{"id":2225368,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2225368/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/20260420215359.1773044-1-jason@redhat.com/","project":{"id":17,"url":"http://patchwork.ozlabs.org/api/1.1/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},"msgid":"<20260420215359.1773044-1-jason@redhat.com>","date":"2026-04-20T21:52:04","name":"[RFC] c++: constexpr union with no active member [PR124910]","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"e6b23c85966391db973d1dc361e80f342c60185b","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/1.1/people/4337/?format=json","name":"Jason Merrill","email":"jason@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/20260420215359.1773044-1-jason@redhat.com/mbox/","series":[{"id":500690,"url":"http://patchwork.ozlabs.org/api/1.1/series/500690/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=500690","date":"2026-04-20T21:52:04","name":"[RFC] c++: constexpr union with no active member [PR124910]","version":1,"mbox":"http://patchwork.ozlabs.org/series/500690/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2225368/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2225368/checks/","tags":{},"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=gjGQ7WRP;\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\"; x-default=true","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":"Tested x86_64-pc-linux-gnu. Since this isn't a regression I'm inclined to\nleave it for 17.\n\n-- 8< --\n\nPatrick pointed out that while r16-8767 made a union constant after\ndestroying its active member, we still weren't treating a union that never\nhad an active member as constant; the difference is the\nCONSTRUCTOR_NO_CLEARING flag, and what that means to\nreduced_constant_expression_p.\n\nIt seems to me that since P2686 [expr.const] says whether a prvalue\nexpression is a constant expression depends on the constituent values, and\n[intro.object] says that only the active member is a constituent value of a\nunion, so a union with no active member has no constituent values and so is\nvacuously constant, like an object of empty type. P2686 as a whole is not a\nDR, but the draft was previously unclear, and CWG2658 also clarified that\ncopying a union is equivalent to copying the active member *if any*.\n\nI was somewhat surprised that none of the existing tests needed to be\nchanged.\n\n\tPR c++/124910\n\tDR 2658\n\ngcc/cp/ChangeLog:\n\n\t* constexpr.cc (reduced_constant_expression_p): Allow a union\n\twith no active member.\n\ngcc/testsuite/ChangeLog:\n\n\t* g++.dg/cpp2a/constexpr-union12.C: New test.\n---\n gcc/cp/constexpr.cc | 7 ++++---\n gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C | 8 ++++++++\n 2 files changed, 12 insertions(+), 3 deletions(-)\n create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C\n\n\nbase-commit: 8e6b7c97232489557e376bdc134ad69aaeb35794","diff":"diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc\nindex a593b66dd41..1926b64cbb7 100644\n--- a/gcc/cp/constexpr.cc\n+++ b/gcc/cp/constexpr.cc\n@@ -4747,9 +4747,10 @@ reduced_constant_expression_p (tree t, tree sz /* = NULL_TREE */)\n \t\t && TREE_CODE (TREE_TYPE (t)) == UNION_TYPE)\n \t {\n \t if (CONSTRUCTOR_NELTS (t) == 0)\n-\t\t/* An initialized union has a constructor element. */\n-\t\treturn false;\n-\t /* And it only initializes one member. */\n+\t\t/* A union with no active member has no constituent values, so\n+\t\t all constituent values are constant. */\n+\t\tgoto ok;\n+\t /* A union can have at most one active member. */\n \t field = NULL_TREE;\n \t }\n \t else\ndiff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C\nnew file mode 100644\nindex 00000000000..42ddfaaf775\n--- /dev/null\n+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C\n@@ -0,0 +1,8 @@\n+// PR c++/124910\n+// { dg-do compile { target c++20 } }\n+\n+union U {\n+ int n;\n+ constexpr U() { }\n+};\n+constexpr U u; // { dg-bogus \"\" } U() is not a constant expression\n","prefixes":["RFC"]}