{"id":2219522,"url":"http://patchwork.ozlabs.org/api/patches/2219522/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/ac9_k-LZ0JWy7Dwc@tucnak/","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":"","list_archive_url":null,"date":"2026-04-03T08:51:31","name":"c++: Implement CWG3135 - constexpr structured bindings with prvalues from tuples","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"26096aa0f8b1fcb2c6753b462845f8dbc6e7e19d","submitter":{"id":671,"url":"http://patchwork.ozlabs.org/api/people/671/?format=json","name":"Jakub Jelinek","email":"jakub@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/ac9_k-LZ0JWy7Dwc@tucnak/mbox/","series":[{"id":498605,"url":"http://patchwork.ozlabs.org/api/series/498605/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=498605","date":"2026-04-03T08:51:31","name":"c++: Implement CWG3135 - constexpr structured bindings with prvalues from tuples","version":1,"mbox":"http://patchwork.ozlabs.org/series/498605/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2219522/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2219522/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=Y4K6j+p7;\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=us-ascii","Content-Disposition":"inline","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 ","Reply-To":"Jakub Jelinek ","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"Hi!\n\nThe following patch implements another CWG issue, added e.g. for the\nP1789R3 (Library Support for Expansion Statements) APIs where get returns\na prvalue rather than a reference.\n\nThe gomp testcase was using get returning prvalue and the changes\ncause some changes in lines on which stuff is reported.\n\nBootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?\n\n2026-04-03 Jakub Jelinek \n\n\t* decl.cc (cp_finish_decomp): Implement CWG3135 - constexpr structured\n\tbindings with prvalues from tuples. Don't call cp_build_reference_type\n\tif init is prvalue for C++26.\n\n\t* g++.dg/cpp26/decomp30.C: New test.\n\t* g++.dg/gomp/pr108503.C: Expect diagnostics in C++26 on a different\n\tline.\n\n\n\tJakub","diff":"--- gcc/cp/decl.cc.jj\t2026-04-01 20:10:43.899063964 +0200\n+++ gcc/cp/decl.cc\t2026-04-02 16:26:49.865791353 +0200\n@@ -11011,7 +11011,8 @@ cp_finish_decomp (tree decl, cp_decomp *\n \t\t maybe_push_decl (t);\n \t\t /* Save the decltype away before reference collapse. */\n \t\t hash_map_safe_put (decomp_type_table, t, eltype);\n-\t\t eltype = cp_build_reference_type (eltype, !lvalue_p (init));\n+\t\t if (cxx_dialect < cxx26 || glvalue_p (init))\n+\t\t eltype = cp_build_reference_type (eltype, !lvalue_p (init));\n \t\t TREE_TYPE (t) = eltype;\n \t\t layout_decl (t, 0);\n \t\t DECL_HAS_VALUE_EXPR_P (t) = 0;\n@@ -11056,7 +11057,8 @@ cp_finish_decomp (tree decl, cp_decomp *\n \t }\n \t /* Save the decltype away before reference collapse. */\n \t hash_map_safe_put (decomp_type_table, v[i], eltype);\n-\t eltype = cp_build_reference_type (eltype, !lvalue_p (init));\n+\t if (cxx_dialect < cxx26 || glvalue_p (init))\n+\t eltype = cp_build_reference_type (eltype, !lvalue_p (init));\n \t TREE_TYPE (v[i]) = eltype;\n \t layout_decl (v[i], 0);\n \t if (DECL_HAS_VALUE_EXPR_P (v[i]))\n--- gcc/testsuite/g++.dg/cpp26/decomp30.C.jj\t2026-04-02 16:55:02.549367082 +0200\n+++ gcc/testsuite/g++.dg/cpp26/decomp30.C\t2026-04-02 16:54:45.142659067 +0200\n@@ -0,0 +1,37 @@\n+// CWG3135 - constexpr structured bindings with prvalues from tuples\n+// { dg-do compile { target c++11 } }\n+// { dg-options \"\" }\n+\n+namespace std {\n+ template struct tuple_size;\n+ template struct tuple_element;\n+}\n+\n+struct A {\n+ template constexpr int get () const { return I + 6; }\n+};\n+\n+template <> struct std::tuple_size { static const int value = 2; };\n+template struct std::tuple_element { using type = int; };\n+template <> struct std::tuple_size { static const int value = 2; };\n+template struct std::tuple_element { using type = const int; };\n+\n+constexpr A\n+bar ()\n+{\n+ return A {};\n+}\n+\n+template \n+constexpr int\n+foo ()\n+{\n+ constexpr auto [...i] = bar ();\t// { dg-warning \"structured binding packs only available with\" \"\" { target { c++17 && c++23_down } } }\n+\t\t\t\t\t// { dg-warning \"structured binding declaration can be 'constexpr' only with\" \"\" { target c++23_down } .-1 }\n+\t\t\t\t\t// { dg-warning \"structured bindings only available with\" \"\" { target c++14_down } .-2 }\n+\t\t\t\t\t// { dg-error \"'' is not a constant expression\" \"\" { target c++23_down } .-3 }\n+ return i...[0] + i...[1];\t\t// { dg-warning \"pack indexing only available with\" \"\" { target c++23_down } }\n+}\n+\n+static_assert (foo <42> () == 13);\t// { dg-error \"non-constant condition for static assertion\" \"\" { target c++23_down } }\n+\t\t\t\t\t// { dg-error \"called in a constant expression\" \"\" { target c++23_down } .-1 }\n--- gcc/testsuite/g++.dg/gomp/pr108503.C.jj\t2023-01-26 14:11:48.505599103 +0100\n+++ gcc/testsuite/g++.dg/gomp/pr108503.C\t2026-04-03 10:33:02.591824046 +0200\n@@ -20,8 +20,8 @@ struct B {\n void\n foo (B a)\n {\n- #pragma omp for collapse(2)\n- for (auto [i, j, k] : a)\n- for (int l = i; l < j; l += k)\t// { dg-error \"initializer expression refers to iteration variable 'i'\" }\n+ #pragma omp for collapse(2)\t\t// { dg-error \"increment expression refers to iteration variable 'k'\" \"\" { target c++26 } .+1 }\n+ for (auto [i, j, k] : a)\t\t// { dg-error \"initializer expression refers to iteration variable 'i'\" \"\" { target c++26 } }\n+ for (int l = i; l < j; l += k)\t// { dg-error \"initializer expression refers to iteration variable 'i'\" \"\" { target c++23_down } }\n ;\t\t\t\t\t// { dg-error \"condition expression refers to iteration variable 'j'\" \"\" { target *-*-* } .-1 }\n-}\t\t\t\t\t// { dg-error \"increment expression refers to iteration variable 'k'\" \"\" { target *-*-* } .-2 }\n+}\t\t\t\t\t// { dg-error \"increment expression refers to iteration variable 'k'\" \"\" { target c++23_down } .-2 }\n","prefixes":[]}