contravariant argument types: wanna?

[Walter Bright]

Andrei Alexandrescu wrote:
> Walter Bright wrote:
>> Andrei Alexandrescu wrote:
>>> Today D does not support contravariant arguments, but Walter told me 
>>> once he'd be quite willing to implement them. It is definitely the 
>>> right thing to do, but Walter would want to see a compelling example 
>>> before getting to work.
>>
>> The stumbling block to contravariant parameters (not arguments) is how 
>> it throws a monkey wrench into the overloading rules. The problem is 
>> deciding if a function overrides or overloads, when its parameter 
>> types are different.
>>
>> The current rule is simple: if the parameter types are identical, it 
>> overrides. If they are not identical, it overloads.
> 
> I don't see that as a huge problem. First off, right now the override 
> keyword is required (and for good reason, I might add).

That is a good point.

> Second, the new 
> rule is simple: if the overriding function can be called with the 
> overriden function's arguments, it is overriding it. True, things get 
> more complicated when the base class also defines a corresponding overload:
> 
> class A {
>     void fun(A);
>     void fun(B);
> }
> 
> class B : A {
>     override void fun(A);
> }
> 
> This must be either an error, or the case that B.fun overrides both 
> overloads of fun in A.

I would really want to get away from the notion of selecting which 
function is overridden based on being a "better" match.