How to befriend a templated class function?

[alikim]

I want to keep func variable private, so I'm trying to befriend it and the code below doesn't compile in VS2017 with an error C2248: 'wrapper::_Instance::func': cannot access private member declared in class 'wrapper::_Instance'.

How do I make it work?

#include "pch.h"
#include <iostream>

class wrapper
{
private:
// ...

public:


template <class T>
void FetchStructList(T);

// instance
class _Instance {

int (*func)(const char*, int*, double*);

public:


_Instance() {

// func = &externalFunction;

};

template <class T>
// friend void wrapper::FetchStructList<T>(T); doesn't compile either
friend void wrapper::FetchStructList(T);

} Instance;

void FetchInstanceExtentions() { FetchStructList(Instance); };
};

template <class T>
void wrapper::FetchStructList(T s)
{
int size = 0;
int res = 0;

if constexpr (std::is_same<T, _Instance>::value)
res = s.func(nullptr, &size, nullptr);
// ...
else {
// ...
return;
}

//...
}

int main()
{
wrapper w;

w.FetchInstanceExtentions();
}

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