bug#69968: Case-folding of Mathematical Alphanumeric Symbols

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Date: Sat, 23 Mar 2024 20:27:45 +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.

> Is it because the Unicode standard doesn't provide information
> about their case-folding?  And indeed they are missing from
> https://unicode.org/Public/UNIDATA/CaseFolding.txt

Unicode doesn't consider them letters.

> But OTOH, I can't find the file CaseFolding.txt in admin/unidata.
> This means Emacs doesn't use this file?

We don't.  We use the case-conversion information in UnicodeData.txt,
as it tells us everything we need to know.

> Then should we add more case-folding information explicitly
> for this Unicode block?

What is the rationale for doing so?  It's against Unicode, so we need
to have a good reason, as this will have to be maintained by hand, and
also because some users might be surprised.

> 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.

> 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?