[Bug libstdc++/109536] New: Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[Bug libstdc++/109536] New: Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[terra at gnome dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

            Bug ID: 109536
           Summary: Failure to compile constexpr std::vector with
                    -D_GLIBCXX_DEBUG
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: terra at gnome dot org
  Target Milestone: ---

Created attachment 54875
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54875&action=edit
Preprocessed source code

The following program fails to compile with -std=gnu++20 -D_GLIBCXX_DEBUG
due to the debug version of std::vector::~vector not being constexpr.

It compiles just fine without -D_GLIBCXX_DEBUG.  FWIW, clang 15 sees no problem
(without -D...) other than "farm" being unused.

-------------------------------------------------------------

#include <vector>

constexpr int
foo ()
{
  std::vector<int> v { 1, 2, 3 };
  return v.size();
}

constexpr int farm = foo ();

-------------------------------------------------------------

# /usr/local/products/gcc/12.1.0/bin/g++ -D_GLIBCXX_DEBUG  -std=gnu++20 -Wall
-c www.C
www.C: In function ‘constexpr int foo()’:
www.C:7:20: error: variable ‘v’ of non-literal type ‘std::__debug::vector<int>’
in ‘constexpr’ function only available with ‘-std=c++2b’ or ‘-std=gnu++2b’
    7 |   std::vector<int> v { 1, 2, 3 };
      |                    ^
In file included from
/usr/local/products/gcc/12.1.0/include/c++/12.1.0/vector:74,
                 from www.C:1:
/usr/local/products/gcc/12.1.0/include/c++/12.1.0/debug/vector:117:11: note:
‘std::__debug::vector<int>’ is not literal because:
  117 |     class vector
      |           ^~~~~~
/usr/local/products/gcc/12.1.0/include/c++/12.1.0/debug/vector:117:11: note:  
‘std::__debug::vector<int>’ does not have ‘constexpr’ destructor
www.C: At global scope:
www.C:11:26: error: ‘constexpr int foo()’ called in a constant expression
   11 | constexpr int farm = foo ();
      |                      ~~~~^~
www.C:5:1: note: ‘constexpr int foo()’ declared here
    5 | foo ()
      | ^~~

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
12-5187-g1ae8edf5f73ca5c3 acknowledged that:
libstdc++: Implement constexpr std::vector for C++20

This implements P1004R2 ("Making std::vector constexpr") for C++20.

For now, debug mode vectors are not supported in constant expressions.
To make that work we might need to disable all attaching/detaching of
safe iterators. That can be fixed later.

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
r12-5187-g1ae8edf5f73ca5c3 even.

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Right - using std::vector in constant expressions isn't supported by debug
mode. Somebody needs to work on that.

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-12-05

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Patch posted:
https://gcc.gnu.org/pipermail/gcc-patches/2023-December/639592.html

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

--- Comment #5 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:7d00a59229ee17af009a3c6c6208b0611740ed49

commit r14-6553-g7d00a59229ee17af009a3c6c6208b0611740ed49
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Dec 6 13:39:52 2023 +0000

    libstdc++: Make __gnu_debug::vector usable in constant expressions
[PR109536]

    This makes constexpr std::vector (mostly) work in Debug Mode. All safe
    iterator instrumentation and checking is disabled during constant
    evaluation, because it requires mutex locks and calls to non-inline
    functions defined in libstdc++.so. It should be OK to disable the safety
    checks, because most UB should be detected during constant evaluation
    anyway.

    We could try to enable the full checking in constexpr, but it would mean
    wrapping all the non-inline functions like _M_attach with an inline
    _M_constexpr_attach that does the iterator housekeeping inline without
    mutex locks when called for constant evaluation, and calls the
    non-inline function at runtime. That could be done in future if we find
    that we've lost safety or useful checking by disabling the safe
    iterators.

    There are a few test failures in C++20 mode, which I'm unable to
    explain. The _Safe_iterator::operator++() member gives errors for using
    non-constexpr functions during constant evaluation, even though those
    functions are guarded by std::is_constant_evaluated() checks. The same
    code works fine for C++23 and up.

    libstdc++-v3/ChangeLog:

            PR libstdc++/109536
            * include/bits/c++config (__glibcxx_constexpr_assert): Remove
            macro.
            * include/bits/stl_algobase.h (__niter_base, __copy_move_a)
            (__copy_move_backward_a, __fill_a, __fill_n_a, __equal_aux)
            (__lexicographical_compare_aux): Add constexpr to overloads for
            debug mode iterators.
            * include/debug/helper_functions.h (__unsafe): Add constexpr.
            * include/debug/macros.h (_GLIBCXX_DEBUG_VERIFY_COND_AT): Remove
            macro, folding it into ...
            (_GLIBCXX_DEBUG_VERIFY_AT_F): ... here. Do not use
            __glibcxx_constexpr_assert.
            * include/debug/safe_base.h (_Safe_iterator_base): Add constexpr
            to some member functions. Omit attaching, detaching and checking
            operations during constant evaluation.
            * include/debug/safe_container.h (_Safe_container): Likewise.
            * include/debug/safe_iterator.h (_Safe_iterator): Likewise.
            * include/debug/safe_iterator.tcc (__niter_base, __copy_move_a)
            (__copy_move_backward_a, __fill_a, __fill_n_a, __equal_aux)
            (__lexicographical_compare_aux): Add constexpr.
            * include/debug/vector (_Safe_vector, vector): Add constexpr.
            Omit safe iterator operations during constant evaluation.
            * testsuite/23_containers/vector/bool/capacity/constexpr.cc:
            Remove dg-xfail-if for debug mode.
            * testsuite/23_containers/vector/bool/cmp_c++20.cc: Likewise.
            * testsuite/23_containers/vector/bool/cons/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/element_access/1.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/element_access/constexpr.cc:
            Likewise.
            *
testsuite/23_containers/vector/bool/modifiers/assign/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/modifiers/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/capacity/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/cmp_c++20.cc: Likewise.
            * testsuite/23_containers/vector/cons/constexpr.cc: Likewise.
            * testsuite/23_containers/vector/data_access/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/element_access/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/modifiers/assign/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/modifiers/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/modifiers/swap/constexpr.cc:
            Likewise.
            * testsuite/23_containers/vector/cons/destructible_debug_neg.cc:
            Adjust dg-error line number.

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |14.0

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 14

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:3d3145e9e1461e92bef02d55d36990261dd0444d

commit r14-8252-g3d3145e9e1461e92bef02d55d36990261dd0444d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jan 18 10:36:07 2024 -0500

    libstdc++/debug: Fix constexpr _Safe_iterator in C++20 mode [PR109536]

    Some _Safe_iterator member functions define a variable of non-literal
    type __gnu_cxx::__scoped_lock, which automatically disqualifies them from
    being constexpr in C++20 mode even if that code path is never constant
    evaluated.  This restriction was lifted by P2242R3 for C++23, but we
    need to work around it in C++20 mode.  To that end this patch defines
    a pair of macros that encapsulate the lambda-based workaround mentioned
    in that paper and uses it to make these functions valid C++20 constexpr
    functions.  The augmented std::vector test element_access/constexpr.cc
    now successfully compiles in C++20 mode with -D_GLIBCXX_DEBUG (and it
    should test all member functions modified by this patch).

            PR libstdc++/109536

    libstdc++-v3/ChangeLog:

            * include/debug/safe_base.h (_Safe_sequence_base::_M_swap):
            Remove _GLIBCXX20_CONSTEXPR from non-inline member function.
            * include/debug/safe_iterator.h
            (_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN): Define.
            (_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END): Define.
            (_Safe_iterator::operator=): Use them around the code path that
            defines a variable of type __gnu_cxx::__scoped_lock.
            (_Safe_iterator::operator++): Likewise.
            (_Safe_iterator::operator--): Likewise.
            (_Safe_iterator::operator+=): Likewise.
            (_Safe_iterator::operator-=): Likewise.
            * testsuite/23_containers/vector/element_access/constexpr.cc
            (test_iterators): Test more iterator operations.
            * testsuite/23_containers/vector/bool/element_access/constexpr.cc
            (test_iterators): Likewise.
            * testsuite/std/ranges/adaptors/all.cc (test08) [_GLIBCXX_DEBUG]:
            Remove.

    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

[Bug libstdc++/109536] Failure to compile constexpr std::vector with -D_GLIBCXX_DEBUG

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109536

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to GCC Commits from comment #5)
>     There are a few test failures in C++20 mode, which I'm unable to
>     explain. The _Safe_iterator::operator++() member gives errors for using
>     non-constexpr functions during constant evaluation, even though those
>     functions are guarded by std::is_constant_evaluated() checks. The same
>     code works fine for C++23 and up.

This part is fixed by Patrick's commit.