[{"id":3684132,"web_url":"http://patchwork.ozlabs.org/comment/3684132/","msgid":"","list_archive_url":null,"date":"2026-04-29T15:28:12","subject":"Re: [PATCH] c++: Delete default ctor for unions with all const\n members","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 4/29/26 5:43 AM, Jakub Jelinek wrote:\n> Hi!\n> \n> I've started looking at P3074R7/P3726R2 papers.\n> The former removes two bullets which describe when default ctor should be\n> deleted:\n> - X is a union and all of its variant members are of const-qualified type (or\n> possibly multi-dimensional array thereof),\n> - X is a non-union class and all members of any anonymous union member are of\n> const-qualified type (or possibly multi-dimensional array thereof).\n> These have been in [class.default.ctor] (or earlier in [class.ctor]) since\n> C++11 till C++23, but we apparently ignore these and accept it, while\n> clang++ deletes them.\n\nI'm inclined to treat dropping these bullets as a DR back to C++20, to \ngo along with the changes in P1330, P1331, and (particularly) CWG2424.\n\n> The following patch implements it for C++11 (I can't find similar wording\n> in C++98, but in C++98 one couldn't use NSDMIs and so gets other diagnostics\n> in such cases) through C++23 inclusive, not for C++26 because P3074R7 drops\n> those.\n> \n> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?\n> \n> 2026-04-28 Jakub Jelinek \n> \n> \t* method.cc (walk_field_subobs): Add ALL_CONST_P argument. Determine\n> \tif all variant members are const qualified. Diagnose and make deleted\n> \tfor C++11 to C++23 if all anonymous union members are const qualified.\n> \t(synthesized_method_walk): Diagnose and make deleted for C++11 to\n> \tC++23 if all variant members of a union are const qualified.\n> \n> \t* g++.dg/cpp0x/defaulted70.C: New test.\n> \t* g++.dg/init/pr25811.C: Expect another error for C++11 to C++23.\n> \t* g++.dg/init/pr43719.C: Likewise.\n> \n> --- gcc/cp/method.cc.jj\t2026-03-27 10:17:14.024332534 +0100\n> +++ gcc/cp/method.cc\t2026-04-28 19:12:37.903067733 +0200\n> @@ -2680,7 +2680,7 @@ walk_field_subobs (tree fields, special_\n> \t\t int quals, tree *spec_p, bool *trivial_p,\n> \t\t bool *deleted_p, bool *constexpr_p,\n> \t\t bool diag, int flags, tsubst_flags_t complain,\n> -\t\t bool dtor_from_ctor)\n> +\t\t bool dtor_from_ctor, int *all_const_p)\n> {\n> if (!fields)\n> return;\n> @@ -2712,6 +2712,16 @@ walk_field_subobs (tree fields, special_\n> \t && TREE_CODE (ctx) == UNION_TYPE)\n> \tbreak;\n> \n> + mem_type = strip_array_types (TREE_TYPE (field));\n> +\n> + if (all_const_p && !ANON_AGGR_TYPE_P (mem_type))\n> +\t{\n> +\t if (*all_const_p == -1 && CP_TYPE_CONST_P (mem_type))\n> +\t *all_const_p = 1;\n> +\t else if (!CP_TYPE_CONST_P (mem_type))\n> +\t *all_const_p = 0;\n> +\t}\n> +\n> if (only_dmi_mem != no)\n> \t{\n> \t if (DECL_INITIAL (field))\n> @@ -2721,7 +2731,6 @@ walk_field_subobs (tree fields, special_\n> \t continue;\n> \t}\n> \n> - mem_type = strip_array_types (TREE_TYPE (field));\n> if (SFK_ASSIGN_P (sfk))\n> \t{\n> \t bool bad = true;\n> @@ -2831,9 +2840,28 @@ walk_field_subobs (tree fields, special_\n> \n> if (ANON_AGGR_TYPE_P (mem_type))\n> \t{\n> +\t int *this_all_const_p = all_const_p, all_const = -1;\n> +\t if (!all_const_p\n> +\t && ANON_UNION_TYPE_P (mem_type)\n> +\t && (sfk == sfk_constructor || sfk == sfk_inheriting_constructor)\n> +\t && (cxx_dialect >= cxx11 && cxx_dialect <= cxx23))\n> +\t this_all_const_p = &all_const;\n> \t walk_field_subobs (TYPE_FIELDS (mem_type), sfk, fnname, quals,\n> \t\t\t spec_p, trivial_p, deleted_p, constexpr_p,\n> -\t\t\t diag, flags, complain, dtor_from_ctor);\n> +\t\t\t diag, flags, complain, dtor_from_ctor,\n> +\t\t\t this_all_const_p);\n> +\t if (all_const == 1)\n> +\t {\n> +\t /* [class.default.ctor]/(2.6)\n> +\t\t X is a non-union class and all members of any anonymous union\n> +\t\t member are of const-qualified type (or array thereof). */\n> +\t if (diag)\n> +\t\terror_at (DECL_SOURCE_LOCATION (field),\n> +\t\t\t \"all members of anonymous union member are \"\n> +\t\t\t \"%-qualified\");\n> +\t if (deleted_p)\n> +\t\t*deleted_p = true;\n> +\t }\n> \t continue;\n> \t}\n> \n> @@ -3130,14 +3158,31 @@ synthesized_method_walk (tree ctype, spe\n> }\n> \n> /* Now handle the non-static data members. */\n> + int *all_const_p = NULL, all_const = -1;\n> + if ((sfk == sfk_constructor || sfk == sfk_inheriting_constructor)\n> + && (cxx_dialect >= cxx11 && cxx_dialect <= cxx23)\n> + && TREE_CODE (ctype) == UNION_TYPE)\n> + all_const_p = &all_const;\n> walk_field_subobs (TYPE_FIELDS (ctype), sfk, fnname, quals,\n> \t\t spec_p, trivial_p, deleted_p, constexpr_p,\n> -\t\t diag, flags, complain, /*dtor_from_ctor*/false);\n> +\t\t diag, flags, complain, /*dtor_from_ctor*/false,\n> +\t\t all_const_p);\n> + if (all_const == 1)\n> + {\n> + /* [class.default.ctor]/(2.5)\n> +\t X is a union and all of its variant members are of const-qualified\n> +\t type (or array thereof). */\n> + if (diag)\n> +\terror_at (DECL_SOURCE_LOCATION (TYPE_NAME (ctype)),\n> +\t\t \"all variant members are %-qualified\");\n> + if (deleted_p)\n> +\t*deleted_p = true;\n> + }\n> if (SFK_CTOR_P (sfk))\n> walk_field_subobs (TYPE_FIELDS (ctype), sfk_destructor,\n> \t\t complete_dtor_identifier, TYPE_UNQUALIFIED,\n> \t\t NULL, NULL, deleted_p, NULL,\n> -\t\t false, flags, complain, /*dtor_from_ctor*/true);\n> +\t\t false, flags, complain, /*dtor_from_ctor*/true, NULL);\n> \n> pop_scope (scope);\n> \n> --- gcc/testsuite/g++.dg/cpp0x/defaulted70.C.jj\t2026-04-28 19:17:37.949029070 +0200\n> +++ gcc/testsuite/g++.dg/cpp0x/defaulted70.C\t2026-04-28 19:18:27.894190340 +0200\n> @@ -0,0 +1,16 @@\n> +// { dg-do compile { target c++11 } }\n> +\n> +union A { const int a = 42; const long b; } a;\n> +// { dg-error \"use of deleted function 'A::A\\\\\\(\\\\\\)'\" \"\" { target c++23_down } .-1 }\n> +// { dg-message \"note: 'A::A\\\\\\(\\\\\\)' is implicitly deleted because the default definition would be ill-formed:\" \"\" { target c++23_down } .-2 }\n> +// { dg-error \"all variant members are 'const'-qualified\" \"\" { target c++23_down } .-3 }\n> +struct B { union { const int a = 42; const long b; }; } b;\n> +// { dg-error \"use of deleted function 'B::B\\\\\\(\\\\\\)'\" \"\" { target c++23_down } .-1 }\n> +// { dg-message \"note: 'B::B\\\\\\(\\\\\\)' is implicitly deleted because the default definition would be ill-formed:\" \"\" { target c++23_down } .-2 }\n> +// { dg-error \"all members of anonymous union member are 'const'-qualified\" \"\" { target c++23_down } .-3 }\n> +union C { const int a = 42; const long b; int c; } c;\n> +struct D { union { const int a = 42; const long b; int c; }; union { const int d = 42; const short e; }; } d;\n> +// { dg-error \"use of deleted function 'D::D\\\\\\(\\\\\\)'\" \"\" { target c++23_down } .-1 }\n> +// { dg-message \"note: 'D::D\\\\\\(\\\\\\)' is implicitly deleted because the default definition would be ill-formed:\" \"\" { target c++23_down } .-2 }\n> +// { dg-error \"all members of anonymous union member are 'const'-qualified\" \"\" { target c++23_down } .-3 }\n> +struct E { union { const int a = 42; const long b; int c; }; } e;\n> --- gcc/testsuite/g++.dg/init/pr25811.C.jj\t2026-03-27 10:17:15.871302394 +0100\n> +++ gcc/testsuite/g++.dg/init/pr25811.C\t2026-04-28 19:32:52.723677165 +0200\n> @@ -126,7 +126,7 @@ struct Z // { dg-error \"deleted\" \"\" { ta\n> \n> union U // { dg-message \"implicitly deleted\" \"\" { target c++11 } }\n> \t// { dg-error \"uninitialized\" \"\" { target c++11 } .-1 }\n> -{\n> +{\t// { dg-error \"all variant members are 'const'-qualified\" \"\" { target { c++11 && c++23_down } } .-2 }\n> int const i; // { dg-message \"should be initialized\" }\n> };\n> \n> --- gcc/testsuite/g++.dg/init/pr43719.C.jj\t2026-03-27 10:17:15.871302394 +0100\n> +++ gcc/testsuite/g++.dg/init/pr43719.C\t2026-04-28 19:33:21.594192733 +0200\n> @@ -110,7 +110,7 @@ struct Z\t\t// { dg-error \"deleted\" \"\" { t\n> };\n> \n> union U // { dg-error \"uninitialized\" \"\" { target c++11 } }\n> -{\n> +{\t// { dg-error \"all variant members are 'const'-qualified\" \"\" { target { c++11 && c++23_down } } .-1 }\n> int const i; // { dg-message \"should be initialized\" }\n> };\n> \n> \n> \tJakub\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=HC3InfwR;\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; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.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=HC3InfwR","sourceware.org; dmarc=pass (p=quarantine dis=none)\n header.from=redhat.com","sourceware.org; spf=pass smtp.mailfrom=redhat.com","server2.sourceware.org;\n arc=none smtp.remote-ip=170.10.129.124"],"Received":["from vm01.sourceware.org (vm01.sourceware.org [38.145.34.32])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature ECDSA (secp384r1) server-digest SHA384)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g5Lmx1Sdyz1yHZ\n\tfor ; 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charset=UTF-8; format=flowed","Content-Transfer-Encoding":"7bit","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"}},{"id":3684136,"web_url":"http://patchwork.ozlabs.org/comment/3684136/","msgid":"","list_archive_url":null,"date":"2026-04-29T15:34:41","subject":"Re: [PATCH] c++: Delete default ctor for unions with all const\n members","submitter":{"id":671,"url":"http://patchwork.ozlabs.org/api/people/671/","name":"Jakub Jelinek","email":"jakub@redhat.com"},"content":"On Wed, Apr 29, 2026 at 11:28:12AM -0400, Jason Merrill wrote:\n> On 4/29/26 5:43 AM, Jakub Jelinek wrote:\n> > Hi!\n> > \n> > I've started looking at P3074R7/P3726R2 papers.\n> > The former removes two bullets which describe when default ctor should be\n> > deleted:\n> > - X is a union and all of its variant members are of const-qualified type (or\n> > possibly multi-dimensional array thereof),\n> > - X is a non-union class and all members of any anonymous union member are of\n> > const-qualified type (or possibly multi-dimensional array thereof).\n> > These have been in [class.default.ctor] (or earlier in [class.ctor]) since\n> > C++11 till C++23, but we apparently ignore these and accept it, while\n> > clang++ deletes them.\n> \n> I'm inclined to treat dropping these bullets as a DR back to C++20, to go\n> along with the changes in P1330, P1331, and (particularly) CWG2424.\n\nSo shall I change the patch then to diagnose it for C++11, C++14 and C++17\nor throw it away?\n\n\tJakub","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=Ti+TIe9X;\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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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"}},{"id":3684280,"web_url":"http://patchwork.ozlabs.org/comment/3684280/","msgid":"<604b805a-6f58-4532-99f2-c80dc219b300@redhat.com>","list_archive_url":null,"date":"2026-04-29T21:24:37","subject":"Re: [PATCH] c++: Delete default ctor for unions with all const\n members","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 4/29/26 11:34 AM, Jakub Jelinek wrote:\n> On Wed, Apr 29, 2026 at 11:28:12AM -0400, Jason Merrill wrote:\n>> On 4/29/26 5:43 AM, Jakub Jelinek wrote:\n>>> Hi!\n>>>\n>>> I've started looking at P3074R7/P3726R2 papers.\n>>> The former removes two bullets which describe when default ctor should be\n>>> deleted:\n>>> - X is a union and all of its variant members are of const-qualified type (or\n>>> possibly multi-dimensional array thereof),\n>>> - X is a non-union class and all members of any anonymous union member are of\n>>> const-qualified type (or possibly multi-dimensional array thereof).\n>>> These have been in [class.default.ctor] (or earlier in [class.ctor]) since\n>>> C++11 till C++23, but we apparently ignore these and accept it, while\n>>> clang++ deletes them.\n>>\n>> I'm inclined to treat dropping these bullets as a DR back to C++20, to go\n>> along with the changes in P1330, P1331, and (particularly) CWG2424.\n> \n> So shall I change the patch then to diagnose it for C++11, C++14 and C++17\n> or throw it away?\n\nI lean toward dropping it; the value of additional strictness only in \nversions before the default seems limited.\n\nJason","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=ecabaclr;\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; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.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=ecabaclr","sourceware.org; dmarc=pass (p=quarantine dis=none)\n header.from=redhat.com","sourceware.org; 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