Consider the following code:

#include<iostream>


// #### This is my code, I can alter it.

struct A;

template<typename T> struct B;

template<typename T>
A operator=(B<T> const&);


// think of this as my custom vec3 implementation.
template<typename T>
class B{
    friend A operator=(B<T> const&);
    T j;
public:
    B(T j):j(j){}
};


// #### This is code of somebody else's github library. I cannot alter it. Think of it as GLM.

// think of this as glm::vec3 . 
struct A{
    int i;
    A(int i=0):i(i){}
    A()=default;
    A(A&&)=default;
    A(A const&)=default;
    constexpr A& operator=(A const&)=default;
    constexpr A& operator=(A&&)=default;
};


// #### This is my code, I can alter it.

// usually, I cannot cast my custom vec3, so it will be empty
template<typename T> A operator=(B<T> const& b){ return A(); }

// but under special cases it is just fine
A operator=(B<int> const& b){ return A(b.j); }


int main(){
    B<int> b(3);
    A a;
    
    a = b;  // as consumer I want to be able to simply assign like so.
    
    std::cout << a.i << "n";
}

Question

What is the best problem to realizing the intended consumer semantics, given I am prohibited from defining the assignment operator like so? Notice I have no way to affect the library GLM. In my actual scenario, I wrote a custom quaternion and want to assign it to glm::quat.