Windows warning C4244 in fmt/chrono.h

[Tradias]

Compiling the following on Windows:

#include <fmt/chrono.h>

int main()
{
    auto s = fmt::format("{}", std::chrono::seconds(1));
    return 0;
}

using

Microsoft (R) C/C++ Optimizing Compiler Version 19.25.28614 for x64
Windows 10 SDK (10.0.18362.0)
fmt v6.2.0

and compiler flags: /W1 -DFMT_LOCALE

produces the warning:

[build] C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\include\xutility(3606): warning C4244: 'argument': conversion from 'const wchar_t' to 'const char', possible loss of data
[build] C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\include\xutility(3643): note: see reference to function template instantiation '_OutIt std::_Copy_unchecked<_InIt,_OutIt>(_InIt,_InIt,_OutIt)' being compiled
[build]         with
[build]         [
[build]             _OutIt=std::back_insert_iterator<fmt::v6::basic_memory_buffer<char,500,std::allocator<char>>>,
[build]             _InIt=const wchar_t *
[build]         ]
[build] ...\fmt\include\fmt/chrono.h(778): note: see reference to function template instantiation '_OutIt std::copy<const wchar_t*,OutputIt>(_InIt,_InIt,_OutIt)' being compiled
[build]         with
[build]         [
[build]             _OutIt=std::back_insert_iterator<fmt::v6::basic_memory_buffer<char,500,std::allocator<char>>>,
[build]             OutputIt=std::back_insert_iterator<fmt::v6::basic_memory_buffer<char,500,std::allocator<char>>>,
[build]             _InIt=const wchar_t *
[build]         ]
[build] ...\fmt\include\fmt/chrono.h(1105): note: see reference to function template instantiation 'OutputIt fmt::v6::internal::format_duration_unit<Char,Period,std::back_insert_iterator<fmt::v6::basic_memory_buffer<char,500,std::allocator<char>>>>(OutputIt)' being compiled
[build]         with
[build]         [
[build]             OutputIt=std::back_insert_iterator<fmt::v6::basic_memory_buffer<char,500,std::allocator<char>>>,
[build]             Char=char,
[build]             Period=std::ratio<1,1>
[build]         ]
[build] ...\fmt\include\fmt\core.h(898): note: see reference to function template instantiation 'std::back_insert_iterator<fmt::v6::internal::buffer<char>> fmt::v6::formatter<Arg,char,void>::format<Context>(const std::chrono::duration<__int64,std::ratio<1,1>> &,FormatContext &)' being compiled
[build]         with
[build]         [
[build]             Arg=std::chrono::duration<__int64,std::ratio<1,1>>,
[build]             Context=fmt::v6::format_context,
[build]             FormatContext=fmt::v6::format_context
[build]         ]
[build] ...\fmt\include\fmt\core.h(898): note: see reference to function template instantiation 'std::back_insert_iterator<fmt::v6::internal::buffer<char>> fmt::v6::formatter<Arg,char,void>::format<Context>(const std::chrono::duration<__int64,std::ratio<1,1>> &,FormatContext &)' being compiled
[build]         with
[build]         [
[build]             Arg=std::chrono::duration<__int64,std::ratio<1,1>>,
[build]             Context=fmt::v6::format_context,
[build]             FormatContext=fmt::v6::format_context
[build]         ]
[build] ...\fmt\include\fmt\core.h(885): note: see reference to function template instantiation 'void fmt::v6::internal::value<Context>::format_custom_arg<T,fmt::v6::formatter<T,char,void>>(const void *,fmt::v6::basic_format_parse_context<char,fmt::v6::internal::error_handler> &,Context &)' being compiled
[build]         with
[build]         [
[build]             Context=fmt::v6::format_context,
[build]             T=std::chrono::duration<__int64,std::ratio<1,1>>
[build]         ]
[build] ...\fmt\include\fmt\core.h(882): note: see reference to function template instantiation 'void fmt::v6::internal::value<Context>::format_custom_arg<T,fmt::v6::formatter<T,char,void>>(const void *,fmt::v6::basic_format_parse_context<char,fmt::v6::internal::error_handler> &,Context &)' being compiled
[build]         with
[build]         [
[build]             Context=fmt::v6::format_context,
[build]             T=std::chrono::duration<__int64,std::ratio<1,1>>
[build]         ]
[build] ...\fmt\include\fmt\core.h(1213): note: see reference to function template instantiation 'fmt::v6::internal::value<Context>::value<T>(const T &)' being compiled
[build]         with
[build]         [
[build]             Context=fmt::v6::format_context,
[build]             T=std::chrono::duration<__int64,std::ratio<1,1>>
[build]         ]
[build] ...\fmt\include\fmt\core.h(1213): note: see reference to function template instantiation 'fmt::v6::internal::value<Context>::value<T>(const T &)' being compiled
[build]         with
[build]         [
[build]             Context=fmt::v6::format_context,
[build]             T=std::chrono::duration<__int64,std::ratio<1,1>>
[build]         ]
[build] ...\fmt\include\fmt\core.h(1344): note: see reference to function template instantiation 'fmt::v6::internal::value<Context> fmt::v6::internal::make_arg<true,Context,std::chrono::duration<__int64,std::ratio<1,1>>,0>(const T &)' being compiled
[build]         with
[build]         [
[build]             Context=fmt::v6::format_context,
[build]             T=std::chrono::duration<__int64,std::ratio<1,1>>
[build]         ]
[build] ...\fmt\include\fmt\core.h(1344): note: while compiling class template member function 'fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>,std::chrono::duration<__int64,std::ratio<1,1>>>::format_arg_store(const std::chrono::duration<__int64,std::ratio<1,1>> &)'
[build] ...\fmt\include\fmt\core.h(1649): note: see reference to function template instantiation 'fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>,std::chrono::duration<__int64,std::ratio<1,1>>>::format_arg_store(const std::chrono::duration<__int64,std::ratio<1,1>> &)' being compiled
[build] ...\fmt\include\fmt\core.h(1737): note: see reference to class template instantiation 'fmt::v6::format_arg_store<fmt::v6::basic_format_context<std::back_insert_iterator<fmt::v6::internal::buffer<char>>,char>,std::chrono::duration<__int64,std::ratio<1,1>>>' being compiled
[build] ..\src\main.cpp(13): note: see reference to function template instantiation 'std::basic_string<char,std::char_traits<char>,std::allocator<char>> fmt::v6::format<char[3],std::chrono::seconds,char>(const S (&),std::chrono::seconds &&)' being compiled
[build]         with
[build]         [
[build]             S=char [3]
[build]         ]

[vitaut]

Fixed in f2ed03b.

[Tradias]

Cool thanks, I wonder how it didn't work correctly before. According to the log the type of Char in the format_duration_unit function is char yet the condition:

if (const_check(std::is_same<Char, wchar_t>()))

evaluated to true.

[vitaut]

AFAICS it is evaluated to false: https://godbolt.org/z/XR3fXD. In any case it's gone now.