question on type checking

["Stephen Weeks" <sweeks@sweeks.com>]

Why does one of the programs below fail to type check while the other two minor
variants succeed?  I would expect all three to succeed.

----------------------------------------------------------------------
(* FAILS *)
module F (Z : sig end) = struct
  type t = A
end

module S = struct
  module Z = struct end
  module M = F (Z)
  include M
end

let f x =
  let module S = S in
  match x with
  | S.A -> ()
----------------------------------------------------------------------
(* WORKS *)
module F (Z : sig end) = struct
  type t = A
end

module S = struct
  module M = F (struct end)
  include M
end

let f x =
  let module S = S in
  match x with
  | S.A -> ()
----------------------------------------------------------------------
(* WORKS *)
module F (Z : sig end) = struct
  type t = A
end

module S = struct
  module Z = struct end
  module M = F (Z)
  include M
end

let f x =
  match x with
  | S.A -> ()
----------------------------------------------------------------------