Why can copy elision not optimise all copy operations?

Question

I read about copy elision and how it can fasten up Programms by giving possibilities to write code more straight foreword without thinking about references of variables. In a small example I tried to find the limits of this technique.

#include <iostream>

class A
{
public:
   A(){}
   A(const A &a) {std::cout << "Copy" << std::endl;}
};

A foo(A a)
{
   return a;
}

int main(void)
{
   A a = foo(foo(A()));
   std::cout << std::endl;
   A b;
   b = foo(foo(b));

   return 0;
}

https://godbolt.org/z/xo88jq

Output:

Copy
Copy

Copy
Copy
Copy

The compiler already elides many of the copies in this example. But why is the compiler not able to elide all those copies leading to a or in the modification of b?