bug#69968: Case-folding of Mathematical Alphanumeric Symbols

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 69968@debbugs.gnu.org
> Date: Sun, 24 Mar 2024 19:09:10 +0200
> 
> >> I wonder why case-folding is not supported for letters from
> >> the Unicode block "Mathematical Alphanumeric Symbols":
> >> https://en.wikipedia.org/wiki/Mathematical_Alphanumeric_Symbols
> >
> > These are not letters, they are symbols.  And letter-case is not
> > defined for symbols.
> 
> 𝘋𝘰 𝘺𝘰𝘶 𝘳𝘦𝘢𝘭𝘭𝘺 𝘵𝘩𝘪𝘯𝘬 𝘵𝘩𝘪𝘴 𝘵𝘦𝘹𝘵 𝘪𝘴 𝘯𝘰𝘵 𝘸𝘳𝘪𝘵𝘵𝘦𝘯 𝘸𝘪𝘵𝘩 𝙡𝙚𝙩𝙩𝙚𝙧𝙨?

What does that prove?  The fact that the glyphs look like normal
letters doesn't mean they are.  Like ℵ and ℶ are not Hebrew letters
they look like (and have left-to-right directionality).  And similarly
with 𞸀, 𞸁 and other mathematical symbols in that block aren't Arabic
letters, and in particular don't shape like Arabic letters.

> >> Case-folding is already supported for some characters from other
> >> Unicode blocks such e.g. FULLWIDTH LATIN CAPITAL LETTERs,
> >> CIRCLED LATIN CAPITAL LETTERs, etc.
> >
> > That's because UnicodeData.txt defines their letter-case conversions.
> 
> Ok, then it's very strange that the Unicode standard doesn't define
> letter-case conversions for other letters.  But what can we do.

We can define case-conversions for them if we decide to do so.
Moreover, Lisp programs which for some reason need that can do that
themselves, even if by default there are no case-conversions defined
for them.  The question is when and why is this needed?

> >> But e.g. PARENTHESIZED LATIN CAPITAL LETTERs are missing too.
> >> What is worse is that in Emacs ⒜ doesn't have even a word syntax
> >> like its counterpart 🄐.
> >
> > I think the fact that 🄐 has the word syntax might be a mistake.  These
> > are both symbols, so why would we want them to have the word syntax?
> 
> Because they look like letters with diacritics.

Not sure I agree.