Re: Odd behavior of operator=() in diamond inheritance situation

 Consider the following code:

//---------------------------------------------------------------
#include <iostream>

struct Base
{
   Base() { std::cout << "Base constructor\n"; }
   Base(const Base&) { std::cout << "Base copy constructor\n"; }
   Base& operator=(const Base&)
   { std::cout << "Base operator=\n"; return *this;}
};

struct M1: virtual public Base {};
struct M2: virtual public Base {};
struct Derived: public M1, public M2 {};

int main()
{
   Derived d1;
   Derived d2(d1);
   d1 = d2;
}
//---------------------------------------------------------------

One would assume that this program prints one message of
each type. However, a bit surprisingly, this is the result:

Base constructor
Base copy constructor
Base operator=
Base operator=

 Why is that?