Wrong enum comparisons

[bearophile]

In some code I have created a small bug that can be reduced to 
something like this, that the D compiler has not caught at 
compile-time:

enum E1 { A, B }
enum E2 { C, D }
void main() {
    E1[2] a;
    with (E2)
        assert(a[0] == D);
}


Why isn't D able to statically tell when you compare values of 
different enums?
How much work is implementing this compile-time test inside the D 
front-end?

Thank you,
bye,
bearophile