Why is my class non default-constructible?

Question

I have those classes:

#include 

template 
class A {
public:
    static_assert(std::is_default_constructible_v)         


        

Answer 1

Undefined behavior it is:

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.