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From: "gnaggnoyil at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110216] New: Outdated implementation of tuple_size
 requirements for structured binding
Date: Mon, 12 Jun 2023 09:09:35 +0000
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X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110216

            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           Summary: Outdated implementation of tuple_size requirements for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=3Dc++20` as of GCC 13.1:

#include <tuple>

struct A { int x; };

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

auto [x] =3D A{};

Clang 16.0 on the other hand accepts this code with `-std=3Dc++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/498092ced200140801180dcc9c2878a49=
350f946=