message: Make sure that count keys are integral
diff: @@ -91,6 +91,11 @@ template <
  >
  void counting_sort(InputIt first, InputIt last, OutputIt out, Key key = Key())
  {
  + static_assert(
  + std::is_integral<decltype(key(*first))>::value,
  + "counting_sort requires integral key type"
  + );
  +
  unsigned cnt[K] = { 0 };
  for (auto it = first; it != last; ++it)
  