How to detect whether there is a specific PRIVATE member variable in class?

Question

This question is based on this post.

Goal: I would like to know if a class has the member variable x. I would like to receive true

Answer 1

The following seems to work. Please tell me if it needs improvement. Live example.

class Haser {
public:
template 
static constexpr bool HasX = false;

template  
static constexpr bool HasX = true;
};

struct A { 
private:    
    int x;
    friend Haser;
};

Haser::HasX is true.