boom.lisp

(defpackage :dat-application
  (:use :cl)
  (:export #:application))

(in-package :dat-application)

(defvar *application*)

(defclass application ()
  ((data :initform 0)))

(defmethod initialize-instance :after ((app application) &key &allow-other-keys)
  (setf *application* app))


(defpackage :sasapp
  (:use :cl))

(in-package :sasapp)

(defclass sas-application (dat-application:application)
  ())

(let ((inst (make-instance 'sas-application)))
  (setf (slot-value inst 'data) 10)
  (assert (= 10 (slot-value inst 'data))))

(in-package :dat-application)
(describe *application*)

https://gist.github.com/pkulev/4da8e07a1a1cedd6770938e8bf20832a#file-boom-lisp-L25

When attempting to
set the slot's value to 10 (SETF of SLOT-VALUE), the slot DATA
is missing from the object #<SAS-APPLICATION {100394E333}>.
   [Condition of type SIMPLE-ERROR]