I still lack some kind of sum type that is always checked for exhaustiveness. When I'm creating a class hierarchy for a domain, 9 times out of 10 I'd rather have a simple sum type. When I have to make a class hierarchy in C# anyway (because there are no simple sum types). I'd like to be able to write something like this, and have this match/switch fail to compile if I add a new type of shape.

    match(shape) 
    {
      case Rectangle r
        return r.With*r.Height;
      case Circle c
        return pi*c.Radius*c.Radius;
    }