[PATCH v1] libstdc++: Fix bug in default ctor of extents.

[Luc Grosheintz]

The array that stores the dynamic extents used to be default
initialized. The standard requires value intialization. This
commit fixes the bug and adds a test.

libstdc++-v3/ChangeLog:

	* include/std/mdspan: Value initialize the array storing the
	dynamic extents.
	* testsuite/23_containers/mdspan/extents/ctor_default.cc: New
	test.

Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
---
 libstdc++-v3/include/std/mdspan               |  2 +-
 .../mdspan/extents/ctor_default.cc            | 41 +++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 47cfa405e44..bcf2fa60fea 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -146,7 +146,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       private:
 	using _S_storage = __array_traits<_IndexType, _S_rank_dynamic>::_Type;
-	[[no_unique_address]] _S_storage _M_dynamic_extents;
+	[[no_unique_address]] _S_storage _M_dynamic_extents{};
       };
 
     template<typename _OIndexType, typename _SIndexType>
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc b/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc
new file mode 100644
index 00000000000..eec300f6896
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_default.cc
@@ -0,0 +1,41 @@
+// { dg-do run { target c++23 } }
+#include <mdspan>
+
+#include <cstdint>
+#include <testsuite_hooks.h>
+
+constexpr auto dyn = std::dynamic_extent;
+
+template<typename Extents>
+  constexpr void
+  test_default_ctor()
+  {
+    Extents exts;
+    for(size_t i = 0; i < Extents::rank(); ++i)
+      if(exts.static_extent(i) == std::dynamic_extent)
+	VERIFY(exts.extent(i) == 0);
+      else
+	VERIFY(exts.extent(i) == Extents::static_extent(i));
+  }
+
+constexpr bool
+test_default_ctor_all()
+{
+  test_default_ctor<std::extents<int, 1>>();
+  test_default_ctor<std::extents<int, dyn>>();
+  test_default_ctor<std::extents<int, 1, 2>>();
+  test_default_ctor<std::extents<int, dyn, 2>>();
+  test_default_ctor<std::extents<int, dyn, dyn>>();
+  test_default_ctor<std::extents<int, 1, 2, 3>>();
+  test_default_ctor<std::extents<int, dyn, 2, dyn>>();
+  test_default_ctor<std::extents<int, dyn, dyn, dyn>>();
+  return true;
+}
+
+int
+main()
+{
+  test_default_ctor_all();
+  static_assert(test_default_ctor_all());
+  return 0;
+}
-- 
2.49.0