Unions in C++11: default constructor seems to be deleted

Question

I am trying to understand how unions were extended by C++11. One thing that changed is the ability to use now non-static data members with non-trivial special member functions.

Answer 1

The cppreference quote is unclear. What happens is that if ANY memeber of the union defines ANY of those non-trivial special member functions, then ALL of them will be deleted by default in the union.

So since you have a non-trivial destructor for X, the U default constructor is deleted.