Re: BUG #18771: ICU custom collations with rules ignore collator strength option. - Mailing list pgsql-bugs

I think in this case it's not really related, as I'm not trying to copy options from the base locale.

It all seems to come from some missing information on the official icu4c docs. When describing the parameters of ucol_openRules() it says:

"strength: The default collation strength; one of UCOL_PRIMARY, UCOL_SECONDARY, UCOL_TERTIARY, UCOL_IDENTICAL,UCOL_DEFAULT_STRENGTH - can be also set in the rules"

And one could easily assume that if it "can also be set in the rules", you could pass UCOL_DEFAULT_STRENGTH and the rules would take precedence. In no place it does mention that UCOL_DEFAULT is a valid value for that parameter, although it is mentioned for the normalizationMode. But, if you look at icu4c sources (https://github.com/unicode-org/icu/blob/f8aa68b0c1c9584633e7a61157185f1a2c275f58/icu4c/source/i18n/collationbuilder.cpp#L182), you can find this:

RuleBasedCollator::internalBuildTailoring(const UnicodeString &rules,
                                          int32_t strength,
                                          UColAttributeValue decompositionMode,
                                          UParseError *outParseError, UnicodeString *outReason,
                                          UErrorCode &errorCode) {

...

    // Set attributes after building the collator,
    // to keep the default settings consistent with the rule string.
    if(strength != UCOL_DEFAULT) {
        setAttribute(UCOL_STRENGTH, static_cast<UColAttributeValue>(strength), errorCode);
    }

...

}

Which not only implies that UCOL_DEFAULT is a valid argument, but also that if you don't pass UCOL_DEFAULT any 'strength' options will be overridden. So it seems that the 'make_icu_collator' function inside postgres should use UCOL_DEFAULT, to allow the rules to set the desired strength level, instead of the current UCOL_DEFAULT_STRENGTH argument.