libstdc++: Implement std::dims from <mdspan>.

This commit implements the C++26 feature std::dims described in P2389R2.
It sets the feature testing macro to 202406 and adds tests.

Also fixes the test mdspan/version.cc

libstdc++-v3/ChangeLog:

	* include/bits/version.def (mdspan): Set value for C++26.
	* include/bits/version.h: Regenerate.
	* include/std/mdspan (dims): Add.
	* src/c++23/std.cc.in (dims): Add.
	* testsuite/23_containers/mdspan/extents/misc.cc: Add tests.
	* testsuite/23_containers/mdspan/version.cc: Update test.

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

6 files changed, 32 insertions, 5 deletions

diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index dbe2cb8..e9830d9 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1008,6 +1008,10 @@ ftms = {
 ftms = {
   name = mdspan;
   values = {
+    v = 202406;
+    cxxmin = 26;
+  };
+  values = {
     v = 202207;
     cxxmin = 23;
   };
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index 7bb6016..59b0cfa1 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -1125,7 +1125,12 @@
 #undef __glibcxx_want_span
 
 #if !defined(__cpp_lib_mdspan)
-# if (__cplusplus >= 202100L)
+# if (__cplusplus >  202302L)
+#  define __glibcxx_mdspan 202406L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_mdspan)
+#   define __cpp_lib_mdspan 202406L
+#  endif
+# elif (__cplusplus >= 202100L)
 #  define __glibcxx_mdspan 202207L
 #  if defined(__glibcxx_want_all) || defined(__glibcxx_want_mdspan)
 #   define __cpp_lib_mdspan 202207L
diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 4db4fd9..b7b4375 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -534,6 +534,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     using dextents = decltype(__mdspan::__build_dextents_type<_IndexType>(
 	make_index_sequence<_Rank>()));
 
+#if __glibcxx_mdspan >= 202406L
+  template<size_t _Rank, typename _IndexType = size_t>
+    using dims = dextents<_IndexType, _Rank>;
+#endif
+
   template<typename... _Integrals>
     requires (is_convertible_v<_Integrals, size_t> && ...)
     explicit extents(_Integrals...) ->
diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index 405bb6e..aa57707 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -1858,6 +1858,9 @@ export namespace std
 {
   using std::extents;
   using std::dextents;
+#if __glibcxx_mdspan >= 202406L
+  using std::dims;
+#endif
   using std::layout_left;
   using std::layout_right;
   using std::layout_stride;
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
index bca8901..8a43a68 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
@@ -159,6 +159,13 @@ static_assert(std::extents<int, 1, dyn>::static_extent(1) == dyn);
 static_assert(std::extents<int, dyn, dyn>::static_extent(0) == dyn);
 static_assert(std::extents<int, dyn, dyn>::static_extent(1) == dyn);
 
+// dims
+#if __glibcxx_mdspan >= 202406L
+static_assert(std::is_same_v<std::dims<0>, std::dextents<size_t, 0>>);
+static_assert(std::is_same_v<std::dims<3>, std::dextents<size_t, 3>>);
+static_assert(std::is_same_v<std::dims<3, int>, std::dextents<int, 3>>);
+#endif
+
 // extent
 template<typename Extent>
   constexpr void
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/version.cc b/libstdc++-v3/testsuite/23_containers/mdspan/version.cc
index 106ee40..7520602 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/version.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/version.cc
@@ -1,9 +1,12 @@
-// { dg-do compile { target c++23 } }
+// { dg-do preprocess { target c++23 } }
+// { dg-add-options no_pch }
+
 #include <mdspan>
 
 #ifndef __cpp_lib_mdspan
 #error "Feature test macro __cpp_lib_mdspan is missing for <mdspan>"
-#if __cpp_lib_mdspan < 202207
-#error "Feature test macro __cpp_lib_mdspan has the wrong value"
-#endif
+#elif __cplusplus <= 202302L && __cpp_lib_mdspan != 202207L
+#error "Feature test macro __cpp_lib_mdspan has the wrong value for C++23"
+#elif __cplusplus > 202302L && __cpp_lib_mdspan != 202406L
+#error "Feature test macro __cpp_lib_mdspan has the wrong value for C++26"
 #endif