On Wed, May 4, 2022 at 4:47 PM Vladimir Grigoriev via Std-Discussion <std-discussion@lists.isocpp.org> wrote:

The following program

namespace S
{

It looks like the "namespace S\n{" before this line is a typo. Is that so?

I removed them, and the rest of the program compiles fine in GCC and Clang. https://godbolt.org/z/fYd8fhrjb

#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
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Brian Bi