bug#12579: Emacs 24.2 crashes or freezes

[Stefan Monnier]

> instead of a more natural syntax (directory, then file):
>   (expand-file-name windows-program-files-dir "SumatraPDF/SumatraPDF.exe")
> I know this may sound stupid, and purely syntactic...

I definitely understand your point, and it is well taken.
But expand-file-name is also often called with a single argument (the
relative file name), so it would be rather odd/inconvenient to combine
the two since Elisp optional args traditionally come at the end rather
than at the beginning.

OTOH, maybe we could do that for a new `concat-file-name-elements' which
would be like `expand-file-name' except that it leaves the result
relative if the directory argument is relative (so it'd be more like
a "concat that adds a slash when needed").


        Stefan