Hi. Thanks for the patch. [I don't have time to look at it right now, but hopefully me or someone else will shortly].

Additionally, for a reason that is to me inexplicable, IcuCollation, as well as
my derivation, insists on sorting articles starting with "A" under "⅍". I have
no idea why this happens or how to change it properly.

This is a guess, but: For the IcuCollation, there is a first-letter.ser (named something like that) file which determines how things are sorted under first letter. This file needs to be kept consist with the collation. Presumably you're collation is using that file, when a new one needs to be generated for it (There's scripts to generate the file in the maintinance sub directory. Off the top of my head I'm unsure how tied they are to ICUCollation or if they would work directly with yours).

I just noticed that my patch has a pretty stupid bug – it only handles first letters of words for sorting, instead of every letter. Some unicode-aware preg_replace would fix this.

Additionally, for a reason that is to me inexplicable, IcuCollation, as well as
my derivation, insists on sorting articles starting with "A" under "⅍". I have
no idea why this happens or how to change it properly.

btw, for anyone else following along - that issue is split to bug 43740

Actual patch:

I don't think this is the way that extending uca-default was envisioned of working. I believe the intention was more to use intl's built in support for different language collations [via new IcuCollation( 'pl' ) ] and add a first-letters-pl.ser file. This would probably integrate the sorting of polish letters with other types of letters more seamlessly. However, how to generate a first-letters-pl.ser file is a bit of an open question at the moment, and probably requires a much expanded maintenance/languages/generateCollationData.php file. [Although I have a vague idea how to make one that would imperfectly, but probably acceptably by hand]

With the actual patch, the getFirstLetter takes a binary string. There's no guarantee that the icu collation won't use a binary code that is also a code point for one of the "polish" letters (in practise that's probably rare though). Additionally since its a binary string, its not guaranteed to be valid UTF-8, and I'm not sure how mb_substr would handle invalid utf-8.

(In reply to comment #3)

I believe the intention was more to use intl's built in support for
different language collations [via new IcuCollation( 'pl' ) ] and add a
first-letters-pl.ser file.

As noted in the zeroth comment, IcuCollation doesn't seem to accept a 'pl' or 'pl_PL' argument for me. I tried that first, of course.

Does either of these work for you? Maybe I just need more PHP libs...

(In reply to comment #4)

As noted in the zeroth comment, IcuCollation doesn't seem to accept a 'pl' or
'pl_PL' argument for me. I tried that first, of course.

Per out IRC discussion, it turns out it actually does, but it also needs the .ser file with a 'pl' suffix. This makes the attachment essentially useless.

I just realized I said some incorrect things in comment 3.

(In reply to comment #3)

With the actual patch, the getFirstLetter takes a binary string. There's no
guarantee that the icu collation won't use a binary code that is also a code
point for one of the "polish" letters (in practise that's probably rare
though). Additionally since its a binary string, its not guaranteed to be
valid
UTF-8, and I'm not sure how mb_substr would handle invalid utf-8.

This is all incorrect. getFirstLetter takes the human readable sortkey.

This makes the attachment essentially useless.

Not entirely useless, after all the patch works now, where we don't have .ser files for pl yet. But Ideally we'd be doing this with the .ser file.

So the patch here is crap, but I submitted I838484b9 to fix this properly.

Merged.