constrain templated format_to on proper format string type.

Signed-off-by: Daniela Engert <dani@ngrt.de>

include/fmt/color.h

@@ -266,10 +266,6 @@ template <>
 inline void reset_color<wchar_t>(FILE *stream) FMT_NOEXCEPT {
   fputs(internal::data::WRESET_COLOR, stream);
 }
-
-template <typename S>
-struct is_format_string: std::integral_constant<
-    bool, std::is_empty<format_string_traits<S>>::value> {};
 } // namespace internal
 
 template <

include/fmt/core.h

@@ -1285,6 +1285,9 @@ struct format_string_traits<
          basic_string_view<typename S::char_type>, S>::value>::type> :
     format_string_traits_base<typename S::char_type> {};
 
+template <typename S>
+struct is_format_string : std::is_empty<format_string_traits<S>> {};
+
 template <typename S>
 struct is_compile_string :
     std::integral_constant<bool, std::is_base_of<compile_string, S>::value> {};

include/fmt/format.h

@@ -3507,7 +3507,8 @@ inline OutputIt vformat_to(
  \endrst
  */
 template <typename OutputIt, typename String, typename... Args>
-inline OutputIt format_to(OutputIt out, const String &format_str,
+inline typename std::enable_if<internal::is_format_string<String>::value, OutputIt>::type
+  format_to(OutputIt out, const String &format_str,
                           const Args &... args) {
   internal::check_format_string<Args...>(format_str);
   typedef typename format_context_t<OutputIt, FMT_CHAR(String) >::type context_t;