Can anyone explain why this does not compile:

module rec A : sig
  type t
end = struct
  type z = { x : B.t }

  type t = z

  let foo t = B.foo t.x t (* Error: This expression has type t but an
expression was expected of type A.t *)
end

and B : sig
  type t
  val foo : B.t -> A.t -> unit
end = struct
  type t
  let foo _ _ = ()
end

Note that if there's no type aliasing in A, it will succeed:


module rec A : sig
  type t
end = struct
  type t = { x : B.t }

  let foo t = B.foo t.x t
end

and B : sig
  type t
  val foo : B.t -> A.t -> unit
end = struct
  type t
  let foo _ _ = ()
end

Thanks!