bug#26272: default-directory: Remark that it must be a directory name

[Tino Calancha]

On Tue, 28 Mar 2017, Eli Zaretskii wrote:

> > From: Tino Calancha <tino.calancha@gmail.com>
> > Date: Tue, 28 Mar 2017 10:54:36 +0900 (JST)
> > cc: Eli Zaretskii <eliz@gnu.org>, Tino Calancha <tino.calancha@gmail.com>,
> >     26272@debbugs.gnu.org
> >
> > III) Wrong
> > M-x cd RET /tmp// RET
> > M-: (default-directory) RET
> > => "/"
>
> Why is this wrong?  It's a standard Emacs feature.
I don't think is wrong, indeed is consistent with the behaviour
in Bug#21454.
I was just a bit sorprise that the result is different in
interactive calls compared with:
M-: (cd "/tmp//") RET
M-: (default-directory) RET
=> "/tmp/"