[PATCH] libstdc++: Constrain views::adjacent(_transform)?<0> to forward_ranges.

[Jonathan Wakely]

On Fri, 24 Apr 2026 at 11:05, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
> This resolves LWG 4098, "views::adjacent<0> should reject non-forward ranges"
> which was approved in Sofia 2024.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (_AdjacentTransform::operator())
>         (_Adjacent::operator()): Require forward_range for N == 0.
>         * testsuite/std/ranges/adaptors/adjacent/1.cc: Test if input_ranges
>         are rejected.
>         * testsuite/std/ranges/adaptors/adjacent_transform/1.cc: Likewise.
> ---
> Testing on x86_64-linux. *adaptors/adjacent* test passed.
> OK for trunk when all tests passes?
>
>  libstdc++-v3/include/std/ranges                        | 10 ++++++++--
>  .../testsuite/std/ranges/adaptors/adjacent/1.cc        |  7 +++++++
>  .../std/ranges/adaptors/adjacent_transform/1.cc        |  7 +++++++
>  3 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 9c7d639a154..ba4c9f60ab2 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -5859,8 +5859,11 @@ namespace views::__adaptor
>      template<size_t _Nm>
>        struct _Adjacent : __adaptor::_RangeAdaptorClosure<_Adjacent<_Nm>>
>        {
> +       // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +       // 4098. views::adjacent<0> should reject non-forward ranges
>         template<viewable_range _Range>
> -         requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> +         requires (_Nm == 0) && forward_range<_Range>

Please add parentheses around these to make the operator precedence clear.

> +                  || __detail::__can_adjacent_view<_Nm, _Range>
>           constexpr auto
>           operator() [[nodiscard]] (_Range&& __r) const
>           {
> @@ -6197,8 +6200,11 @@ namespace views::__adaptor
>      template<size_t _Nm>
>        struct _AdjacentTransform : __adaptor::_RangeAdaptor<_AdjacentTransform<_Nm>>
>        {
> +       // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +       // 4098. views::adjacent<0> should reject non-forward ranges
>         template<viewable_range _Range, typename _Fp>
> -         requires (_Nm == 0) || __detail::__can_adjacent_transform_view<_Nm, _Range, _Fp>
> +         requires (_Nm == 0) && forward_range<_Range>
> +                  || __detail::__can_adjacent_transform_view<_Nm, _Range, _Fp>
>           constexpr auto
>           operator() [[nodiscard]] (_Range&& __r, _Fp&& __f) const
>           {
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
> index 0a5c67f5614..57322922e5e 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc
> @@ -9,6 +9,13 @@
>  namespace ranges = std::ranges;
>  namespace views = std::views;
>
> +template<typename Range>
> +concept can_adjacent0 = requires (Range& rg)
> +{ views::adjacent<0>(rg); };
> +
> +static_assert( !can_adjacent0<__gnu_test::test_input_range<int>> );
> +static_assert( can_adjacent0<__gnu_test::test_forward_range<int>> );
> +
>  constexpr bool
>  test01()
>  {
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc
> index 6890618754b..a75d02efbb8 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc
> @@ -9,6 +9,13 @@
>  namespace ranges = std::ranges;
>  namespace views = std::views;
>
> +template<typename Range>
> +concept can_adjacent0 = requires (Range& rg)
> +{ views::adjacent_transform<0>(rg, []() { return 10; }); };
> +
> +static_assert( !can_adjacent0<__gnu_test::test_input_range<int>> );
> +static_assert( can_adjacent0<__gnu_test::test_forward_range<int>> );
> +
>  constexpr bool
>  test01()
>  {
> --
> 2.53.0
>