File "pr7620_bad.ml", line 10, characters 17-19:
10 |   (function `A | `B -> () : 'a) (`A : [`A]);
                      ^^
Error: This pattern matches values of type "[? `B ]"
       but a pattern was expected which matches values of type "[ `A ]"
       The second variant type does not allow tag(s) "`B"