Hi,

I have a problem with this test-case which doesn't compile:

module M : sig
  type a = [`A]
  type 'a b = [`B of 'a]

  val a : unit -> a
  val b : 'a -> 'a b
end = struct
  type a = [`A]
  type 'a b = [`B of 'a ]

  let a () = `A
  let b x = `B x
end;;

let rec f = function `B x -> f x | `A -> ();;

f (M.b (M.a ()));;

and produce the following error:

Error: This expression has type M.a M.b = [ `B of M.a ]
       but an expression was expected of type [< `B of 'a ] as 'a
       Type M.a = [ `A ] is not compatible with type M.a M.b = [ `B of M.a ]
       These two variant types have no intersection

The thing is, if we define M.a and M.b with « unit -> [> a ] » and « 'a -> [> 'a b ] », it does compile but I don't know why the previous code doesn't.

Does somebody have any hints ?