https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103904
--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-10 branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:22b86cdc4d7fddb4991a08515e47e66fe5c41def commit r10-10808-g22b86cdc4d7fddb4991a08515e47e66fe5c41def Author: Patrick Palka <ppa...@redhat.com> Date: Tue May 31 14:38:11 2022 -0400 libstdc++: Implement P2325 changes to default-constructibility of views NB: This backport of r12-1606 to the 10 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. And in the 10 branch we we don't have __non_propagating_cache or the partial specialization of __box, so the changes to them are omitted too. Finally, we don't update __cpp_lib_ranges to 202106L because many significant 202106 Ranges changes still aren't implemented in the 10 branch, e.g. P2281R1 and P2210R2. === 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/stl_iterator.h (common_iterator): Constrain the default ctor. (counted_iterator): Likewise. * include/std/ranges (ranges::view): Remove default_initializable requirement. (subrange): Constrain the default ctor. (__detail::__box::operator=): Handle self-assignment. (single_view): Constraint the default ctor. (iota_view): Relax semiregular constraint to copyable. Constrain the default ctor. (iota_view::_Iterator): Constraint the default ctor. (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): Constrain the default ctor. (common_view): Likewise. (reverse_view): Likewise. (elements_view): Likewise. (elements_view::_Iterator): Likewise. * include/std/span (enable_view<span<_ElementType, _Extent>>): Define this partial specialization to true unconditionally. * 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)
