user-defined types that have overloaded operator<<

[jzablot]

Hi; I'm hitting an issue where user-defined types which overload operator<< are causing seemingly wrongly formatted output if using format_to_n. fmt::format seems to work okay.

Here's an example; I tried against fmtlib 5.1.0 (seems to behave similarly for older versions). I'm building with Visual Studio 2015 on windows, but I think I might be seeing similar behaviour with Xcode (I can confirm if needed).

#include "stdafx.h"

#define FMT_HEADER_ONLY 1
#include <fmt/format.h>
#include <fmt/ostream.h> 


class Test
{
public:

   friend std::ostream& operator<<(std::ostream& strm, const Test& cpstr)
   {
      return strm << "555";
   }
};
 

int main()
{   
   Test a;

   char buf[256];
   fmt::format_to_n_result<char*> res = fmt::format_to_n(buf, 256, "Test {} end", a);
   // buf contains Test  end

   std::string s = fmt::format("Test {} end", a);
   s = s;
   // s contains  Test 555 end   
}

If I switch to use the other style (struct formatter per docs), things start working. But that's not ideal for me since I already have a bunch of classes with stream operators overridden already in various bits of my project.

[vitaut]

This was fixed in #806.