Fallbacks to DefaultMergeCalendarFields should be removed (or made unconditional)

[gibson042]

CalendarMergeFields looks up a mergeFields method on its input calendar instance to invoke, but substitutes use of the DefaultMergeCalendarFields operation if no value is found. mergeFields itself exists on Temporal.Calendar.prototype in the primordial state, thinly wrapping DefaultMergeCalendarFields in the absence of ECMA-402 and extended in ECMA-402 to include built-in support for non-ISO-8601 calendars.

This has the net effect of delete Temporal.Calendar.prototype.mergeFields continuing to "work" but removing built-in calendar-sensitive behavior, which a) bad in its own right, and b) divergent from similar manipulation of other built-in methods (e.g., CalendarDateFromFields unconditionally invokes dateFromFields and will therefore fail if the built-in method is removed).

I see two general options for resolution:

1. Remove the fallback and instead always look up and invoke methods.
2. Remove the fallback and instead always always use built-in operations for built-in calendars, effectively ignoring the prototype (but presumably leaving the methods as thin wrappers around built-in operations for use by user-defined subclasses).

I think option 2 would be most consistent with the push in #1808 to minimize action at a distance.

[ptomato]

Didn't we just decide this yesterday already in #2310?

[gibson042]

The decision of #2310 requires the presence of (w.l.o.g.) mergeFields on every custom calendar instance, but does not cover what to do regarding built-in calendars when the built-in method has been removed or replaced.

[ptomato]

I would say that's covered by #1808, the builtin-calendars-are-strings behaviour would imply option (2).

[gibson042]

I don't think that implication holds because built-in calendar instances will presumably always inherit from Temporal.Calendar.prototype and could fail to override any methods there, but as noted above, option 2 is most consistent with a goal to which #1808 contributes.

[ptomato]

This has been addressed somewhere along the line, probably by #2519. The fallback does not exist anymore.