(declaim (ftype (function (simple-vector)) foo))
(defun foo (x)
  (declare (optimize (speed 3)))
  (length x))

; file: /tmp/slimezkgdmK
; in: DEFUN FOO
;     (LENGTH X)
; 
; note: unable to
;   optimize
; due to type uncertainty:
;   The first argument is a SEQUENCE, not a VECTOR.
; 
; compilation unit finished
;   printed 1 note

CL-USER> (describe #'foo)
#<FUNCTION FOO>
  [compiled function]


Lambda-list: (X)
Declared type: (FUNCTION (SIMPLE-VECTOR) *)
Derived type: (FUNCTION (T) (VALUES (UNSIGNED-BYTE 44) &OPTIONAL))
Source file: /tmp/slimezkgdmK