libstdc++: Implement submdspan and submdspan_mapping for layout_left. [PR110352]

Implements `submdspan` and `submdspan_mapping` for layout_left as
described in P3663 (Future proofing mdspan).

When computing the offset of the submdspan, one must check that the
lower bound of the slice range isn't out-of-range. There's a few
cases when the lower bound is never out-of-range:

  - full_extent and exts.extent(k) != 0,
  - collapsing slice types.

If those conditions are known to hold, no checks are generated.

Similarly, if all slices are full_extent, there's no need to call
mapping(0,...,0) for standardized mappings.

The implementation prepares to use the symmetry between layout_left and
layout_right and introduces concepts like a "layout side", i.e. left,
right or unknown/strided.

The tests use an iterator to replace nested for-loops. Which also makes
it easier to write the core test logic in a rank-independent manner.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mdspan::__is_submdspan_mapping_result)
	(__mdspan::__submdspan_mapping_result, __mdspan::__fwd_prod)
	(__mdspan::__acceptable_slice_type, __mdspan::__slice_begin)
	(__mdspan::__suboffset, __mdspan::_LayoutSide, __mdspan::__mapping_side)
	(__mdspan::_StridesTrait, __mdspan::__substrides_generic)
	(__mdspan::__substrides_standardized, __mdspan::__substrides)
	(__mdspan::__is_unit_stride_slice, __mdspan::_SliceKind)
	(__mdspan::__make_slice_kind, __mdspan::__make_slice_kind_array)
	(__mdspan::__is_block, __mdspan::__padded_block_begin_generic)
	(__mdspan::__padded_block_begin, __mpdspan::_SubMdspanMapping)
	(__mdspan::__submdspan_mapping_impl): Define.
	(__mdspan::__dynamic_slice_extent, __mdspan::__static_slice_extent)
	(__mdspan::__subextents): Move earlier in the file.
	(layout_left::mapping::submdspan_mapping, __mdspan::__sliceable_mapping)
	(__mdspan::__submapping, submdspan): Define.
	* src/c++23/std.cc.in: Add submdspan.
	* testsuite/23_containers/mdspan/submdspan/generic.cc: New test.
	* testsuite/23_containers/mdspan/submdspan/selections/left.cc:
	Instantiate selection tests for layout_left.
	* testsuite/23_containers/mdspan/submdspan/selections/testcases.h: Generic
	tests different selections.
	* testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc: New test.
	* testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

0 files changed, 0 insertions, 0 deletions