template <typename t> class A;
template <typename t> class B
{
   public:
     friend template <typename t> class A; // <-- ошибка
};

template <typename t> class A;
template <typename t> class B
{
public:
    friend class A<t>;
};

template <typename t> class A;
template <typename t> class B
{
public:
    template<typename>
    friend class A;
};