open Base

(* [id] can operate on global arguments *)

let f : 'a. 'a -> 'a = Fn.id

[%%expect {| |}]

(* [id] can operate on local arguments *)

let f : 'a. local_ 'a -> local_ 'a = Fn.id

[%%expect {| |}]

(* [id] cannot make a local argument global; this would be unsound *)

let f : 'a. local_ 'a -> 'a = Fn.id

[%%expect
  {|
Line _, characters _-_:
Error: This expression has type local_ 'b -> local_ 'b
       but an expression was expected of type local_ 'a -> 'a
|}]

(* [id] cannot make a global argument local; this is unexpected.
   If this following code gets accepted, it means that the meaning of
   [@local_opt] may have changed. However, the [f] below is not unsound. *)

let f : 'a. 'a -> local_ 'a = Fn.id

[%%expect
  {|
Line _, characters _-_:
Error: This expression has type 'b -> 'b
       but an expression was expected of type 'a -> local_ 'a
|}]