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

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.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)