foo.ml

type 'a foo = Foo : {
  f : 'a -> int;
  mutable x : string;
} -> 'a foo

let some_foo : 'a. 'a foo = Foo {
  f = (fun _ -> 1);
  x = "";
}

(*
File "foo.ml", line 6, characters 28-65:
Error: This definition has type 'a foo which is less general than
         'a0. 'a0 foo
*)