putting more smarts into a == b

[Andrei Alexandrescu]

Frank Benoit wrote:
> Andrei Alexandrescu schrieb:
>> Consider two objects a and b with a of class type. Currently, the
>> expression a == b is blindly rewritten as a.opEquals(b). I argue it
>> should be rewritten into a call to an (imaginary/inlined) function
>> equalObjects(a, b), with the following definition:
>>
>> bool equalObjects(T, U)(T a, U b) if (is(T == class))
>> {
>>     static if (is(U == class))
>>     {
>>         if (b is null) return a is null;
>>         if (a is null) return b is null;
>>     }
>>     else
>>     {
>>         enforce(a !is null);
>>     }
>>     return a.opEquals(b);
>> }
>>
>> This hoists the identity test outside the opEquals call and also deals
>> with null references. What do you think?
>>
>>
>> Andrei
> 
> What about interfaces?

Good question! What do they do now? I ran this:

interface A {}
class Widget : A {}

void main() {
     auto a = cast(A) new Widget;
     A b = null;
     writeln(a == b);
     writeln(b == a);
}

To my surprise, the program printed false twice. If I replace A with 
Widget inside main, the program prints false then crashes with the 
mythical segfault :o).

So how are interfaces compared?


Andrei