What if calendar.fields() returns "calendar" as one of the fields?

[ptomato]

Follow up from #1014 (comment)

This is something we'll probably have to test in the test262 tests, and make sure that something sensible happens.

[ptomato]

Also mentioned in #1053 (comment) - what if it returns one of the built-in fields that wasn't in the input array?

[justingrant]

Related is #1054:

1. Why does fields require returning all fields? Are calendars allowed to remove built-in fields? If not, why not just additional ones, so implementation would be simpler and it'd avoid error cases because we could simply ignore or throw when we see a built-in fields.
2. Why is fields a method instead of a property returning an object like below? See Calendar.prototype.fields feedback #1054 for reasons why this could be helpful, e.g. reduce errors, easier debugging, etc.

{
  plainDate: ['era'],
  plainYearMonth: ['era'],
  // plainMonthDay: undefined, 
  // plainTime: undefined, 
  zonedDateTime:  ['era'],
  plainDateTime: ['era']
}

3. Why is it called fields instead of another name that isn't confusable with getFields and that clarifies that only names, not values, are returned? For example, extraFieldNames.

[ptomato]

I think we can stick to #1054 for those, the questions above still apply even if those changes are made.

[ptomato]

Pretty sure the answer is that we do something sensible in that case. Verify and close

[gibson042]

It looks like the "fields" method is only invoked from CalendarFields, whose output is always fed to PrepareTemporalFields which propagates it back to end up in CalendarYearMonthFromFields, CalendarMonthDayFromFields, CalendarDateFromFields, or InterpretTemporalDateTimeFields. The first three send fields right back to the corresponding calendar method, while the last sends fields to ToTemporalTimeRecord (which invokes PrepareTemporalFields itself and then looks only for specific fields in the result) and then to the already-covered CalendarDateFromFields. I think we're good.

[ptomato]

Thanks for looking into it. Closing 😄