[Bug c++/110216] New: Outdated implementation of tuple_size requirements for structured binding

["gnaggnoyil at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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           Summary: Outdated implementation of tuple_size requirements for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