Ambiguous call to overloaded function arg_mapper when xchars header is included

[odygrd]

After upgrading to fmt v8.0.1 I am getting a compilation error on visual studio.
I haven't really looked in depth but it looks like it is coming from lib fmt.
It only happens for specific versions of visual studio and not for every version it seems.

It is reproduced here :
https://github.com/odygrd/quill/runs/3413814682?check_suite_focus=true

I could also reproduce it myself on Visual Studio 2019 16.8.1

Anyone has seen this before on windows ? I can compile fine on macos and on linux

It looks like one of the tests that is using fmt causes this compiler error since the workflow where the tests are no being compiled compiles fine. I am still trying to dig deeper and find exactly where it is caused

[vitaut]

It looks like you are mixing wide and narrow character strings which is not supported:

fmt::v8::detail::arg_mapper<fmt::v8::basic_format_context<fmt::v8::appender,char>>::map<_Iter>(T)' [D:\a\quill\build\quill\test\TEST_Log.vcxproj]
          with
          [
              _Iter=const wchar_t *,
              T=const wchar_t *
          ]

Notice that the basic_format_context<..., char> is narrow (char) while T is wide (const wchar_t *).

[odygrd]

It seems that including xchars.h makes the function ambigious.
The code below worked in previous version. In this version it fails to compile because xchars.h is included.
If you comment out #include <fmt/xchar.h> the code compiles

https://godbolt.org/z/3T6E1fsGa

[Brainy0207]

https://godbolt.org/z/3T6E1fsGa

But you are still mixing wide and single byte characters:

[odygrd]

I checked and see it works for LOG(L"test", arg_1);

Prior to 8.0.1 and without including xchars.h it was possible to use a non-wide char format string with wide char arguments. The only requirement was not to mix non-wide with wide arguments.

When importing xchars.h tho it looks like this is not the case anymore and that a wide char argument also requires a wide char format string.

Is that expected behaviour ?

[Brainy0207]

#include <fmt/format.h>

int main()
{
    const wchar_t* arg_1 = L"Hello?";
    fmt::format(FMT_STRING("{}"), arg_1);
    return 0;
}

without including xchar.h, 7.1.3 and 8.0.1 (trunk) give me:

[odygrd]

Thanks for the response. I am referring to the above code specifically when using check_format_string

Another example (with xchars header included)

works with wstring and non wide string format
https://godbolt.org/z/jjE71KjoY

does not work anymore if you change wstring to wchar_t
https://godbolt.org/z/3T6E1fsGa

[vitaut]

Fixed in 6d597e3. Now it gives the expected error (since mixing of narrow and wide strings is disallowed) regardless of whether xchar.h is included or not:

include/fmt/core.h:2481:12: error: call to deleted constructor of 'conditional_t<has_formatter<mapped_type, context>::value, formatter<mapped_type>, fallback_formatter<const wchar_t *>>' (aka 'fmt::v8::detail::fallback_formatter<const wchar_t *>')
  auto f = conditional_t<has_formatter<mapped_type, context>::value,
           ^
include/fmt/core.h:2727:23: note: in instantiation of function template specialization 'fmt::v8::detail::parse_format_specs<const wchar_t *, fmt::v8::detail::compile_parse_context<char>>' requested here
        parse_funcs_{&parse_format_specs<Args, parse_context_type>...} {}
                      ^
include/fmt/core.h:2768:37: note: in instantiation of member function 'fmt::v8::detail::format_string_checker<char, fmt::v8::detail::error_handler, const wchar_t *>::format_string_checker' requested here
      (parse_format_string<true>(s, checker(s, {})), true);
                                    ^
test1.cc:8:16: note: in instantiation of function template specialization 'fmt::v8::detail::check_format_string<const wchar_t *, FMT_COMPILE_STRING, 0>' requested here
  fmt::detail::check_format_string<std::remove_reference_t<Args>...>(
               ^
test1.cc:28:3: note: in instantiation of function template specialization 'log<FMT_COMPILE_STRING, const wchar_t *&>' requested here
  LOG("test", arg_1);
  ^
test1.cc:23:5: note: expanded from macro 'LOG'
    log(FMT_STRING(fmt), ##__VA_ARGS__); \
    ^
include/fmt/core.h:979:3: note: 'fallback_formatter' has been explicitly marked deleted here
  fallback_formatter() = delete;
  ^

Thanks for reporting!