Comment 3 for bug 309475

I apparently encountered this same problem in (https://bugs.launchpad.net/sbcl/+bug/1558695).

My case was the following:

(in-package :cl-user)

(defclass B ()
  ())

;; test-function-subtype takes an argument which is a
;; unary function B->B
(defun test-function-subtype (f)
  (declare (type (function (B) B) f)
    (optimize (debug 3) (speed 0) (safety 3)))
  (let ((ret-val (funcall f (make-instance 'B))))
    (format t "ret-val = ~A (typep ret-val 'B)=~A~%" ret-val (typep ret-val 'B))
    (assert (typep ret-val 'B))))

;; But I cannot make the return less specific
(test-function-subtype #'(lambda (b)
      (declare (type B b))
      12))

----------------------------

The compiler apparently replaces (typep ret-val 'B) with t. And at run-time
the value of ret-val is 12, which is NOT of type B.