[{"id":3679959,"web_url":"http://patchwork.ozlabs.org/comment/3679959/","msgid":"","list_archive_url":null,"date":"2026-04-21T15:52:31","subject":"Re: [PATCH v2] c++/reflection: bogus -Wmissing-field-initializers\n with [PR124950]","submitter":{"id":48004,"url":"http://patchwork.ozlabs.org/api/people/48004/","name":"Jonathan Wakely","email":"jwakely@redhat.com"},"content":"On Tue, 21 Apr 2026 at 15:49, Marek Polacek wrote:\n>\n> On Tue, Apr 21, 2026 at 07:19:13AM +0100, Jonathan Wakely wrote:\n> > On Tue, 21 Apr 2026, 02:43 Tim Song, wrote:\n> >\n> > > Might be better to leave out the initializer for name since not specifying\n> > > it is pretty unusual? That matches what Barry proposed in some version of\n> > > P3795 before it was removed in an LWG telecon (for lack of normative\n> > > effect).\n> > >\n> >\n> > Yes, I think that was what LWG agreed with, but couldn't say so in the\n> > standard wording.\n>\n> That means that we'll emit a warning with\n>\n> data_member_spec (^^int, { .bit_width = 8 })\n\nYes, you can do .name = {} to make it explicit that you want an\nanonymous bitfield. I don't expect that to be common anyway.\n\n> which I don't like, but if that's what's been decided, here's an\n> updated patch.\n\nOK for trunk, thanks.\n\n>\n> > And would it be simpler to initialize alignment and bit_width with {}\n> > instead of nullopt? There's only one constructor that's viable for {} so no\n> > need to do resolution to find the best match for =nullopt.\n>\n> Done.\n>\n> Tested reflect/*, ok for trunk?\n>\n> -- >8 --\n> We emit -Wmissing-field-initializers warnings for code like\n>\n> data_member_spec (^^int, { .name = \"dms\" })\n>\n> which seems undesirable. We can initialize the members of\n> std::meta::data_member_options to suppress that warning (clang's \n> has these initializers too).\n>\n> PR c++/124950\n>\n> libstdc++-v3/ChangeLog:\n>\n> * include/std/meta (std::meta::data_member_options): Initialize\n> alignment, bit_width, and annotations members.\n>\n> gcc/testsuite/ChangeLog:\n>\n> * g++.dg/reflect/data_member_spec7.C: New test.\n> ---\n> gcc/testsuite/g++.dg/reflect/data_member_spec7.C | 9 +++++++++\n> libstdc++-v3/include/std/meta | 6 +++---\n> 2 files changed, 12 insertions(+), 3 deletions(-)\n> create mode 100644 gcc/testsuite/g++.dg/reflect/data_member_spec7.C\n>\n> diff --git a/gcc/testsuite/g++.dg/reflect/data_member_spec7.C b/gcc/testsuite/g++.dg/reflect/data_member_spec7.C\n> new file mode 100644\n> index 00000000000..057db043bf8\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/reflect/data_member_spec7.C\n> @@ -0,0 +1,9 @@\n> +// PR c++/124950\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection -Wmissing-field-initializers\" }\n> +// Test that we don't emit bogus -Wmissing-field-initializers warnings.\n> +\n> +#include \n> +using namespace std::meta;\n> +constexpr auto dms = data_member_spec (^^int, { .name = \"dms\" });\n> +constexpr auto dms2 = data_member_spec (^^int, { .bit_width = 8 }); // { dg-warning \"missing initializer\" }\n> diff --git a/libstdc++-v3/include/std/meta b/libstdc++-v3/include/std/meta\n> index 22b4da44cdd..b12f49a2096 100644\n> --- a/libstdc++-v3/include/std/meta\n> +++ b/libstdc++-v3/include/std/meta\n> @@ -424,10 +424,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION\n> };\n>\n> optional<_Name> name;\n> - optional alignment;\n> - optional bit_width;\n> + optional alignment = {};\n> + optional bit_width = {};\n> bool no_unique_address = false;\n> - vector annotations;\n> + vector annotations = {};\n> };\n> consteval info data_member_spec(info, data_member_options);\n> consteval bool is_data_member_spec(info);\n>\n> base-commit: 6adc49fc08cb87b0e7c8ead3176b45dae2ff4e2a\n> --\n> 2.53.0\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=bEfMX1rt;\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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