[{"id":3679499,"web_url":"http://patchwork.ozlabs.org/comment/3679499/","msgid":"<0283d9bc-ad1f-4f65-e95e-e403833d2782@idea>","list_archive_url":null,"date":"2026-04-20T17:39:53","subject":"Re: [pushed] c++: std::optional reset and constexpr [PR124910]","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Mon, 20 Apr 2026, Jason Merrill wrote:\n\n> Tested x86_64-pc-linux-gnu, applying to trunk.\n> \n> -- 8< --\n> \n> Constant evaluation didn't recognize that destroying _M_value made it no\n> longer the active member of the anonymous union, so we were treating the\n> result as containing an out-of-lifetime value. Instead we should treat the\n> union as no longer having an active member.\n> \n> \tPR c++/124910\n> \n> gcc/cp/ChangeLog:\n> \n> \t* constexpr.cc (cxx_eval_store_expression): Ending the\n> \tlifetime of the active member means no active member.\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* g++.dg/cpp2a/constexpr-union11.C: New test.\n> ---\n> gcc/cp/constexpr.cc | 10 +++++++\n> .../g++.dg/cpp2a/constexpr-union11.C | 29 +++++++++++++++++++\n> 2 files changed, 39 insertions(+)\n> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union11.C\n> \n> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc\n> index 5b7d385fa97..a593b66dd41 100644\n> --- a/gcc/cp/constexpr.cc\n> +++ b/gcc/cp/constexpr.cc\n> @@ -8207,6 +8207,16 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,\n> \t no_zero_init = true;\n> \t}\n> \n> + /* Ending the lifetime of the active union member means the union no\n> +\t longer has an active member. */\n> + if (code == UNION_TYPE && refs->is_empty ()\n> +\t && TREE_CLOBBER_P (init)\n> +\t && CLOBBER_KIND (init) >= CLOBBER_OBJECT_END)\n> +\t{\n> +\t vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);\n> +\t return void_node;\n> +\t}\n> +\n> ctors.safe_push (valp);\n> vec_safe_push (indexes, index);\n> \n> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union11.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union11.C\n> new file mode 100644\n> index 00000000000..56919715f55\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union11.C\n> @@ -0,0 +1,29 @@\n> +// PR c++/124910\n> +// { dg-do compile { target c++20 } }\n> +\n> +struct nullopt_t { } nullopt;\n> +using _Stored_type = int;\n> +struct _Empty { };\n> +struct _Optional_payload_base {\n> + union {\n> + int _M_value = 1;\n> + _Empty _M_empty;\n> + };\n> + constexpr void _M_reset() {\n> + _M_value.~_Stored_type();\n> + _M_empty = {};\n\nI think we should strengthen this testcase by removing the _M_empty\nassignment/activation?\n\nIt's curious that\n\n union U {\n int n;\n constexpr U() { }\n };\n constexpr U u; // error: U() is not a constant expression\n\nis still rejected even though the roundabout\n\n union U {\n int n;\n constexpr U() { n = 42; using type = int; n.~type(); }\n };\n constexpr U u;\n\nis now accepted.\n\n\n> + }\n> +};\n> +struct _Optional_payload : _Optional_payload_base {};\n> +struct _Optional_base {\n> + _Optional_payload _M_payload;\n> +};\n> +struct optional : _Optional_base {\n> + constexpr void operator=(nullopt_t) { _M_payload._M_reset(); }\n> +};\n> +constexpr auto f() {\n> + optional result;\n> + result = nullopt;\n> + return result;\n> +}\n> +constexpr auto v = f();\n> \n> base-commit: b40bb34c3f5e9942d083efdee393c6e2d25e5981\n> -- \n> 2.53.0\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=X8h6Q/5a;\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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