On Tue, Jul 7, 2026 at 12:21 PM Tomasz Kamiński <[email protected]> wrote:
> 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.
> ---
> Tested on x86_64-linux locally. OK for trunk?
>
> libstdc++-v3/include/std/mdspan | 3 +-
> .../mdspan/layouts/stride_neg.cc | 31 +++++++++++++++++++
> 2 files changed, 33 insertions(+), 1 deletion(-)
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index f5556f35fa1..5e1c4d8b1a4 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1891,7 +1891,8 @@ _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>(__strides[__i]);
>
Before committing, I will restore as_const call here, to match types that
do not have const-conversion,
to match standard to the letter.
> }
>
> 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 00000000000..153560bc82f
> --- /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()" }
> --
> 2.54.0
>
>
