type t = A | B
type u = C of t
Line 1, characters 18-54:
1 | let print_t out = function A -> Format.fprintf out "A";;
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 8 [partial-match]: this pattern-matching is not exhaustive.
  Here is an example of a case that is not matched: B

val print_t : Format.formatter -> t -> unit = <fun>
- : t =
<printer print_t raised an exception: File "//toplevel//", line 1, characters 18-23: Pattern matching failed>
- : u =
C
 <printer print_t raised an exception: File "//toplevel//", line 1, characters 18-23: Pattern matching failed>