gistfile1.lisp

(defclass my-metaclass (standard-class)
  ((slot-metaclass :initform  nil
                   :initarg   :slot-metaclass
                   :accessor  slot-metaclass))
  (:documentation "metaclass"))

(defmethod validate-superclass ((class my-metaclass) (super-class standard-class))
  t)

(defmethod make-instance ((class direct-my-arg-slot) &rest initargs &key my-arg)
  (error "123213213"))

(defclass direct-my-arg-slot (standard-direct-slot-definition)
  ((my-arg :initarg :my-arg :reader my-arg))
  (:documentation "Type of slots expanded my-arg"))

(defmethod direct-slot-definition-class ((class my-metaclass) &rest initargs &key my-arg &allow-other-keys)
  (declare (ignore initargs))
  (if my-arg
      (find-class 'direct-my-arg-slot)
      (call-next-method)))

(defclass my-class ()
  ((slot-class :initform  nil
	       :my-arg    :the-arg
	       :initarg   :slot-class
	       :accessor  :slot-class))
  (:metaclass my-metaclass))

(make-instance 'my-class :slot-class "111")