some friend functions don't follow the rule

For the following snippet:

class A{
    friend void f(){};
    public:
        A(){f();} //error
};

class B{
    friend void f(void* ptr){};
    public:
        B(){f(this);} //no error
};

According to the rule that although friend functions can be defined inside a class, yet they are not visible until they are declared somewhere outside the class scope, the error in the definition of class A is explained.

But I am confused why the snippet for class B doesn't produce the same error as class A's.

Please can anyone tell me about this?