[regression?] format_n(wchar_t*) results in warning C4244

[TheQwertiest]

Repro code:

#define FMT_HEADER_ONLY
#include <fmt/format.h>

#include <vector>

int main()
{
    std::vector<wchar_t> buffer(40);
    fmt::format_to_n( buffer.data(), buffer.size(), L"(unknown)" );
    return 0;
}

Compiler: VS 19.26.28805 x64
Compile cmd: cl.exe test.cpp -Ipath_to_fmt_include /std:c++17 /EHsc
Error/warning message:

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Microsoft (R) C/C++ Optimizing Compiler Version 19.26.28805 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cpp
xmemory(204): warning C4244: 'initializing': conversion from 'const Char' to '_Ty', possible loss of data
        with
        [
            Char=wchar_t
        ]
        and
        [
            _Ty=char
        ]
xmemory(1427): note: see reference to function template instantiation 'void std::_Construct_in_place<T,const Char&>(_Ty &,const Char &) noexcept' being compiled
        with
        [
            T=char,
            Char=wchar_t,
            _Ty=char
        ]
xmemory(1593): note: see reference to function template instantiation 'void std::_Uninitialized_backout<_NoThrowFwdIt>::_Emplace_back<const Char&>(const Char &)' being compiled
        with
        [
            _NoThrowFwdIt=char *,
            Char=wchar_t
        ]
xmemory(1593): note: see reference to function template instantiation 'void std::_Uninitialized_backout<_NoThrowFwdIt>::_Emplace_back<const Char&>(const Char &)' being compiled
        with
        [
            _NoThrowFwdIt=char *,
            Char=wchar_t
        ]
fmt\format.h(1568): note: see reference to function template instantiation '_NoThrowFwdIt std::uninitialized_copy<const Char*,T*>(const _InIt,const _InIt,_NoThrowFwdIt)' being compiled
        with
        [
            _NoThrowFwdIt=char *,
            Char=wchar_t,
            T=char,
            _InIt=const wchar_t *
        ]
fmt\format.h(1534): note: while compiling class template member function 'void fmt::v6::detail::int_writer<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t,uint_type>::on_num(void)'
        with
        [
            OutputIt=wchar_t *
        ]
fmt\format.h(1240): note: see reference to function template instantiation 'void fmt::v6::detail::int_writer<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t,uint_type>::on_num(void)' being compiled
        with
        [
            OutputIt=wchar_t *
        ]
fmt\format.h(1746): note: see reference to class template instantiation 'fmt::v6::detail::int_writer<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t,uint_type>' being compiled
        with
        [
            OutputIt=wchar_t *
        ]
fmt\format.h(1822): note: see reference to function template instantiation 'void fmt::v6::detail::arg_formatter_base<Range,fmt::v6::detail::error_handler>::write_int<T>(T,const fmt::v6::basic_format_specs<wchar_t> &)' being compiled
        with
        [
            Range=range,
            T=__int64
        ]
fmt\format.h(1822): note: see reference to function template instantiation 'void fmt::v6::detail::arg_formatter_base<Range,fmt::v6::detail::error_handler>::write_int<T>(T,const fmt::v6::basic_format_specs<wchar_t> &)' being compiled
        with
        [
            Range=range,
            T=__int64
        ]
fmt\core.h(1170): note: see reference to function template instantiation 'fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>> fmt::v6::detail::arg_formatter_base<Range,fmt::v6::detail::error_handler>::operator ()<__int64,0>(T)' being compiled
        with
        [
            OutputIt=wchar_t *,
            Range=range,
            T=__int64
        ]
fmt\core.h(1170): note: see reference to function template instantiation 'fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>> fmt::v6::detail::arg_formatter_base<Range,fmt::v6::detail::error_handler>::operator ()<__int64,0>(T)' being compiled
        with
        [
            OutputIt=wchar_t *,
            Range=range,
            T=__int64
        ]
fmt\format.h(3176): note: see reference to function template instantiation 'fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>> fmt::v6::visit_format_arg<ArgFormatter,Context>(Visitor &&,const fmt::v6::basic_format_arg<Context> &)' being compiled
        with
        [
            OutputIt=wchar_t *,
            ArgFormatter=fmt::v6::arg_formatter<range>,
            Context=context,
            Visitor=fmt::v6::arg_formatter<range>
        ]
fmt\format.h(3395): note: see reference to function template instantiation 'fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>> fmt::v6::vformat_to<fmt::v6::arg_formatter<range>,wchar_t,fmt::v6::basic_format_context<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t>>(fmt::v6::detail::output_range<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t>,fmt::v6::basic_string_view<wchar_t>,fmt::v6::basic_format_args<fmt::v6::basic_format_context<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t>>,fmt::v6::detail::locale_ref)' being compiled
        with
        [
            OutputIt=wchar_t *
        ]
fmt\format.h(3449): note: see reference to function template instantiation 'OutputIt fmt::v6::vformat_to<fmt::v6::basic_string_view<wchar_t>,fmt::v6::detail::truncating_iterator<wchar_t*,std::integral_constant<bool,false>>,,0>(OutputIt,const S &,fmt::v6::basic_format_args<fmt::v6::basic_format_context<OutputIt,wchar_t>>)' being compiled
        with
        [
            OutputIt=fmt::v6::detail::truncating_iterator<wchar_t *,std::integral_constant<bool,false>>,
            S=fmt::v6::basic_string_view<wchar_t>
        ]
fmt\format.h(3467): note: see reference to function template instantiation 'fmt::v6::format_to_n_result<_Ty *> fmt::v6::vformat_to_n<OutputIt,wchar_t,,0>(OutputIt,size_t,fmt::v6::basic_string_view<wchar_t>,fmt::v6::basic_format_args<fmt::v6::basic_format_context<fmt::v6::detail::truncating_iterator<OutputIt,std::integral_constant<bool,false>>,wchar_t>>)' being compiled
        with
        [
            _Ty=wchar_t,
            OutputIt=wchar_t *
        ]
test.cpp(9): note: see reference to function template instantiation 'fmt::v6::format_to_n_result<_Ty *> fmt::v6::format_to_n<_Ty*,wchar_t[10],,0>(OutputIt,size_t,const S (&))' being compiled
        with
        [
            _Ty=wchar_t,
            OutputIt=wchar_t *,
            S=wchar_t [10]
        ]
xutility(3778): warning C4244: '=': conversion from '_Elem' to 'T', possible loss of data
        with
        [
            _Elem=wchar_t
        ]
        and
        [
            T=char
        ]
xutility(3814): note: see reference to function template instantiation '_OutIt std::_Copy_unchecked<_Iter,_NoThrowFwdIt>(_InIt,_InIt,_OutIt)' being compiled
        with
        [
            _OutIt=char *,
            _Iter=wchar_t *,
            _NoThrowFwdIt=char *,
            _InIt=wchar_t *
        ]
fmt\format.h(563): note: see reference to function template instantiation '_OutIt std::copy<InputIt,OutputIt>(_InIt,_InIt,_OutIt)' being compiled
        with
        [
            _OutIt=char *,
            InputIt=wchar_t *,
            OutputIt=char *,
            _InIt=wchar_t *
        ]
fmt\format.h(929): note: see reference to function template instantiation 'OutputIt fmt::v6::detail::copy_str<Char,Char*,Iterator,0>(InputIt,InputIt,OutputIt)' being compiled
        with
        [
            OutputIt=char *,
            Char=wchar_t,
            Iterator=char *,
            InputIt=wchar_t *
        ]
fmt\format.h(1550): note: see reference to function template instantiation 'Iterator *fmt::v6::detail::format_decimal<Char,UInt,char*>(Iterator,UInt,int)' being compiled
        with
        [
            Iterator=char *,
            Char=wchar_t,
            UInt=uint_type
        ]

[vitaut]

What library version do you use?

[TheQwertiest]

I used the latest HEAD available at the time of the posting.

[vitaut]

What's the commit hash?

[TheQwertiest]

The latest 16aec06 also has this issue (have tried just now).
Full commit hash: 16aec06

[vitaut]

Fixed in 21409cf. Thanks for reporting.