Container Contravariance was Re: [typing] Type-erasure re generics

[Jesse Phillips]

Jonathan M Davis Wrote:

> > Ok, then let us actually test that and show it is valid[1]:
> > 
> > class A {}
> > class B:A {}
> > 
> > void main() {
> >     A[] a = new B[6];
> >     a[0] = new A();
> > }
> > 
> > 1. http://ideone.com/5sUZt
> > 
> > I have shown that what I am talking about is valid for Arrays (a container)
> > but not templates. The issues of actually implementing it is not related
> > to what I was replying to. I introduced it as a benefit to forgetting type
> > information in "generics." I was then told "Having a container of one type
> > should not be castable to a container of another type." So I am pointing
> > out we need to remove this feature from arrays right now!
> 
> In Java, you can't use generics with arrays, so it's not an issue.
> 
> But if the D code you give here compiles, then that's a bug and needs to be 
> reported.
> 
> - Jonathan M Davis

Java you will see that it has no issue converting the container, but throws an exception if inserting the wrong type:

http://ideone.com/EZRvn


Ok, I see the point:

class A {}
class B:A { void fly() {} }
 
void main() {
    B[] b = new B[6];
    A[] a = b;
    a[0] = new A();
    
    b[0].fly();
}

http://ideone.com/rLiVL

Which is why others were saying you could have B converted to a const container of A as this conversion is good for passing a container of B to a function that takes a container of A.