Why do we need to use virtual ~A() = default; instead of virtual ~A() {} in C++11?

Question

In Stack Overflow post Checking the object type in C++11, I have the comment:

In C++11 you'll actually want to do virtual ~A() = default; Otherwise, you'll lose the implict move constructors.

What is virtual ~A() = default; for? How come implicit move constructors lost with virtual ~A() {}?

Answer 1

The comment is not correct.

Both:

virtual ~A() = default; 

and

virtual ~A() {} 

are user declared. And the implicit move members are inhibited if the destructor is user declared.

[dcl.fct.def.default]/p4 discusses user-declared and user-provided special members:

A special member function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.