[Bug libstdc++/105505] New: P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[Bug libstdc++/105505] New: P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105505

            Bug ID: 105505
           Summary: P1951R1 (Default Arguments for pair's Forwarding
                    Constructor) is unimplemented
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
                CC: redi at gcc dot gnu.org
  Target Milestone: ---

https://en.cppreference.com/w/cpp/compiler_support claims that P1951R1 (Default
Arguments for pair's Forwarding Constructor) is supported in GCC libstdc++ 12,
but it seems that libstdc++ 12 doesn't actually add default template arguments
for said forwarding constructor.

For example, the following still does not work with libstdc++ (but works with
latest libc++ and MSVC STL: https://godbolt.org/z/6oY3G77oT).

```
#include <utility>

struct MoveOnly {
    MoveOnly();
    MoveOnly(MoveOnly&&);
};

std::pair<int, MoveOnly> p{42, {}};
```

[Bug libstdc++/105505] P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105505

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-05-06
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I thought I'd pushed that change but apparently not.

I've fixed the cppreference page.

[Bug libstdc++/105505] P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105505

Jonathan Wakely <redi at gcc dot gnu.org> changed:

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

[Bug libstdc++/105505] P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105505

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.0

[Bug libstdc++/105505] P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105505

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:35f8c661d67f53486f2f3e25887323476a7d0c2b

commit r14-7173-g35f8c661d67f53486f2f3e25887323476a7d0c2b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jan 11 22:38:02 2024 +0000

    libstdc++: Implement C++23 P1951R1 (Default Args for pair's Forwarding
Ctor) [PR105505]

    This was approved for C++23 at he June 2021 virtual meeting.

    This allows more efficient construction of std::pair members when {} is
    used as a constructor argument. The perfect forwarding constructor can
    be used, so that the member variables are constructed from forwarded
    arguments instead of being copy constructed from temporaries.

    libstdc++-v3/ChangeLog:

            PR libstdc++/105505
            * include/bits/stl_pair.h (pair::pair(U1&&, U2&&)) [C++23]: Add
            default template arguments, as per P1951R1.
            * testsuite/20_util/pair/cons/default_tmpl_args.cc: New test.

[Bug libstdc++/105505] P1951R1 (Default Arguments for pair's Forwarding Constructor) is unimplemented

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105505

Jonathan Wakely <redi at gcc dot gnu.org> changed:

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I really have pushed it now