{"id":2231567,"url":"http://patchwork.ozlabs.org/api/1.1/patches/2231567/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/20260430205935.2360603-1-jwakely@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":"<20260430205935.2360603-1-jwakely@redhat.com>","date":"2026-04-30T20:58:54","name":"libstdc++: Make std::unique_ptr::operator* SFINAE-friendly","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"a143e4b46d5a1807f84bbae8c3e4acdcf4db8966","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/1.1/people/48004/?format=json","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/20260430205935.2360603-1-jwakely@redhat.com/mbox/","series":[{"id":502384,"url":"http://patchwork.ozlabs.org/api/1.1/series/502384/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=502384","date":"2026-04-30T20:58:54","name":"libstdc++: Make std::unique_ptr::operator* SFINAE-friendly","version":1,"mbox":"http://patchwork.ozlabs.org/series/502384/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2231567/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2231567/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=NIdakQp5;\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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Thu, 30 Apr 2026 20:59:36 +0000 (UTC)"],"DKIM-Filter":["OpenDKIM Filter v2.11.0 sourceware.org 98A984143927","OpenDKIM Filter v2.11.0 sourceware.org 24B924310D63"],"DMARC-Filter":"OpenDMARC Filter v1.4.2 sourceware.org 24B924310D63","ARC-Filter":"OpenARC Filter v1.0.0 sourceware.org 24B924310D63","ARC-Seal":"i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1777582781; cv=none;\n b=V4LftH707h93eofCjFLAd8MaeZGp54XAd9XtRo+JF+ZCPqEBFfgp3UD6/FvWGctejdqqZVsyt6iIm3f+E2dc1TqSvwyZBGCcQduFVRbK1DT3li7JEGp8uxAIYObkfxvaTgaWXlEcP/KyNodKkyCQEir3LhD2GLoIet+pbPoDSdc=","ARC-Message-Signature":"i=1; a=rsa-sha256; d=sourceware.org; s=key;\n t=1777582781; c=relaxed/simple;\n bh=Pn9EvC7sOO96UM8C7slcuV7DS3EhzOKTIih8mH5p6Jo=;\n h=DKIM-Signature:From:To:Subject:Date:Message-ID:MIME-Version;\n b=GLKhsVTbm+1N5FZdj8VCNlIDDkywyNi2jj2yaRNPMhjAmaSKxUkXWhxYaPzjlL4pFvtwMJ6wwA/p5OrnOEeBPh3ym9p8aWcBXLbbf1RbZwdE2TtC0bXiao6COAmzxAxyHufSrAZNvFbnCrom2gREbXWg1ibrFp9YzTSvAIn1/tA=","ARC-Authentication-Results":"i=1; server2.sourceware.org","DKIM-Signature":"v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com;\n s=mimecast20190719; t=1777582780;\n h=from:from:reply-to:subject:subject:date:date:message-id:message-id:\n to:to:cc:mime-version:mime-version:content-type:content-type:\n content-transfer-encoding:content-transfer-encoding;\n bh=uImX6T6zszvH57pzdXKF1N05UxISADNK4yrB2LxGo3w=;\n b=NIdakQp5UfQlzkne8FzL6OfbYmAxd5r1+ZCNXZeTJvwn3fFJpSFJ2J8UtHUsLPVRfdS7Nt\n EahVEyZtVH3dXbPikMEBy/0fKKQB1rF9W4fu9YhdLkEuoVVJdGNCGHCMZvuKhxk5aiZH4t\n mBJF1JHR+NIoAx1Otx6ExHjT45wZKiQ=","X-MC-Unique":"qzTL_qPFNfGqH4p3puBOIA-1","X-Mimecast-MFC-AGG-ID":"qzTL_qPFNfGqH4p3puBOIA_1777582778","From":"Jonathan Wakely ","To":"gcc-patches@gcc.gnu.org,\n\tlibstdc++@gcc.gnu.org","Subject":"[PATCH] libstdc++: Make std::unique_ptr::operator* SFINAE-friendly","Date":"Thu, 30 Apr 2026 21:58:54 +0100","Message-ID":"<20260430205935.2360603-1-jwakely@redhat.com>","MIME-Version":"1.0","X-Scanned-By":"MIMEDefang 3.4.1 on 10.30.177.111","X-Mimecast-Spam-Score":"0","X-Mimecast-MFC-PROC-ID":"Lw5kIAhntC-uiDgJgNWtpTUjRlv9iFa4D879ou8l51o_1777582778","X-Mimecast-Originator":"redhat.com","Content-Type":"text/plain","Content-Transfer-Encoding":"8bit","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":"This implements LWG 4324, \"unique_ptr::operator* is not\nSFINAE-friendly\", approved in Croydon, 2026.\n\nThe noexcept-specifier added to C++23 by LWG 2762 is ill-formed if the\npointer type cannot be dereferenced, which means that code which was\nchecking whether the function exists (e.g. in a SFINAE context) no\nlonger works. Such code was always questionable, because the function\nbody was ill-formed if the pointer isn't dereferenceable, so the SFINAE\ncheck was probably giving the wrong answer.\n\nLWG 4324 adds a constraint to the function, so that it doesn't\nparticipate in overload resolution if it would be ill-formed. That's\neasy to implement for C++20 because we can just add a requires-clause.\nFor C++11/14/17 we can't constrain it easily, so just adjust the\nnoexcept-specifier so that it's not ill-formed.\n\nlibstdc++-v3/ChangeLog:\n\n\t* include/bits/unique_ptr.h (unique_ptr::_Can_deref)\n\t(unique_ptr::_S_nothrow_deref): New helpers for pre-C++20.\n\t(operator*): Either constrain or use _S_nothrow_deref.\n\t* testsuite/20_util/unique_ptr/lwg4324.cc: New test.\n---\n\nTested x86_64-linux.\n\n libstdc++-v3/include/bits/unique_ptr.h | 42 ++++++++++++++++++-\n .../testsuite/20_util/unique_ptr/lwg4324.cc | 22 ++++++++++\n 2 files changed, 63 insertions(+), 1 deletion(-)\n create mode 100644 libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc","diff":"diff --git a/libstdc++-v3/include/bits/unique_ptr.h b/libstdc++-v3/include/bits/unique_ptr.h\nindex 5b7c8938ff74..222e61d221ed 100644\n--- a/libstdc++-v3/include/bits/unique_ptr.h\n+++ b/libstdc++-v3/include/bits/unique_ptr.h\n@@ -289,6 +289,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n \t __not_>\n >;\n \n+#if ! __cpp_concepts\n+ template\n+\tstruct _Can_deref\n+\t: false_type { };\n+\n+ template\n+\tstruct _Can_deref<_Ptr, __void_t())>>\n+\t: true_type { };\n+\n+#if __cpp_constexpr >= 201304L // C++14 relaxed constexpr\n+#pragma GCC diagnostic push\n+#pragma GCC diagnostic ignored \"-Wc++17-extensions\" // if constexpr\n+ template\n+\tstatic constexpr bool\n+\t_S_nothrow_deref(int)\n+\t{\n+\t if constexpr (_Can_deref<_Ptr>::value)\n+\t return noexcept(*std::declval<_Ptr>());\n+\t return false;\n+\t}\n+#pragma GCC diagnostic pop\n+#else\n+ template())>\n+\tstatic constexpr bool\n+\t_S_nothrow_deref(int) { return noexcept(*std::declval<_Ptr>()); }\n+ template\n+\tstatic constexpr bool\n+\t_S_nothrow_deref(...) { return false; }\n+#endif\n+#endif\n+\n public:\n // Constructors.\n \n@@ -442,7 +473,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n /// Dereference the stored pointer.\n _GLIBCXX23_CONSTEXPR\n typename add_lvalue_reference::type\n- operator*() const noexcept(noexcept(*std::declval()))\n+ operator*() const\n+ // _GLIBCXX_RESOLVE_LIB_DEFECTS\n+ // 2762. unique_ptr operator*() should be noexcept\n+ // 4324. unique_ptr::operator* is not SFINAE-friendly\n+#if __cpp_concepts\n+ noexcept(noexcept(*std::declval()))\n+ requires requires { *std::declval(); }\n+#else\n+ noexcept(_S_nothrow_deref(0))\n+#endif\n {\n #if _GLIBCXX_USE_BUILTIN_TRAIT(__reference_converts_from_temporary)\n \t// _GLIBCXX_RESOLVE_LIB_DEFECTS\ndiff --git a/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc b/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc\nnew file mode 100644\nindex 000000000000..13ebfb929fca\n--- /dev/null\n+++ b/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4324.cc\n@@ -0,0 +1,22 @@\n+// { dg-do compile { target c++11 } }\n+\n+// LWG 4324. unique_ptr::operator* is not SFINAE-friendly\n+\n+#include \n+\n+template\n+struct nothrow_dereference\n+: std::false_type { };\n+\n+template\n+struct nothrow_dereference())>\n+: std::true_type { };\n+\n+static_assert( nothrow_dereference>::value, \"\");\n+static_assert( ! nothrow_dereference>::value, \"\");\n+\n+#if __cplusplus >= 202002L // C++20\n+template concept dereferenceable = requires(T& t) { *t; };\n+static_assert( dereferenceable>);\n+static_assert(!dereferenceable>);\n+#endif\n","prefixes":[]}