contravariant argument types: wanna?

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> On Wed, 23 Sep 2009 11:13:26 -0400, Andrei Alexandrescu 
> <SeeWebsiteForEmail at erdani.org> wrote:
>> class AutoVehicle { ... }
>> class Car : AutoVehicle { ... }
>> class Truck : AutoVehicle { ... }
>>
>> class Driver {
>>      // A driver is licensed to drive a car
>>      void drive(Car c);
>> }
>>
>> class TruckDriver : Driver {
>>      // A truck driver is licensed to drive a car...
>>      override void drive(Car c);
>>      // ... and a truck
>>      void drive(Truck c);
>>      // No contravariance needed yet
>> }
>>
>> class JamesBond : Driver {
>>      // James Bond can drive any auto vehicle
>>      // Contravariance needed here
>>      override void drive(AutoVehicle c) { ... }
>> }
>>
>> Now if what you have is a JamesBond and a Truck, you need 
>> contravariance to have him drive it. (A HotGirl may or may not be 
>> present in the scene.)
> 
> Your example just triggered a possible problem with contravariance.  
> Consider this class:
> 
> class Bad : TruckDriver {
>    override void drive(AutoVehicle c) { ...}
> }
> 
> What does this override, drive(Truck) or drive(Car), or both?  What if 
> you didn't want to override both?  My instinct is that this should be an 
> error, to keep things simple.  But it might be very annoying for some 
> designs...

It should override both. It is already the case that one method 
overrides several others (e.g. with multiple inheritance of interfaces).

Andrei