Steel Bank Common Lisp

(declaim (ftype (function (symbol) (values symbol &optional))
                foo))
(defun foo (x)
  (check-type x symbol)
  x)

; in: DEFUN FOO
;     (DEFUN FOO (X) (CHECK-TYPE X SYMBOL) X)
; --> PROGN EVAL-WHEN 
; ==>
;   (SB-IMPL::%DEFUN 'FOO
;                    (SB-INT:NAMED-LAMBDA FOO
;                        (X)
;                      (BLOCK FOO (CHECK-TYPE X SYMBOL) X))
;                    NIL 'NIL (SB-C:SOURCE-LOCATION))
; 
; note: Assignment to argument: X
;   prevents use of assertion from function type proclamation:
;   SYMBOL
; 
; 
; compilation unit finished
;   printed 1 note

Can anyone explain this one? The check-type is there for implementations
which do not interpret declarations as assertions.

  -T.