Is the yearAbsent assertion in ParseISODateTime correct?

[trflynn89]

In ParseISODateTime, step 4.a.ii.4 states:

4. If goal is TemporalMonthDayString and parseResult does not contain a DateYear Parse Node, then
   a. Assert: goal is the last element of allowedFormats.
   b. Set yearAbsent to true.

In the PlainMonthDay/from/argument-propertybag-calendar-iso-string.js test262 test, we create a Temporal.PlainMonthDay of the form:

Temporal.PlainMonthDay.from({ monthCode: "M11", day: 18, calendar: "01-01" });

Since calendar is a String, we enter ParseTemporalCalendarString. Its first step is:

1. Let parseResult be Completion(ParseISODateTime(string, « TemporalDateTimeString[+Zoned], TemporalDateTimeString[~Zoned], TemporalInstantString, TemporalTimeString, TemporalMonthDayString, TemporalYearMonthString »)).

So the last item in allowedFormats is TemporalYearMonthString. However, the calendar string "01-01" can be parsed as a TemporalMonthDayString, and in that case, the DateYear parse node will be empty. We will then fail the assertion that the parsed goal is the last allowed format.

[trflynn89]

Aha, the string "01-1" can actually also be parsed as a TemporalTimeString, with hour "01" and UTC offset "-01". So we then don't hit the assertion.

[trflynn89]

Actually, I'm back to thinking this is potentially a bug - "01-01" shouldn't be parsed as a TemporalTimeString due to the ambiguity rules:

https://tc39.es/proposal-temporal/#sec-temporal-iso8601grammar-static-semantics-early-errors

* It is a Syntax Error if ParseText(Time DateTimeUTCOffset[~Z], DateSpecMonthDay) is a Parse Node.

[ptomato]

I agree with this reading, thanks for catching it. This is a regression from ce94055. It could be that we need to swap the order of TemporalYearMonthString and TemporalMonthDayString in ParseTemporalCalendarString and ParseTemporalTimeZoneString, but on the other hand I'm not sure I remember what the assertion is guaranteeing in the first place. Maybe it was to ensure that the yearAbsent code path is only hit in cases where a month-day string is really supposed to be accepted?