Why is it undefined to derefrence a pointer to extern union?

Question

I have the following code. Why its undefined to derefrence union pointers in the following way ?

extern union MyUn *P1;          
extern union MyUn *P2;

extern void myfunc(void)
{
      *P1 = *P2;                
}

Answer 1

That has nothing to do with unions in particular, and it's not "undefined", either: It's simply a compiler error if you try to dereference a pointer to an incomplete type (for obvious reasons).

Answer 2

If you haven't also defined the union in this source file, the compiler doesn't know how much to copy.

What is the size of the union?