[{"id":3682055,"web_url":"http://patchwork.ozlabs.org/comment/3682055/","msgid":"<80063e98-c765-7b32-d582-92b20c013f5d@idea>","list_archive_url":null,"date":"2026-04-24T14:22:11","subject":"Re: [PATCH] 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 in all standard\n> 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> I initially tought that we should backport it, but the only impact of\n> the issues is you cannot rely on constraint check on views::elements\n> of such range. Any use of such elements_views remain ill-formed in\n> all modes.\n> \n> Testing on x86-64_linux. All *elements* test passed in all standard\n> modes. OK for trunk when all test passes?\n> \n> libstdc++-v3/include/std/ranges | 15 ++++++++++-----\n> .../testsuite/std/ranges/adaptors/elements.cc | 15 +++++++++++++++\n> 2 files changed, 25 insertions(+), 5 deletions(-)\n> \n> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges\n> index 55d3c520ba4..4eb0a1a25bf 100644\n> --- a/libstdc++-v3/include/std/ranges\n> +++ b/libstdc++-v3/include/std/ranges\n> @@ -4354,20 +4354,25 @@ namespace views::__adaptor\n> \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 =\n> +#if __cpp_lib_tuple_like // >= C++23\n> + __tuple_like<_Tp> && _Nm < tuple_size_v<_Tp>\n> #else\n> - template\n> - concept __has_tuple_element = requires(_Tp __t)\n> + requires\n> {\n> \ttypename tuple_size<_Tp>::type;\n> \trequires _Nm < tuple_size_v<_Tp>;\n> \ttypename tuple_element_t<_Nm, _Tp>;\n> + }\n> +#endif\n> + && requires(_Tp __t)\n> + {\n> \t{ std::get<_Nm>(__t) }\n> \t -> convertible_to&>;\n> };\n> -#endif\n\nIt seems more readable to me to keep the C++23 version completely\nseparate from the C++20 version, and just copy/paste the std::get\nrequirement into the C++23 version. Intertwining the two versions\ndoesn't seem beneficial as the C++20 version is unlikely to ever\nget changed at this point?\n\n> \n> template\n> concept __returnable_element\n> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> index 5237d787e7b..1b15f79c5be 100644\n> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\n> @@ -27,6 +27,21 @@\n> namespace ranges = std::ranges;\n> namespace views = ranges::views;\n> \n\nCan you mention the LWG issue in the added tests?\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> +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=eHN44NC8;\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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