[Bug c++/95407] New: G++ allows access to base class members from a friend of a derived class

[Bug c++/95407] New: G++ allows access to base class members from a friend of a derived class

[dragondreamer at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95407

            Bug ID: 95407
           Summary: G++ allows access to base class members from a friend
                    of a derived class
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dragondreamer at live dot com
  Target Milestone: ---

G++ (at least trunk, 10.1.0, 9.3, 9.2) successfully compiles the following
code:

// ===================================
#include <iostream>

struct test_base
{
protected:
    static constexpr int value = 123;
};

template<typename Base>
struct ignored_friend_declaration
{
    static void bug()
    {
        std::cout << Base::value;
    }
};

struct test_child : test_base
{
    using ignored_friend_declaration_t = ignored_friend_declaration<test_base>;
    friend ignored_friend_declaration_t;

    static void test()
    {
        ignored_friend_declaration_t::bug();
    }
};

int main()
{
    test_child::test();
}
// ===================================

According to http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1873,
access to Base::value should be disallowed. Clang produces the following error
message: <source>:14:28: error: 'value' is a protected member of 'test_base'.

See also https://bugs.llvm.org/show_bug.cgi?id=46036

[Bug c++/95407] [DR 1873] G++ allows access to base class members from a friend of a derived class

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95407

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|G++ allows access to base   |[DR 1873] G++ allows access
                   |class members from a friend |to base class members from
                   |of a derived class          |a friend of a derived class
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-06-04

[Bug c++/95407] [DR 1873] G++ allows access to base class members from a friend of a derived class

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95407

Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arthur.j.odwyer at gmail dot com

--- Comment #1 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
According to Richard's comment on https://bugs.llvm.org/show_bug.cgi?id=46036 ,
http://cwg-issue-browser.herokuapp.com/cwg1873 indicates that Clang may be
correct according to the Standard as written. HOWEVER...

The "misbehavior" here isn't really about `friend`; it's about the interaction
between `static` and `protected`. If you change your test case to make `value`
non-static, then all vendors agree that the non-static `value` is inaccessible.

With a static protected member (Clang rejects, GCC and MSVC accept):
https://godbolt.org/z/essofs
With a non-static protected member (all three correctly reject):
https://godbolt.org/z/T1GWTP

And here's a test case that doesn't use `friend` at all, but rather uses a
private inheritance path, so that `C` is derived from `A`, but `C` itself is
not aware of the fact. This case came up on Slack on 2020-08-14:
https://cpplang.slack.com/archives/C5GN4SP41/p1597391677014800

With a static protected member (GCC and Clang accept, MSVC rejects):
https://godbolt.org/z/6d786M
With a non-static protected member (all three correctly reject):
https://godbolt.org/z/MeTxeW

// https://godbolt.org/z/6d786M
struct A {
protected:
    static int sf();
};
struct B : private A {};

struct C : public B {
    static void test();
};

void C::test() { &::A::sf; }

[Bug c++/95407] [DR 1873] G++ allows access to base class members from a friend of a derived class

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95407

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think PR 67943 is also related. But then DR 472 is also related to that one.