Duration rounding throws an error about proposal-temporal HOT 4 OPEN

This is a regression from #2884. So while the bad news is that we may have inadvertently introduced a bug, the good news is that there wasn't TC39 consensus on the buggy behaviour, so we can just make it work again without having to go to plenary.

I'll investigate whether the best fix is a revert or something else, because there is definitely some stuff from #2884 that we should keep.

from proposal-temporal.

Temporal.PlainDate.from("2023-05-31").until("2024-04-30", {largestUnit: "month"}).toString()

returns "P10M30D", but used to return "P11M" at some point. SpiderMonkey still uses the old definition of DifferenceISODate, which is probably why it doesn't throw a RangeError for the round call.

Temporal.Duration.prototype.round calls DifferencePlainDateTimeWithRounding with one = 2023-05-31 and two = 2024-04-30. DifferenceISODateTime(one, two, largestUnit = "months") returns P10M30D. So RoundRelativeDuration is called with duration = P10M30D and increment = 1. In NudgeToCalendarUnit, r1 = 10 (months from P10M30D) and r2 = 11. Adding r2 months to the start date ("2023-05-31") returns the end date ("2024-04-30"), which leads to evaluating true for destEpochNs ≥ endEpochNs in NudgeToCalendarUnit, step 13.a.

If DifferenceISODateTime had returned "P11M" instead of "P10M30D", then r1 = 11 and r2 = 12, and everything would have worked correctly.

from proposal-temporal.

We probably just need to relax the assertions. See #2924

from proposal-temporal.

No, we can't just relax these range checks, because they ensure that ApplyUnsignedRoundingMode can be called without hitting assertions within ApplyUnsignedRoundingMode, specifically the Assert: r1 < x < r2. step.

Edit: I didn't see that you also propose to change ApplyUnsignedRoundingMode. I don't have enough time to check if ApplyUnsignedRoundingMode can be changed to allow r1 ≤ x ≤ r2 instead of the current requirement r1 ≤ x < r2.

That still leaves the question unanswered if Temporal.PlainDate.from("2023-05-31").until("2024-04-30", {largestUnit: "month"}).toString() should return "P10M30D" or "P11M". I'd prefer if this question is first answered before applying #2924.

from proposal-temporal.