mysterious compile-time type error

[Christophe Rhodes]

 affects sbcl
 done

The below code generates a compile-time type error in the compilation of
SHIFT-FIELD, but I can't for the life of me see why. It is true that
the compiler is clever enough to spot that the call to
LINEAR-POS-IN-FIELD never returns (the asserts are the wrong way round)
but that should /help/, because MAKE-FIELD is then known to be a
(FUNCTION NIL NIL). Baffled.

(declaim (optimize (debug 3) (safety 3)))

(deftype element-type () '(unsigned-byte 8))
(deftype field-type () '(array element-type *))

(defparameter *width* 0)
(defparameter *height* 0)
(deftype coord-type () '(integer -1 100))
(declaim (type coord-type *width* *height*))

(declaim (inline linear-pos-in-field))
(defun linear-pos-in-field (x y)
  ;(declare (type coord-type x y))
  (assert (>= 0 x))
  (assert (>= 0 y))
  (assert (< x *width*))
  (assert (< y *height*))
  (+ (* (1+ y) *width*)
     (1+ x)))

(defun make-field ()
  ;; we make an array that's "big" enough.
  ;; The right and left borders are the same elements.
  (make-array
    (+ 1 ;; +1 is the top-left-most element
       (linear-pos-in-field (+ 2 *height*) (+ 1 *width*)))
    :element-type 'element-type
    :initial-element 0))

(defun shift-field (orig-f &optional (f (make-field)))
  (declare (type field-type orig-f f)))