Basic units fmt example fails to compile on MSVC

[Cazadorro]

Given the very basic example, compilation fails, and it fails when using fmt::format. for example, everything is fine, up until line 57:

std::cout << UNITS_STD_FMT::format("{}", v3) << '\n';

where I get:

C:\PROGRA~2\MICROS~2\2022\BUILDT~1\VC\Tools\MSVC\1434~1.319\bin\Hostx86\x64\cl.exe  /nologo /TP -DUNITS_DOWNCAST_MODE=1 -DUNITS_USE_LIBFMT=1 -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\gsl-lite\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-io\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\fmt\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\isq\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\isq-iec80000\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\isq-natural\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-cgs\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-fps\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-international\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-hep\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-iau\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-imperial\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-typographic\include -IC:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\systems\si-uscs\include /DWIN32 /D_WINDOWS /EHsc /Zi /Ob0 /Od /RTC1 -MDd /std:c++20 /utf-8 -std:c++20 /showIncludes /FoCMakeFiles\cpptesting.dir\main.cpp.obj /FdCMakeFiles\cpptesting.dir\ /FS -c C:\Users\user\Documents\GitRepositories\cpptesting\main.cpp

C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<fmt::v9::monostate>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<char>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<bool>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<float>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<double>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<long double>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<char const *>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<fmt::v9::basic_string_view<char> >': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<void const *>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(107) : error C4716: 'units::detail::width_checker::operator()<fmt::v9::basic_format_arg<fmt::v9::basic_format_context<fmt::v9::appender,char> >::handle>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<bool>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<fmt::v9::monostate>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<char>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<float>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<double>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<long double>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<fmt::v9::basic_string_view<char> >': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<char const *>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<void const *>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(122) : error C4716: 'units::detail::precision_checker::operator()<fmt::v9::basic_format_arg<fmt::v9::basic_format_context<fmt::v9::appender,char> >::handle>': must return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(231) : warning C4715: 'units::detail::do_parse_arg_id<char const *,char const *,`units::detail::parse_width<char const *,char const *,fmt::v9::formatter<units::quantity<units::isq::si::dim_speed,units::isq::si::kilometre_per_hour,__int64>,char,void>::spec_handler &>'::`2'::width_adapter &>': not all control paths return a value
C:\Users\user\Documents\GitRepositories\cpptesting\external\units\src\core-fmt\include\units\bits\fmt.h(231) : warning C4715: 'units::detail::do_parse_arg_id<char const *,char const *,`units::detail::parse_precision<char const *,char const *,fmt::v9::formatter<units::quantity<units::isq::si::dim_speed,units::isq::si::kilometre_per_hour,__int64>,char,void>::spec_handler &>'::`2'::precision_adapter &>': not all control paths return a value
ninja: build stopped: subcommand failed.

If I look at the relevant portion of code causing the error, I indeed see this:

struct width_checker {
  template<typename T>
  [[nodiscard]] constexpr unsigned long long operator()(T value) const
  {
    if constexpr (is_integer<T>) {
      if constexpr (std::numeric_limits<T>::is_signed) {
        if (value < 0) UNITS_THROW(UNITS_STD_FMT::format_error("negative width"));
      }
      return static_cast<unsigned long long>(value);
    } else {
      UNITS_THROW(UNITS_STD_FMT::format_error("width is not integer"));
    }
  }
};

which if the errors are correct, mean that it 100% makes sense why there would be errors here, there's floats and chars being put into this thing with no return path at compile time.

It looks like there's some sort of issue with some sort of std::variant based on the monostate present? But it's not letting me see exactly where the issue is (looks like there's some sort of monkey patching going on, making it impossible for me to diagnose on my own).