[{"id":3678937,"web_url":"http://patchwork.ozlabs.org/comment/3678937/","msgid":"<6c56e77d-5d4f-f02c-9323-cd03024df001@idea>","list_archive_url":null,"date":"2026-04-18T01:59:29","subject":"Re: [PATCH] libstdc++: Fix constantness of engaged -> disengaged\n std::optional [PR124910]","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Fri, 17 Apr 2026, Patrick Palka wrote:\n\n> Tested on x86_64-pc-linux-gnu, does this look OK for trunk? Perhaps\n> backports as well?\n> \n> -- >8 --\n> \n> We implement std::optional for trivially destructible T as a union\n> with an empty dummy member (_M_empty) indicating the disengaged state.\n> When we disengage such an std::optional that's already engaged we call\n> _M_destroy, which clears the _M_engaged flag and invokes the contained\n> objects's destructor, ending the lifetime of the corresponding union\n> member (_M_value) and leaving the union with no active member. While\n> benign at runtime, a union subobject with no active member violates\n> core constant expression requirements. Consequently the resulting\n> value can't be used as a constant initializer, which Clang and recent\n> GCC (r16-3022) correctly diagnose.\n> \n> To fix this, this patch makes _M_destroy activate the _M_empty union\n> member after ending the destroying and deactivating _M_value. We use\n> std::construct_at instead of simple assignment to work around a front\n> end bug (see comment #6 of the PR).\n> \n> \tPR c++/124910\n> \n> libstdc++-v3/ChangeLog:\n> \n> \t* include/std/optional (_Optional_payload_base::_M_destroy):\n> \tDuring constant evaluation, after invoking destructor of\n> \t_M_value, call construct_at to activate _M_empty.\n> \t* testsuite/20_util/optional/constexpr/124910.cc: New test.\n> ---\n> libstdc++-v3/include/std/optional | 2 +\n> .../20_util/optional/constexpr/124910.cc | 63 +++++++++++++++++++\n> 2 files changed, 65 insertions(+)\n> create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> \n> diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\n> index 0f4cf0bd1ef6..0524222eab97 100644\n> --- a/libstdc++-v3/include/std/optional\n> +++ b/libstdc++-v3/include/std/optional\n> @@ -321,6 +321,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> {\n> \t_M_engaged = false;\n> \t_M_payload._M_value.~_Stored_type();\n> +\tif (std::__is_constant_evaluated())\n> +\t std::construct_at(std::__addressof(_M_payload._M_empty));\n\nWhoops, forgot to test with GLIBCXX_TESTSUITE_STDS that includes 17.\nconstruct_at is C++20-only, and so is full constexpr optional support.\nSo we should guard this added code with __cpp_lib_optional >= 202106L\nlike so. Retesting thoroughly overnight...\n\n-- >8 --\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, call construct_at to\n\tactivate _M_empty.\n\t* testsuite/20_util/optional/constexpr/124910.cc: New test.\n---\n libstdc++-v3/include/std/optional | 4 ++\n .../20_util/optional/constexpr/124910.cc | 63 +++++++++++++++++++\n 2 files changed, 67 insertions(+)\n create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n\ndiff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\nindex 0f4cf0bd1ef6..8370fac08109 100644\n--- a/libstdc++-v3/include/std/optional\n+++ b/libstdc++-v3/include/std/optional\n@@ -321,6 +321,10 @@ _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 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..f7facdec020d\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n@@ -0,0 +1,63 @@\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+ constexpr A(int m) : m(m) { }\n+ int m;\n+};\n+\n+struct B {\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 do_test() {\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+\n+int main() {\n+ do_test();\n+ do_test();\n+ do_test();\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=Tm98HMop;\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-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":3678938,"web_url":"http://patchwork.ozlabs.org/comment/3678938/","msgid":"","list_archive_url":null,"date":"2026-04-18T02:25:50","subject":"Re: [PATCH] libstdc++: Fix constantness of engaged -> disengaged\n std::optional [PR124910]","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Fri, 17 Apr 2026, Patrick Palka wrote:\n\n> On Fri, 17 Apr 2026, Patrick Palka wrote:\n> \n> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk? Perhaps\n> > backports as well?\n> > \n> > -- >8 --\n> > \n> > We implement std::optional for trivially destructible T as a union\n> > with an empty dummy member (_M_empty) indicating the disengaged state.\n> > When we disengage such an std::optional that's already engaged we call\n> > _M_destroy, which clears the _M_engaged flag and invokes the contained\n> > objects's destructor, ending the lifetime of the corresponding union\n> > member (_M_value) and leaving the union with no active member. While\n> > benign at runtime, a union subobject with no active member violates\n> > core constant expression requirements. Consequently the resulting\n> > value can't be used as a constant initializer, which Clang and recent\n> > GCC (r16-3022) correctly diagnose.\n> > \n> > To fix this, this patch makes _M_destroy activate the _M_empty union\n> > member after ending the destroying and deactivating _M_value. We use\n> > std::construct_at instead of simple assignment to work around a front\n> > end bug (see comment #6 of the PR).\n> > \n> > \tPR c++/124910\n> > \n> > libstdc++-v3/ChangeLog:\n> > \n> > \t* include/std/optional (_Optional_payload_base::_M_destroy):\n> > \tDuring constant evaluation, after invoking destructor of\n> > \t_M_value, call construct_at to activate _M_empty.\n> > \t* testsuite/20_util/optional/constexpr/124910.cc: New test.\n> > ---\n> > libstdc++-v3/include/std/optional | 2 +\n> > .../20_util/optional/constexpr/124910.cc | 63 +++++++++++++++++++\n> > 2 files changed, 65 insertions(+)\n> > create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> > \n> > diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\n> > index 0f4cf0bd1ef6..0524222eab97 100644\n> > --- a/libstdc++-v3/include/std/optional\n> > +++ b/libstdc++-v3/include/std/optional\n> > @@ -321,6 +321,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> > {\n> > \t_M_engaged = false;\n> > \t_M_payload._M_value.~_Stored_type();\n> > +\tif (std::__is_constant_evaluated())\n> > +\t std::construct_at(std::__addressof(_M_payload._M_empty));\n> \n> Whoops, forgot to test with GLIBCXX_TESTSUITE_STDS that includes 17.\n> construct_at is C++20-only, and so is full constexpr optional support.\n> So we should guard this added code with __cpp_lib_optional >= 202106L\n> like so. Retesting thoroughly overnight...\n\nIn C++17 optional was only partially constexpr, supporting only simple\nconstruction and assignment, and I don't think we can end up with an\noptional in such a state using only those operations. So it should be\nfine to use construct_at and restrict ourselves to C++20, I think.\n\n> \n> -- >8 --\n> \n> \tPR c++/124910\n> \n> libstdc++-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, call construct_at to\n> \tactivate _M_empty.\n> \t* testsuite/20_util/optional/constexpr/124910.cc: New test.\n> ---\n> libstdc++-v3/include/std/optional | 4 ++\n> .../20_util/optional/constexpr/124910.cc | 63 +++++++++++++++++++\n> 2 files changed, 67 insertions(+)\n> create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> \n> diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\n> index 0f4cf0bd1ef6..8370fac08109 100644\n> --- a/libstdc++-v3/include/std/optional\n> +++ b/libstdc++-v3/include/std/optional\n> @@ -321,6 +321,10 @@ _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 std::construct_at(std::__addressof(_M_payload._M_empty));\n> +#endif\n> }\n> \n> #if __cplusplus >= 202002L\n> diff --git a/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> new file mode 100644\n> index 000000000000..f7facdec020d\n> --- /dev/null\n> +++ b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> @@ -0,0 +1,63 @@\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> + constexpr A(int m) : m(m) { }\n> + int m;\n> +};\n> +\n> +struct B {\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 do_test() {\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> +\n> +int main() {\n> + do_test();\n> + do_test();\n> + do_test();\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 (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=eqVo71U8;\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","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":3678961,"web_url":"http://patchwork.ozlabs.org/comment/3678961/","msgid":"","list_archive_url":null,"date":"2026-04-18T11:51:49","subject":"Re: [PATCH] libstdc++: Fix constantness of engaged -> disengaged\n std::optional [PR124910]","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/people/48004/","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"content":"On Sat, 18 Apr 2026 at 03:00, Patrick Palka wrote:\n>\n> On Fri, 17 Apr 2026, Patrick Palka wrote:\n>\n> > Tested on x86_64-pc-linux-gnu, does this look OK for trunk? Perhaps\n> > backports as well?\n> >\n> > -- >8 --\n> >\n> > We implement std::optional for trivially destructible T as a union\n> > with an empty dummy member (_M_empty) indicating the disengaged state.\n> > When we disengage such an std::optional that's already engaged we call\n> > _M_destroy, which clears the _M_engaged flag and invokes the contained\n> > objects's destructor, ending the lifetime of the corresponding union\n> > member (_M_value) and leaving the union with no active member. While\n> > benign at runtime, a union subobject with no active member violates\n> > core constant expression requirements. Consequently the resulting\n> > value can't be used as a constant initializer, which Clang and recent\n> > GCC (r16-3022) correctly diagnose.\n> >\n> > To fix this, this patch makes _M_destroy activate the _M_empty union\n> > member after ending the destroying and deactivating _M_value. We use\n\n\"after ending the destroying and deactivating\" seems to have got garbled.\n\nOK for trunk with that corrected.\n\n> > std::construct_at instead of simple assignment to work around a front\n> > end bug (see comment #6 of the PR).\n> >\n> > PR c++/124910\n> >\n> > libstdc++-v3/ChangeLog:\n> >\n> > * include/std/optional (_Optional_payload_base::_M_destroy):\n> > During constant evaluation, after invoking destructor of\n> > _M_value, call construct_at to activate _M_empty.\n> > * testsuite/20_util/optional/constexpr/124910.cc: New test.\n> > ---\n> > libstdc++-v3/include/std/optional | 2 +\n> > .../20_util/optional/constexpr/124910.cc | 63 +++++++++++++++++++\n> > 2 files changed, 65 insertions(+)\n> > create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> >\n> > diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\n> > index 0f4cf0bd1ef6..0524222eab97 100644\n> > --- a/libstdc++-v3/include/std/optional\n> > +++ b/libstdc++-v3/include/std/optional\n> > @@ -321,6 +321,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> > {\n> > _M_engaged = false;\n> > _M_payload._M_value.~_Stored_type();\n> > + if (std::__is_constant_evaluated())\n> > + std::construct_at(std::__addressof(_M_payload._M_empty));\n>\n> Whoops, forgot to test with GLIBCXX_TESTSUITE_STDS that includes 17.\n> construct_at is C++20-only, and so is full constexpr optional support.\n> So we should guard this added code with __cpp_lib_optional >= 202106L\n> like so. Retesting thoroughly overnight...\n>\n> -- >8 --\n>\n> PR c++/124910\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/std/optional (_Optional_payload_base::_M_destroy)\n> [__cpp_lib_optional >= 202106L]: During constant evaluation,\n> after invoking destructor of _M_value, call construct_at to\n> activate _M_empty.\n> * testsuite/20_util/optional/constexpr/124910.cc: New test.\n> ---\n> libstdc++-v3/include/std/optional | 4 ++\n> .../20_util/optional/constexpr/124910.cc | 63 +++++++++++++++++++\n> 2 files changed, 67 insertions(+)\n> create mode 100644 libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n>\n> diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional\n> index 0f4cf0bd1ef6..8370fac08109 100644\n> --- a/libstdc++-v3/include/std/optional\n> +++ b/libstdc++-v3/include/std/optional\n> @@ -321,6 +321,10 @@ _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 (std::is_constant_evaluated())\n> + std::construct_at(std::__addressof(_M_payload._M_empty));\n> +#endif\n> }\n>\n> #if __cplusplus >= 202002L\n> diff --git a/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> new file mode 100644\n> index 000000000000..f7facdec020d\n> --- /dev/null\n> +++ b/libstdc++-v3/testsuite/20_util/optional/constexpr/124910.cc\n> @@ -0,0 +1,63 @@\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> + constexpr A(int m) : m(m) { }\n> + int m;\n> +};\n> +\n> +struct B {\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 do_test() {\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> +\n> +int main() {\n> + do_test();\n> + do_test();\n> + do_test();\n> +}\n> --\n> 2.54.0.rc1.54.g60f07c4f5c\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=iIegRYkk;\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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