[Bug c++/111018] New: lexical interpretation of friendship rules depends on whether the friend function has a definition

[Bug c++/111018] New: lexical interpretation of friendship rules depends on whether the friend function has a definition

[eric.niebler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111018

            Bug ID: 111018
           Summary: lexical interpretation of friendship rules depends on
                    whether the friend function has a definition
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com
  Target Milestone: ---

Starting in gcc 12, transitive friendship is extended to friend functions that
are defined lexically within the body of the friend class. E.g.:


struct S;

struct T {
  friend struct S;
private:
  template <class T>
  static void foo(T) {}
};

struct S {
  template <class Self>
  friend auto bar(Self s) -> decltype(::T::foo(s)) { // (1)
    return ::T::foo(s);
  }
};

Prior to gcc-12, the commented line would have been rejected, but now it is
accepted. Great, it brings gcc in line with clang and is arguably more
sensible.

HOWEVER, it does NOT work if the friend function is merely declared but not
defined. For instance, this is still an error:

struct S;

struct T {
  friend struct S;
private:
  template <class T>
  static void foo(T) {}
};

struct S {
  template <class Self>
  friend auto bar(Self s) -> decltype(::T::foo(s)); // NO FN DEFINITION
};

int main() {
  S s;
  using T = decltype(bar(s)); // ERROR: T::foo is private
}


This is very confusing and inconsistent behavior.

See: https://godbolt.org/z/WT9P37Wba

[Bug c++/111018] lexical interpretation of friendship rules depends on whether the friend function has a definition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111018

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So if we make a definition outside of the class:
```
  template <class Self>
  auto bar(Self s) -> decltype(::T::foo(s))
  {

  }
```
Then clang rejects the code too ...
So ....

[Bug c++/111018] lexical interpretation of friendship rules depends on whether the friend function has a definition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111018

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|rejects-valid               |

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC rejects both cases ....

[Bug c++/111018] lexical interpretation of friendship rules depends on whether the friend function has a definition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111018

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://wg21.link/cwg1699

[Bug c++/111018] lexical interpretation of friendship rules depends on whether the friend function has a definition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111018

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=58993

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am going to leave this one open only because of what is mentioned in the
commit that changed behavior here:
r13-465-g4df735e01e3199978