Workaround bugs in gcc 8

include/fmt/color.h

@@ -589,10 +589,11 @@ OutputIt vformat_to(
   \endrst
 */
 template <typename OutputIt, typename S, typename... Args,
-          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, char_t<S>>::value&&
-                            detail::is_string<S>::value)>
-inline OutputIt format_to(OutputIt out, const text_style& ts,
-                          const S& format_str, Args&&... args) {
+          bool enable = (detail::is_output_iterator<OutputIt, char_t<S>>::value&&
+                             detail::is_string<S>::value)>
+inline auto format_to(OutputIt out, const text_style& ts,
+                      const S& format_str, Args&&... args) ->
+    typename std::enable_if<enable, OutputIt>::type {
   return vformat_to(out, ts, to_string_view(format_str),
                     fmt::make_args_checked<Args...>(format_str, args...));
 }

include/fmt/compile.h

@@ -667,14 +667,14 @@ OutputIt format_to(OutputIt out, const S&, const Args&... args) {
   return format_to(out, compiled, args...);
 }
 
-template <typename OutputIt, typename CompiledFormat, typename... Args,
-          FMT_ENABLE_IF(detail::is_output_iterator<
-                        OutputIt, typename CompiledFormat::char_type>::value&&
-                            std::is_base_of<detail::basic_compiled_format,
-                                            CompiledFormat>::value)>
-format_to_n_result<OutputIt> format_to_n(OutputIt out, size_t n,
-                                         const CompiledFormat& cf,
-                                         const Args&... args) {
+template <typename OutputIt, typename CompiledFormat, typename... Args>
+typename std::enable_if<(detail::is_output_iterator<
+                         OutputIt, typename CompiledFormat::char_type>::value&&
+                             std::is_base_of<detail::basic_compiled_format,
+                                             CompiledFormat>::value),
+                        format_to_n_result<OutputIt>
+>::type format_to_n(OutputIt out, size_t n,
+                    const CompiledFormat& cf, const Args&... args) {
   auto it =
       format_to(detail::truncating_iterator<OutputIt>(out, n), cf, args...);
   return {it.base(), it.count()};

include/fmt/core.h

@@ -1968,10 +1968,11 @@ inline void vprint_mojibake(std::FILE*, string_view, format_args) {}
 // GCC 8 and earlier cannot handle std::back_insert_iterator<Container> with
 // vformat_to<ArgFormatter>(...) overload, so SFINAE on iterator type instead.
 template <typename OutputIt, typename S, typename Char = char_t<S>,
-          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value)>
-OutputIt vformat_to(
+          bool enable = detail::is_output_iterator<OutputIt, Char>::value>
+auto vformat_to(
     OutputIt out, const S& format_str,
-    basic_format_args<buffer_context<type_identity_t<Char>>> args) {
+    basic_format_args<buffer_context<type_identity_t<Char>>> args) ->
+    typename std::enable_if<enable, OutputIt>::type {
   decltype(detail::get_buffer<Char>(out)) buf(detail::get_buffer_init(out));
   detail::vformat_to(buf, to_string_view(format_str), args);
   return detail::get_iterator(buf);
@@ -2023,10 +2024,10 @@ inline format_to_n_result<OutputIt> vformat_to_n(
  \endrst
  */
 template <typename OutputIt, typename S, typename... Args,
-          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, char_t<S>>::value)>
-inline format_to_n_result<OutputIt> format_to_n(OutputIt out, size_t n,
-                                                const S& format_str,
-                                                const Args&... args) {
+          bool enable = detail::is_output_iterator<OutputIt, char_t<S>>::value>
+inline auto format_to_n(OutputIt out, size_t n,
+                        const S& format_str, const Args&... args) ->
+    typename std::enable_if<enable, format_to_n_result<OutputIt>>::type {
   const auto& vargs = fmt::make_args_checked<Args...>(format_str, args...);
   return vformat_to_n(out, n, to_string_view(format_str), vargs);
 }

include/fmt/locale.h

@@ -51,10 +51,10 @@ inline OutputIt vformat_to(
 }
 
 template <typename OutputIt, typename S, typename... Args,
-          typename Char = char_t<S>,
-          FMT_ENABLE_IF(detail::is_output_iterator<OutputIt, Char>::value)>
-inline OutputIt format_to(OutputIt out, const std::locale& loc,
-                          const S& format_str, Args&&... args) {
+          bool enable = detail::is_output_iterator<OutputIt, char_t<S>>::value>
+inline auto format_to(OutputIt out, const std::locale& loc,
+                      const S& format_str, Args&&... args) ->
+    typename std::enable_if<enable, OutputIt>::type {
   const auto& vargs = fmt::make_args_checked<Args...>(format_str, args...);
   return vformat_to(out, loc, to_string_view(format_str), vargs);
 }