{"id":2224860,"url":"http://patchwork.ozlabs.org/api/patches/2224860/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/20260418232807.3364700-1-ppalka@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":"<20260418232807.3364700-1-ppalka@redhat.com>","list_archive_url":null,"date":"2026-04-18T23:28:07","name":"[committed,v2] libstdc++: Fix constantness of engaged -> disengaged std::optional [PR124910]","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"01ffe762de044e7abd06f9e34ce6bb687642d40c","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/?format=json","name":"Patrick Palka","email":"ppalka@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/20260418232807.3364700-1-ppalka@redhat.com/mbox/","series":[{"id":500477,"url":"http://patchwork.ozlabs.org/api/series/500477/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=500477","date":"2026-04-18T23:28:07","name":"[committed,v2] libstdc++: Fix constantness of engaged -> disengaged std::optional [PR124910]","version":2,"mbox":"http://patchwork.ozlabs.org/series/500477/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2224860/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2224860/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=f26mptY3;\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":"Changes in v2:\n - Added code comment\n - Clarified commit message, removed red herring about trivial\n destructibility.\n - Extended test with 'x6' direct-initialization that avoids invoking\n optional's move constructor. This testcase, when instantiated with T=B,\n demonstrates that non-trivially-copyable optional is also affected\n and we must set _M_empty even in that case.\n\n-- >8 --\n\nWhen an optional that contains a value is cleared, _M_destroy invokes the\ndestructor of the contained value _Storage::_M_value, leaving the union\n_Storage without an active member. While this is benign at runtime, a\nunion suboject with no active member violates core constant expression\nrequirements and in turn an optional in this state can't be used as a\nconstant initializer, which Clang and recent GCC (since r16-3022) correctly\ndiagnose.\n\nTo fix this, this patch makes _M_destroy activate the dummy union member\n_M_empty after destroying _M_value to ensure that the union always has\nan active member throughout its lifetime. We use std::construct_at\ninstead of simple assignment to work around a front end bug (comment #3\nin the PR). Doing so means we don't activate the member in C++17 mode,\nwhich should be fine; I don't think it's possible to disengage an engaged\noptional using only the C++17 constexpr optional operations.\n\n\tPR c++/124910\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/std/optional (_Optional_payload_base::_M_destroy)\n\t[__cpp_lib_optional >= 202106L]: During constant evaluation,\n\tafter invoking destructor of _M_value, use construct_at to\n\tactivate _M_empty.\n\t* testsuite/20_util/optional/constexpr/124910.cc: New test.\n\nReviewed-by: Jonathan Wakely \n---\n libstdc++-v3/include/std/optional | 6 ++\n .../20_util/optional/constexpr/124910.cc | 74 +++++++++++++++++++\n 2 files changed, 80 insertions(+)\n create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc","diff":"diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\nindex 0f4cf0bd1ef6..49ba7b6b45dd 100644\n--- a/libstdc++-v3/include/std/optional\n+++ b/libstdc++-v3/include/std/optional\n@@ -321,6 +321,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n {\n \t_M_engaged = false;\n \t_M_payload._M_value.~_Stored_type();\n+#if __cpp_lib_optional >= 202106L // full constexpr support\n+\tif (std::is_constant_evaluated())\n+\t // Ensure union _M_payload always has an active member, for sake\n+\t // of the core constant expression requirements.\n+\t std::construct_at(std::__addressof(_M_payload._M_empty));\n+#endif\n }\n \n #if __cplusplus >= 202002L\ndiff --git a/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\nnew file mode 100644\nindex 000000000000..2f61f7e4e775\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n@@ -0,0 +1,74 @@\n+// { dg-do compile { target c++20 } }\n+\n+// PR124910 - bogus 'std::optional{...}' is not a constant expression error\n+// after resetting it via '= nullopt'\n+\n+#include \n+\n+struct A\n+{\n+ constexpr A(int m) : m(m) { }\n+ int m;\n+};\n+\n+struct B\n+{\n+ constexpr B(int m) : m(m) { }\n+ constexpr ~B() { }\n+ int m;\n+};\n+\n+static_assert( std::is_trivially_destructible_v );\n+static_assert( std::is_trivially_destructible_v );\n+static_assert( ! std::is_trivially_destructible_v );\n+\n+template\n+void\n+do_test()\n+{\n+ constexpr std::optional x1 = [] {\n+ std::optional o = 1;\n+ o = std::nullopt;\n+ return o;\n+ }();\n+\n+ constexpr std::optional x2 = [] {\n+ std::optional o = 1;\n+ o.reset();\n+ return o;\n+ }();\n+\n+ constexpr std::optional x3 = [] {\n+ std::optional o1 = 1;\n+ std::optional o2;\n+ o1 = o2;\n+ return o1;\n+ }();\n+\n+ constexpr std::optional x4 = [] {\n+ std::optional o1 = 1;\n+ std::optional o2;\n+ o1 = std::move(o2);\n+ return o1;\n+ }();\n+\n+ constexpr std::optional x5 = [] {\n+ std::optional o1 = 1;\n+ std::optional o2;\n+ std::swap(o1, o2);\n+ return o1;\n+ }();\n+\n+ struct C : std::optional {\n+ constexpr C() : std::optional(1) { this->reset(); }\n+ };\n+ constexpr C x6;\n+}\n+\n+int\n+main()\n+{\n+ do_test();\n+ do_test();\n+ do_test();\n+}\n","prefixes":["committed","v2"]}