[Bug c++/110216] New: Outdated implementation of tuple_size requirements for structured binding

[Bug c++/110216] New: Outdated implementation of tuple_size requirements for structured binding

[gnaggnoyil at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110216

            Bug ID: 110216
           Summary: Outdated implementation of tuple_size requirements for
                    structured binding
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gnaggnoyil at gmail dot com
  Target Milestone: ---

The following code is rejected with `g++ -std=c++20` as of GCC 13.1:

#include <tuple>

struct A { int x; };

namespace std
{
    template<>
    struct tuple_size<::A> {};
}

auto [x] = A{};

Clang 16.0 on the other hand accepts this code with `-std=c++20` flag.

Wording of N4849 (and also the latest working draft as of today) specifies the
expected behavior in [dcl.struct.bind]#4 as follows:

    Otherwise, if the qualified-id std::tuple_size<E> names a complete class
type with a member named value, ...

Therefore, the above code shouldn't follow the requirements specified in
[dcl.struct.bind]#4 as tuple-like types. Instead, it should follow those in
[dcl.struct.bind]#5 and do the binding to data members.

This behavior change was introduced by CWG 2386 and corresponding wording
change can be found at
https://github.com/cplusplus/draft/commit/498092ced200140801180dcc9c2878a49350f946

[Bug c++/110216] Outdated implementation of tuple_size requirements for structured binding

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110216

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Alias|                            |cwg2386
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-06-12

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/110216] tuple_size requirements for structured binding has not been updated after DR 2386

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110216

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/projects/cxx-dr-status.html

listed as ? there.

[Bug c++/110216] tuple_size requirements for structured binding has not been updated after DR 2386

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110216

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> https://gcc.gnu.org/projects/cxx-dr-status.html
> 
> listed as ? there.

Updated to say "No".

[Bug c++/110216] tuple_size requirements for structured binding has not been updated after DR 2386

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110216

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:1a43af04dd62b80f45700f94ed241347263ed773

commit r14-3204-g1a43af04dd62b80f45700f94ed241347263ed773
Author: gnaggnoyil <gnaggnoyil@gmail.com>
Date:   Sat Aug 12 16:16:52 2023 +0800

    c++: follow DR 2386 and update implementation of get_tuple_size [PR110216]

    DR 2386 updated the tuple_size requirements for structured binding and
    it now requires tuple_size to be considered only if
    std::tuple_size<TYPE> names a complete class type with member value. GCC
    before this patch does not follow the updated requrements, and this
    patch is intended to implement it.

    (jason) Accepting pseudonym sign-off because a change this small is not
    legally significant for copyright.

            DR 2386
            PR c++/110216

    gcc/cp/ChangeLog:

            * decl.cc (get_tuple_size): Update implementation for DR 2386.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/decomp10.C: Update expected error for DR 2386.
            * g++.dg/cpp1z/pr110216.C: New test.

    Signed-off-by: gnaggnoyil <gnaggnoyil@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

[Bug c++/110216] tuple_size requirements for structured binding has not been updated after DR 2386

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110216

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

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

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Seems to be fixed; I'm updating cxx-dr-status.html.