{"id":2227946,"url":"http://patchwork.ozlabs.org/api/patches/2227946/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/20260424143332.2133358-1-tkaminsk@redhat.com/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/projects/17/?format=json","name":"GNU Compiler Collection","link_name":"gcc","list_id":"gcc-patches.gcc.gnu.org","list_email":"gcc-patches@gcc.gnu.org","web_url":null,"scm_url":null,"webscm_url":null,"list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"<20260424143332.2133358-1-tkaminsk@redhat.com>","list_archive_url":null,"date":"2026-04-24T14:31:38","name":"[v2] libsdc++: Restore check for validity of std::get for elements_view.","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"9e7f1707367415fb74827db3b91cee1f87f50bde","submitter":{"id":90409,"url":"http://patchwork.ozlabs.org/api/people/90409/?format=json","name":"Tomasz Kamiński","email":"tkaminsk@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/20260424143332.2133358-1-tkaminsk@redhat.com/mbox/","series":[{"id":501377,"url":"http://patchwork.ozlabs.org/api/series/501377/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=501377","date":"2026-04-24T14:31:38","name":"[v2] libsdc++: Restore check for validity of std::get for elements_view.","version":2,"mbox":"http://patchwork.ozlabs.org/series/501377/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2227946/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2227946/checks/","tags":{},"related":[],"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=ZvcpxHk/;\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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charset=UTF-8","Content-Transfer-Encoding":"8bit","X-BeenThere":"gcc-patches@gcc.gnu.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Gcc-patches mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"Resolves LWG3797, \"elements_view insufficiently constrained\".\n\nWhen P2165R4 updated __has_tuple_element in C++23 to reuse __tuple_like\nconcept, it dropped the requirement of validity of get, assuming that for\ntuple_like type with size of N, get on lvalue is well-formed for any I < N.\nThis however does not hold for ranges::subrange (tuple-like of size 2) with\nmove-only iterator, for which get can only be applied on rvalue. In consequence\nconstrains allowed instantiating elements_view for range of such subrange,\nbut instantiating it's iterator lead to hard error from iterator_category\ncomputation.\n\nThis patch applies the requirements on validity of get also in C++23 and\nlater standard modes.\n\nlibstdc++-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\nReviewed-by: Patrick Palka \nSigned-off-by: Tomasz Kamiński \n---\nv2 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\nTested 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(-)","diff":"diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges\nindex 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)\ndiff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc\nindex 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","prefixes":["v2"]}