type _ tree =
  | Node  : 'a tree * 'a tree -> 'a tree
  | Leaf  : [ `good ] tree
  | Taint : [< `good | `bad ] tree -> [ `bad ] tree

let taint : [< `good | `bad ] tree -> [ `bad ] tree =
  fun tr -> match tr with
    | Taint tr -> Taint tr
    | _ -> Taint tr

let rec untaint (type t) (tr : t tree) : [ `good ] tree =
  match tr with
    | Taint _ -> failwith "Cannot untaint this tree"
    | Leaf -> Leaf
    | Node (l, r) -> Node (untaint l, untaint r)
(*                                 ^
Error: This expression has type t tree
       but an expression was expected of type t tree
       The type constructor t would escape its scope
*)