in src/Windows/libraries/multi_byte/include/m/multi_byte/convert.h [302:312]
void utf16_to_acp(InputIt front, InputIt end, std::string& out)
{
auto const view = std::basic_string_view<Utf16CharT, CharTraitsT>(front, end);
auto const chars_needed = utf16_to_acp_length(view);
out.resize_and_overwrite(chars_needed,
[view](auto buffer, auto buffer_size) -> std::size_t {
auto span = m::make_span(buffer, buffer_size);
return utf16_to_acp(view, span);
});
}