[Bug c++/68089] New: c++ friend with template

[Bug c++/68089] New: c++ friend with template

[dushistov at mail dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68089

            Bug ID: 68089
           Summary: c++ friend with template
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dushistov at mail dot ru
  Target Milestone: ---

Here is code:
#include <iostream>

template<typename T>
class C
{
    template<typename U> // ????????
    friend class C;

    template<typename U>
    friend bool operator == (C const& c1, C<U> const& c2)
    {
      return c1.i == c2.i;
    }

    int i;

  public:

    C(): i {0} {}
    C(int val): i {val} {}
}; // template class C

int main()
{
  C<int>  c1 {5};
  C<char> c2 {5};

  if (c1 == c2) std::cout << "All ok" << std::endl;
  else          std::cout << "Very bad" << std::endl;



  return 0;
}

compile line:
g++ -Wall -std=c++14 test4.cpp
g++ -Wall -std=c++14 test4.cpp
test4.cpp: In instantiation of 'bool operator==(const C<T>&, const C<U>&) [with
U = char; T = int]':
test4.cpp:28:13:   required from here
test4.cpp:15:9: error: 'int C<char>::i' is private
     int i;
         ^
test4.cpp:12:19: error: within this context
       return c1.i == c2.i;

so even if bool operator == (C const& c1, C<U> const& c2) is friend of
template<typename T> class C; it not allowed to access C<>::i,
but clang compiles this code without errors.

[Bug c++/68089] c++ friend with template

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68089

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The error seems correct to me, the function is only a friend of its first
argument, not its second. Friendship is not transitive, just because the
function friends with C<T> and C<T> is friends with C<U> that does not mean the
function is friends with C<U>.