val it = () : unit
signature S =
  sig
    datatype t = A | B of t
    val test : t -> unit
  end
structure A : S
functor F(X: sig
               datatype t = A | B of t
               val test : t -> unit
             end) :
         sig
           strange tycon: [E]
           val f : X.t -> bool
         end
structure B :
  sig
    datatype t = A | B of X.t
    val f : X.t -> bool
  end
val x = true : bool