[{"id":3681940,"web_url":"http://patchwork.ozlabs.org/comment/3681940/","msgid":"","list_archive_url":null,"date":"2026-04-24T10:11:44","subject":"Re: [PATCH] libstdc++: Constrain views::adjacent(_transform)?<0> to\n forward_ranges.","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/people/48004/","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"content":"On Fri, 24 Apr 2026 at 11:05, Tomasz Kamiński wrote:\n>\n> This resolves LWG 4098, \"views::adjacent<0> should reject non-forward ranges\"\n> which was approved in Sofia 2024.\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/std/ranges (_AdjacentTransform::operator())\n> (_Adjacent::operator()): Require forward_range for N == 0.\n> * testsuite/std/ranges/adaptors/adjacent/1.cc: Test if input_ranges\n> are rejected.\n> * testsuite/std/ranges/adaptors/adjacent_transform/1.cc: Likewise.\n> ---\n> Testing on x86_64-linux. *adaptors/adjacent* test passed.\n> OK for trunk when all tests passes?\n>\n> libstdc++-v3/include/std/ranges | 10 ++++++++--\n> .../testsuite/std/ranges/adaptors/adjacent/1.cc | 7 +++++++\n> .../std/ranges/adaptors/adjacent_transform/1.cc | 7 +++++++\n> 3 files changed, 22 insertions(+), 2 deletions(-)\n>\n> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges\n> index 9c7d639a154..ba4c9f60ab2 100644\n> --- a/libstdc++-v3/include/std/ranges\n> +++ b/libstdc++-v3/include/std/ranges\n> @@ -5859,8 +5859,11 @@ namespace views::__adaptor\n> template\n> struct _Adjacent : __adaptor::_RangeAdaptorClosure<_Adjacent<_Nm>>\n> {\n> + // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> + // 4098. views::adjacent<0> should reject non-forward ranges\n> template\n> - requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>\n> + requires (_Nm == 0) && forward_range<_Range>\n\nPlease add parentheses around these to make the operator precedence clear.\n\n> + || __detail::__can_adjacent_view<_Nm, _Range>\n> constexpr auto\n> operator() [[nodiscard]] (_Range&& __r) const\n> {\n> @@ -6197,8 +6200,11 @@ namespace views::__adaptor\n> template\n> struct _AdjacentTransform : __adaptor::_RangeAdaptor<_AdjacentTransform<_Nm>>\n> {\n> + // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> + // 4098. views::adjacent<0> should reject non-forward ranges\n> template\n> - requires (_Nm == 0) || __detail::__can_adjacent_transform_view<_Nm, _Range, _Fp>\n> + requires (_Nm == 0) && forward_range<_Range>\n> + || __detail::__can_adjacent_transform_view<_Nm, _Range, _Fp>\n> constexpr auto\n> operator() [[nodiscard]] (_Range&& __r, _Fp&& __f) const\n> {\n> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc\n> index 0a5c67f5614..57322922e5e 100644\n> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc\n> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent/1.cc\n> @@ -9,6 +9,13 @@\n> namespace ranges = std::ranges;\n> namespace views = std::views;\n>\n> +template\n> +concept can_adjacent0 = requires (Range& rg)\n> +{ views::adjacent<0>(rg); };\n> +\n> +static_assert( !can_adjacent0<__gnu_test::test_input_range> );\n> +static_assert( can_adjacent0<__gnu_test::test_forward_range> );\n> +\n> constexpr bool\n> test01()\n> {\n> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc\n> index 6890618754b..a75d02efbb8 100644\n> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc\n> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc\n> @@ -9,6 +9,13 @@\n> namespace ranges = std::ranges;\n> namespace views = std::views;\n>\n> +template\n> +concept can_adjacent0 = requires (Range& rg)\n> +{ views::adjacent_transform<0>(rg, []() { return 10; }); };\n> +\n> +static_assert( !can_adjacent0<__gnu_test::test_input_range> );\n> +static_assert( can_adjacent0<__gnu_test::test_forward_range> );\n> +\n> constexpr bool\n> test01()\n> {\n> --\n> 2.53.0\n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=MYv86tx9;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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};\n> > +\n> > +static_assert( !can_adjacent0<__gnu_test::test_input_range> );\n> > +static_assert( can_adjacent0<__gnu_test::test_forward_range> );\n> > +\n> > constexpr bool\n> > test01()\n> > {\n> > diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc\n> > index 6890618754b..a75d02efbb8 100644\n> > --- a/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc\n> > +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/adjacent_transform/1.cc\n> > @@ -9,6 +9,13 @@\n> > namespace ranges = std::ranges;\n> > namespace views = std::views;\n> >\n> > +template\n> > +concept can_adjacent0 = requires (Range& rg)\n> > +{ views::adjacent_transform<0>(rg, []() { return 10; }); };\n> > +\n> > +static_assert( !can_adjacent0<__gnu_test::test_input_range> );\n> > +static_assert( can_adjacent0<__gnu_test::test_forward_range> );\n> > +\n> > constexpr bool\n> > test01()\n> > {\n> > --\n> > 2.53.0\n> >","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=EobFqaRY;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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