Bruce Leban wrote:

If you change the semantics so that it either (1) it always always includes a trailing / or (2) it includes a trailing slash if the two paths have it in common, then you don't have the weirdness that in this case it returns a slash and in others it doesn't. I am slightly inclined to (1) at this point.

But then the common prefix of "/a/b" and "/a/c" would be "/a/",
which would be very unexpected -- usually the dirname of a path is
not considered to include a trailing slash.

The special treatment of the root directory is no weirder than it
is anywhere else. It's already special, since in unix it's the
only case where a trailing slash is semantically significant.
(To the kernel, at least -- a few command line utilities break this
rule, but they're screwy.)