copying an object of non-trivial type error

[federico-busato]

The fix addresses the following error related to fmt-9.0.0 uint128_fallback, reproducible with GCC-8.3.0

fmt/format.h:290:12: error: 'void* memcpy(void*, const void*, size_t)' copying an object of non-trivial type 'class fmt::v9::detail::uint128_fallback' from an array of 'const long double' [-Werror=class-memaccess]
   std::memcpy(&to, &from, sizeof(to));
   ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
fmt/format.h:309:7: note: 'class fmt::v9::detail::uint128_fallback' declared here
 class uint128_fallback {

[vitaut]

Thanks for the PR but please revert unrelated changes.

[vitaut]

Also could you post the full diagnostic to see where the problematic call comes from?

[federico-busato]

you are right. Sorry, I messed up the commits.
This is the full log:

fmt/include/fmt/format.h: In instantiation of 'To fmt::v9::detail::bit_cast(const From&) [with To = fmt::v9::detail::uint128_fallback; From = long double; typename std::enable_if<(sizeof (To) == sizeof (From)), int>::type <anonymous> = 0]':
fmt/include/fmt/format.h:1395:75:   required from 'bool fmt::v9::detail::basic_fp<F>::assign(Float) [with Float = long double; typename std::enable_if<(! std::integral_constant<bool, (std::numeric_limits<Float>::digits == 106)>::value), int>::type <anonymous> = 0; F = fmt::v9::detail::uint128_fallback]'
fmt/include/fmt/format.h:3137:47:   required from 'int fmt::v9::detail::format_float(Float, int, fmt::v9::detail::float_specs, fmt::v9::detail::buffer<char>&) [with Float = long double]'
fmt/include/fmt/format.h:3205:23:   required from 'OutputIt fmt::v9::detail::write(OutputIt, T, fmt::v9::basic_format_specs<Char>, fmt::v9::detail::locale_ref) [with Char = char; OutputIt = fmt::v9::appender; T = long double; typename std::enable_if<std::integral_constant<bool, (std::is_floating_point<T>::value || std::is_same<T, void>::value)>::value, int>::type <anonymous> = 0]'
fmt/include/fmt/format.h:3369:21:   required from 'fmt::v9::detail::arg_formatter<Char>::iterator fmt::v9::detail::arg_formatter<Char>::operator()(T) [with T = long double; Char = char; fmt::v9::detail::arg_formatter<Char>::iterator = fmt::v9::appender]'
fmt/include/fmt/core.h:1637:11:   required from 'decltype (vis(0)) fmt::v9::visit_format_arg(Visitor&&, const fmt::v9::basic_format_arg<Context>&) [with Visitor = fmt::v9::detail::arg_formatter<char>&; Context = fmt::v9::basic_format_context<fmt::v9::appender, char>; decltype (vis(0)) = fmt::v9::appender]'
fmt/include/fmt/format.h:4107:38:   required from 'const Char* fmt::v9::detail::vformat_to(fmt::v9::detail::buffer<T>&, fmt::v9::basic_string_view<Char>, fmt::v9::basic_format_args<fmt::v9::basic_format_context<typename std::conditional<std::is_same<typename fmt::v9::type_identity<T>::type, char>::value, fmt::v9::appender, std::back_insert_iterator<fmt::v9::detail::buffer<typename fmt::v9::type_identity<T>::type> > >::type, typename fmt::v9::type_identity<T>::type> >, fmt::v9::detail::locale_ref)::format_handler::on_format_specs(int, const Char*, const Char*) [with Char = char]'
fmt/include/fmt/format.h:4110:1:   required from 'void fmt::v9::detail::vformat_to(fmt::v9::detail::buffer<T>&, fmt::v9::basic_string_view<Char>, fmt::v9::basic_format_args<fmt::v9::basic_format_context<typename std::conditional<std::is_same<typename fmt::v9::type_identity<T>::type, char>::value, fmt::v9::appender, std::back_insert_iterator<fmt::v9::detail::buffer<typename fmt::v9::type_identity<T>::type> > >::type, typename fmt::v9::type_identity<T>::type> >, fmt::v9::detail::locale_ref) [with Char = char; typename fmt::v9::type_identity<T>::type = char; typename std::conditional<std::is_same<typename fmt::v9::type_identity<T>::type, char>::value, fmt::v9::appender, std::back_insert_iterator<fmt::v9::detail::buffer<typename fmt::v9::type_identity<T>::type> > >::type = fmt::v9::appender]'
fmt/include/fmt/core.h:3179:19:   required from 'OutputIt fmt::v9::vformat_to(OutputIt, fmt::v9::string_view, fmt::v9::format_args) [with OutputIt = std::back_insert_iterator<fmt::v9::basic_memory_buffer<char, 2048> >; typename std::enable_if<fmt::v9::detail::is_output_iterator<OutputIt, char>::value, int>::type <anonymous> = 0; fmt::v9::string_view = fmt::v9::basic_string_view<char>; fmt::v9::format_args = fmt::v9::basic_format_args<fmt::v9::basic_format_context<fmt::v9::appender, char> >]'
fmt/include/fmt/core.h:3199:18:   required from 'OutputIt fmt::v9::format_to(OutputIt, fmt::v9::format_string<T ...>, T&& ...) [with OutputIt = std::back_insert_iterator<fmt::v9::basic_memory_buffer<char, 2048> >; T = {const tm&}; typename std::enable_if<fmt::v9::detail::is_output_iterator<OutputIt, char>::value, int>::type <anonymous> = 0; fmt::v9::format_string<T ...> = fmt::v9::basic_format_string<char, const tm&>]'
file.h:204:69:   required from here
fmt/include/fmt/format.h:290:12: error: 'void* memcpy(void*, const void*, size_t)' copying an object of non-trivial type 'class fmt::v9::detail::uint128_fallback' from an array of 'const long double' [-Werror=class-memaccess]
   std::memcpy(&to, &from, sizeof(to));
   ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
fmt/format.h:309:7: note: 'class fmt::v9::detail::uint128_fallback' declared here
 class uint128_fallback {
       ^~~~~~~~~~~~~~~~

from: fmt::format_to(std::back_inserter(out), "[{:%Y-%m-%d %H:%M:%S}]", tm);, where tm is an instance of *std::tm

[phprus]

Please provide a reproduction code and compiler options.

On godbolt without error:
https://godbolt.org/z/bhGWxxcbh

[vitaut]

One more thing: do you by any chance have a godbolt repro for this warning? I tried to make one in https://godbolt.org/z/7Ybn5zrr9 but to no avail.

[federico-busato]

unfortunately, I cannot expose much as the code comes from an internal build pipeline. I also tried it on my side on the nearest configurations Arm64 GCC-8.5.0 and x86-64 GCC-8.3.0 and I'm not able to reproduce the error. Maybe it is an issue specifically related to Arm64 GCC-8.3-2019.03 Linux-GNU. If you think that this fix is not relevant, please discard it. I see that uint128_fallback is trivially copiable

[vitaut]

If you think that this fix is not relevant, please discard it.

Yeah, considering that uint128_fallback is trivially copyable I don't think we should merge this in unless the warning reproduces more widely. But thanks anyway.

[vitaut]

I ended up suppressing the warning since it appears to be more wide spread (see e.g. #3000).