test.ml

(* Compiles *)
module rec One : sig
  val a : int -> int
end = struct
  let a x = x + (One.a x)
end

(* Compiles *)
module rec Two : sig
  val a : int -> int
  module B : sig end
end = struct
  let a x = x + (Two.a x)
  module B = struct end
end

(* Does not compile *)
module rec Three : sig
  val a : int -> int
  module B : functor (X : sig end) -> sig end
end = struct
  let a x = x + (Three.a x)
  module B (X : sig end) = struct end
end