Sorry, the previous link was false. 
The patch is available at:
http://pauillac.inria.fr/~lefessan/src/patch.cast.gz

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I have made a small patch to ocaml-2.02 to allow safe casts of objects.
The patch adds two new keywords "implements" and "cast". 

 - "implements" ("implements c1 c2") is used to declare that objects
from a class c1 can be cast to the type of another class
c2. Implements checks at compile time that such a cast is
safe. Polymorphic classes (class ['a] ...)  are not allowed as
"implements" parameters. However, derived classes are allowed 
(class b = [int] t). By default, casting objects to their original class
is always allowed without using "implements", but all other casts must be
precedeed by an "implements" which allows them.

- "cast" ("cast o1 c1") is used to cast an object o1 to the type of a class c1.
A runtime check is performed to verify that the original class of the object
can be cast to the new class (thanks to an "implements" instruction). If
not correct, a (Failure "Cast failure") is raised.

Here is an example of use. Such a cast is interesting to retrieve the
original type of an object after it has been subtyped to be stored in
a generic structure.

class t1 () =
  object 
  method a = 0
  end;;

class t2 () = 
  object
  method b = 1
  method a = 0
  end;;

let o1 = new t1 ();;
let o2 = new t2 ();; (* val o2 : t2 *)

let x = (o2 :> t1);; (* val x : t1 *)
let y = cast x t2;; (* cast x to its original class, val y : t2 *)

implements t2 t1;; (* t2 implements the interface of t1 *)
let x = (o2 :> < >);; (* val x: < > no methods *)
let y = cast x t1;; (* cast x to the interface of t1, val y : t1 *)

let z = cast o1 t2;;  (* ERROR: Failure "Cast failure" *)
implements t1 t2;;    (* ERROR: not (t1 :> t2) *)


- Fabrice

Homepage: http://pauillac.inria.fr/~lefessan