Warning for moving to an abstract type

Learning
2

Private types can help a bit:

module M : sig
  type t = private string * int

  val v : string -> int -> t
end = struct
  type t = string * int

  let v a b = a, b
end

let f (_ : string * int) = ()

let () = f (M.v "a" 1)

Error: This expression has type M.t but an expression was expected of type
         string * int

However, that’s a as-breaking change as just making the type abstract.

Edit: Alright, that’s not a warning but an error.