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This commit adds a new layout layout_left_padded as standardized in N5014. It adds a purely internal feature testing macro padded_layouts and registers layout_left_padded in the std module. This commit implements LWG4372, because without it's not possible to properly test padded layouts with a dynamic padding value. It also implements LWG4314, for consistency with prior layouts. The implementation uses a _PaddedStorage to deduplicate most of the code shared between left- and right-padded layouts. It's implemented through aggregation rather than inheritence, because of a bug related to inheriting conditionally explicit ctors. The tests are written such that the canonical version works for layout_left_padded. A version for layout_right_padded is derived essentially by reversing the order of the extents. PR libstdc++/110352 libstdc++-v3/ChangeLog: * include/bits/version.def (padded_layouts): Add new internal feature testing macro. * include/bits/version.h: Regenerate. * include/std/mdspan (__fwd_prod): New overload. (layout_left_padded): Add declaration and implementation. (layout_right_padded): Add declaration only. (layout_left::mapping::mapping): New overload for left padded mappings. (__index_type_cast): New function that performs a checked cast to index_type. (__is_left_padded_mapping): New concept. (__is_right_padded_mapping): Ditto. (__standardized_mapping): Recognize left and right padded mappings. (_LeftPaddedIndices): Traits for left padded details. (_PaddedStorage): New class for implementing padded layouts. * src/c++23/std.cc.in (layout_left_padded): Add. * testsuite/23_containers/mdspan/layouts/class_mandate_neg.cc: Refactor and add tests for layout_left_padded. * testsuite/23_containers/mdspan/layouts/ctors.cc: Ditto. * testsuite/23_containers/mdspan/layouts/empty.cc: Ditto. * testsuite/23_containers/mdspan/layouts/mapping.cc: Ditto. * testsuite/23_containers/mdspan/layouts/padded.cc: Ditto. * testsuite/23_containers/mdspan/layouts/padded_neg.cc: Ditto. * testsuite/23_containers/mdspan/layouts/padded_traits.h: New traits. Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
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