[{"id":3679691,"web_url":"http://patchwork.ozlabs.org/comment/3679691/","msgid":"","list_archive_url":null,"date":"2026-04-21T06:27:17","subject":"Re: [PATCH] libstdc++: Restrict engaged -> disengaged std::optional\n fix [PR124910]","submitter":{"id":4329,"url":"http://patchwork.ozlabs.org/api/people/4329/","name":"Jonathan Wakely","email":"jwakely.gcc@gmail.com"},"content":"On Mon, 20 Apr 2026, 22:25 Patrick Palka, wrote:\n\n> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?\n>\n\nOK\n\n(We could use addressof instead of __addressof as they're equally efficient\nnowadays, and I think we could just use & here? But it's fine as is.)\n\n\n> -- >8 --\n>\n> It turns out a union without an active member does not violate C++20 core\n> constant expression rules and r16-8748 was really just a workaround for\n> a front end bug. The actual underlying problem -- that the constexpr\n> evaluator treated an explicitly destroyed union member as still active\n> -- has been fixed by r16-8767 which makes this workaround unnecessary\n> for GCC. Rather than remove the workaround, restrict it to Clang which\n> seems to have a similar bug.\n>\n> PR c++/124910\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/std/optional (_Optional_payload_base::_M_destroy):\n> Restrict r16-8748 workaround to Clang, and adjust comment.\n> ---\n> libstdc++-v3/include/std/optional | 5 ++---\n> 1 file changed, 2 insertions(+), 3 deletions(-)\n>\n> diff --git a/libstdc++-v3/include/std/optional\n> b/libstdc++-v3/include/std/optional\n> index 49ba7b6b45dd..b9cc019feff7 100644\n> --- a/libstdc++-v3/include/std/optional\n> +++ b/libstdc++-v3/include/std/optional\n> @@ -321,10 +321,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> {\n> _M_engaged = false;\n> _M_payload._M_value.~_Stored_type();\n> -#if __cpp_lib_optional >= 202106L // full constexpr support\n> +#if defined(__clang__) && __cpp_lib_optional >= 202106L // full constexpr\n> support\n> if (std::is_constant_evaluated())\n> - // Ensure union _M_payload always has an active member, for sake\n> - // of the core constant expression requirements.\n> + // Work around PR124910 for Clang.\n> std::construct_at(std::__addressof(_M_payload._M_empty));\n> #endif\n> }\n> --\n> 2.54.0.rc1.54.g60f07c4f5c\n>\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 (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=fw1bohVR;\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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Mon, 20 Apr 2026\n 23:27:29 -0700 (PDT)","MIME-Version":"1.0","References":"<20260420212533.3862595-1-ppalka@redhat.com>","In-Reply-To":"<20260420212533.3862595-1-ppalka@redhat.com>","From":"Jonathan Wakely ","Date":"Tue, 21 Apr 2026 07:27:17 +0100","X-Gm-Features":"AQROBzAy-kDHpWKVmCL0V8y9BmaaHGS-we90b6WZbjlfeDSv_c7TX-ILnSgT0ug","Message-ID":"\n ","Subject":"Re: [PATCH] libstdc++: Restrict engaged -> disengaged std::optional\n fix [PR124910]","To":"Patrick Palka ","Cc":"gcc-patches , \"libstdc++\" ","Content-Type":"multipart/alternative; boundary=\"0000000000007683d2064ff28223\"","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":3679890,"web_url":"http://patchwork.ozlabs.org/comment/3679890/","msgid":"<5f997c24-e834-3872-8c2d-3e3e9419476f@idea>","list_archive_url":null,"date":"2026-04-21T13:26:00","subject":"Re: [PATCH] libstdc++: Restrict engaged -> disengaged std::optional\n fix [PR124910]","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Tue, 21 Apr 2026, Jonathan Wakely wrote:\n\n> \n> \n> On Mon, 20 Apr 2026, 22:25 Patrick Palka, wrote:\n> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?\n> \n> \n> OK\n> \n> (We could use addressof instead of __addressof as they're equally efficient nowadays, and I think we could just use & here? But it's fine as is.)\n\nTrue, I went with __addressof to be locally consisten, and to minimize\ncognitive overhead (using & would at first glance seem potentially buggy\nwhereas addressof is definitely safe).\n\n> \n> \n> -- >8 --\n> \n> It turns out a union without an active member does not violate C++20 core\n> constant expression rules and r16-8748 was really just a workaround for\n> a front end bug.  The actual underlying problem -- that the constexpr\n> evaluator treated an explicitly destroyed union member as still active\n> -- has been fixed by r16-8767 which makes this workaround unnecessary\n> for GCC.  Rather than remove the workaround, restrict it to Clang which\n> seems to have a similar bug.\n> \n>         PR c++/124910\n> \n> libstdc++-v3/ChangeLog:\n> \n>         * include/std/optional (_Optional_payload_base::_M_destroy):\n>         Restrict r16-8748 workaround to Clang, and adjust comment.\n> ---\n>  libstdc++-v3/include/std/optional | 5 ++---\n>  1 file changed, 2 insertions(+), 3 deletions(-)\n> \n> diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\n> index 49ba7b6b45dd..b9cc019feff7 100644\n> --- a/libstdc++-v3/include/std/optional\n> +++ b/libstdc++-v3/include/std/optional\n> @@ -321,10 +321,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n>        {\n>         _M_engaged = false;\n>         _M_payload._M_value.~_Stored_type();\n> -#if __cpp_lib_optional >= 202106L // full constexpr support\n> +#if defined(__clang__) && __cpp_lib_optional >= 202106L // full constexpr support\n>         if (std::is_constant_evaluated())\n> -         // Ensure union _M_payload always has an active member, for sake\n> -         // of the core constant expression requirements.\n> +         // Work around PR124910 for Clang.\n>           std::construct_at(std::__addressof(_M_payload._M_empty));\n>  #endif\n>        }\n> --\n> 2.54.0.rc1.54.g60f07c4f5c\n> \n> \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=DxqDzB0w;\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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