Why can an expression a + b find operator+ defined as a friend inside the class of a and …

Why can an expression a + b find operator+ defined as a friend inside the class of a and b?

1. Argument-dependent lookup adds the hidden friend as a candidate.
2. Every friend function is a private member function.
3. The compiler searches every class definition in the program.
4. friend changes the operator into a built-in operation.