Warning C4244 when using wtime_input_facet with MSVC

Start with boost/date_time/time_facet.hpp at time_input_facet::check_special_value and follow its call to convert_string_type in boost/date_time/string_convert.hpp. Reproduce the minimal wtime_input_facet example with MSVC 2022 at /W4, then verify that the diagnostic conversion no longer emits C4244 while preserving the reported invalid-time behavior.

Description

When using boost::posix_time::wtime_input_facet with a wide-character stream, MSVC 2022 reports warning C4244 because time_input_facet::check_special_value converts a std::wstring to std::string through convert_string_type<wchar_t, char>.

The conversion appears to construct a std::string directly from const wchar_t* iterators, which causes a narrowing conversion and may result in data loss.

Minimal example

#include <boost/date_time/posix_time/posix_time.hpp>

#include <locale>
#include <sstream>

int main()
{
    std::wistringstream input(L"not-a-valid-ptime");

    input.imbue(std::locale(
        input.getloc(),
        new boost::posix_time::wtime_input_facet));

    boost::posix_time::ptime value;
    input >> value;

    return 0;
}

Environment

• Visual Studio 2022 Professional
• MSVC 14.44.35207
• Boost installed through vcpkg
• Target: x64-windows
• Warning level: /W4

Actual result

The project builds successfully, but MSVC reports:

C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.44.35207\include\xutility(4813,18):
warning C4244: '=': conversion from 'const wchar_t' to 'char', possible loss of data

The template instantiation trace points to:

boost/date_time/time_facet.hpp(1117,31):
time_input_facet<ptime, wchar_t>::check_special_value

boost/date_time/time_facet.hpp(1260,29):
convert_string_type<wchar_t, char>

boost/date_time/string_convert.hpp(26,12):
std::basic_string<char>::insert(...)

The relevant code path is equivalent to:

std::string tmp = convert_string_type<char_type, char>(mr.cache);

where char_type is wchar_t.

Expected result

Using wtime_input_facet should not produce a narrowing-conversion warning when compiling with /W4.

The conversion used to create the diagnostic message should either:

1. perform an explicit, well-defined wide-to-narrow conversion, or
2. preserve the original character type until the diagnostic is formatted.

Could this be fixed or suppressed safely for the wchar_t instantiation?