message: fix: invariant too strict
diff: @@ -16,7 +16,7 @@ namespace details {
  invariant_greater_equal(start, 0);
  invariant_less(start, s.size());
  invariant_greater_equal(length, 0);
  - invariant_less(start + length, s.size());
  + invariant_less_equal(start + length, s.size());
  return s.substr(safe_cast<size_t>(start), safe_cast<size_t>(length));
  }
  