implimenting interface function by inheriting from other class

[Alexey via Digitalmars-d-learn]

Hello
```D
interface Int
{
    void coolFunc();
}

class C1
{
    void coolFunc()
    {
        return;
    }
}
class C2 : C1, Int
{

}

void main()
{
    auto c = new C2;
}
```
dmd says it's not Ok:
t.d(14): Error: class `t.C2` interface function `void coolFunc()` 
is not implemented

how to make dmd happy?