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.
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 (back_insert_iterator): Remove the
default ctor.
(front_insert_iterator): Likewise.
(insert_iterator): Likewise. Remove NSDMIs.
(common_iterator): Constrain the default ctor.
(counted_iterator): Likewise.
* include/bits/stream_iterator.h (ostream_iterator): Remove the
default ctor.
* 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.
(basic_istream_view): Remove the default ctor. Remove NSDMIs.
Remove redundant checks for empty _M_stream.
(basic_istream_view::_Iterator): Likewise.
(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/24_iterators/back_insert_iterator/constexpr.cc:
Don't attempt to default construct a back_insert_iterator.
* testsuite/24_iterators/front_insert_iterator/constexpr.cc:
Don't attempt to default construct a front_insert_iterator.
* testsuite/24_iterators/insert_iterator/constexpr.cc:
Don't attempt to default construct an insert_iterator.
* testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc:
Remove this test for default constructibility of ostream_iterator.
* testsuite/std/ranges/97600.cc: Don't attempt to default
construct a basic_istream_view.
* 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.
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