Armed
            Bear Common Lisp 1.9.0

Java
            17.0.3 Azul Systems, Inc.

OpenJDK
            64-Bit Server VM

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CL-USER(1):
            (use-package :mop)

(defclass
            my-effective-slot-definition
            (standard-effective-slot-definition)

())

(defclass
            my-class (standard-class)

())

(defmethod
            validate-superclass

  
                    ((class my-class)

 
                      (superclass standard-class))

t)

(defmethod
            effective-slot-definition-class

  
                    ((class my-class) &rest initargs)

 
            (declare (ignore initargs))

 
            (find-class 'my-effective-slot-definition))

(defclass
            my-object ()

 
            ((my-slot :allocation :class))

 
            (:metaclass my-class))

T

CL-USER(2):
            CL-USER(2): #<STANDARD-CLASS MY-EFFECTIVE-SLOT-DEFINITION
            {731A05FD}>

CL-USER(3):
            CL-USER(3): #<STANDARD-CLASS MY-CLASS {42C6245B}>

CL-USER(4):
            CL-USER(4): #<STANDARD-METHOD VALIDATE-SUPERCLASS
            (MY-CLASS STANDARD-CLASS) {4959790F}>

CL-USER(5):
            CL-USER(5): #<STANDARD-METHOD
            EFFECTIVE-SLOT-DEFINITION-CLASS (MY-CLASS) {2AAA2CBF}>

CL-USER(6):
            CL-USER(6): #<THREAD "interpreter" {25793D3C}>:
            Debugger invoked on condition of type UNBOUND-SLOT

 
            The slot LOCATION is unbound in the object
            #<MY-EFFECTIVE-SLOT-DEFINITION {32EB5C37}>.

Restarts:

 
            0: TOP-LEVEL Return to top level.

[1]
            CL-USER(7): 