While addressing ASDF issue #140 I stumbled on what seems to me to be an error. That is that

(ensure-directory-pathname "")

at least on SBCL, returns a pathname that has NIL in all its slots except HOST.

To me, this seemed wrong, and looking further, I found that directory-pathname-p returns T when given this pathname, because directory-pathname-p is defined (in its docstring) as:

A directory-pathname is a pathname without a filename.

I would have assumed that a directory pathname was also a pathname with a non-NIL directory component.

I am considering making this (incompatible) change, but am interested in any feedback about whether this is the correct thing to do.