[Bug libstdc++/100770] New: Incorrect if constexpr statement in ranges::unique_copy

[Bug libstdc++/100770] New: Incorrect if constexpr statement in ranges::unique_copy

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

            Bug ID: 100770
           Summary: Incorrect if constexpr statement in
                    ranges::unique_copy
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

In ranges::unique_copy in ranges_algo.h#L1435:


  else if constexpr (input_iterator<_Out>
                     && same_as<iter_value_t<_Iter>, iter_value_t<_Out>>)


Since there is no short circuit in if constexpr statement, this will
incorrectly abort compilation when iter_value_t<_Out> is ill-formed. 
(thanks T.C. for pointing that out).



#include <algorithm>
#include <iostream>
#include <sstream>

int main() {
  std::istringstream str("42 42 42");
  std::ranges::unique_copy(
    std::istream_iterator<int>(str), 
    std::istream_iterator<int>(),
    std::ostream_iterator<int>(std::cout, " "));
}

https://godbolt.org/z/PM58b3KrE

[Bug libstdc++/100770] Incorrect if constexpr statement in ranges::unique_copy

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
   Last reconfirmed|                            |2021-05-26
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug libstdc++/100770] Incorrect if constexpr statement in ranges::unique_copy

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

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

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

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
We should use a requires expression here instead.

[Bug libstdc++/100770] Incorrect if constexpr statement in ranges::unique_copy

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed on trunk so far by r12-1195:

Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu, 3 Jun 2021 12:30:29 -0400

    libstdc++: Avoid hard error in ranges::unique_copy [PR100770]

    Here, in the constexpr if condition within ranges::unique_copy, when
    input_iterator<_Out> isn't satisfied we must avoid substituting into
    iter_value_t<_Out> because the latter isn't necessarily well-formed
    then.  To that end, this patch factors out the condition into a concept
    and uses it throughout.

    This patch also makes the definition of our testsuite
    output_iterator_wrapper more minimal by setting its value_type, pointer
    and reference member types to void.  This means our existing tests for
    unique_copy already exercise the fix for this bug, so we don't need
    to add another test.  The only other fallout of this testsuite iterator
    change appears in std/ranges/range.cc, where the use of range_value_t
    on a test_output_range is now ill-formed.

    libstdc++-v3/ChangeLog:

        * include/bits/ranges_algo.h (__detail::__can_reread_output):
        Factor out this concept from ...
        (__unique_copy_fn::operator()): ... here.  Use the concept
        throughout.
        * testsuite/std/ranges/range.cc: Remove now ill-formed use
        of range_value_t on an output_range.
        * testsuite/util/testsuite_iterators.h (output_iterator_wrapper):
        Define value_type, pointer and reference member types to void.

[Bug libstdc++/100770] Incorrect if constexpr statement in ranges::unique_copy

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

--- 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:a35c5a5fc3eb610b77833855048163486c2fdb49

commit r11-8546-ga35c5a5fc3eb610b77833855048163486c2fdb49
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 3 12:30:29 2021 -0400

    libstdc++: Avoid hard error in ranges::unique_copy [PR100770]

    Here, in the constexpr if condition within ranges::unique_copy, when
    input_iterator<_Out> isn't satisfied we must avoid substituting into
    iter_value_t<_Out> because the latter isn't necessarily well-formed
    then.  To that end, this patch factors out the condition into a concept
    and uses it throughout.

    This patch also makes the definition of our testsuite
    output_iterator_wrapper more minimal by setting its value_type, pointer
    and reference member types to void.  This means our existing tests for
    unique_copy already exercise the fix for this bug, so we don't need
    to add another test.  The only other fallout of this testsuite iterator
    change appears in std/ranges/range.cc, where the use of range_value_t
    on a test_output_range is now ill-formed.

            PR libstdc++/100770

    libstdc++-v3/ChangeLog:

            * include/bits/ranges_algo.h (__detail::__can_reread_output):
            Factor out this concept from ...
            (__unique_copy_fn::operator()): ... here.  Use the concept
            throughout.
            * testsuite/std/ranges/range.cc: Remove now ill-formed use
            of range_value_t on an output_range.
            * testsuite/util/testsuite_iterators.h (output_iterator_wrapper):
            Define value_type, pointer and reference member types to void.

    (cherry picked from commit 0614bbbe59c6953e088cffa39dfe63d86fc1fa96)

[Bug libstdc++/100770] Incorrect if constexpr statement in ranges::unique_copy

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

--- 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:bee80d74f7ebc8e1ada913aa059dedc0976f28c0

commit r10-9908-gbee80d74f7ebc8e1ada913aa059dedc0976f28c0
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 3 12:30:29 2021 -0400

    libstdc++: Avoid hard error in ranges::unique_copy [PR100770]

    Here, in the constexpr if condition within ranges::unique_copy, when
    input_iterator<_Out> isn't satisfied we must avoid substituting into
    iter_value_t<_Out> because the latter isn't necessarily well-formed
    then.  To that end, this patch factors out the condition into a concept
    and uses it throughout.

    This patch also makes the definition of our testsuite
    output_iterator_wrapper more minimal by setting its value_type, pointer
    and reference member types to void.  This means our existing tests for
    unique_copy already exercise the fix for this bug, so we don't need
    to add another test.  The only other fallout of this testsuite iterator
    change appears in std/ranges/range.cc, where the use of range_value_t
    on a test_output_range is now ill-formed.

            PR libstdc++/100770

    libstdc++-v3/ChangeLog:

            * include/bits/ranges_algo.h (__detail::__can_reread_output):
            Factor out this concept from ...
            (__unique_copy_fn::operator()): ... here.  Use the concept
            throughout.
            * testsuite/std/ranges/range.cc: Remove now ill-formed use
            of range_value_t on an output_range.
            * testsuite/util/testsuite_iterators.h (output_iterator_wrapper):
            Define value_type, pointer and reference member types to void.

    (cherry picked from commit 0614bbbe59c6953e088cffa39dfe63d86fc1fa96)

[Bug libstdc++/100770] Incorrect if constexpr statement in ranges::unique_copy

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100770

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

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

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