[PATCH] libstdc++: Properly handle forwarding references in __mdspan::__index_type_cast.

[Tomasz Kaminski]

On Tue, Mar 10, 2026 at 4:48 PM Tomasz Kamiński <tkaminsk@redhat.com> wrote:

> The _OIndexType template parameter will maybe deduced to reference, when
> invoked from constructor and operator[]/at (after r16-7645-g66e60479e97640)
> accepting array or span. We need to use forward for class-type and
> remove_cvref_t for is_integral checks.
>
> This resolves LWG4020: extents::index-cast weirdness.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (__mdspan::__index_type_cast): Handle
>         forwarding referene and use forward for class types.
>         * testsuite/23_containers/mdspan/int_like.h (ConstLValueInt)
>         (CustomIndexKind::ConstLValue): Define.
>         (CustomIndexType): Handle CustomIndexKind::ConstLValue.
>         * testsuite/23_containers/mdspan/at.cc: Test with ConstLValueInt.
>         * testsuite/23_containers/mdspan/extents/custom_integer.cc:
> Likewise.
>         * testsuite/23_containers/mdspan/mdspan.cc: Likewise.
>
> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
> ---
> Tested *mdspan* for all standard mode. Testing on x86_64-linux.
> OK for trunk when all tests passes?
>
>  libstdc++-v3/include/std/mdspan                       |  7 ++++---
>  libstdc++-v3/testsuite/23_containers/mdspan/at.cc     |  1 +
>  .../23_containers/mdspan/extents/custom_integer.cc    |  2 ++
>  .../testsuite/23_containers/mdspan/int_like.h         | 11 +++++++++++
>  libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc |  2 ++
>  5 files changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index f8847642c80..5e0d52ff695 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -80,10 +80,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        return true;
>      }
>
> -    template<typename _IndexType, typename _OIndexType>
> +    template<typename _IndexType, typename _OIndexTypeRef>
>        constexpr _IndexType
> -      __index_type_cast(_OIndexType&& __other)
> +      __index_type_cast(_OIndexTypeRef&& __other)
>        {
>
Locally I have added:
 // _GLIBCXX_RESOLVE_LIB_DEFECTS

 // 4020. extents::index-cast weirdnes


> +       using _OIndexType = std::remove_cvref_t<_OIndexTypeRef>;
>         if constexpr (std::is_integral_v<_OIndexType>)
>           {
>             constexpr _IndexType __index_type_max
> @@ -100,7 +101,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           }
>         else
>           {
> -           auto __ret = static_cast<_IndexType>(std::move(__other));
> +           auto __ret =
> static_cast<_IndexType>(std::forward<_OIndexTypeRef>(__other));
>             if constexpr (std::is_signed_v<_IndexType>)
>               __glibcxx_assert(__ret >= 0);
>             return __ret;
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/at.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/at.cc
> index c5a12d12ccc..367398dd078 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/at.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/at.cc
> @@ -116,4 +116,5 @@ main()
>    test_at<ThrowingInt, false, false>();
>    test_at<MutatingInt, true, false>();
>    test_at<RValueInt, true, false>();
> +  test_at<ConstLValueInt, false, true>();
>  }
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/extents/custom_integer.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/custom_integer.cc
> index 99de4015ef3..4e9669c98d8 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/custom_integer.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/custom_integer.cc
> @@ -86,11 +86,13 @@ main()
>    test_shape_all<ThrowingInt, false>();
>    test_shape_all<MutatingInt, false>();
>    test_shape_all<RValueInt, false>();
> +  test_shape_all<ConstLValueInt, true>();
>
>    test_pack_all<int, true>();
>    test_pack_all<IntLike, true>();
>    test_pack_all<ThrowingInt, false>();
>    test_pack_all<MutatingInt, true>();
>    test_pack_all<RValueInt, true>();
> +  test_pack_all<ConstLValueInt, false>();
>    return 0;
>  }
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> index b839be73ae9..99ecd155ca4 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
> @@ -7,6 +7,7 @@ enum class CustomIndexKind
>    Throwing,
>    Mutating,
>    RValue,
> +  ConstLValue,
>  };
>
>  template<CustomIndexKind Kind, bool Copyable = false>
> @@ -57,6 +58,15 @@ template<CustomIndexKind Kind, bool Copyable = false>
>      requires (Kind == CustomIndexKind::RValue)
>      { return value; }
>
> +    constexpr
> +    operator int() const& noexcept
> +    requires (Kind == CustomIndexKind::ConstLValue)
> +    { return value; }
> +
> +    operator int() const&&
> +    requires (Kind == CustomIndexKind::ConstLValue)
> +    = delete;
> +
>    private:
>      int value;
>    };
> @@ -65,6 +75,7 @@ using IntLike = CustomIndexType<CustomIndexKind::Const>;
>  using ThrowingInt = CustomIndexType<CustomIndexKind::Throwing>;
>  using MutatingInt = CustomIndexType<CustomIndexKind::Mutating>;
>  using RValueInt = CustomIndexType<CustomIndexKind::RValue>;
> +using ConstLValueInt = CustomIndexType<CustomIndexKind::ConstLValue>;
>
>  struct NotIntLike
>  { };
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> index a92a0554417..270b3f18daa 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> @@ -755,6 +755,7 @@ main()
>    test_from_int_like<ThrowingInt, false, false>();
>    test_from_int_like<MutatingInt, true, false>();
>    test_from_int_like<RValueInt, true, false>();
> +  test_from_int_like<ConstLValueInt, false, true>();
>
>    test_from_opaque_accessor();
>    test_from_base_class_accessor();
> @@ -767,6 +768,7 @@ main()
>    test_access<ThrowingInt, false, false>();
>    test_access<MutatingInt, true, false>();
>    test_access<RValueInt, true, false>();
> +  test_access<ConstLValueInt, false, true>();
>
>    test_swap();
>    static_assert(test_swap());
> --
> 2.53.0
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20260318/dc4be9e1/attachment.htm>