bug#13553: 24.3.50; incorrect usage of IS_DIRECTORY_SEP

[Eli Zaretskii <eliz@gnu.org>]

> From: Shigeru Fukaya <shigeru.fukaya@gmail.com>
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>,
>  13553@debbugs.gnu.org
> Date: Sun, 27 Jan 2013 17:36:48 +0900
> 
> CharNextExA/CharPrevExA work based on dynamically specified codepage.
> 
> And, _mbspbrk/_mbslwr are also used in w32.c.  They work based on MS
> Window's locale setting.
> 
> Isn't it inconsistent?

It is, but I don't know of any practical way of eliminating that
inconsistency.  In the normal case, where the current
file-name-coding-system is the same as the ANSI codepage, there is no
inconsistency, so using CharNextExA/CharPrevExA is never worse than
using _mbsinc/_mbsdec.  It is sometimes better, because the code which
calls the _mbs* functions is executed only by some functions in w32.c.

> _mbsinc/_mbsdec would simply be sufficient?

That would preclude code like this:

  (let ((file-name-coding-system 'cp932))
    (expand-file-name "SOMETHING" "C:/")

I think it's reasonable to expect this to be supported, even if the
default file-name encoding is not cp932.

> (Besides, I don't know well, _mbslwr seems to me lower latin
> characters' case on some locale.  Is it true, or intended behavior?)

The intended behavior is to downcase letters for which lower-case is
defined.  I don't know if this is limited to Latin characters.