Workaround broken numeric_limites, part 2 (#1787)

include/fmt/format.h

@@ -724,13 +724,18 @@ class FMT_API format_error : public std::runtime_error {
 
 namespace detail {
 
+template <typename T>
+constexpr bool is_signed() {
+  return std::numeric_limits<T>::is_signed || std::is_same<T, int128_t>();
+}
+
 // Returns true if value is negative, false otherwise.
 // Same as `value < 0` but doesn't produce warnings if T is an unsigned type.
-template <typename T, FMT_ENABLE_IF(std::numeric_limits<T>::is_signed)>
+template <typename T, FMT_ENABLE_IF(is_signed<T>())>
 FMT_CONSTEXPR bool is_negative(T value) {
   return value < 0;
 }
-template <typename T, FMT_ENABLE_IF(!std::numeric_limits<T>::is_signed)>
+template <typename T, FMT_ENABLE_IF(!is_signed<T>())>
 FMT_CONSTEXPR bool is_negative(T) {
   return false;
 }