{"id":2226646,"url":"http://patchwork.ozlabs.org/api/patches/2226646/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhunfc5mlg.gcc.gcc-TEST.peppe.36.1.1@forge-stage.sourceware.org/","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":"","list_archive_url":null,"date":"2026-04-22T17:47:28","name":"[v1,1/1] libstdc++: constrain std::atomic's default constructor","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"fd5a6e9eefb4400839b07487aa265a9dc88c10ec","submitter":{"id":93222,"url":"http://patchwork.ozlabs.org/api/people/93222/?format=json","name":"peppe via Sourceware Forge","email":"forge-bot+peppe@forge-stage.sourceware.org"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/bmm.hhunfc5mlg.gcc.gcc-TEST.peppe.36.1.1@forge-stage.sourceware.org/mbox/","series":[{"id":501056,"url":"http://patchwork.ozlabs.org/api/series/501056/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=501056","date":"2026-04-22T17:47:28","name":"WIP: libstdc++: constrain std::atomic's default constructor","version":1,"mbox":"http://patchwork.ozlabs.org/series/501056/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2226646/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2226646/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 spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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server2.sourceware.org","From":"peppe via Sourceware Forge ","Date":"Wed, 22 Apr 2026 17:47:28 +0000","Subject":"[PATCH v1 1/1] libstdc++: constrain std::atomic's default constructor","To":"gcc-patches mailing list ","Message-ID":"\n ","X-Mailer":"batrachomyomachia","X-Requested-Reviewer":"redi","X-Pull-Request-Organization":"gcc","X-Pull-Request-Repository":"gcc-TEST","X-Pull-Request":"https://forge.sourceware.org/gcc/gcc-TEST/pulls/36","References":"\n ","In-Reply-To":"\n ","X-Patch-URL":"\n https://forge.sourceware.org/peppe/gcc-TEST/commit/50befa32f7cc696850607d9c09b81a4de529597a","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":"gcc-patches mailing list ,\n peppe@noreply.localhost","Errors-To":"gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org"},"content":"From: Giuseppe D'Angelo \n\nThis commit implements the proposed resolution to LWG4169, which is\nto constrain std::atomic's default constructor based on whether\nT itself is default constructible.\n\nAt the moment, std::atomic's primary template in libstdc++ has a\ndefaulted default constructor. Value-initialization of the T member\n(since C++20 / P0883R2) is done via a NSDMI (= T()).\n\nGCC already considers the defaulted constructor constrained/deleted,\nhowever this behavior is non-standard (see the discussion in PR116769):\nthe presence of a NSDMI should not make the constructor unavailable to\noverload resolution/deleted ([class.default.ctor]/2.5 does not apply).\nWhen using libstdc++ on Clang, this causes build issues as the\nconstructor is *not* deleted there -- the interpretation of\n[class.default.ctor]/4 seems to match Clang's behavior.\n\nTherefore, although there would be \"nothing to do\" with GCC+libstdc++,\nthis commit changes the code as to stop relying on the GCC language\nextension. In C++ >= 20 modes, std::atomic's defaulted default\nconstructor is changed to be a non-defaulted one, with a constraint\nadded as per LWG4169; value-initialization of the data member is moved\nfrom the NSDMI to the member init list. The new signature matches the\none in the Standard as per [atomics.types.operations]/1.\n\nIn pre-C++20 modes, the constructor is left defaulted. This ensures\ncompatibility with C++11/14/17 behavior. In other words: we are not\nbackporting P0883R2 to earlier language modes here.\n\nAmend an existing test to check that a std::atomic wrapping a\nnon-default constructible type is always non-default constructible:\nfrom C++20, because of the constraint; before C++20, because we\nare removing the NSDMI, and therefore [class.default.ctor]/2.5\napplies.\n\nAdd another test that checks that std::atomic is trivially default\nconstructible in pre-C++20 modes, and it isn't afterwards.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/bits/version.def (atomic_value_initialization):\n\tGuard the FTM with the language concepts FTM.\n\t* include/bits/version.h: Regenerate.\n\t* include/std/atomic (atomic): When atomic value init is\n\tdefined, change the defaulted default constructor to\n\ta non-defaulted one, constraining it as per LWG4169.\n\tOtherwise, keep the existing constructor.\n\tRemove the NSDMI for the _M_i member.\n\t(_GLIBCXX20_INIT): Drop the macro, as it is not needed any more.\n\t* testsuite/29_atomics/atomic/69301.cc: Test that\n\tan atomic wrapping a non-default-constructible type is\n\talways itself non-default-constructible (in all language\n\tmodes).\n\t* testsuite/29_atomics/atomic/cons/trivial.cc: New test.\n---\n libstdc++-v3/include/bits/version.def | 1 +\n libstdc++-v3/include/bits/version.h | 2 +-\n libstdc++-v3/include/std/atomic | 22 +++++-----\n .../testsuite/29_atomics/atomic/69301.cc | 2 +\n .../29_atomics/atomic/cons/trivial.cc | 41 +++++++++++++++++++\n 5 files changed, 57 insertions(+), 11 deletions(-)\n create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/cons/trivial.cc","diff":"diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def\nindex 2af5a54bff28..56638759539b 100644\n--- a/libstdc++-v3/include/bits/version.def\n+++ b/libstdc++-v3/include/bits/version.def\n@@ -770,6 +770,7 @@ ftms = {\n values = {\n v = 201911;\n cxxmin = 20;\n+ extra_cond = \"__cpp_concepts >= 201907L\";\n };\n };\n \ndiff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h\nindex 9833023cfdc8..29e1535298cb 100644\n--- a/libstdc++-v3/include/bits/version.h\n+++ b/libstdc++-v3/include/bits/version.h\n@@ -856,7 +856,7 @@\n #undef __glibcxx_want_atomic_ref\n \n #if !defined(__cpp_lib_atomic_value_initialization)\n-# if (__cplusplus >= 202002L)\n+# if (__cplusplus >= 202002L) && (__cpp_concepts >= 201907L)\n # define __glibcxx_atomic_value_initialization 201911L\n # if defined(__glibcxx_want_all) || defined(__glibcxx_want_atomic_value_initialization)\n # define __cpp_lib_atomic_value_initialization 201911L\ndiff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic\nindex cd08df34ba77..9b1aca0fc09a 100644\n--- a/libstdc++-v3/include/std/atomic\n+++ b/libstdc++-v3/include/std/atomic\n@@ -51,6 +51,7 @@\n \n #include \n #include \n+#include \n \n namespace std _GLIBCXX_VISIBILITY(default)\n {\n@@ -189,14 +190,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n #endif // __cpp_lib_atomic_wait\n };\n \n-/// @cond undocumented\n-#if __cpp_lib_atomic_value_initialization\n-# define _GLIBCXX20_INIT(I) = I\n-#else\n-# define _GLIBCXX20_INIT(I)\n-#endif\n-/// @endcond\n-\n /**\n * @brief Generic atomic type, primary class template.\n *\n@@ -216,7 +209,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n static constexpr int _S_alignment\n = _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);\n \n- alignas(_S_alignment) _Tp _M_i _GLIBCXX20_INIT(_Tp());\n+ alignas(_S_alignment) _Tp _M_i;\n \n static_assert(__is_trivially_copyable(_Tp),\n \t\t \"std::atomic requires a trivially copyable type\");\n@@ -232,7 +225,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n #endif\n \n public:\n+#if __cpp_lib_atomic_value_initialization\n+ // _GLIBCXX_RESOLVE_LIB_DEFECTS\n+ // 4169. std::atomic's default constructor should be constrained\n+ constexpr atomic() noexcept(is_nothrow_default_constructible_v<_Tp>)\n+\trequires is_default_constructible_v<_Tp>\n+\t: _M_i()\n+ {}\n+#else\n atomic() = default;\n+#endif\n+\n ~atomic() noexcept = default;\n atomic(const atomic&) = delete;\n atomic& operator=(const atomic&) = delete;\n@@ -414,7 +417,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n #endif // __cpp_lib_atomic_wait\n \n };\n-#undef _GLIBCXX20_INIT\n \n /// Partial specialization for pointer types.\n template\ndiff --git a/libstdc++-v3/testsuite/29_atomics/atomic/69301.cc b/libstdc++-v3/testsuite/29_atomics/atomic/69301.cc\nindex da54b0e259ef..72bf4af6af30 100644\n--- a/libstdc++-v3/testsuite/29_atomics/atomic/69301.cc\n+++ b/libstdc++-v3/testsuite/29_atomics/atomic/69301.cc\n@@ -29,6 +29,8 @@ struct NonDefaultConstructible\n \n template class std::atomic;\n \n+static_assert(!std::is_default_constructible>::value);\n+\n void\n test01()\n {\ndiff --git a/libstdc++-v3/testsuite/29_atomics/atomic/cons/trivial.cc b/libstdc++-v3/testsuite/29_atomics/atomic/cons/trivial.cc\nnew file mode 100644\nindex 000000000000..bb578db362df\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/trivial.cc\n@@ -0,0 +1,41 @@\n+// { dg-do compile { target c++11 } }\n+\n+#include \n+#include \n+\n+// C++20 / P0883R2 makes std::atomic value-initialize, so it's no\n+// longer going to be trivially default constructible; check that it\n+// still is in earlier language modes.\n+// (We are not applying that paper as a DR.)\n+#if __cpp_lib_atomic_value_initialization\n+constexpr bool atomic_default_ctor_is_trivial = false;\n+#else\n+constexpr bool atomic_default_ctor_is_trivial = true;\n+#endif\n+\n+template\n+using isTDC = std::is_trivially_default_constructible;\n+\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+\n+struct DefaultConstructible\n+{\n+ int a;\n+ long long b;\n+ char* p;\n+};\n+static_assert(isTDC>::value == atomic_default_ctor_is_trivial);\n+\n+struct NonDefaultConstructible\n+{\n+ NonDefaultConstructible(int i) : val(i) { }\n+ int val;\n+};\n+// Not default constructible, therefore not trivially default constructible\n+static_assert(isTDC>::value == false);\n","prefixes":["v1","1/1"]}