[Bug libstdc++/108952] New: Regression in uses_allocator_construction_args for pair of rvalue references

[Bug libstdc++/108952] New: Regression in uses_allocator_construction_args for pair of rvalue references

[christian.mazakas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

            Bug ID: 108952
           Summary: Regression in uses_allocator_construction_args for
                    pair of rvalue references
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: christian.mazakas at gmail dot com
  Target Milestone: ---

Consider the following example as demonstrated on Compiler Explorer:
https://godbolt.org/z/jhxxK9hT3

The C++20 version of uses_allocator_construction_args seems to inappropriately
forward the pair's data members and when they happen to be rvalue references,
they decay to an lvalue reference which breaks constructibility tests.

This same code will pass for previous standards versions.

I'm not sure about the range of compilers affected by this but it seems to
follow wherever C++20 support is enabled.

[Bug libstdc++/108952] Regression in uses_allocator_construction_args for pair of rvalue references

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 54546
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54546&action=edit
testcase

Please next time also attach or put inline the testcase

[Bug libstdc++/108952] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-02-27

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem is that std::move(pr).first always yields an lvalue, even if the
type of pr is pair<T&&, U&&>.

I tried to simplify uses_allocator_construction_args to avoid indirection,
which I think is the source of this issue.

[Bug libstdc++/108952] Regression in uses_allocator_construction_args for pair of rvalue references

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #3 from TC <rs2740 at gmail dot com> ---
https://cplusplus.github.io/LWG/issue3527

[Bug libstdc++/108952] Regression in uses_allocator_construction_args for pair of rvalue references

[pdimov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

Peter Dimov <pdimov at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pdimov at gmail dot com

--- Comment #4 from Peter Dimov <pdimov at gmail dot com> ---
An easy fix is to use `std::get<0>` instead of `.first`.

[Bug libstdc++/108952] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Peter Dimov from comment #4)
> An easy fix is to use `std::get<0>` instead of `.first`.

Yeah I tried that, but I think there's an additional change needed (or I messed
something up).

[Bug libstdc++/108952] [10/11/12/13 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Regression in               |[10/11/12/13 Regression]
                   |uses_allocator_construction |Regression in
                   |_args for pair of rvalue    |uses_allocator_construction
                   |references                  |_args for pair of rvalue
                   |                            |references
           Keywords|                            |rejects-valid
      Known to fail|                            |10.1.0, 11.1.0, 12.1.0,
                   |                            |13.0, 9.2.0
      Known to work|                            |8.5.0, 9.1.0

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Aha, I broke std::get(pair<T1, T2>&&) for C++20 mode.

[Bug libstdc++/108952] [10/11/12/13 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #7 from CVS 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:8e342c04550466ab088c33746091ce7f3498ee44

commit r13-6373-g8e342c04550466ab088c33746091ce7f3498ee44
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Feb 27 22:34:57 2023 +0000

    libstdc++: Fix uses_allocator_construction_args for pair<T&&, U&&>
[PR108952]

    This implements LWG 3527 which fixes the handling of pair<T&&, U&&> in
    std::uses_allocator_construction_args.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108952
            * include/bits/uses_allocator_args.h
            (uses_allocator_construction_args): Implement LWG 3527.
            * testsuite/20_util/pair/astuple/get-2.cc: New test.
            * testsuite/20_util/scoped_allocator/108952.cc: New test.
            * testsuite/20_util/uses_allocator/lwg3527.cc: New test.

[Bug libstdc++/108952] [10/11/12 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[10/11/12/13 Regression]    |[10/11/12 Regression]
                   |Regression in               |Regression in
                   |uses_allocator_construction |uses_allocator_construction
                   |_args for pair of rvalue    |_args for pair of rvalue
                   |references                  |references

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug libstdc++/108952] [10/11/12 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.5
           Priority|P3                          |P2

[Bug libstdc++/108952] [10/11/12 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:858ff5c919fc1af31e399a7aea118c87f855972a

commit r12-9487-g858ff5c919fc1af31e399a7aea118c87f855972a
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Feb 27 22:34:57 2023 +0000

    libstdc++: Fix uses_allocator_construction_args for pair<T&&, U&&>
[PR108952]

    This implements LWG 3527 which fixes the handling of pair<T&&, U&&> in
    std::uses_allocator_construction_args.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108952
            * include/bits/uses_allocator_args.h
            (uses_allocator_construction_args): Implement LWG 3527.
            * testsuite/20_util/pair/astuple/get-2.cc: New test.
            * testsuite/20_util/scoped_allocator/108952.cc: New test.
            * testsuite/20_util/uses_allocator/lwg3527.cc: New test.

    (cherry picked from commit 8e342c04550466ab088c33746091ce7f3498ee44)

[Bug libstdc++/108952] [10/11 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[10/11/12 Regression]       |[10/11 Regression]
                   |Regression in               |Regression in
                   |uses_allocator_construction |uses_allocator_construction
                   |_args for pair of rvalue    |_args for pair of rvalue
                   |references                  |references

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 12.3 too.

[Bug libstdc++/108952] [10/11 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:6930165acd05c8eef88a99b9546742b108e0c84e

commit r11-10656-g6930165acd05c8eef88a99b9546742b108e0c84e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Feb 27 22:34:57 2023 +0000

    libstdc++: Fix uses_allocator_construction_args for pair<T&&, U&&>
[PR108952]

    This implements LWG 3527 which fixes the handling of pair<T&&, U&&> in
    std::uses_allocator_construction_args.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108952
            * include/bits/uses_allocator_args.h
            (uses_allocator_construction_args): Implement LWG 3527.
            * testsuite/20_util/pair/astuple/get-2.cc: New test.
            * testsuite/20_util/scoped_allocator/108952.cc: New test.
            * testsuite/20_util/uses_allocator/lwg3527.cc: New test.

    (cherry picked from commit 8e342c04550466ab088c33746091ce7f3498ee44)

[Bug libstdc++/108952] [10/11 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:bd0179f31dfb9b009cb561d336f3f1c6fbdd4ceb

commit r10-11319-gbd0179f31dfb9b009cb561d336f3f1c6fbdd4ceb
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Feb 27 22:34:57 2023 +0000

    libstdc++: Fix uses_allocator_construction_args for pair<T&&, U&&>
[PR108952]

    This implements LWG 3527 which fixes the handling of pair<T&&, U&&> in
    std::uses_allocator_construction_args.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108952
            * include/std/memory (uses_allocator_construction_args):
            Implement LWG 3527.
            * testsuite/20_util/pair/astuple/get-2.cc: New test.
            * testsuite/20_util/scoped_allocator/108952.cc: New test.
            * testsuite/20_util/uses_allocator/lwg3527.cc: New test.

    (cherry picked from commit 8e342c04550466ab088c33746091ce7f3498ee44)

[Bug libstdc++/108952] [10/11 Regression] Regression in uses_allocator_construction_args for pair of rvalue references

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108952

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

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

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 12.3, 11.4 and 10.5