Improve UTF-8 support

include/fmt/format-inl.h

@@ -1336,11 +1336,20 @@ FMT_FUNC void report_system_error(int error_code,
   report_error(format_system_error, error_code, message);
 }
 
+#ifndef FMT_UNICODE
+#  define FMT_UNICODE 0
+#endif
+
 FMT_FUNC void vprint(std::FILE* f, string_view format_str, format_args args) {
   memory_buffer buffer;
   internal::vformat_to(buffer, format_str,
                        basic_format_args<buffer_context<char>>(args));
+#if FMT_UNICODE
+  internal::utf8_to_utf16 u16(string_view(buffer.data(), buffer.size()));
+  std::fputws(u16.c_str(), f);
+#else
   internal::fwrite_fully(buffer.data(), 1, buffer.size(), f);
+#endif
 }
 
 FMT_FUNC void vprint(string_view format_str, format_args args) {