Making an undefined class as friend, and defining it later

Question

Making an unknown friend

template
class List
{
protected:

    class a {
        int x;
        int y;
    private:
        friend class b;         


        

Answer 1

The code is ill-formed and Comeau rejects it giving the following error

error: invalid redeclaration of type name "b" (declared at
      line 11)

I think this is a bug in g++. Intel C++ rejects it too. You can fix the code by defining class B above A.

template  class b { 
        int z;
        U y;
};
class a {
        int x;
        int y;
    private:
        friend class b;  
};