[Bug c++/57593] Friendship not extended into friend member-declaration

[Bug c++/57593] Friendship not extended into friend member-declaration

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57593

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> ---
If a defect report has been submitted, then we should add its number to the
Subject of this bug and suspend it immediately. Do you have the number?

[Bug c++/57593] Friendship not extended into friend member-declaration

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57593

--- Comment #2 from David Krauss <potswa at mac dot com> ---
The maintainer (Mike Miller at EDG) said simply that he will add it to the
list, but as far as I know a number isn't assigned until the new list is
drafted. The title is "access granted by friendship to class-scope friend
declaration." (But I suppose he could take a liberty and remove the redundant
"class-scope".)

Regardless of any possible defect, GCC is inconsistent. The change in access
applies to certain declarations, not others, but GCC is applying it to parts of
certain declarations (seemingly only the function body is excluded). To be
consistent with the current standard, the only choices are to allow all 3
examples or disallow all 3. The latter choice would be a nasty breaking change.

If it helps, here is the text of DR submission.
----

Friendship is not transitive, but it does grant access to "member declarations
of the befriended class." ([class.friend]/2) And a friend is declared by a
member-declaration. Is a grammatical member-declaration with the friend
specifier still a member declaration?

class c {
    class n {};
    friend struct s;
};

struct s {
    friend class c::n; // 1
    friend c::n g(); // 2
    friend void f() { c::n(); } // 3
};

If a friend member-declaration is not a member declaration, then it is
impossible to grant access to a private nested class (1) or a function composed
from a type specified using a private nested class (2). Both GCC and Clang
allow these examples.

If a friend member-declaration is a member declaration, then it implements one
degree of transitive friendship, if it is also a definition (3). Clang accepts
this example, but GCC rejects it.

Friends are enough like members that the access should apply. One common
application is operator<< . A manipulator proxy class may need access from its
insertion operator to its client's restricted members. Another application is
to define a function that would otherwise be a nonstatic member but receives a
container or smart pointer, not a direct reference to the object. These are the
most common cases for the inline friend definition syntax. They are found by
ADL so the namespace-scope declaration may be omitted.

Working around by befriending the pseudo-member friend may be troublesome as it
may violate separation of concerns, form undesired associated classes for ADL,
or require forming a declaration which may use inconvenient nested types or
reduplicated SFINAE.

Although access control isn't really a security mechanism, no "exploit" would
be opened by granting access because a friend can always delegate to a static
member anyway.

Proposed resolution:

Modify [class.friend]:

… can be accessed in the base-specifiers, member-declarations, and definitions
of members of the befriended class.

Is there any context within a class-specifier where access control is performed
besides its base-specifiers and member-declarations? It doesn't seem to apply
to name lookup in the template-name of a partial or explicit specialization,
but I can't find the spec. Perhaps it should be:

… can be accessed in the class-specifier and definitions of members of the
befriended class.
>From gcc-bugs-return-424351-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Fri Jun 14 13:31:38 2013
Return-Path: <gcc-bugs-return-424351-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Delivered-To: listarch-gcc-bugs@gcc.gnu.org
Received: (qmail 25116 invoked by alias); 14 Jun 2013 13:31:38 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
Delivered-To: mailing list gcc-bugs@gcc.gnu.org
Received: (qmail 25075 invoked by uid 48); 14 Jun 2013 13:31:32 -0000
From: "paolo.carlini at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/57593] Friendship not extended into friend member-declaration
Date: Fri, 14 Jun 2013 13:31:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 4.9.0
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: paolo.carlini at oracle dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields:
Message-ID: <bug-57593-4-aAn21fEzDM@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-57593-4@http.gcc.gnu.org/bugzilla/>
References: <bug-57593-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2013-06/txt/msg00730.txt.bz2
Content-length: 331

http://gcc.gnu.org/bugzilla/show_bug.cgi?idW593

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Ok. Please, when you have a number, send it here. In any case, I suppose we'd
rather prefer to have a resolution in order to act, otherwise we are not sure
in which way we want to resolve any inconsistency.