(defun foo (x)
  (if (car x)
      (if (consp x)
          'ok
          (cos x))))

;; foo.lisp:5:11:
;;   note: deleting unreachable code
;;   warning: 
;;     Derived type of COMMON-LISP-USER::X is
;;       (VALUES NULL &OPTIONAL),
;;     conflicting with its asserted type
;;       NUMBER.
;;     See also:
;;       SBCL Manual, Handling of Types [:node]
;; 
;; Compilation failed.