I'm sorry would you mind going into a bit more depth about that example? It's not obvious to me why the compiler can't figure out the the visitor is no longer exhaustive.
Visitor is just a collection of functions for all type's subtypes. The problem is that visitor knows nothing about class' subtypes, just like the base class knows nothing about it's children, so you have to add visit cases manually for each child class. The only way to detect that visitor is non exhaustive is to apply it to the child class it has no visit function for.
