The following program
 
namespace S
{
#include <iostream>
 
namespace S
{
 
    template <unsigned N>
    class X
    {
    public:
        int i = N;
 
        friend X operator+( const X &a, const X &b ) noexcept
        {
            return { .i = a.i + b.i };
        }
    };
 
    inline X<256> operator+( const X<256> &a, const X<256> &b ) noexcept;
 
}
 
int main()
{
        S::X<256>( *ptr )( const S::X<256>&a, const S::X<256>&b ) noexcept = &::S::operator+;
 
        std::cout << ptr( S::X<256>(), S::X<256>() ).i << '\n';
}
 
compiles using MS VC++ (2019) but does not compile using gcc or clang.
 
I suspect that the problem is that the friend function is a templated entity that is not instantiated when the class granting friendship is instantiated.
 
On the other hand, this behavior contradicts the behavior when the class is not a template class. Should the friend function be instantiated in such a case?. 
 
With best regards
(Vlad from Moscow)
You can meet me at http://cpp.forum24.ru/ or www.stackoverflow.com or http://ru.stackoverflow.com