type (_, _) eq = Refl : ('a, 'a) eq
val cast : ('a, 'b) eq -> 'a -> 'b = <fun>
Line 3, characters 30-55:
3 |   let rec (p : (int, a) eq) = match p with Refl -> Refl in
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This kind of expression is not allowed as right-hand side of let rec