fmt/printf.h with wchar_t* loses int type

[deadem]

This code works just fine:
https://godbolt.org/z/x2BA0m

#include <fmt/format.h>
#include <boost/date_time/gregorian/gregorian.hpp>
...
boost::gregorian::greg_year year(2015);
std::wcout << fmt::format(L"{:d}", year);

But if I include <fmt/printf.h> instead of <format.h> I've got a runtime error:
https://godbolt.org/z/e2x3Sp

terminate called after throwing an instance of 'fmt::v6::format_error'
  what():  invalid type specifier

With char* formatting there is no error:
https://godbolt.org/z/Z80SwD

std::cout << fmt::format("{:d}", year);

[vitaut]

The problem is that fmt/printf.h includes fmt/ostream.h which enables formatting of boost::gregorian::greg_year via operator<< and overrides implicit conversion to an integer. A condiiton should be added in

fmt/include/fmt/core.h

Line 861 in f7aedc5

FMT_ENABLE_IF(!is_string<T>::value && !is_char<T>::value &&

disabling this overload if T is convertible to an integral type (or floating point type for that matter).

[vitaut]

On a second thought, the current behavior is correct because one should be able to override implicit conversion. To fix the issue either don't include fmt/ostream.h or provide a specialization of formatter (https://godbolt.org/z/9D_krW):

#include <iostream>
#define FMT_HEADER_ONLY 1
#include <fmt/printf.h> // this should really be <fmt/format.h>
#include <boost/date_time/gregorian/gregorian.hpp>

namespace fmt {
template <typename Char>
struct formatter<boost::gregorian::greg_year, Char> : formatter<int, Char> {
  template <typename Context>
  auto format(boost::gregorian::greg_year y, Context& ctx) {
    return formatter<int, Char>::format(y, ctx);
  }
};
}

int main() {
  boost::gregorian::greg_year year(2015);
  std::wcout << fmt::format(L"{:d}", year);
}

[deadem]

@vitaut: I have same thoughts until I discover that error disappears when I use char* instead of wchar_t*.

So

fmt::format(L"{:d}", year);

fails, but

fmt::format("{:d}", year);

not.

So I think there are question of consistency. I'am expect that both cases work or both cases fail. Why the problem exists only with wchar_t pointer?

[deadem]

And second question is why fmt try to deduce argument explicitly requested as int "{:d}" through operator<< even if int type can be inferred.

[vitaut]

Why the problem exists only with wchar_t pointer?

Great question. As it turned out there is a difference in standard overload sets for char and wchar_t which together with a way operator<< detection worked in {fmt} caused the difference in behavior.

And second question is why fmt try to deduce argument explicitly requested as int "{:d}" through operator<< even if int type can be inferred.

It's mostly for enums since people may want to override implicit conversion with operator<<. In this particular case there is no user-defined operator<< and therefore formatting should go through the implicit conversion.

Should be fixed in ccc8f5d.