C++ global extern "C" friend can't reach private member on namespaced class

Please consider the code:

#include    <iostream>

using namespace std;

extern  "C"
void    foo( void );

namespace   A
{
    template< int No >
    class   Bar
    {
    private:
        friend  void    ::foo( void );

        static void private_func( int n );
    };

    template< int No >
    void    Bar< No >::private_func( int n )
    {
        cout << "A:Bar< " << No << ">::private_func( " << n << " )" << endl;
    }
}

extern  "C"
void    foo( void )
{
    A::Bar< 0 >::private_func( 1 );
}

int main( )
{
    cout << " ---- " << endl;
    foo( );
}

G++ gives:

> g++ -Wall -o extern_c extern_c.cpp
extern_c.cpp: In function ‘void foo()’:
extern_c.cpp:20:7: error: ‘static void A::Bar<No>::private_func(int) [with int No = 0]’ is private
extern_c.cpp:29:31: error: within this context

If I comment the namspace A, it will compile and run correctly.

What am I missing?

I looked related topics, but could not find any that fits in my problem.

• C++: namespace conflict between extern "C" and class member
• Why this friend function can't access a private member of the class?

Thanks people.

EDIT:

I am now convinced that extern "C" has nothing to do with the problem. Please ignore it.