[gcc r16-9361] libstdc++: Validate user-provided stride values for layout_stride.

https://gcc.gnu.org/g:f13f153acb97ac491efa6c0df854f3de2dc68b51

commit r16-9361-gf13f153acb97ac491efa6c0df854f3de2dc68b51
Author: Tomasz Kamiński <[email protected]>
Date:   Fri Jul 3 14:41:02 2026 +0200

    libstdc++: Validate user-provided stride values for layout_stride.
    
    Converting the __strides values using __index_type_cast asserts
    that they are non-negative and each value is representable as
    index_type.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/mdspan
            (layout_stride::mapping::mapping(const extent_type&, span<...>)):
            Convert strides using __index_type_cast, which bring asserts
            for negative and unrepresentable values.
            * testsuite/23_containers/mdspan/layouts/stride_neg.cc: New test.
    
    Reviewed-by: Jonathan Wakely <[email protected]>
    Signed-off-by: Tomasz Kamiński <[email protected]>
    (cherry picked from commit 0bcb57a2294fbadfa8388bdc3a5be356ad3cf91c)

Diff:
---
 libstdc++-v3/include/std/mdspan                    |  3 ++-
 .../23_containers/mdspan/layouts/stride_neg.cc     | 31 ++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 7ee0c351ad96..b47b4de67b2b 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -1897,7 +1897,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: _M_extents(__exts)
 	{
 	  for (size_t __i = 0; __i < extents_type::rank(); ++__i)
-	    _M_strides[__i] = index_type(as_const(__strides[__i]));
+	    _M_strides[__i] =
+	      __mdspan::__index_type_cast<index_type>(as_const(__strides[__i]));
 	}
 
       template<typename _OIndexType>
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
new file mode 100644
index 000000000000..153560bc82ff
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
@@ -0,0 +1,31 @@
+// { dg-do compile { target c++23 } }
+#include <mdspan>
+
+#include "../layout_traits.h"
+#include <cstdint>
+
+constexpr size_t dyn = std::dynamic_extent;
+
+constexpr bool
+test_stride_overflow()
+{
+  auto exts = std::extents<uint8_t, dyn, dyn>(1, 3);
+  auto n = size_t(1) << 9;
+  auto m = std::layout_stride::mapping(exts, std::array{n, 1zu}); // { dg-error "expansion of" }
+  (void) m;
+  return true;
+}
+static_assert(test_stride_overflow()); // { dg-error "expansion of" }
+
+constexpr bool
+test_stride_negative()
+{
+  auto exts = std::extents<std::size_t, dyn, dyn>(1, 3);
+  auto m = std::layout_stride::mapping(exts, std::array{1, -4}); // { dg-error "expansion of" } 
+  (void) m;
+  return true;
+}
+static_assert(test_stride_negative()); // { dg-error "expansion of" } 
+
+// { dg-prune-output "non-constant condition for static assertion" }
+// { dg-prune-output "__glibcxx_assert_fail()" }