DifferenceTemporalZonedDateTime: Move time zone equality check next to calendar check?

[anba]

Step 3:

3. If ? CalendarEquals(zonedDateTime.[[Calendar]], other.[[Calendar]]) is false, then
   a. Throw a RangeError exception.

and step 9:

9. If ? TimeZoneEquals(zonedDateTime.[[TimeZone]], other.[[TimeZone]]) is false, then
   a. Throw a RangeError exception.

should probably be moved next to each other.

[gibson042]

This might not be important to change, but having an early return in between two checks that don't affect is undesirable. Will revisit after #2519.

[ptomato]

I wrote a comment just now in favour of changing this, and then deleted it, because I realized something that I completely missed on the first read-through.

The time zone check is intentionally done after processing the options argument, because the options determine whether you take the exact-time difference (for which the time zone isn't relevant) or the wall-time difference (for which the time zones must be equal). So I now think we should not make a change here.

[gibson042]

Ah, right. The polyfill explains it like «When calculating difference between time zones, largestUnit must be 'hours' or smaller because day lengths can vary between time zones due to DST or time zone offset changes.». That should probably be explained in the algorithm with a note.