[PATCH v2 2/5] libstdc++: Check prerequite of extents::extents.

[Tomasz Kaminski]

On Fri, Jun 27, 2025 at 11:29 AM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> Previously the prerequite of the extents ctors that
>
>     static_extent(i) == dynamic_extent || extent(i) == other.extent(i).
>
> was not checked. This commit add the __glibcxx_assert and test it.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (extents): Check prerequite of the ctor that
>         static_extent(i) == dynamic_extent || extent(i) == other.extent(i).
>         * testsuite/23_containers/mdspan/extents/class_mandates_neg.cc:
>         Test the implemented prerequite.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM. As in case of mdspan, I would put the constexpr _neg test in other
file.

>  libstdc++-v3/include/std/mdspan               | 13 ++++++++
>  .../mdspan/extents/class_mandates_neg.cc      | 31 +++++++++++++++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 39d02ac08df..e198d65bba3 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -110,10 +110,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>             return __se;
>         }
>
> +       template<size_t _OtherRank, typename _GetOtherExtent>
> +         constexpr bool
> +         _S_is_compatible_extents(_GetOtherExtent __get_extent) noexcept
> +         {
> +           if constexpr (_OtherRank == _S_rank)
> +             for (size_t __i = 0; __i < _S_rank; ++__i)
> +               if (_Extents[__i] != dynamic_extent
> +                   && !cmp_equal(_Extents[__i],
> _S_int_cast(__get_extent(__i))))
> +                 return false;
> +           return true;
> +         }
> +
>         template<size_t _OtherRank, typename _GetOtherExtent>
>           constexpr void
>           _M_init_dynamic_extents(_GetOtherExtent __get_extent) noexcept
>           {
> +
>  __glibcxx_assert(_S_is_compatible_extents<_OtherRank>(__get_extent));
>             for (size_t __i = 0; __i < _S_rank_dynamic; ++__i)
>               {
>                 size_t __di = __i;
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
> index f9c1c019666..8179ff39962 100644
> ---
> a/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/class_mandates_neg.cc
> @@ -3,10 +3,41 @@
>
>  #include <cstdint>
>
> +constexpr size_t dyn = std::dynamic_extent;
> +
>  std::extents<uint8_t, size_t(1) << 9> e1; // { dg-error "from here" }
>  std::extents<char, 1> e2;                 // { dg-error "from here" }
>  std::extents<bool, 1> e3;                 // { dg-error "from here" }
>  std::extents<double, 1> e4;               // { dg-error "from here" }
> +
> +constexpr bool
> +test_dyn2sta_extents_mismatch_00()
> +{
> +  auto e0 = std::extents<int, dyn>{1};
> +  [[maybe_unused]] auto e1 = std::extents<int, 2>{e0};        // {
> dg-error "expansion of" }
> +  return true;
> +}
> +static_assert(test_dyn2sta_extents_mismatch_00());            // {
> dg-error "expansion of" }
> +
> +constexpr bool
> +test_dyn2sta_extents_mismatch_01()
> +{
> +  [[maybe_unused]] auto e = std::extents<int, 1, dyn>{2, 2}; // {
> dg-error "expansion of" }
> +  return true;
> +}
> +static_assert(test_dyn2sta_extents_mismatch_01());           // {
> dg-error "expansion of" }
> +
> +constexpr bool
> +test_dyn2sta_extents_mismatch_02()
> +{
> +  std::array<int, 2> exts{2, 2};
> +  [[maybe_unused]] auto e = std::extents<int, 1, dyn>{exts}; // {
> dg-error "expansion of" }
> +  return true;
> +}
> +static_assert(test_dyn2sta_extents_mismatch_02());           // {
> dg-error "expansion of" }
> +
>  // { dg-prune-output "dynamic or representable as IndexType" }
>  // { dg-prune-output "signed or unsigned integer" }
>  // { dg-prune-output "invalid use of incomplete type" }
> +// { dg-prune-output "non-constant condition for static assertion" }
> +// { dg-prune-output "__glibcxx_assert" }
> --
> 2.49.0
>
>
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