template<typename T, typename S>
class Enclosing{
public:
    class Nested;
    Nested foo();
};
 
template<typename T, typename S>
class Enclosing<T,S>::Nested{
public:
    T field;
    void some_method();
    friend class Enclosing;    // instead of this line I also tried:
                               // friend class Enclosing<T,S> 
                               // and it didn't work either
};
 
template<typename T, typename S>
typename Enclosing<T,S>::Nested Enclosing<T,S>::foo (){
    Nested nes;
    nes.some_method();         // the problem appears here
    return nes;
}

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