CollationsOfLocale: Reconsider calling DefaultLocale

[anba]

CollationsOfLocale calls DefaultLocale, because that's what ICU4C returns when calling ucol_getKeywordValuesForLocale("collation", ...). This was noted in:

• Define fallback behaviour in {Calendars,Collations,HourCycles,NumberingSystems,CharacterDirection,WeekInfo}OfLocale proposal-intl-locale-info#76 (comment)
• Normative: Add more explicit algorithms for lookups proposal-intl-locale-info#92

For example in current implementations:

V8:

$ LANG=en ./d8 -e 'print(new Intl.Locale("abcdefgh").getCollations())'
emoji,eor
$ LANG=de ./d8 -e 'print(new Intl.Locale("abcdefgh").getCollations())'
emoji,eor,phonebk
$ LANG=sv ./d8 -e 'print(new Intl.Locale("abcdefgh").getCollations())'
emoji,eor,trad
$ LANG=zh ./d8 -e 'print(new Intl.Locale("abcdefgh").getCollations())'
emoji,eor,pinyin,stroke,zhuyin
$ LANG=ko ./d8 -e 'print(new Intl.Locale("abcdefgh").getCollations())'
emoji,eor,searchjl,unihan

JSC:

~$ LANG=en ./jsc -e 'print(new Intl.Locale("abcdefgh").getCollations())'
emoji,eor
~$ LANG=de ./jsc -e 'print(new Intl.Locale("abcdefgh").getCollations())'
phonebk,emoji,eor
~$ LANG=sv ./jsc -e 'print(new Intl.Locale("abcdefgh").getCollations())'
trad,emoji,eor
~$ LANG=zh ./jsc -e 'print(new Intl.Locale("abcdefgh").getCollations())'
pinyin,stroke,unihan,zhuyin,emoji,eor
~$ LANG=ko ./jsc -e 'print(new Intl.Locale("abcdefgh").getCollations())'
searchjl,unihan,emoji,eor

While it's possible to mirror the ICU4C behaviour when implementing this in ICU4X, maybe the spec should be changed to not call DefaultLocale?

It's easy for ICU4C-based implementations to detect when the fallback to the default locale was used (ucol_getKeywordValuesForLocale returns the status code U_USING_DEFAULT_WARNING), so no large code changes will be needed.

Proposed spec:

1. If loc.[[Collation]] is not undefined, then
   1. Return CreateArrayFromList(« loc.[[Collation]] »).
2. Let language be GetLocaleLanguage(loc.[[Locale]]).
3. Let r be LookupMatchingLocaleByPrefix(%Intl.Collator%.[[AvailableLocales]], « loc.[[Locale]] »).
4. If r is not undefined, then
   1. Let foundLocale be r.[[locale]].
   2. Let foundLocaleData be %Intl.Collator%.[[SortLocaleData]].[[]].
   3. Let list be a copy of foundLocaleData.[[co]].
   4. Assert: list[0] is null.
   5. Remove the first element from list.
5. Else,
   1. Let list be « "emoji", "eor" ».
6. Let sorted be a copy of list, sorted according to lexicographic code unit order.
7. Return CreateArrayFromList(sorted).

[sffc]

CC @hsivonen

[hsivonen]

It looks to me like this would make the behavior of CollationsOfLocale more consistent with the other "OfLocale" operations that back Intl.Locale.

[sffc]

This spec was just recently landed as part of Intl Locale Info. I'm not sure why it uses DefaultLocale here but not in the other operations. CC @FrankYFTang