[libc++] P2389R2: dextents Index Type Parameter (#97393)

This patch implements P2389R2, which was adopted at the St. Louis meeting.
It builds upon previous enhancements from P2299R3, which introduced deduction
guides and the `dextents` alias template.

Author: Xiaoyang Liu

libcxx/docs/FeatureTestMacroTable.rst

@@ -446,6 +446,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_linalg``                                       *unimplemented*
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_mdspan``                                       ``202406L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_optional_range_support``                       *unimplemented*
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_out_ptr``                                      *unimplemented*

libcxx/docs/ReleaseNotes/19.rst

@@ -54,6 +54,7 @@ Implemented Papers
 - P2713R1 - Escaping improvements in ``std::format``
 - P2231R1 - Missing ``constexpr`` in ``std::optional`` and ``std::variant``
 - P0019R8 - ``std::atomic_ref``
+- P2389R2 - Alias template ``dims`` for the ``extents`` of ``mdspan``
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx2cPapers.csv

@@ -65,7 +65,7 @@
 "","","","","","",""
 "`P2747R2 <https://wg21.link/P2747R2>`__","CWG","``constexpr`` placement new","St. Louis June 2024","","",""
 "`P2997R1 <https://wg21.link/P2997R1>`__","LWG","Removing the common reference requirement from the indirectly invocable concepts","St. Louis June 2024","","",""
-"`P2389R2 <https://wg21.link/P2389R2>`__","LWG","``dextents`` Index Type Parameter","St. Louis June 2024","","",""
+"`P2389R2 <https://wg21.link/P2389R2>`__","LWG","``dextents`` Index Type Parameter","St. Louis June 2024","|Complete|","19.0",""
 "`P3168R2 <https://wg21.link/P3168R2>`__","LWG","Give ``std::optional`` Range Support","St. Louis June 2024","","","|ranges|"
 "`P3217R0 <https://wg21.link/P3217R0>`__","LWG","Adjoints to 'Enabling list-initialization for algorithms': find_last","St. Louis June 2024","","",""
 "`P2985R0 <https://wg21.link/P2985R0>`__","LWG","A type trait for detecting virtual base classes","St. Louis June 2024","","",""

libcxx/include/__mdspan/extents.h

@@ -454,6 +454,12 @@ struct __make_dextents< _IndexType, 0, extents<_IndexType, _ExtentsPack...>> {
 template <class _IndexType, size_t _Rank>
 using dextents = typename __mdspan_detail::__make_dextents<_IndexType, _Rank>::type;
 
+#  if _LIBCPP_STD_VER >= 26
+// [mdspan.extents.dims], alias template `dims`
+template <size_t _Rank, class _IndexType = size_t>
+using dims = dextents<_IndexType, _Rank>;
+#  endif
+
 // Deduction guide for extents
 #  if _LIBCPP_STD_VER >= 26
 template <class... _IndexTypes>

libcxx/include/mdspan

@@ -20,6 +20,10 @@ namespace std {
   template<class IndexType, size_t Rank>
     using dextents = see below;
 
+  // [mdspan.extents.dims], alias template dims
+  template<size_t Rank, class IndexType = size_t>
+    using dims = see below; // since C++26
+
   // [mdspan.layout], layout mapping
   struct layout_left;
   struct layout_right;

libcxx/include/version

@@ -159,7 +159,8 @@ __cpp_lib_make_unique                                   201304L <memory>
 __cpp_lib_map_try_emplace                               201411L <map>
 __cpp_lib_math_constants                                201907L <numbers>
 __cpp_lib_math_special_functions                        201603L <cmath>
-__cpp_lib_mdspan                                        202207L <mdspan>
+__cpp_lib_mdspan                                        202406L <mdspan>
+                                                        202207L // C++23
 __cpp_lib_memory_resource                               201603L <memory_resource>
 __cpp_lib_move_iterator_concept                         202207L <iterator>
 __cpp_lib_move_only_function                            202110L <functional>
@@ -530,6 +531,8 @@ __cpp_lib_void_t                                        201411L <type_traits>
 // # define __cpp_lib_is_virtual_base_of                   202406L
 // # define __cpp_lib_is_within_lifetime                   202306L
 // # define __cpp_lib_linalg                               202311L
+# undef  __cpp_lib_mdspan
+# define __cpp_lib_mdspan                               202406L
 // # define __cpp_lib_optional_range_support               202406L
 # undef  __cpp_lib_out_ptr
 // # define __cpp_lib_out_ptr                              202311L

libcxx/test/std/containers/views/mdspan/extents/dims.pass.cpp

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <mdspan>
+
+// template<size_t Rank, class IndexType = size_t>
+//     using dims = see below;
+//
+// Result: A type E that is a specialization of extents such that
+//         E::rank() == Rank && E::rank() == E::rank_dynamic() is true,
+//         and E::index_type denotes IndexType.
+
+#include <mdspan>
+#include <cstddef>
+
+#include "test_macros.h"
+
+template <class IndexType>
+void test_alias_template_dims() {
+  constexpr size_t D = std::dynamic_extent;
+  ASSERT_SAME_TYPE(std::dims<0, IndexType>, std::extents<IndexType>);
+  ASSERT_SAME_TYPE(std::dims<1, IndexType>, std::extents<IndexType, D>);
+  ASSERT_SAME_TYPE(std::dims<2, IndexType>, std::extents<IndexType, D, D>);
+  ASSERT_SAME_TYPE(std::dims<3, IndexType>, std::extents<IndexType, D, D, D>);
+  ASSERT_SAME_TYPE(std::dims<9, IndexType>, std::extents<IndexType, D, D, D, D, D, D, D, D, D>);
+}
+
+template <>
+void test_alias_template_dims<size_t>() {
+  constexpr size_t D = std::dynamic_extent;
+  ASSERT_SAME_TYPE(std::dims<0>, std::extents<size_t>);
+  ASSERT_SAME_TYPE(std::dims<1>, std::extents<size_t, D>);
+  ASSERT_SAME_TYPE(std::dims<2>, std::extents<size_t, D, D>);
+  ASSERT_SAME_TYPE(std::dims<3>, std::extents<size_t, D, D, D>);
+  ASSERT_SAME_TYPE(std::dims<9>, std::extents<size_t, D, D, D, D, D, D, D, D, D>);
+}
+
+int main(int, char**) {
+  test_alias_template_dims<int>();
+  test_alias_template_dims<unsigned int>();
+  test_alias_template_dims<size_t>();
+  return 0;
+}

libcxx/test/std/language.support/support.limits/support.limits.general/mdspan.version.compile.pass.cpp

@@ -18,6 +18,7 @@
 /*  Constant                         Value
     __cpp_lib_freestanding_mdspan    202311L [C++26]
     __cpp_lib_mdspan                 202207L [C++23]
+                                     202406L [C++26]
     __cpp_lib_submdspan              202306L [C++26]
 */
 
@@ -115,8 +116,8 @@
 # ifndef __cpp_lib_mdspan
 #   error "__cpp_lib_mdspan should be defined in c++26"
 # endif
-# if __cpp_lib_mdspan != 202207L
-#   error "__cpp_lib_mdspan should have the value 202207L in c++26"
+# if __cpp_lib_mdspan != 202406L
+#   error "__cpp_lib_mdspan should have the value 202406L in c++26"
 # endif
 
 # if !defined(_LIBCPP_VERSION)

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

@@ -147,6 +147,7 @@
     __cpp_lib_math_constants                                201907L [C++20]
     __cpp_lib_math_special_functions                        201603L [C++17]
     __cpp_lib_mdspan                                        202207L [C++23]
+                                                            202406L [C++26]
     __cpp_lib_memory_resource                               201603L [C++17]
     __cpp_lib_modules                                       202207L [C++23]
     __cpp_lib_move_iterator_concept                         202207L [C++20]
@@ -7289,8 +7290,8 @@
 # ifndef __cpp_lib_mdspan
 #   error "__cpp_lib_mdspan should be defined in c++26"
 # endif
-# if __cpp_lib_mdspan != 202207L
-#   error "__cpp_lib_mdspan should have the value 202207L in c++26"
+# if __cpp_lib_mdspan != 202406L
+#   error "__cpp_lib_mdspan should have the value 202406L in c++26"
 # endif
 
 # if !defined(_LIBCPP_VERSION) || _LIBCPP_AVAILABILITY_HAS_PMR

libcxx/utils/generate_feature_test_macro_components.py

@@ -868,7 +868,7 @@ def add_version_header(tc):
             "name": "__cpp_lib_mdspan",
             "values": {
                 "c++23": 202207,
-                # "c++26": 202406, # P2389R2 dextents Index Type Parameter
+                "c++26": 202406,  # P2389R2 dextents Index Type Parameter
             },
             "headers": ["mdspan"],
         },