[Bug c++/112744] New: Nested name specifier wrongly produces ambiguity in accessing static field

[Bug c++/112744] New: Nested name specifier wrongly produces ambiguity in accessing static field

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112744

            Bug ID: 112744
           Summary: Nested name specifier wrongly produces ambiguity in
                    accessing static field
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This program

struct A { constexpr static int a = 0; };
struct B : A {};
struct C : A {};
struct D : B, C {};

int main() {
    (void)D{}.a;   //ok everywhere
    (void)D{}.A::a; //error in GCC
}

is accepted in Clang and MSVC, but GCC fails on the line with nested name
specifier:

error: 'A' is an ambiguous base of 'D'

Online demo: https://godbolt.org/z/vY6rjd9fx

[Bug c++/112744] Nested name specifier wrongly produces ambiguity in accessing static field

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112744

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-28
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed, I think.  Since a is static, there is only one copy.  So it should
not matter that A is an ambiguous base of D (which it is).  See [class]/note-3.

For the error line, in finish_class_member_access_expr scope will be A, so we
do
 3496           /* Find the base of OBJECT_TYPE corresponding to SCOPE.  */
 3497           access_path = lookup_base (object_type, scope, ba_check,
 3498                                      NULL, complain);
where object_type=D, scope=A.  But the ba_check means we give an error.

We don't know at this point that name refers to a static data member.  But we
can look it up, and maybe use ba_any.

Not a regression.

[Bug c++/112744] Nested name specifier wrongly produces ambiguity in accessing static field

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112744

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/112744] Nested name specifier wrongly produces ambiguity in accessing static field

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112744

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:725c68c54c265fe7f6fc7babff7139f3161bdfa6

commit r14-6024-g725c68c54c265fe7f6fc7babff7139f3161bdfa6
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Nov 28 14:44:24 2023 -0500

    c++: wrong ambiguity in accessing static field [PR112744]

    Given

      struct A { constexpr static int a = 0; };
      struct B : A {};
      struct C : A {};
      struct D : B, C {};

    we give the "'A' is an ambiguous base of 'D'" error for

      D{}.A::a;

    which seems wrong: 'a' is a static data member so there is only one copy
    so it can be unambiguously referred to even if there are multiple A
    objects.  clang++/MSVC/icx agree.

    This patch uses ba_any: [class.access.base] requires conversion to a unique
    base subobject for non-static data members, but it does not require that
the
    base be unique or accessible for static data members.

            PR c++/112744

    gcc/cp/ChangeLog:

            * typeck.cc (finish_class_member_access_expr): When accessing
            a static data member, use ba_any for lookup_base.

    gcc/testsuite/ChangeLog:

            * g++.dg/lookup/scoped11.C: New test.
            * g++.dg/lookup/scoped12.C: New test.
            * g++.dg/lookup/scoped13.C: New test.
            * g++.dg/lookup/scoped14.C: New test.
            * g++.dg/lookup/scoped15.C: New test.

[Bug c++/112744] Nested name specifier wrongly produces ambiguity in accessing static field

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112744

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed for GCC 14.