GCC14 warning for subobject linkage

[dinomight]

While messing around with compiling {fmt}, I came across this warning when building with GCC14. This warning is not generated under GCC13. I've also noticed that the PR actions don't include GCC14 or some of the more modern Clang compilers. Do those need to be added? I'll likely do some digging into this warning and may propose a PR later for it, if I'm able to understand it. Wanted to make sure this was documented.

In file included from /mnt/development/fmt/include/fmt/ranges.h:20,
                 from /mnt/development/fmt/test/ranges-test.cc:8:
/mnt/development/fmt/include/fmt/format.h: In instantiation of ‘struct fmt::v11::formatter<views_filter_view_test::codec_mask, char, void>’:
/mnt/development/fmt/include/fmt/base.h:1101:52:   required by substitution of ‘template<class T> struct fmt::v11::detail::use_format_as_member<T, std::integral_constant<bool, std::is_arithmetic<typename std::remove_cv<typename std::remove_reference<decltype (fmt::v11::formatter<T>::format_as(declval<const T&>()))>::type>::type>::value> > [with T = views_filter_view_test::codec_mask]’
 1101 |     remove_cvref_t<decltype(formatter<T>::format_as(std::declval<const T&>()))>;
      |                             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/development/fmt/include/fmt/base.h:1124:47:   required by substitution of ‘template<class T, typename std::enable_if<std::integral_constant<bool, (((((((std::is_class<T>::value || std::is_enum<_Tp>::value) || std::is_union<T>::value) || std::is_array<T>::value) && (! fmt::v11::detail::has_to_string_view<T>::value)) && (! fmt::v11::detail::is_named_arg<T>::value)) && (! fmt::v11::detail::use_format_as<T>::value)) && (! fmt::v11::detail::use_format_as_member<typename std::remove_const<_Tp>::type, std::integral_constant<bool, true> >::value))>::value, int>::type <anonymous> > static fmt::v11::conditional_t<((bool)has_formatter<T, char>()), T&, void> fmt::v11::detail::type_mapper<char>::map(T&) [with T = views_filter_view_test::codec_mask; typename std::enable_if<std::integral_constant<bool, (((((((std::is_class<T>::value || std::is_enum<_Tp>::value) || std::is_union<T>::value) || std::is_array<T>::value) && (! fmt::v11::detail::has_to_string_view<T>::value)) && (! fmt::v11::detail::is_named_arg<T>::value)) && (! fmt::v11::detail::use_format_as<T>::value)) && (! fmt::v11::detail::use_format_as_member<typename std::remove_const<_Tp>::type, std::integral_constant<bool, true> >::value))>::value, int>::type <anonymous> = <missing>]’
 1124 |                   !use_format_as<T>::value && !use_format_as_member<U>::value>;
      |                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/development/fmt/include/fmt/base.h:1197:57:   required by substitution of ‘template<class T, class Char> using fmt::v11::detail::mapped_type_constant = fmt::v11::detail::type_constant<decltype (fmt::v11::detail::type_mapper<Char>::map(declval<T&>())), Char> [with T = views_filter_view_test::codec_mask&; Char = char]’
 1197 | using mapped_t = decltype(detail::type_mapper<Char>::map(std::declval<T&>()));
      |                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
/mnt/development/fmt/include/fmt/base.h:1676:47:   required from ‘constexpr fmt::v11::detail::format_string_checker<Char, NUM_ARGS, NUM_NAMED_ARGS, DYNAMIC_NAMES>::format_string_checker(fmt::v11::basic_string_view<Char>, fmt::v11::detail::arg_pack<T ...>) [with T = {views_filter_view_test::codec_mask&}; Char = char; int NUM_ARGS = 1; int NUM_NAMED_ARGS = 0; bool DYNAMIC_NAMES = false]’
 1676 |       : types_{mapped_type_constant<T, Char>::value...},
      |                                               ^~~~~
/mnt/development/fmt/include/fmt/base.h:2708:57:   required from here
 2708 |     if (FMT_USE_CONSTEVAL) parse_format_string<char>(s, checker(s, arg_pack()));
      |                                                         ^~~~~~~~~~~~~~~~~~~~~~
/mnt/development/fmt/test/ranges-test.cc:747:5:   in ‘constexpr’ expansion of ‘fmt::v11::fstring<views_filter_view_test::codec_mask&>("{}")’
/mnt/development/fmt/include/fmt/format.h:3810:8: error: ‘fmt::v11::formatter<views_filter_view_test::codec_mask, char, void>’ has a base ‘fmt::v11::formatter<std::ranges::filter_view<std::ranges::ref_view<const std::array<int, 4> >, views_filter_view_test::format_as(codec_mask)::<lambda(auto:38)> >, char, void>’ which has internal linkage [-Werror=subobject-linkage]
 3810 | struct formatter<T, Char, void_t<detail::format_as_result<T>>>
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[vitaut]

Please provide a godbolt repro.

[dinomight]

I tried to snip out the test code that's causing the problem, but it isn't reproducing over on godbolt. This is what I tried: https://godbolt.org/z/7xWKbbxPv

I'm not sure if that means it has something to do with the test framework or the fact that there's more content in the compilation unit. I can confirm that removing the test entirely resolves the issues. I can also confirm that removing things from the asserts also doesn't solve it. Setting up godbolt as a gtest setup doesn't replicate either: https://godbolt.org/z/coKTTx7Yb