template <class Derived, class ...Dependencies>
class System {
    static_assert(
        all<std::is_base_of<Component, Dependencies>{}...>{}>,
        "Every type in Dependencies should inherit from Component"
    );
}

@Retention(RetentionPolicy.RUNTIME)
@Target(value = {ElementType.FIELD})
public @interface Message {
    Class<?>[] signature() default {};
}