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Description
This is similar to #961, but about calendars.
There could be two sources for calendars: the input type being formatted, and the message string. (Actually, there is a third, the locale of the message formatter.)
The spec should be more clear about resolution when these are in conflict with one another.
As a reference point, in Temporal, we throw an exception if two of the calendars differ and they are not ISO-8601. For example:
| Calendar 1 | Calendar 2 | Resolved |
|---|---|---|
| iso8601 | iso8601 | iso8601 |
| iso8601 | buddhist | buddhist |
| buddhist | chinese | ERROR |