libstdc++: Implement submdspan_mapping for layout_right. [PR110352]

Adds submdspan_mapping for layout_right as described in P3663.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan
	(__mdspan::_SubMdspanMapping<_LayoutSide::__right>): Define.
	(layout_right::mapping::submdspan_mapping): New friend function.
	* testsuite/23_containers/mdspan/submdspan/selections/right.cc:
	Instantiate tests for layout_right.
	* testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
	Ditto.
	* testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc:
	Ditto.

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

libstdc++-v3/include/std/mdspan

@@ -1303,6 +1303,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  { return __mdspan::__is_block(__slice_kinds, __sub_rank); }
       };
 
+    template<>
+      struct _SubMdspanMapping<_LayoutSide::__right>
+      {
+	using _Layout = layout_right;
+	template<size_t _Pad> using _PaddedLayout = layout_right_padded<_Pad>;
+
+	template<typename _Mapping, size_t _Us>
+	  static consteval size_t
+	  _S_pad()
+	  {
+	    using _Extents = typename _Mapping::extents_type;
+	    constexpr auto __rank = _Extents::rank();
+	    constexpr auto __sta_exts
+	      = __mdspan::__static_extents<_Extents>(_Us + 1, __rank);
+	    if constexpr (!__mdspan::__all_static(__sta_exts))
+	      return dynamic_extent;
+	    else
+	      return __fwd_prod(__sta_exts);
+	  }
+
+	template<size_t _Nm>
+	  static consteval bool
+	  _S_is_unpadded_submdspan(span<const _SliceKind, _Nm> __slice_kinds,
+				   size_t __sub_rank)
+	  {
+	    auto __rev_slice_kinds = array<_SliceKind, _Nm>{};
+	    for(size_t __i = 0; __i < _Nm; ++__i)
+	      __rev_slice_kinds[__i] = __slice_kinds[_Nm - 1 - __i];
+	    return __mdspan::__is_block(span(__rev_slice_kinds), __sub_rank);
+	  }
+      };
+
     template<typename _Mapping>
       constexpr auto
       __submdspan_mapping_impl(const _Mapping& __mapping)
@@ -1678,6 +1710,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents));
 	}
 
+#if __glibcxx_submdspan
+      template<typename... _Slices>
+	requires (extents_type::rank() == sizeof...(_Slices))
+	friend constexpr auto
+	submdspan_mapping(const mapping& __mapping, _Slices... __slices)
+	{ return __mdspan::__submdspan_mapping_impl(__mapping, __slices...); }
+#endif // __glibcxx_submdspan
+
 	[[no_unique_address]] extents_type _M_extents{};
     };
 

libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right.cc

@@ -0,0 +1,9 @@
+// { dg-do run { target c++26 } }
+#include "testcases.h"
+
+int
+main()
+{
+  test_all<std::layout_right>();
+  return 0;
+}

libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc

@@ -129,8 +129,14 @@ main()
   test_layout_unpadded_return_types<std::layout_left>();
   static_assert(test_layout_unpadded_return_types<std::layout_left>());
 
+  test_layout_unpadded_return_types<std::layout_right>();
+  static_assert(test_layout_unpadded_return_types<std::layout_right>());
+
   test_layout_unpadded_padding_value<std::layout_left>();
   static_assert(test_layout_unpadded_padding_value<std::layout_left>());
+
+  test_layout_unpadded_padding_value<std::layout_right>();
+  static_assert(test_layout_unpadded_padding_value<std::layout_right>());
   return 0;
 }
 

libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc

@@ -24,6 +24,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_int_under<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_int_under<std::layout_right>());  // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -33,6 +34,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_int_over<std::layout_left>());    // { dg-error "expansion of" }
+static_assert(test_int_over<std::layout_right>());   // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -42,6 +44,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_tuple_under<std::layout_left>());     // { dg-error "expansion of" }
+static_assert(test_tuple_under<std::layout_right>());    // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -51,6 +54,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_tuple_reversed<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_tuple_reversed<std::layout_right>());  // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -60,6 +64,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_tuple_over<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_tuple_over<std::layout_right>());  // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -69,6 +74,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_strided_slice_zero<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_strided_slice_zero<std::layout_right>());  // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -78,6 +84,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_strided_slice_offset_under<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_strided_slice_offset_under<std::layout_right>());  // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -87,6 +94,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_strided_slice_offset_over<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_strided_slice_offset_over<std::layout_right>());  // { dg-error "expansion of" }
 
 template<typename Layout>
   constexpr bool
@@ -96,6 +104,7 @@ template<typename Layout>
     return true;
   }
 static_assert(test_strided_slice_extent_over<std::layout_left>());   // { dg-error "expansion of" }
+static_assert(test_strided_slice_extent_over<std::layout_right>());  // { dg-error "expansion of" }
 
 // { dg-prune-output "static assertion failed" }
 // { dg-prune-output "__glibcxx_assert_fail" }