Multiple inheritance and covariant return types.

[Frank Fischer]

Bruno Medeiros wrote:

> Frank Fischer wrote:

>> 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()
>> 
> 
> 
> It's not a bug.
> Classes are *convertible* to interfaces that it implements but they are
> not *covariant* with said interfaces. The same thing happens for an
> interface and it's super interfaces. The interface is not covariant with
> is super interfaces, only convertible to them. (exception maybe the
> first interface). Thus in the example above AB is not both an A and B.
> This behavior is different than Java and other OO languages, and is so
> for performance reasons. There are some tricks the compiler does to
> allows override as if the types were covariant, but it only works in
> some situations.

Thanks for the explanations. So I have to live with that behaviour and
handle the problem in a different way.

Frank