[PATCH 2/2] libstdc++: Add ranges::borrowed_range specialization for optional<T&> [PR122425]

[Jonathan Wakely]

OK thanks

On Thu, 6 Nov 2025 at 16:07, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
>         PR libstdc++/122425
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/optional
>         (ranges::enable_borrowed_range<optional<_Tp&>>): Define.
>         * testsuite/20_util/optional/range.cc: Update tests.
> ---
>  libstdc++-v3/include/std/optional                | 6 ++++++
>  libstdc++-v3/testsuite/20_util/optional/range.cc | 6 +++++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
> index fad5f57a94c..5407fc74a41 100644
> --- a/libstdc++-v3/include/std/optional
> +++ b/libstdc++-v3/include/std/optional
> @@ -2178,6 +2178,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      inline constexpr bool
>        ranges::enable_view<optional<_Tp>> = true;
>
> +#if __cpp_lib_optional >= 202506L // C++26
> +  template<typename _Tp>
> +    constexpr bool
> +    ranges::enable_borrowed_range<optional<_Tp&>> = true;
> +#endif
> +
>    template<typename _Tp>
>      inline constexpr range_format
>        format_kind<optional<_Tp>> = range_format::disabled;
> diff --git a/libstdc++-v3/testsuite/20_util/optional/range.cc b/libstdc++-v3/testsuite/20_util/optional/range.cc
> index 1cb3eb6ceff..981969cb614 100644
> --- a/libstdc++-v3/testsuite/20_util/optional/range.cc
> +++ b/libstdc++-v3/testsuite/20_util/optional/range.cc
> @@ -19,12 +19,16 @@ test_range_concepts()
>    static_assert(std::ranges::contiguous_range<O>);
>    static_assert(std::ranges::sized_range<O>);
>    static_assert(std::ranges::common_range<O>);
> -  static_assert(!std::ranges::borrowed_range<O>);
> +
> +  // an optional<T&> is borrowed range
> +  constexpr bool is_ref_opt = std::is_reference_v<T>;
> +  static_assert(std::ranges::borrowed_range<O> == is_ref_opt);
>
>    // an optional<const T> is not assignable, and therefore does not satisfy ranges::view
>    constexpr bool is_const_opt = std::is_const_v<T>;
>    static_assert(std::ranges::view<O> == !is_const_opt);
>    static_assert(std::ranges::viewable_range<O> == !is_const_opt);
> +
>  }
>
>  template<typename O>
> --
> 2.51.0
>