[{"id":3682057,"web_url":"http://patchwork.ozlabs.org/comment/3682057/","msgid":"<3d3ae93e-7946-5e6f-3f2f-0a49274c8d1c@idea>","list_archive_url":null,"date":"2026-04-24T14:36:17","subject":"Re: [PATCH v2] libsdc++: Restore check for validity of std::get for\n elements_view.","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Fri, 24 Apr 2026, Tomasz Kamiński wrote:\n\n> Resolves LWG3797, \"elements_view insufficiently constrained\".\n> \n> When P2165R4 updated __has_tuple_element in C++23 to reuse __tuple_like\n> concept, it dropped the requirement of validity of get, assuming that for\n> tuple_like type with size of N, get on lvalue is well-formed for any I < N.\n> This however does not hold for ranges::subrange (tuple-like of size 2) with\n> move-only iterator, for which get can only be applied on rvalue. In consequence\n> constrains allowed instantiating elements_view for range of such subrange,\n> but instantiating it's iterator lead to hard error from iterator_category\n> computation.\n> \n> This patch applies the requirements on validity of get also in C++23 and\n> later standard modes.\n> \n> libstdc++-v3/ChangeLog:\n> \n> \t* include/std/ranges (__detail::__has_tuple_element): Check\n> \tif std::get<_Nm>(__t) returns referenceable type also for C++23\n> \tand later.\n> \t* testsuite/std/ranges/adaptors/elements.cc: Add test covering\n> \tvector of ranges::subrange with move-only iterator.\n> \n> Reviewed-by: Patrick Palka \n> Signed-off-by: Tomasz Kamiński \n> ---\n> v2 applies suggestion from Patrick:\n> - keep seprate defintion of __has_tuple_element for pre/post C++23\n> - add mention of LWG3797 in comment\n> \n> Tested on x86_654-linux. OK for trunk?\n\nLGTM!\n\n> \n> libstdc++-v3/include/std/ranges | 9 ++++++++-\n> .../testsuite/std/ranges/adaptors/elements.cc | 16 ++++++++++++++++\n> 2 files changed, 24 insertions(+), 1 deletion(-)\n> \n> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges\n> index 55d3c520ba4..7d8b37f8c5c 100644\n> --- a/libstdc++-v3/include/std/ranges\n> +++ b/libstdc++-v3/include/std/ranges\n> @@ -4355,8 +4355,15 @@ namespace views::__adaptor\n> namespace __detail\n> {\n> #if __cpp_lib_tuple_like // >= C++23\n> + // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> + // 3797. elements_view insufficiently constrained\n> template\n> - concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>;\n> + concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>\n> + && requires(_Tp __t)\n> +\t {\n> +\t { std::get<_Nm>(__t) }\n> +\t -> convertible_to&>;\n> +\t };\n> #else\n> template\n> concept __has_tuple_element = requires(_Tp __t)\n> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> index 5237d787e7b..d384cfd5adf 100644\n> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> @@ -27,6 +27,22 @@\n> namespace ranges = std::ranges;\n> namespace views = ranges::views;\n> \n> +template\n> +concept can_elements = requires (Rg&& rg)\n> +{ views::elements<0>(rg); };\n> +\n> +static_assert( !can_elements<0, std::vector> );\n> +static_assert( can_elements<0, std::tuple[4]> );\n> +static_assert( can_elements<1, std::vector>> );\n> +\n> +// Test LWG 3797. elements_view insufficiently constrained\n> +using move_only_iter_range = __gnu_test::test_input_range_nocopy;\n> +using move_only_iter_subrange = ranges::subrange<\n> + ranges::iterator_t,\n> + ranges::sentinel_t>;\n> +static_assert( can_elements<0, std::vector>> );\n> +static_assert( !can_elements<0, std::vector> );\n> +\n> void\n> test01()\n> {\n> -- \n> 2.53.0\n> \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=OUQKtPGp;\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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In consequence\n> constrains allowed instantiating elements_view for range of such subrange,\n> but instantiating it's iterator lead to hard error from iterator_category\n> computation.\n>\n> This patch applies the requirements on validity of get also in C++23 and\n> later standard modes.\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/std/ranges (__detail::__has_tuple_element): Check\n> if std::get<_Nm>(__t) returns referenceable type also for C++23\n> and later.\n> * testsuite/std/ranges/adaptors/elements.cc: Add test covering\n> vector of ranges::subrange with move-only iterator.\n>\n> Reviewed-by: Patrick Palka \n> Signed-off-by: Tomasz Kamiński \n> ---\n> v2 applies suggestion from Patrick:\n> - keep seprate defintion of __has_tuple_element for pre/post C++23\n> - add mention of LWG3797 in comment\n>\n> Tested on x86_654-linux. OK for trunk?\n>\n> libstdc++-v3/include/std/ranges | 9 ++++++++-\n> .../testsuite/std/ranges/adaptors/elements.cc | 16 ++++++++++++++++\n> 2 files changed, 24 insertions(+), 1 deletion(-)\n>\n> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges\n> index 55d3c520ba4..7d8b37f8c5c 100644\n> --- a/libstdc++-v3/include/std/ranges\n> +++ b/libstdc++-v3/include/std/ranges\n> @@ -4355,8 +4355,15 @@ namespace views::__adaptor\n> namespace __detail\n> {\n> #if __cpp_lib_tuple_like // >= C++23\n> + // _GLIBCXX_RESOLVE_LIB_DEFECTS\n> + // 3797. elements_view insufficiently constrained\n> template\n> - concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>;\n> + concept __has_tuple_element = __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>\n> + && requires(_Tp __t)\n> + {\n> + { std::get<_Nm>(__t) }\n> + -> convertible_to&>;\n> + };\n\nI like Patrick's suggestion, I agree this seems more maintainable.\n\nOK for trunk, thanks.\n\n\n> #else\n> template\n> concept __has_tuple_element = requires(_Tp __t)\n> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> index 5237d787e7b..d384cfd5adf 100644\n> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> @@ -27,6 +27,22 @@\n> namespace ranges = std::ranges;\n> namespace views = ranges::views;\n>\n> +template\n> +concept can_elements = requires (Rg&& rg)\n> +{ views::elements<0>(rg); };\n> +\n> +static_assert( !can_elements<0, std::vector> );\n> +static_assert( can_elements<0, std::tuple[4]> );\n> +static_assert( can_elements<1, std::vector>> );\n> +\n> +// Test LWG 3797. elements_view insufficiently constrained\n> +using move_only_iter_range = __gnu_test::test_input_range_nocopy;\n> +using move_only_iter_subrange = ranges::subrange<\n> + ranges::iterator_t,\n> + ranges::sentinel_t>;\n> +static_assert( can_elements<0, std::vector>> );\n> +static_assert( !can_elements<0, std::vector> );\n> +\n> void\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=U4dNB7Lx;\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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