bug#34834: 26.1; Remote `eshell/mv' and `eshell/cp' on Windows: Opening output file: Invalid argument, c:/home/ ...

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: jordan.t.wilson@gmx.com,  34834@debbugs.gnu.org
> Date: Thu, 28 Mar 2019 18:57:49 +0100
> 
> Remote file names shouldn't get a drive letter. But this happened:

Can you describe how this is supposed to work?  AFAIR, Tramp's
file-name handler gets called, and then turns around and calls back
into expand-file-name after inhibiting file-name handlers?  Or
something like that.  How does Emacs expand remote file names
correctly on Posix platforms?  If that somehow depends on the fact
that remote file names begin with a slash, and thus look like absolute
file names, then that's not enough for Windows.

Also, is this use case somehow special?  If not, how come Tramp works
at all on Windows?

> >> M-: (expand-file-name "123" "tmp/") ;; 123 is a file, tmp is a directory 
> >> there.
> >>
> >> => "c:/plinkx:detlefx:/home/albinus/tmp/123"
> 
> And the traces I've shown indicate, that it isn't Tramp who adds the
> drive letter. The rest to check are C sources, I'm unable to debug on MS
> Windows.

My problem is not to debug Emacs, my problem is where to debug.
Please help me focus my search for the culprit.

Meanwhile, the same problem happens in Emacs 26, so this is not new.