This should compile but does not.

class A {
    class I1{
    }
}

class B : A {
    class I2 : I1 { // line 9
    }

    I1 getI(){
        return new I2;
    }
}
void main(){}

test.d(9): class test.B.I2 is nested within B, but super class I1 is nested within A