bug#12517: 24.2; (file-name-directory "~") returns nil

[Eli Zaretskii]

> From: Leo <sdl.web@gmail.com>
> Date: Wed, 26 Sep 2012 10:35:10 +0800
> 
> I was bitten by this corner case that (file-name-directory "~") returns
> nil.

As does (file-name-directory "a").

> I wonder if this is a bug/oversight?

I think it is neither.  file-name-directory works on the syntax of its
argument, not on its semantics.  The ELisp manual says:

 -- Function: file-name-directory filename
     This function returns the directory part of FILENAME, as a
     directory name (*note Directory Names::), or `nil' if FILENAME
     does not include a directory part.

"FILENAME does not include a directory part" means literally that: the
string FILENAME must _include_ a directory part for
file-name-directory to return a non-nil result.

If you want the directory of its argument, even if the argument is
given as any kind of shorthand, run that argument through
expand-file-name first.