public class Client {
    // Compiled against Shape v1, where the only permitted subtypes are Circle and Square.
    public static double area(Shape s) {
        return switch (s) {
            case Circle c -> Math.PI * c.r() * c.r();
            case Square q -> q.side() * q.side();
        };
    }

    public static void main(String[] args) {
        System.out.println("circle: " + area(new Circle(1)));
        System.out.println("square: " + area(new Square(2)));
    }
}