libstdc++: Fix bug in default ctor of extents.

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>

2 files changed, 42 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 47cfa40..bcf2fa6 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 0000000..eec300f
--- /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;
+}