Multiple inheritance and covariant return types.

[BCS]

Reply to Frank,

> Hi,
> 
> I have another question on multiple inheritance. The following code
> does not work (tested with DMD 1.020 and 2.003)
> 
> ---
> interface A {}
> interface B {}
> 
> interface AB: A, B {}
> 
> abstract class X {
> abstract A func();
> }
> interface Y {
> B func();
> }
> class XY: X, Y {
> AB func() { return null; }
> }
> ---
> The compiler complains about the line 'AB func() { return null; }':
> t.d(38): function t.XY.func incompatible covariant types A() and B()
> 
> but I think it should work, because AB is both, an A and a B, so the
> return type is covariant with both overridden methods. In fact, the
> example works fine, if X is an interface instead of an abstract class.
> 
> Is this a bug or is it my mistake?
> 
> Thanks,
> Frank

The issues is that casting from a class that implements A, B and AB to each 
of these results in a slightly different result.

IIRC when a class is cast to an interface, what you get is a pointer to a 
v-table inside of the class (not the normal one). However because AB inherits 
from A and B, you end up getting two different v-tables. So returning an 
AB interface instance would give you something that is an AB and might be 
an A or B but not both. (This is all at the binary level of course)