Templated vec

[bearophile]

This is an array struct template, this code compiles:

struct Vec(size_t N) {
    void opBinary(string op:"~", size_t M)(int[M]) {}
}
void main() {
    Vec!2 a1;
    int[3] a2;
    a1 ~ a2; // OK
}



But this code doesn't compile:

struct Vec(size_t N) {
    void opBinary(string op:"~", size_t M)(Vec!M) {}
}
void main() {
    Vec!2 a1;
    Vec!3 a2;
    a1 ~ a2; // line 7, Error
}


DMD 2.049 shows:
test.d(7): Error: cannot implicitly convert expression (a2) of type Vec!(3) to Vec!(M)

Can you show me how to fix the second program?

Bye and thank you,
bearophile