Make a class template a friend of itself for different instantiation

Question

I want to make A<T> a friend of A<T2> for any types T and T2.

Is this possible?

Thanks.

Test (also at godbolt.org):

template <class T>
class A {
public:
    template <typename T2> void test(A<T2>& a) { a.v_ = 2;}
private:
    int v_;
    template <typename T2> friend A;
};

int main() {
    A<int> a;
    A<int> b;
    b.test(a);
    return 0;
}

Compiler Error:

<source>:7:28: error: friend type templates must use an elaborated type

    template <typename T2> friend A;

                           ^~~~~~~~

Answer 1

It should be

template <typename T2> friend class A;