bug#12581: 24.2.50: redo+ fails to modify edit menu

[Stefan Monnier <monnier@iro.umontreal.ca>]

> I have tracked this down to the command
> (setcar (cdr (memq :enable (assq 'undo menu-bar-edit-menu))) nil)
[...]
> that fails with the same error. Actually redo+ attempts to setcar to
> something else but obviously the problem is that for some reason
[...]
> Is this a bug or expected behavior? Should redo+ be modified?

It's not a bug, AFAIC.  You can change redo+ to use something like:

   (define-key menu-bar-edit-menu [undo]
     ;; We use `cdr+assq' instead of `lookup-key' because lookup-key
     ;; would return the actual binding inside the menu-item, whereas
     ;; we want the whole menu-item (IOW we don't want the get_keyelt
     ;; part of lookup-key).  We could use
     ;; (catch 'found
     ;;   (map-keymap (lambda (k v)
     ;;                 (if (eq k 'undo) (throw 'found v)))
     ;;               menu-bar-edit-menu))
     ;; instead.
     (let ((elem (copy-sequence (cdr (assq 'undo menu-bar-edit-menu)))))
       (setcar (cdr (memq :enable elem)) <your-exp-here>)
       elem))

       
-- Stefan