MOP issues with <redefinable-class>

[Thompson, David]

I've been playing around with GOOPS and the metaobject protocol
lately. It has been fun, but I've hit a snag with redefinable classes.
Here's a contrived example program that demonstrates my problem:

    (use-modules (oop goops)
                 (srfi srfi-111))

    (define-class <meta> (<class>))

    (define (boxed-slot? slot)
      (get-keyword #:box? (slot-definition-options slot)))

    (define-method (compute-getter-method (class <meta>) slot)
      (if (boxed-slot? slot)
          (make <method>
            #:specializers (list class)
            #:procedure (let ((slot-name (slot-definition-name slot)))
                          (lambda (obj)
                            (unbox (slot-ref obj slot-name)))))
          (next-method)))

    (define-method (compute-setter-method (class <meta>) slot)
      (if (boxed-slot? slot)
          (make <method>
            #:specializers (list class <top>)
            #:procedure (let ((slot-name (slot-definition-name slot)))
                          (lambda (obj value)
                            (set-box! (slot-ref obj slot-name) value))))
          (next-method)))

    (define-class <foo> ()
      (bar #:accessor bar #:box? #t #:init-form (box 123))
      #:metaclass <meta>)

    (define-class <redefinable-foo> ()
      (bar #:accessor bar #:box? #t #:init-form (box 123))
      #:metaclass <redefinable-class>)

    ;; This works:
    (pk (+ (bar (make <foo>)) 456))

    ;; This throws an error:
    (pk (+ (bar (make <redefinable-foo>)) 456))

The above code introduces a metaclass, <meta>, that supports a special
slot option: #:box? The metaclass arranges for boxed slots to be
boxed/unboxed transparently via the slot's accessor methods when it
detects the presence of #:box? #t in the slot options.  However, the
redefinable class implementation does a transformation of the slot
definitions in its compute-slot method and discards all non-standard
slot options, so the final line throws an error because the bar method
returns a box, not a number.

I see two possibilities:

1) Slots are not designed to be extended with custom options, but it
just happens to work in the case of <class>
2) There's a bug in compute-slots for <redefinable-class>

Is this a bug or as-designed?

Thanks,

- Dave