(stringIdent);
+    if (IsBuiltinCalendar(stringIdent)) return GetBuiltinCalendar(stringIdent);
+    let calendar;
+    try {
+      ({ calendar } = ES.ParseISODateTime(stringIdent, { zoneRequired: false }));
+    } catch {
+      throw new RangeError(`Invalid calendar: ${stringIdent}`);
+    }
+    if (!calendar) calendar = 'iso8601';
+    return GetBuiltinCalendar(calendar);
   }
 }
 
@@ -371,6 +379,9 @@ const BUILTIN_CALENDARS = {
   // To be filled in as builtin calendars are implemented
 };
 
+function IsBuiltinCalendar(id) {
+  return id in BUILTIN_CALENDARS;
+}
 function GetBuiltinCalendar(id) {
   if (!(id in BUILTIN_CALENDARS)) throw new RangeError(`unknown calendar ${id}`);
   return new BUILTIN_CALENDARS[id]();

polyfill/test/calendar.mjs

@@ -10,7 +10,7 @@ import Pretty from '@pipobscure/demitasse-pretty';
 const { reporter } = Pretty;
 
 import { strict as assert } from 'assert';
-const { equal } = assert;
+const { equal, throws } = assert;
 
 import * as Temporal from 'proposal-temporal';
 const { Calendar } = Temporal;
@@ -90,6 +90,33 @@ describe('Calendar', () => {
       equal(typeof Calendar.from, 'function');
     });
   });
+  describe('Calendar.from()', () => {
+    describe('from identifier', () => {
+      test('iso8601');
+      test('gregory');
+      test('japanese');
+      function test(id) {
+        const calendar = Calendar.from(id);
+        it(`Calendar.from(${id}) is a calendar`, () => assert(calendar instanceof Calendar));
+        it(`Calendar.from(${id}) has the correct ID`, () => equal(calendar.id, id));
+      }
+      it('Calendar.from throws with bad identifier', () => {
+        throws(() => Calendar.from('local'));
+        throws(() => Calendar.from('iso-8601'));
+        throws(() => Calendar.from('[c=iso8601]'));
+      });
+    });
+    describe('Calendar.from(ISO string)', () => {
+      test('1994-11-05T08:15:30-05:00', 'iso8601');
+      test('1994-11-05T08:15:30-05:00[c=gregory]', 'gregory');
+      test('1994-11-05T13:15:30Z[c=japanese]', 'japanese');
+      function test(isoString, id) {
+        const calendar = Calendar.from(isoString);
+        it(`Calendar.from(${isoString}) is a calendar`, () => assert(calendar instanceof Calendar));
+        it(`Calendar.from(${isoString}) has ID ${id}`, () => equal(calendar.id, id));
+      }
+    });
+  });
 });
 
 import { normalize } from 'path';

spec/abstractops.html

@@ -740,6 +740,14 @@ <h1>ISO 8601 grammar</h1>
       TemporalYearMonthString :
           DateSpecYearMonth
           DateTime
+
+      TemporalCalendarString :
+          CalendarName
+          TemporalInstantString
+          CalendarDateTime
+          Time
+          DateSpecYearMonth
+          DateSpecMonthDay
     </emu-grammar>
   </emu-clause>
 
@@ -828,6 +836,22 @@ <h1>ParseTemporalInstantString ( _isoString_ )</h1>
     </emu-alg>
   </emu-clause>
 
+  <emu-clause id="sec-temporal-parsetemporalcalendarstring" aoid="ParseTemporalCalendarString">
+    <h1>ParseTemporalCalendarString ( _isoString_ )</h1>
+    <p>
+      The ParseTemporalCalendarString abstract operation interprets an ISO 8601 string and retrieves the calendar identifier from it.
+    </p>
+    <emu-alg>
+      1. Assert: Type(_isoString_) is String.
+      1. If _isoString_ does not satisfy the syntax of a |TemporalCalendarString| (see <emu-xref href="#sec-temporal-iso8601grammar"></emu-xref>), then
+        1. Throw a *RangeError* exception.
+      1. Let _id_ be the part of _isoString_ produced by the |CalendarName| production, or *undefined* if not present.
+      1. If _id_ is *undefined*, then
+        1. Return *"iso8601"*.
+      1. Return _id_.
+    </emu-alg>
+  </emu-clause>
+
   <emu-clause id="sec-temporal-parsetemporaldatestring" aoid="ParseTemporalDateString">
     <h1>ParseTemporalDateString ( _isoString_ )</h1>
     <p>

spec/calendar.html

@@ -8,6 +8,18 @@ <h1>Temporal.Calendar Objects</h1>
   <emu-clause id="sec-temporal-calendar-abstract-ops">
     <h1>Abstract Operations for Temporal.Calendar Objects</h1>
 
+    <emu-clause id="sec-temporal-isbuiltincalendar" aoid="IsBuiltinCalendar">
+      <h1>IsBuiltinCalendar ( _id_ )</h1>
+      <p>
+        An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the *IsBuiltinCalendar* abstract operation as specified in the ECMA-402 specification.
+        If an ECMAScript implementation does not include the ECMA-402 API the following specification of the *IsBuiltinCalendar* abstract operation is used.
+      </p>
+      <emu-alg>
+        1. If _id_ is not *"iso8601"*, return *false*.
+        1. Return *true*.
+      </emu-alg>
+    </emu-clause>
+
     <emu-clause id="sec-temporal-getbuiltincalendar" aoid="GetBuiltinCalendar">
       <h1>GetBuiltinCalendar ( _id_ )</h1>
       <p>An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the *GetBuiltinCalendar* abstract operation as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the *GetBuiltinCalendar* abstract operation is used.</p>
@@ -160,8 +172,10 @@ <h1>Temporal.Calendar.from ( _item_ )</h1>
       <emu-alg>
         1. If Type(_item_) is Object, then
           1. Return _item_.
-        1. Let _stringIdent_ be ? ToString(_item_).
-        1. Return ? GetBuiltinCalendar(_stringIdent_).
+        1. Let _string_ be ? ToString(_item_).
+        1. If ! IsBuiltinCalendar(_string_) is *false*, then
+          1. Let _string_ be ? ParseTemporalCalendarString(_string_).
+        1. Return ? GetBuiltinCalendar(_string_).
       </emu-alg>
       <p>
         This function is the <dfn>%Temporal.Calendar.from%</dfn> intrinsic object.