Devon Sean McCullough <Emacs-hacker2023@jovi.net> writes:

> A better expansion such as
>
> (let* ((v (make-overlay 1 2))
>        (s (- (overlay-end v) 1)))
>   (progn
>     (move-overlay v (overlay-start v) s)
>     s))
>
> looks ok and should pacify the byte compiler.

Agreed.  This should do it: