message: Since combine is no longer destructive, we always need to copy
diff: @@ -70,10 +70,6 @@ template <class T> struct QuantileOperation {
  }
  template <class STATE, class OP> static void Combine(STATE source, STATE *target) {
  - if (target->len == 0) {
  - *target = source;
  - return;
  - }
  resize_state(target, target->pos + source.pos);
  memcpy(target->v + target->pos * sizeof(T), source.v, source.pos * sizeof(T));
  target->pos += source.pos;
  