[Bug c++/100639] New: reverse_view<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[Bug c++/100639] New: reverse_view<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

            Bug ID: 100639
           Summary: reverse_view<I>::reference erroneously uses
                    iterator_traits<I>::reference instead of
                    iter_reference_t<I>
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Short example (from https://stackoverflow.com/q/67573305/2069064):

#include <ranges>

template <typename T> using iota = std::ranges::iota_view<T, T>;
template <typename T> using iota_iter = std::ranges::iterator_t<iota<T>>;

static_assert(std::same_as<
    std::reverse_iterator<iota_iter<int64_t>>::reference,
    int64_t>);

This assertion fails when compiling with -std=c++20 (because the reference type
is 'void') but passes with -std=gnu++20. The direct reason is that the
difference_type of the iota_view iterator is __int128, which is considered a
signed_integral with gnu++20 but not c++20.

But the reason this matters is because std::reverse_iterator<I>::reference is
defined as std::iterator_traits<I>::reference (which checks that I satsifies
cpp17-input-iterator which has the signed_integral constraint) instead of being
defined as std::iter_reference_t<I> (which has no such check). With the latter
implementation, the assertion above would pass on either version. 

The result is that reversing an iota_view<int64_t, int64_t> isn't a range on
-std=c++20.

[Bug c++/100639] reverse_view<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-05-17
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

[Bug libstdc++/100639] reverse_iterator<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
            Summary|reverse_view<I>::reference  |reverse_iterator<I>::refere
                   |erroneously uses            |nce erroneously uses
                   |iterator_traits<I>::referen |iterator_traits<I>::referen
                   |ce instead of               |ce instead of
                   |iter_reference_t<I>         |iter_reference_t<I>

[Bug libstdc++/100639] reverse_iterator<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The assertion also fails when __int128 isn't available (which can be simulated
by compiling with -U__SIZEOF_INT128__), because in that case iota_view's
difference_type is the integer-like class type __max_diff_type, and
integer-like class types aren't integral even in GNU mode.

[Bug libstdc++/100639] reverse_iterator<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

--- Comment #2 from CVS 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:d5cbe0f0d4b7bc11f80b2236521f90ec94e95767

commit r12-946-gd5cbe0f0d4b7bc11f80b2236521f90ec94e95767
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 20 14:08:17 2021 -0400

    libstdc++: Implement missing P0896R4 changes to reverse_iterator [PR100639]

    This implements the P0896R4 changes to reverse_iterator's member types
    value_type, difference_type and reference in C++20 mode, which fixes
    taking the reverse_iterator of an iterator with a non-integral
    difference_type (such as iota_view<long long>).

    libstdc++-v3/ChangeLog:

            PR libstdc++/100639
            * include/bits/stl_iterator.h (reverse_iterator::difference_type):
            In C++20 mode, define in terms of iter_difference_t as per P0896R4.
            (reverse_iterator::reference): Likewise, but with iter_reference_t.
            (reverse_iterator::value_type): Likewise, but with iter_value_t.
            * testsuite/std/ranges/adaptors/reverse.cc (test08): New test.
            * testsuite/24_iterators/reverse_iterator/100639.cc: New test.

[Bug libstdc++/100639] reverse_iterator<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:a50cc70f1b5ce3ae11528095fa3a1feadb206357

commit r11-8540-ga50cc70f1b5ce3ae11528095fa3a1feadb206357
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 20 14:08:17 2021 -0400

    libstdc++: Implement missing P0896R4 changes to reverse_iterator [PR100639]

    This implements the P0896R4 changes to reverse_iterator's member types
    value_type, difference_type and reference in C++20 mode, which fixes
    taking the reverse_iterator of an iterator with a non-integral
    difference_type (such as iota_view<long long>).

    libstdc++-v3/ChangeLog:

            PR libstdc++/100639
            * include/bits/stl_iterator.h (reverse_iterator::difference_type):
            In C++20 mode, define in terms of iter_difference_t as per P0896R4.
            (reverse_iterator::reference): Likewise, but with iter_reference_t.
            (reverse_iterator::value_type): Likewise, but with iter_value_t.
            * testsuite/std/ranges/adaptors/reverse.cc (test08): New test.
            * testsuite/24_iterators/reverse_iterator/100639.cc: New test.

    (cherry picked from commit d5cbe0f0d4b7bc11f80b2236521f90ec94e95767)

[Bug libstdc++/100639] reverse_iterator<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:c8bd39bc10eac3028dbf509b27457172360e6e8c

commit r10-9902-gc8bd39bc10eac3028dbf509b27457172360e6e8c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu May 20 14:08:17 2021 -0400

    libstdc++: Implement missing P0896R4 changes to reverse_iterator [PR100639]

    This implements the P0896R4 changes to reverse_iterator's member types
    value_type, difference_type and reference in C++20 mode, which fixes
    taking the reverse_iterator of an iterator with a non-integral
    difference_type (such as iota_view<long long>).

    libstdc++-v3/ChangeLog:

            PR libstdc++/100639
            * include/bits/stl_iterator.h (reverse_iterator::difference_type):
            In C++20 mode, define in terms of iter_difference_t as per P0896R4.
            (reverse_iterator::reference): Likewise, but with iter_reference_t.
            (reverse_iterator::value_type): Likewise, but with iter_value_t.
            * testsuite/std/ranges/adaptors/reverse.cc (test08): New test.
            * testsuite/24_iterators/reverse_iterator/100639.cc: New test.

    (cherry picked from commit d5cbe0f0d4b7bc11f80b2236521f90ec94e95767)

[Bug libstdc++/100639] reverse_iterator<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I>

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.4
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for 10.4/11.2/12