Understanding the type error: “expected signature Int*Int->Int but got Int*Int->Int”

expected signature Int*Int->Int but got Int*Int->Int

let f x y = x + y;;

val f : int -> int -> int

f(1, 2)

fun f g = g (1, 2);

f (42, fn x => x * 2)

# let f x = x + 1;;
val f : int -> int = <fun>
# type int = Foo of string;;
type int = Foo of string
# f (Foo "hello");;
This expression has type int but is here used with type int

# let f g x y = g(x,y) + x + y;;
val f : (int * int -> int) -> int -> int -> int = <fun>
# type int = Foo of int;;
type int = Foo of int
# let h (Foo a, Foo b) = (Foo a);;
val h : int * int -> int = <fun>
# f h;;
This expression has type int * int -> int but is here used with type
  int * int -> int