Extension resolution for data provider

[jedel1043]

Is there an API to verify if an extension value for a specific LanguageIdentifier in a provider is supported, or plans to provide such API?

Extended:

ECMA-402 defines ResolveLocale, which goes through the process of checking if a language is supported by the provider (solved in #2374), and choosing a valid set of unicode extensions supported by the provider for the selected language.

This process is aid by the [[LocaleData]] internal slot, which has all the information of every supported unicode extension for every "supported" language. However, #2374 (comment) clarifies that providers don't have a bounded list of supported languages because of locale fallback, so I doubt providers would also contain an equivalent to [[LocaleData]].

From some tests I did, I can apparently do data requests like on #2374 to check if an extension is supported by the provider. However, I don't know if this is a reliable method to do this. I also would need to consider default values for every extension, but said values could differ between languages. (would #1833 solve that?) Are there any APIs I'm missing that could be used in this case?

[sffc]

Clients are intended to treat locale fallback like a black box. As long as you are using a properly configured LocaleFallbackProvider in ICU4X, all supported extension keywords will be resolved automatically.

Note that Locale and DataLocale support extension keywords but not LanguageIdentifier.

[jedel1043]

Ah, so locales with fallback automatically return the default value of an extension if the provider doesn't support the requested value?

[sffc]

Correct

See Thai DateTimeFormatter for an example. It defaults to the Buddhist calendar if the -u-ca extension is missing.

[jedel1043]

Nice! That simplifies the logic by a lot, since I just need to pass the locale directly to the component I want to use, then recover the resolved settings and locale from the created component, right?

Though, hc is an exception, because ECMA-402 defines hour12 which forcibly converts the default value to an 11/12 hour format, meaning I'd need to manually fetch the TimeLengthV1 data to get the default time before constructing the DateTimeFormatter. I'm hoping this is the only exception I'd need to handle 😅