>
> If you move a directory over another by renaming, the previous directory 
> is gone, and replaced by the one you moved in its place, right?  I'm not 
> talking about what GNU mv does, I'm talking about what Emacs will do 
> when renaming a directory into another one.
>

Sorry, I don't understand your question.  With the patch, if the output 
directory already exists, an "File already exists" is displayed, as I 
think should be the case.

I attach a slightly improved version, in which a tar file containing a 
single regular file is uncompressed into a directory.