Can't use public nested class as private method parameter

In the following code:

class Outer {
   private:
    void f_private(Outer::Inner in); // Wrong

   public:
    class Inner {};
    void f_public(Outer::Inner in); // OK
};

f_private() cannot use nested class Outer::Inner as parameter type. But it's ok to do so in f_public().

Can someone explain to me in what rule this is based on, and what's the benefit it?

The problem is not that it's public or private, it's the order. This may seem strange, with other class members this is not a problem, but consider that in this case you are declaring a new user defined type, because of that you must declare it before you use it:

class Outer 
{
public:
    class Inner {};   
    void f_public(Outer::Inner in);  // OK

private:
    void f_private(Outer::Inner in); // OK
};

Or:

class Outer
{
public:
    class Inner;                          // declare
    void f_public(Outer::Inner in);       // OK

private:
    void f_private(Outer::Inner in);       // OK
};

class Outer::Inner {};                     // define

void Outer::f_private(Outer::Inner in){}   // method definition after class definition

void Outer::f_public(Outer::Inner in){}    //

If you use the class as pointer or reference parameter then there is no need to define it before, a forward declaration will be enough.