[PATCH] c++/reflection: bogus -Wmissing-field-initializers with <meta> [PR124950]

[Jonathan Wakely]

On Tue, 21 Apr 2026, 02:43 Tim Song, <t.canens.cpp@gmail.com> wrote:

> Might be better to leave out the initializer for name since not specifying
> it is pretty unusual? That matches what Barry proposed in some version of
> P3795 before it was removed in an LWG telecon (for lack of normative
> effect).
>

Yes, I think that was what LWG agreed with, but couldn't say so in the
standard wording.

And would it be simpler to initialize alignment and bit_width with {}
instead of nullopt? There's only one constructor that's viable for {} so no
need to do resolution to find the best match for =nullopt.




> On Mon, Apr 20, 2026 at 4:46 PM Marek Polacek <polacek@redhat.com> wrote:
>
>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>
>> -- >8 --
>> We emit -Wmissing-field-initializers warnings for code like
>>
>>   data_member_spec (^^int, { .name = "dms" })
>>
>> which seems undesirable.  We can initialize the members of
>> std::meta::data_member_options to suppress that warning (clang's <meta>
>> has these initializers too).
>>
>>         PR c++/124950
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/std/meta (std::meta::data_member_options): Initialize
>>         name, alignment, bit_width, and annotations members.
>>
>> gcc/testsuite/ChangeLog:
>>
>>         * g++.dg/reflect/data_member_spec7.C: New test.
>> ---
>>  gcc/testsuite/g++.dg/reflect/data_member_spec7.C | 8 ++++++++
>>  libstdc++-v3/include/std/meta                    | 8 ++++----
>>  2 files changed, 12 insertions(+), 4 deletions(-)
>>  create mode 100644 gcc/testsuite/g++.dg/reflect/data_member_spec7.C
>>
>> diff --git a/gcc/testsuite/g++.dg/reflect/data_member_spec7.C
>> b/gcc/testsuite/g++.dg/reflect/data_member_spec7.C
>> new file mode 100644
>> index 00000000000..bdcdb1ee773
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/reflect/data_member_spec7.C
>> @@ -0,0 +1,8 @@
>> +// PR c++/124950
>> +// { dg-do compile { target c++26 } }
>> +// { dg-additional-options "-freflection -Wmissing-field-initializers" }
>> +// Test that we don't emit bogus -Wmissing-field-initializers warnings.
>> +
>> +#include <meta>
>> +using namespace std::meta;
>> +constexpr auto dms = data_member_spec (^^int, { .name = "dms" });
>> diff --git a/libstdc++-v3/include/std/meta b/libstdc++-v3/include/std/meta
>> index 22b4da44cdd..69016a621ef 100644
>> --- a/libstdc++-v3/include/std/meta
>> +++ b/libstdc++-v3/include/std/meta
>> @@ -423,11 +423,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>         info _M_unused = {};
>>        };
>>
>> -      optional<_Name> name;
>> -      optional<int> alignment;
>> -      optional<int> bit_width;
>> +      optional<_Name> name = nullopt;
>> +      optional<int> alignment = nullopt;
>> +      optional<int> bit_width = nullopt;
>>        bool no_unique_address = false;
>> -      vector<info> annotations;
>> +      vector<info> annotations = {};
>>      };
>>      consteval info data_member_spec(info, data_member_options);
>>      consteval bool is_data_member_spec(info);
>>
>> base-commit: 416fe77efb4ff6adf39c3326d462161217b75b42
>> --
>> 2.53.0
>>
>>
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