Allow property bags in Temporal.Calendar.from()

These are kept distinct from plain-object custom calendars by the absence
of a 'calendar' property. This way, any Temporal type that carries a
calendar, as well as any property bag of that type, can be passed to
Temporal.Calendar.from().

See: #925

docs/calendar.md

@@ -47,6 +47,7 @@ The identifier of a custom calendar must consist of one or more components of be
 
 It's also possible for a plain object to be a custom calendar, without subclassing.
 The object must implement the `Temporal.Calendar` methods and have an `id` property.
+It must not have a `calendar` property, so that it can be distinguished in `Temporal.Calendar.from()` from other Temporal objects that have a calendar.
 It is possible to pass such an object into any Temporal API that would normally take a built-in `Temporal.Calendar`.
 
 ## Constructor
@@ -76,12 +77,13 @@ cal = new Temporal.Calendar('gregory');
 ### Temporal.Calendar.**from**(_thing_: any) : Temporal.Calendar
 
 **Parameters:**
-- `thing`: A `Temporal.Calendar` object or a value from which to create a `Temporal.Calendar`.
+- `thing`: A calendar object, a Temporal object that carries a calendar, or a value from which to create a `Temporal.Calendar`.
 
-**Returns:** a new `Temporal.Calendar` object.
+**Returns:** a calendar object.
 
 This static method creates a new calendar from another value.
-If the value is another `Temporal.Calendar` object, a new object representing the same calendar is returned.
+If the value is another `Temporal.Calendar` object, or object implementing the calendar protocol, the same object is returned.
+If the value is another Temporal object that carries a calendar or an object with a `calendar` property, such as a `Temporal.ZonedDateTime`, the object's calendar is returned.
 
 Any other value is converted to a string, which is expected to be either:
 - a string that is accepted by `new Temporal.Calendar()`; or

polyfill/index.d.ts

@@ -347,6 +347,7 @@ export namespace Temporal {
 
   export interface CalendarProtocol {
     id: string;
+    calendar?: never;
     year(date: Temporal.Date): number;
     month(date: Temporal.Date): number;
     day(date: Temporal.Date): number;
@@ -409,7 +410,7 @@ export namespace Temporal {
    *
    * See https://tc39.es/proposal-temporal/docs/calendar.html for more details.
    */
-  export class Calendar implements Required<CalendarProtocol> {
+  export class Calendar implements Omit<Required<CalendarProtocol>, 'calendar'> {
     static from(item: CalendarProtocol | string): Temporal.Calendar;
     constructor(calendarIdentifier: string);
     readonly id: string;

polyfill/lib/calendar.mjs

@@ -117,7 +117,11 @@ export class Calendar {
     return GetSlot(this, CALENDAR_ID);
   }
   static from(item) {
-    if (ES.Type(item) === 'Object') return item;
+    if (ES.Type(item) === 'Object') {
+      if (!('calendar' in item)) return item;
+      item = item.calendar;
+      if (ES.Type(item) === 'Object' && !('calendar' in item)) return item;
+    }
     const stringIdent = ES.ToString(item);
     if (IsBuiltinCalendar(stringIdent)) return GetBuiltinCalendar(stringIdent);
     let calendar;

polyfill/test/calendar.mjs

@@ -100,10 +100,42 @@ describe('Calendar', () => {
         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]'));
+      it('other types with a calendar are accepted', () => {
+        [
+          Temporal.Date.from('1976-11-18[c=gregory]'),
+          Temporal.DateTime.from('1976-11-18[c=gregory]'),
+          Temporal.MonthDay.from('1972-11-18[c=gregory]'),
+          Temporal.YearMonth.from('1976-11-01[c=gregory]')
+        ].forEach((obj) => {
+          const calFrom = Calendar.from(obj);
+          assert(calFrom instanceof Calendar);
+          equal(calFrom.id, 'gregory');
+        });
+      });
+      it('property bag with calendar object is accepted', () => {
+        const cal = new Calendar('iso8601');
+        const calFrom = Calendar.from({ calendar: cal });
+        assert(calFrom instanceof Calendar);
+        equal(calFrom.id, 'iso8601');
+      });
+      it('property bag with string is accepted', () => {
+        const calFrom = Calendar.from({ calendar: 'iso8601' });
+        assert(calFrom instanceof Calendar);
+        equal(calFrom.id, 'iso8601');
+      });
+      it('property bag with custom calendar is accepted', () => {
+        const custom = { id: 'custom-calendar' };
+        const calFrom = Calendar.from({ calendar: custom });
+        equal(calFrom, custom);
+      });
+      it('throws with bad identifier', () => {
+        throws(() => Calendar.from('local'), RangeError);
+        throws(() => Calendar.from('iso-8601'), RangeError);
+        throws(() => Calendar.from('[c=iso8601]'), RangeError);
+      });
+      it('throws with bad value in property bag', () => {
+        throws(() => Calendar.from({ calendar: 'local' }), RangeError);
+        throws(() => Calendar.from({ calendar: { calendar: 'iso8601' } }), RangeError);
       });
     });
     describe('Calendar.from(ISO string)', () => {

spec/calendar.html

@@ -171,7 +171,9 @@ <h1>Temporal.Calendar.from ( _item_ )</h1>
       </p>
       <emu-alg>
         1. If Type(_item_) is Object, then
-          1. Return _item_.
+          1. If ? HasProperty(_item_, *"calendar"*) is *false*, return _item_.
+          1. Set _item_ to ? Get(_item_, *"calendar"*).
+          1. If Type(_item_) is Object and ? HasProperty(_item_, *"calendar"*) is *false*, return _item_.
         1. Let _string_ be ? ToString(_item_).
         1. If ! IsBuiltinCalendar(_string_) is *false*, then
           1. Let _string_ be ? ParseTemporalCalendarString(_string_).