(declaim (optimize (speed 3)))

(defun arr-fieldi (i ar)
  (aref (the (simple-array (signed-byte 64)
                           (10))
             ar)
        i))

(defun update-arr-fieldi (i v ar)
  (progn (setf (aref ar i) v)
         ar))

(defvar my-ar (make-array 10
                          :element-type '(signed-byte 64)
                          :initial-element '0))

; True:
(typep my-ar 'simple-array)

(update-arr-fieldi 0 (1+ most-positive-fixnum) my-ar)

(assert (equal 537 ; wrong!
               (arr-fieldi 0 my-ar)))

(update-arr-fieldi 0 most-positive-fixnum my-ar)

(assert (equal most-positive-fixnum ; correct
               (arr-fieldi 0 my-ar)))