嵌套类中需要前向声明的友元声明

我正在尝试为具有私有成员的嵌套类编写非侵入性的boost:：序列化例程。不幸的是，我没能让g++相信序列化例程是内部类的朋友。g++似乎需要序列化例程的前向声明，而这反过来又需要嵌套类的前向宣布，而这在C++中是无法实现的。我是错过了什么，还是这不可能？相比之下，clang++不需要前向声明，下面的代码也没有问题。以下代码说明了问题：

#include <boost/archive/text_oarchive.hpp>
class Outer;
//class Outer::Inner;   // Not valid C++
namespace boost
{
    namespace serialization
    {
        template <class Archive>
        void serialize(Archive &ar, Outer& outer, const unsigned int version);
        //template <class Archive>
        //void serialize(Archive &ar, Outer::Inner& inner, const unsigned int version); // Cannot be done since forward declaration of nested class not possible.
    }
}
class Outer
{
    class Inner
    {
        int member_{42};
        template <class Archive>
        friend void boost::serialization::serialize(Archive &ar, Outer::Inner &inner, const unsigned int version);  // This does not work with gcc since the compiler seems to expect a forward declaration, which cannot be done (see above).
    };
    Inner inner_;
    template <class Archive>
    friend void boost::serialization::serialize(Archive &ar, Outer &outer, const unsigned int version);
    template <class Archive>
    friend void boost::serialization::serialize(Archive &ar, Inner &inner, const unsigned int version);
};
namespace boost
{
    namespace serialization
    {
        template <class Archive>
        void serialize(Archive &ar, Outer& outer, const unsigned int version)
        {
            ar & outer.inner_;
        }
        template <class Archive>
        void serialize(Archive &ar, Outer::Inner& inner, const unsigned int version)
        {
            ar & inner.member_;
        }
    }
}
int main()
{
    Outer outer;
    boost::archive::text_oarchive(std::cout) << outer;
}

用-std=c++11和-lboost_serialization编译。使用g++编译抱怨member_是私有的，即使存在友元声明。g++拒绝内部类中的友元声明是否正确？