Intl.DateTimeFormat does not support 'und' locale

[sven-oly]

This seems wrong. Apparently 'und' falls back to 'en', which is different behavior than ICU4C.

Examples:

Welcome to Node.js v18.19.1.
Type ".help" for more information.
> dt = new Intl.DateTimeFormat('und', {"month":"short","weekday":"narrow","day":"numeric","calendar":"gregory","numberingSystem":"latn"})
DateTimeFormat [Intl.DateTimeFormat] {}
> dt.format()
'T, Apr 30'

> Intl.DateTimeFormat.supportedLocalesOf(["und"])
[]
> Intl.DateTimeFormat.supportedLocalesOf(["und", "en"])
[ 'en' ]
 

[sffc]

I thought that "und" was supported in engines, but I guess not?

CC @anba @FrankYFTang @gibson042 @eemeli

[eemeli]

und is not supported in browsers. Supporting it would probably fix some of the use cases of the Stable Formatting proposal, but not all.

[FrankYFTang]

The reason is very simple. there are no locale resources defined for "und".
See
https://github.com/unicode-org/cldr/blob/main/common/main/und.xml
is a 404

Also ref https://tc39.es/ecma402/#available-locales-list

[sffc]

The resources for "und" are stored in root.xml in CLDR.

[FrankYFTang]

In v8, internally we call

uloc_openAvailableByType(ULOC_AVAILABLE_WITH_LEGACY_ALIASES, &status);

to find out what locales are available. neither "und" nor "root" is enumerated

https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/uloc_8h.html#a1d61e1cb6a0d2ad60dc3cd78c931e551
said
"Gets a list of available locales according to the type argument, allowing the user to access different sets of supported locales in ICU."

if "und" and "root" are not reported by ICU as "available locales", then v8 will not treat them as supported.

[FrankYFTang]

sorry, I hit closed by accident.

[sffc]

I made an upstream issue: https://unicode-org.atlassian.net/browse/ICU-22766

Whether or not ICU decides to start including the root locale in the return value of uloc_openAvailableByType, I think Web engines could decide to include that locale in their own lists of supported locales.