void resize(size_type count, T value = T()) {
    if (count <= N) {
      // new `count` of elements completely fit on stack
      if (size_ >= N) {
        // currently, all data on heap
        // move back to stack

        /////////////////////
        // the bug is here //
        /////////////////////
        std::move(heap_.begin(), heap_.end(), stack_.begin());
      } else {
        // all data already on stack
        // just update size
      }
    } else {
      // new `count` of data is going to be on the heap
      // check if data is currently on the stack
      if (size_ < N) {
        // move to heap
        std::move(stack_.begin(), stack_.end(), std::back_inserter(heap_));
      }
      heap_.resize(count, value);
    }
    size_ = count;
  }

struct Elem {
  int data;
  T(int data_) : data{ data_ } {}
};

small_vector<T, 4> elems; // error