bug#16731: 24.3.50; Latin small letter sharp s is not considered lower-case

[Eli Zaretskii]

> Date: Wed, 12 Feb 2014 21:10:57 +0100
> From: Andreas Röhler <andreas.roehler@easy-emacs.de>
> CC: 16731@debbugs.gnu.org
> 
> While upcase1 can't succeed, downcase should - if "ß" is a member of 
> downcase_table.

But which character do you want to downcase in this case?

This whole logic works only for _pairs_ of characters (and the
char-table used here is populated by calls to set-case-syntax-pair).
Such machinery cannot possibly work when there's no pair.

The only way I can see out of this conundrum is to consult the
Lowercase Unicode property of the character as fallback, assuming that
won't slow down regex search too much.