# type 'a a = A : int a | B : float a | C : int a;;
# let f : type v. v a -> int = function A -> 1 | B -> 2 | C -> 1;;
val f : 'v a -> int = <fun>

# let f : type v. v a -> int = function A | C -> 1 | B -> 2;;
Characters 38-39:
  let f : type v. v a -> int = function A | C -> 1 | B -> 2;;
                                        ^
Error: This pattern matches values of type int a
       but a pattern was expected which matches values of type v a
       Type int is not compatible with type v