bug#11309: 24.1.50; Case problems with [:upper:] and Cyrillic, Greek

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 8 Dec 2020 17:57:32 +0100
> Cc: larsi@gnus.org, kehoea@parhasard.net, 11309@debbugs.gnu.org
> 
> This is not about the Lisp (upcase x) function but the C upcase(x) function, 
> which uses the upcase table directly.
> They affect the uppercasep and lowercasep functions which are used in the 
> regexp engine. Thus we get uppercasep(ß)=lowercasep(ß)=false which is wrong.

Why is it wrong, and what practical problems does this cause?

> The logic of 'lowercasep' may need to be changed because its use of upcase 
> and downcase which return their argument if the respective table has no entry 
> for it. Let's see what can be done.

I don't want us to change the logic of such basic functions for the
benefit of a single obscure character.  Let's first see what problems
with this character we have in practice, and then discuss what is the
best way of solving those problems.

TIA