{"id":2219794,"url":"http://patchwork.ozlabs.org/api/patches/2219794/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/patch/adEOglkPuoWDPemf@Thaum.localdomain/","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":"2026-04-04T13:13:38","name":"c++/modules: Handle importing an undeduced auto over a deduced auto [PR124735]","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"9fa79511e2c0884741c5e2750cf3ca8d1a41b2d7","submitter":{"id":85216,"url":"http://patchwork.ozlabs.org/api/people/85216/?format=json","name":"Nathaniel Shead","email":"nathanieloshead@gmail.com"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/gcc/patch/adEOglkPuoWDPemf@Thaum.localdomain/mbox/","series":[{"id":498725,"url":"http://patchwork.ozlabs.org/api/series/498725/?format=json","web_url":"http://patchwork.ozlabs.org/project/gcc/list/?series=498725","date":"2026-04-04T13:13:38","name":"c++/modules: Handle importing an undeduced auto over a deduced auto [PR124735]","version":1,"mbox":"http://patchwork.ozlabs.org/series/498725/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2219794/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2219794/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 (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=pwaZF/OK;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=temperror (SPF Temporary Error: DNS Timeout) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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charset=us-ascii","Content-Disposition":"inline","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":"Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk/15?\n\n-- >8 --\n\nLike with implicit constexpr, we need to handle cases where we import an\nfunction declaration with undeduced auto, but we already have a\ndefinition available with deduced auto. In this case we can just keep\nthe existing declaration's type.\n\n\tPR c++/124735\n\ngcc/cp/ChangeLog:\n\n\t* module.cc (trees_in::is_matching_decl): Handle importing\n\tundeduced auto when existing is already deduced.\n\ngcc/testsuite/ChangeLog:\n\n\t* g++.dg/modules/auto-8_a.H: New test.\n\t* g++.dg/modules/auto-8_b.C: New test.\n\nSigned-off-by: Nathaniel Shead \n---\n gcc/cp/module.cc | 6 +++++-\n gcc/testsuite/g++.dg/modules/auto-8_a.H | 7 +++++++\n gcc/testsuite/g++.dg/modules/auto-8_b.C | 15 +++++++++++++++\n 3 files changed, 27 insertions(+), 1 deletion(-)\n create mode 100644 gcc/testsuite/g++.dg/modules/auto-8_a.H\n create mode 100644 gcc/testsuite/g++.dg/modules/auto-8_b.C","diff":"diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc\nindex 6958388e454..25ffbd9eb5a 100644\n--- a/gcc/cp/module.cc\n+++ b/gcc/cp/module.cc\n@@ -12650,7 +12650,9 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)\n \n /* Similarly if EXISTING has an undeduced return type, but DECL's\n \t is already deduced. */\n- if (undeduced_auto_decl (existing) && !undeduced_auto_decl (decl))\n+ bool e_undeduced = undeduced_auto_decl (existing);\n+ bool d_undeduced = undeduced_auto_decl (decl);\n+ if (e_undeduced && !d_undeduced)\n \t{\n \t dump (dumper::MERGE)\n \t && dump (\"Propagating deduced return type to %N\", existing);\n@@ -12659,6 +12661,8 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)\n \t DECL_SAVED_AUTO_RETURN_TYPE (existing) = TREE_TYPE (e_type);\n \t TREE_TYPE (existing) = change_return_type (TREE_TYPE (d_type), e_type);\n \t}\n+ else if (d_undeduced && !e_undeduced)\n+\t/* EXISTING was deduced, leave it alone. */;\n else if (type_uses_auto (d_ret)\n \t && !same_type_p (TREE_TYPE (d_type), TREE_TYPE (e_type)))\n \t{\ndiff --git a/gcc/testsuite/g++.dg/modules/auto-8_a.H b/gcc/testsuite/g++.dg/modules/auto-8_a.H\nnew file mode 100644\nindex 00000000000..a0a5cacd995\n--- /dev/null\n+++ b/gcc/testsuite/g++.dg/modules/auto-8_a.H\n@@ -0,0 +1,7 @@\n+// PR c++/124735\n+// { dg-additional-options \"-fmodule-header\" }\n+// { dg-module-cmi {} }\n+\n+struct S {\n+ auto foo();\n+};\ndiff --git a/gcc/testsuite/g++.dg/modules/auto-8_b.C b/gcc/testsuite/g++.dg/modules/auto-8_b.C\nnew file mode 100644\nindex 00000000000..bd749228a04\n--- /dev/null\n+++ b/gcc/testsuite/g++.dg/modules/auto-8_b.C\n@@ -0,0 +1,15 @@\n+// PR c++/124735\n+// { dg-additional-options \"-fmodules\" }\n+\n+struct S {\n+ auto foo();\n+};\n+\n+auto S::foo() { return 123; }\n+auto pfn = &S::foo;\n+\n+import \"auto-8_a.H\";\n+\n+int main() {\n+ return S{}.foo() != 123;\n+}\n","prefixes":[]}