unused lexical variable in `cl-loop'

[Emanuel Berg]

This code works but the byte-compiler says the lexical
variable 'num' is unused on the first two occasions it occurs
in the code.

;;; -*- lexical-binding: t -*-
;;
;; this file:
;;   https://dataswamp.org/~incal/emacs-init/math.el

(require 'cl-lib)

(defun interval (&optional min max)
  (interactive "nmin: \nnmax: ")
  (or min (setq min 0))
  (or max (setq max 9))
  (unless (<= min max)
    (error "Bogus interval") )
  (cl-loop for i from min to max
           with num = ()
           collect i into num
           finally return num) )

;; (interval 10 5)
;; (interval)
;; (interval 19 99)

;; Warning: Unused lexical variable `num'
;; Warning: Unused lexical variable `num'

I don't know if that makes sense, it doesn't look like it,
however it gave me an idea:

(defun interval (&optional min max)
  (interactive "nmin: \nnmax: ")
  (or min (setq min 0))
  (or max (setq max 9))
  (unless (<= min max)
    (error "Bogus interval") )
  (cl-loop for i from min to max collect i) )

and indeed that works just as well and there isn't a warning.

So that's good ... but why nothing happens with 'num' in the
first attempt I wonder?

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