I wrote the next code and if the comment line will be removed it will work. But i did the same with ref to int and everything was nice even without constructor.

#include <iostream>

struct B;

struct A
{
    B& b_;
    //A(B& b):b_(b){}
};


struct B:A
{
    int x = 5;

    B():A(*this)
    {
    }
};


int main()
{
    B b;
    std::cout<<b.b_.x<<std::endl;
}

https://godbolt.org/z/7hd7oaPon