Documentation for notdir says:

"A file name that ends with a slash becomes an empty string. This is unfortunate, because it means that the result does not always have the same number of whitespace-separated file names as the argument had; but we do not see any other valid alternative."

Wrong. $(notdir foo/) should return . (a period).

This way, $(dir name)/$(notdir name) = name always.

A path ending in / is only really valid if it is a directory. If it is a directory, then it contains . and .. so returning . should be valid if the original path was valid. And foo/./bar is the same as foo/bar unless a filesystem is screwed up.

So the . should be returned instead of nothing, to keep the number of filenames the same, and to allow join to rejoin them correctly.