(* Structure subtyping *)

signature SIG1 = sig
    type t
    val x : t
end

signature SIG2 = sig
    val x : int
end

signature SIG1AND2 = sig
    type t = int
    val x : t
end

functor F(X : SIG1AND2) = struct
    structure A : SIG1 = X
    structure B : SIG2 = X
end
(* functor F : SIG1AND2 -> (sig structure A : SIG1 and B : SIG2 end) *)

fun f X =
    let structure A as SIG1 = X
        structure B as SIG2 = X (* Type error here *)
    in  ()
    end
(* Type mismatch, X must have both types: [ SIG1 ] and [ SIG2 ] *)