Eli Zaretskii <eliz@gnu.org> writes:

> A better error message would be
>
>   (error "Cannot copy `%s' into its subdirectory `%s'" from to)
Done

Have fixed commented block in `dired-create-files', have a look.

> I don't understand why you use expand-file-name here: file-truename
> does it for you anyway.
Fixed.

> Suggest to modify the doc string as follows:
Done.

Have modified `file-subdir-of-p' according to your advices.
Please have a look. (Tested with success on windows also)

(file-subdir-of-p "/" "/") works now.

> Finally, it looks like this function only works when its two arguments
> already exist; when they don't, it returns nil.  If this is the
> intent, it should be reflected in the doc string.
Fixed docstring.
Fixed `copy-directory' by doing another check of `file-subdir-of-p'
after creation of the non--existing subdir. Thanks for this.