Comment 3 for bug 1893749

In case someone stumbles into this issue, I found a much cleaner solution than a straightlaced eval: eval-when. It can shift evaluation to compile-time. Like this:

(progn ;; 3
  (eval-when (:compile-toplevel :load-toplevel :execute)
    (defclass A () ((x :type boolean))) ;; (x :type boolean) -> (x)
    (c2mop:ensure-finalized (find-class 'A)))
  (gen))