[Bug libstdc++/95322] New: std::list | take | transform, expression does not work cbegin() == end()

[Bug libstdc++/95322] New: std::list | take | transform, expression does not work cbegin() == end()

[gcc-bugs at marehr dot dialup.fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

            Bug ID: 95322
           Summary: std::list | take | transform, expression does not work
                    cbegin() == end()
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-bugs at marehr dot dialup.fu-berlin.de
  Target Milestone: ---

Hello gcc-team,

I'm not sure if this is intended behaviour, or if this is a defect in the
standard, or something that you should never write in a generic context.

ASFAIK you can use that expression on any container and on any view, but not on
some(?!) combinations of views? This is a bit unexpected.

```c++
#include <concepts>
#include <list>
#include <ranges>

int main()
{

    std::list v{0,1 ,2, 3, 4, 5, 5, 6}; // works if std::vector

    auto view1 = v | std::views::take(5);
    auto view2 = v | std::views::transform([] (int i) { return i + 3;});
    auto view3 = view1 | std::views::transform([] (int i) { return i + 3;});

    std::ranges::cbegin(view1) == std::ranges::end(view1); // works
    std::ranges::cbegin(view2) == std::ranges::end(view2); // works
    std::ranges::cbegin(view3) == std::ranges::end(view3); // does not work
    return 0;
}
```

https://godbolt.org/z/4Nzr69

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-05-26
           Keywords|                            |rejects-valid

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think it's a defect in the standard, the sentinel for transform_view can only
be compared to iterators of the same constness:

          friend constexpr bool
          operator==(const _Iterator<_Const>& __x, const _Sentinel& __y)
          { return __y.__equal(__x); }

I think it should work though.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's an implicit conversion from sentinel<false> to sentinel<true>, but that
would only happen if the operator== had been found, and since it's a hidden
friend it can't be found unless one of the operands is already sentinel<true>.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Adding this to transform_view::_Sentinel fixes it:

          friend constexpr bool
          operator==(const _Iterator<true>& __x, const _Sentinel& __y)
          requires (!_Const)
          { return _Sentinel<true>(__y).__equal(__x); }

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

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

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

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Attempting to work around this by using views::common like so

       auto view3 = view1 | std::views::transform([] (int i) { return i + 3;})
| std::views::common;
       /* ... */
       std::ranges::cbegin(view3) == std::ranges::end(view3);

instead results is a compilation failure in stl_iterator.h:

/home/patrick/code/gcc/libstdc++-v3/include/bits/stl_iterator.h:1772:27: error:
‘_Common_iter_proxy’ was not declared in this scope; did you mean
‘std::__detail::_Common_iter_proxy’?         1772 |  return
_Common_iter_proxy(*_M_it);                                                    
      |         ~~~~~~~~~~~~~~~~~~^~~~~~~~                                     
                                                                               
                                    |         std::__detail::_Common_iter_proxy
                                                                               
                                                           
/home/patrick/code/gcc/libstdc++-v3/include/bits/stl_iterator.h:1572:13: note:
‘std::__detail::_Common_iter_proxy’ declared here

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #5 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:3bf5e7657b752cc2352778e8c20ac9cdddca4f93

commit r11-645-g3bf5e7657b752cc2352778e8c20ac9cdddca4f93
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue May 26 16:17:34 2020 -0400

    libstdc++: Fix common_iterator::operator-> [PR95322]

    This patch fixes the definition of common_iterator::operator-> when the
    underlying iterator's operator* returns a non-reference.

    The first problem is that the class __detail::_Common_iter_proxy is used
    unqualified.  Fixing that revealed another problem: the class's template
    friend declaration of common_iterator doesn't match up with the
    definition of common_iterator, because the friend declaration isn't
    constrained.

    If we try to make the friend declaration match up by adding constraints,
    we run into frontend bug PR93467.  So we currently can't correctly
    express this friend relation between __detail::_Common_iter_proxy and
    common_iterator.

    As a workaround to this frontend bug, this patch moves the definition of
    _Common_iter_proxy into the class template of common_iterator so that we
    could instead express the friend relation via the injected-class-name.

    (This bug was found when attempting to use views::common to work around
    the compile failure with the testcase in PR95322.)

    libstdc++-v3/ChangeLog:

            PR libstdc++/95322
            * include/bits/stl_iterator.h (__detail::_Common_iter_proxy):
            Remove and instead define it ...
            (common_iterator::_Proxy): ... here.
            (common_iterator::operator->): Use it.
            * testsuite/24_iterators/common_iterator/2.cc: New test.
            * testsuite/std/ranges/adaptors/95322.cc: New test.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.2
             Status|NEW                         |ASSIGNED

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
A new LWG issue has been submitted, and there is a suggested resolution.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #7 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Jonathan Wakely from comment #6)
> A new LWG issue has been submitted, and there is a suggested resolution.

Will take care and inform in this issue here.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thanks Daniel, I'm just testing Casey's resolution, and extending it to
join_view, so I'll send another mail once that testing is complete.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #9 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:6c2582c0406250c66e2eb3651f8e8638796b7f53

commit r11-673-g6c2582c0406250c66e2eb3651f8e8638796b7f53
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 27 22:08:15 2020 +0100

    libstdc++: Fix view adaptors for mixed-const sentinels and iterators (PR
95322)

    The bug report is that transform_view's sentinel<false> cannot be
    compared to its iterator<true>.  The comparison is supposed to use
    operator==(iterator<Const>, sentinel<Const>) after converting
    sentinel<false> to sentinel<true>. However, the operator== is a hidden
    friend so is not a candidate when comparing iterator<true> with
    sentinel<false>. The required conversion would only happen if we'd found
    the operator, but we can't find the operator until after the conversion
    happens.

    A new LWG issue has been reported, but not yet assigned a number.  The
    solution suggested by Casey Carter is to make the hidden friends of the
    sentinel types work with iterators of any const-ness, so that no
    conversions are required.

    Patrick Palka observed that join_view has a similar problem and a
    similar fix is used for its sentinel.

            PR libstdc++/95322
            * include/std/ranges (transform_view::_Sentinel): Allow hidden
            friends to work with _Iterator<true> and _Iterator<false>.
            (join_view::_Sentinel): Likewise.
            * testsuite/std/ranges/adaptors/95322.cc: New test.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on master so far, backport to follow soon.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #11 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:fc78e991c35a5ee14efafb4e5566a9570fa31dd4

commit r10-8194-gfc78e991c35a5ee14efafb4e5566a9570fa31dd4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue May 26 16:17:34 2020 -0400

    libstdc++: Fix common_iterator::operator-> [PR95322]

    This patch fixes the definition of common_iterator::operator-> when the
    underlying iterator's operator* returns a non-reference.

    The first problem is that the class __detail::_Common_iter_proxy is used
    unqualified.  Fixing that revealed another problem: the class's template
    friend declaration of common_iterator doesn't match up with the
    definition of common_iterator, because the friend declaration isn't
    constrained.

    If we try to make the friend declaration match up by adding constraints,
    we run into frontend bug PR93467.  So we currently can't correctly
    express this friend relation between __detail::_Common_iter_proxy and
    common_iterator.

    As a workaround to this frontend bug, this patch moves the definition of
    _Common_iter_proxy into the class template of common_iterator so that we
    could instead express the friend relation via the injected-class-name.

    (This bug was found when attempting to use views::common to work around
    the compile failure with the testcase in PR95322.)

    libstdc++-v3/ChangeLog:

            PR libstdc++/95322
            * include/bits/stl_iterator.h (__detail::_Common_iter_proxy):
            Remove and instead define it ...
            (common_iterator::_Proxy): ... here.
            (common_iterator::operator->): Use it.
            * testsuite/24_iterators/common_iterator/2.cc: New test.
            * testsuite/std/ranges/adaptors/95322.cc: New test.

    (cherry picked from commit 3bf5e7657b752cc2352778e8c20ac9cdddca4f93)

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #12 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Daniel Krügler from comment #7)
> (In reply to Jonathan Wakely from comment #6)
> > A new LWG issue has been submitted, and there is a suggested resolution.
> 
> Will take care and inform in this issue here.

The new LWG issue exists now:

https://cplusplus.github.io/LWG/issue3448

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #13 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:38250e577e26de7aace65b4d32a94a1404f076a9

commit r10-8450-g38250e577e26de7aace65b4d32a94a1404f076a9
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed May 27 22:08:15 2020 +0100

    libstdc++: Fix view adaptors for mixed-const sentinels and iterators (PR
95322)

    The bug report is that transform_view's sentinel<false> cannot be
    compared to its iterator<true>.  The comparison is supposed to use
    operator==(iterator<Const>, sentinel<Const>) after converting
    sentinel<false> to sentinel<true>. However, the operator== is a hidden
    friend so is not a candidate when comparing iterator<true> with
    sentinel<false>. The required conversion would only happen if we'd found
    the operator, but we can't find the operator until after the conversion
    happens.

    A new LWG issue has been reported, but not yet assigned a number.  The
    solution suggested by Casey Carter is to make the hidden friends of the
    sentinel types work with iterators of any const-ness, so that no
    conversions are required.

    Patrick Palka observed that join_view has a similar problem and a
    similar fix is used for its sentinel.

            PR libstdc++/95322
            * include/std/ranges (transform_view::_Sentinel): Allow hidden
            friends to work with _Iterator<true> and _Iterator<false>.
            (join_view::_Sentinel): Likewise.
            * testsuite/std/ranges/adaptors/95322.cc: New test.

    (cherry picked from commit 6c2582c0406250c66e2eb3651f8e8638796b7f53)

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

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

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

--- Comment #14 from Jonathan Wakely <redi at gcc dot gnu.org> ---
fixed for 10.2

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[gcc-bugs at marehr dot dialup.fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #15 from gcc-bugs at marehr dot dialup.fu-berlin.de ---
Thank you!

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #16 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:3ae0cd94abc15e33dc06ca7a5f76f14b1d74129f

commit r11-2898-g3ae0cd94abc15e33dc06ca7a5f76f14b1d74129f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Aug 26 21:51:48 2020 -0400

    libstdc++: Implement remaining piece of LWG 3448

    Almost all of the proposed resolution for LWG 3448 is already
    implemented; the only part left is to adjust the return type of
    transform_view::sentinel::operator-.

    libstdc++-v3/ChangeLog:

            PR libstdc++/95322
            * include/std/ranges (transform_view::sentinel::__distance_from):
            Give this a deduced return type.
            (transform_view::sentinel::operator-): Adjust the return type so
            that it's based on the constness of the iterator rather than
            that of the sentinel.
            * testsuite/std/ranges/adaptors/95322.cc: Refer to LWG 3488.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #17 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:e066821b6f6b7332c7a67981f7b33c9ba0ccaee7

commit r11-3834-ge066821b6f6b7332c7a67981f7b33c9ba0ccaee7
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Oct 12 13:46:21 2020 -0400

    libstdc++: Apply proposed resolution for LWG 3449 [PR95322]

    Now that the frontend bug PR96805 is fixed, we can cleanly apply the
    proposed resolution for this issue.

    This slightly deviates from the proposed resolution by declaring _CI a
    member of take_view instead of take_view::_Sentinel, since it doesn't
    depend on anything within _Sentinel anymore.

    libstdc++-v3/ChangeLog:

            PR libstdc++/95322
            * include/std/ranges (take_view::_CI): Define this alias
            template as per LWG 3449 and remove ...
            (take_view::_Sentinel::_CI): ... this type alias.
            (take_view::_Sentinel::operator==): Adjust use of _CI
            accordingly.  Define a second overload that accepts an iterator
            of the opposite constness as per LWG 3449.
            (take_while_view::_Sentinel::operator==): Likewise.
            * testsuite/std/ranges/adaptors/95322.cc: Add tests for LWG 3449.

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #18 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:574ab3c85bb393e0ed0171b96eb42e0dd1e91de4

commit r10-8927-g574ab3c85bb393e0ed0171b96eb42e0dd1e91de4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Aug 26 21:51:48 2020 -0400

    libstdc++: Implement remaining piece of LWG 3448

    Almost all of the proposed resolution for LWG 3448 is already
    implemented; the only part left is to adjust the return type of
    transform_view::sentinel::operator-.

    libstdc++-v3/ChangeLog:

            PR libstdc++/95322
            * include/std/ranges (transform_view::sentinel::__distance_from):
            Give this a deduced return type.
            (transform_view::sentinel::operator-): Adjust the return type so
            that it's based on the constness of the iterator rather than
            that of the sentinel.
            * testsuite/std/ranges/adaptors/95322.cc: Refer to LWG 3488.

    (cherry picked from commit 3ae0cd94abc15e33dc06ca7a5f76f14b1d74129f)

[Bug libstdc++/95322] std::list | take | transform, expression does not work cbegin() == end()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95322

--- Comment #19 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:be5f22ebe964244223f178330a66b67d3f58334e

commit r10-9104-gbe5f22ebe964244223f178330a66b67d3f58334e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Oct 12 13:46:21 2020 -0400

    libstdc++: Apply proposed resolution for LWG 3449 [PR95322]

    Now that the frontend bug PR96805 is fixed, we can cleanly apply the
    proposed resolution for this issue.

    This slightly deviates from the proposed resolution by declaring _CI a
    member of take_view instead of take_view::_Sentinel, since it doesn't
    depend on anything within _Sentinel anymore.

    libstdc++-v3/ChangeLog:

            PR libstdc++/95322
            * include/std/ranges (take_view::_CI): Define this alias
            template as per LWG 3449 and remove ...
            (take_view::_Sentinel::_CI): ... this type alias.
            (take_view::_Sentinel::operator==): Adjust use of _CI
            accordingly.  Define a second overload that accepts an iterator
            of the opposite constness as per LWG 3449.
            (take_while_view::_Sentinel::operator==): Likewise.
            * testsuite/std/ranges/adaptors/95322.cc: Add tests for LWG 3449.

    (cherry picked from commit e066821b6f6b7332c7a67981f7b33c9ba0ccaee7)