Are "islamic" and "islamic-rgsa" still supported by Intl.DisplayNames?

[anba]

Are both "islamic" and "islamic-rgsa" still supported by Intl.DisplayNames?

js> new Intl.DisplayNames("en", {type: "calendar"}).of("islamic")
"Hijri Calendar"
js> new Intl.DisplayNames("en", {type: "calendar"}).of("islamic-rgsa")
"Hijri Calendar (Saudi Arabia, sighting)" 

If they are still supported, then https://github.com/tc39/test262/blob/main/test/intl402/Intl/supportedValuesOf/calendars-accepted-by-DisplayNames.js needs to be updated. This assertion checks that Intl.DisplayNames doesn't support any other calendars than the ones returned by Intl.supportedValuesOf. If Intl.supportedValuesOf no longer returns "islamic" and "islamic-rgsa", but Intl.DisplayNames still supports both identifiers, then the assertion will fail.

[ben-allen]

supportedValuesOf should not return "islamic"/"islamic-rgsa" but what Intl.DisplayNames does with them depends on the locale data (and whether it is updated). We'll remove the test for Intl.DisplayNames supporting non-supported calendars, since it's implementation-defined behaviour.

[sffc]

For Intl.DisplayNames, ECMA-402 says

The value of the [[LocaleData]] internal slot is implementation-defined within the constraints described in 9.1 and the following additional constraints:

and

The display name style fields under display name type "calendar" should contain Records with keys corresponding to calendar identifiers that can be matched by the type Unicode locale nonterminal. The value of these fields must be string values.

so it seems that it can support display names beyond those in the AvailableCalendars list.

[sffc]

TG2 discussion: https://github.com/tc39/ecma402/blob/main/meetings/notes-2026-01-08.md#are-islamic-and-islamic-rgsa-still-supported-by-intldisplaynames-91

[ben-allen]

Transferring to 402, per discussion in TG2