[Bug libstdc++/103904] [defect fix] Please backport P2325R3 to 10 and 11

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103904

--- Comment #12 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:92d612cccc1eeccf858cecd29ce4b7e734df7ae9

commit r11-9555-g92d612cccc1eeccf858cecd29ce4b7e734df7ae9
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 11 09:01:51 2022 -0500

    libstdc++: Implement P2325 changes to default-constructibility of views

    NB: This backport of r12-1606 to the 11 branch deliberately omits parts
    of P2325R3 so as to maximize backward compatibility with pre-P2325R3 code.
    In particular, we don't remove the default ctors for back_insert_iterator,
    front_insert_iterator, ostream_iterator, ref_view and basic_istream_view.

    ===

    This implements the wording changes of P2325R3 "Views should not be
    required to be default constructible".  Changes are relatively
    straightforward, besides perhaps those to __box (which now stands
    for copyable-box instead of semiregular-box) and __non_propagating_cache.

    For __box, this patch implements the recommended practice to also avoid
    std::optional when the boxed type is nothrow_move/copy_constructible.

    For __non_propagating_cache, now that it's used by split_view::_M_current,
    we need to add assignment from a value of the underlying type to the
    subset of the std::optional API implemented for the cache (needed by
    split_view::begin()).  Hence the new __non_propagating_cache::operator=
    overload.

    In passing, this fixes the undesirable list-init in the constructors of
    the partial specialization of __box as reported in PR100475 comment #7.

            PR libstdc++/103904

    libstdc++-v3/ChangeLog:

            * include/bits/iterator_concepts.h (weakly_incrementable): Remove
            default_initializable requirement.
            * include/bits/ranges_base.h (ranges::view): Likewise.
            * include/bits/ranges_util.h (subrange): Constrain the default
            ctor.
            * include/bits/stl_iterator.h (common_iterator): Constrain the
            default ctor.
            (counted_iterator): Likewise.
            * include/std/ranges (__detail::__box::operator=): Handle
            self-assignment in the primary template.
            (__detail::__box): In the partial specialization: adjust
            constraints as per P2325.  Add specialized operator= for the
            case when the wrapped type is not copyable.  Constrain the
            default ctor.  Avoid list-initialization.
            (single_view): Constraint the default ctor.
            (iota_view): Relax semiregular constraint to copyable.
            Constrain the default ctor.
            (iota_view::_Iterator): Constraint the default ctor.
            (ref_view): Remove the default ctor.  Remove NSDMIs.
            (ref_view::_Iterator): Constrain the default ctor.
            (__detail::__non_propagating_cache::operator=): Define overload
            for assigning from a value of the underlying type.
            (filter_view): Likewise.
            (filter_view::_Iterator): Likewise.
            (transform_view): Likewise.
            (transform_view::_Iterator): Likewise.
            (take_view): Likewise.
            (take_view::_Iterator): Likewise.
            (take_while_view): Likewise.
            (take_while_view::_Iterator): Likewise.
            (drop_while_view): Likewise.
            (drop_while_view::_Iterator): Likewise.
            (join_view): Likewise.
            (split_view::_OuterIter::__current): Adjust after changing the
            type of _M_current.
            (split_view::_M_current): Wrap it in a __non_propagating_cache.
            (split_view::split_view): Constrain the default ctor.
            (common_view): Constrain the default ctor.
            (reverse_view): Likewise.
            (elements_view): Likewise.
            * include/std/span (enable_view<span<_ElementType, _Extent>>):
            Define this partial specialization to true unconditionally.
            * include/std/version (__cpp_lib_ranges): Adjust value.
            * testsuite/std/ranges/adaptors/detail/semiregular_box.cc:
            Rename to ...
            * testsuite/std/ranges/adaptors/detail/copyable_box.cc: ... this.
            (test02): Adjust now that __box is copyable-box not
            semiregular-box.
            (test03): New test.
            * testsuite/std/ranges/p2325.cc: New test.
            * testsuite/std/ranges/single_view.cc (test06): New test.
            * testsuite/std/ranges/view.cc: Adjust now that view doesn't
            require default_initializable.

    (cherry picked from commit 4b4f5666b4c2f3aab2a9f3d53d394e390b9b682d)