Why does assignment operator not work as expected?

I got stuck with operator assignment problem - it doesn't work as expected. Here's the example code:

#include <iostream>

class TestClass{
private:
    int pop;
public:
    TestClass(){
        std::cout<<"Default Constuctor\n";
    }
    TestClass(int i):pop(i){
        std::cout<<"Param Constuctor\n";
    }

    TestClass &operator=(const TestClass &other){
        std::cout<<"Assignment Operator \n";
        return *this;
    }

    friend std::ostream &operator<<(std::ostream &out, TestClass &x);
};

std::ostream &operator<<(std::ostream &out, TestClass &x){
    out<<" This is the TestClass with pop=" << x.pop <<"\n";
    return out;
}


int main()
{
    TestClass    P0(333);
    TestClass P_foo(555);

    P_foo = P0;

    std::cout << P0;
    std::cout << P_foo;

    return 0;
}

The result of this program is

Param Constuctor
Param Constuctor
Assignment Operator 
 This is the TestClass with pop=333
 This is the TestClass with pop=555

So P_foo object preserves the initialized value 555. If I comment out my custom assignment operator, the program works as expected.

Param Constuctor
Param Constuctor
 This is the TestClass with pop=333
 This is the TestClass with pop=333

What's wrong with my assignment operator function?