Comment 2 for bug 1766003

Why is the behavior of do so different from:

(defun foo2 (startr endr)
  (declare (dynamic-extent startr))
  (let ((acc 0))
    (loop for i fixnum from startr to endr
       do (incf acc i))
    acc))

where, again, the initial value of i is startr, but I see no warning? I'm imagine I'm missing something obvious here.