Polyfill: Change check to assertion in untilCalendar

[catamorphism]

This check should be an assertion, as the condition is checked before calling untilCalendar.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.41%. Comparing base (03f9057) to head (c534e88).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3218      +/-   ##
==========================================
+ Coverage   96.75%   97.41%   +0.65%     
==========================================
  Files          22       22              
  Lines       10398    10399       +1     
  Branches     1859     1826      -33     
==========================================
+ Hits        10061    10130      +69     
+ Misses        289      245      -44     
+ Partials       48       24      -24     

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[ptomato]

I found a code snippet that triggers this assertion:

const d1 = Temporal.PlainDateTime.from('2026-01-06T11:02[u-ca=gregory]');
const d2 = Temporal.PlainDateTime.from('2026-01-07T09:02[u-ca=gregory]');
d1.until(d2, { largestUnit: 'years' });

So probably we should consider it a testing coverage gap in any case.

I guess to fix this, we should decide at what level we want to have this early return. As long as it isn't observable, we can do this editorially.

One possibility is to fix all callers of CalendarDateUntil both in the polyfill and spec text so that we can make the assertion in CalendarDateUntil. Another possibility is to just make CalendarDateUntil definitively return if the two dates are equal and then we can make the assertion here in nonISOHelperBase.untilCalendar.

[catamorphism]

One possibility is to fix all callers of CalendarDateUntil both in the polyfill and spec text so that we can make the assertion in CalendarDateUntil. Another possibility is to just make CalendarDateUntil definitively return if the two dates are equal and then we can make the assertion here in nonISOHelperBase.untilCalendar.

@ptomato I added the early return in CalendarDateUntil; that seems like the simplest thing. I'll also submit a test262 PR with your test.

[catamorphism]

test262 PR: tc39/test262#4813