[PATCH v3] libstdc++: Implement std::dims from <mdspan>.

[Jonathan Wakely]

On 21/07/25 17:50 +0200, Luc Grosheintz wrote:
>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.
>
>Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

OK for trunk.

>---
> libstdc++-v3/include/bits/version.def                 |  4 ++++
> libstdc++-v3/include/bits/version.h                   |  7 ++++++-
> libstdc++-v3/include/std/mdspan                       |  5 +++++
> libstdc++-v3/src/c++23/std.cc.in                      |  3 +++
> .../testsuite/23_containers/mdspan/extents/misc.cc    |  7 +++++++
> .../testsuite/23_containers/mdspan/version.cc         | 11 +++++++----
> 6 files changed, 32 insertions(+), 5 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
>index dbe2cb8f175..e9830d9d685 100644
>--- a/libstdc++-v3/include/bits/version.def
>+++ b/libstdc++-v3/include/bits/version.def
>@@ -1007,6 +1007,10 @@ 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 7bb6016df68..59b0cfa1f92 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 271fdb5d8c7..9afb9304fb5 100644
>--- a/libstdc++-v3/include/std/mdspan
>+++ b/libstdc++-v3/include/std/mdspan
>@@ -414,6 +414,11 @@ _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 9301ed90c00..6d7f3fda060 100644
>--- a/libstdc++-v3/src/c++23/std.cc.in
>+++ b/libstdc++-v3/src/c++23/std.cc.in
>@@ -1851,6 +1851,9 @@ 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 bca8901685d..8a43a682004 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 106ee4010ee..752060262a0 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
>-- 
>2.50.0
>
>