why is observer required to be copyable?

Trying to implement a very simple observer pattern. The observers are not copyable.

Problem: The compiler reports error that the copy constructor of the observer class is deleted and needed here: function(std::forward<_Functor>(__f)).swap(*this);

class subject {
  template <typename T>
  void registerObserver(T&& observer) {
    mNotifyTarget = std::forward<T>(observer);
  }

  std::function<void()> mNotifyTarget;
};

class observer {
  void operator()() { ... }
}

Not having experience with std::function I tried to remove the std::forward<T>() in the assignment, the problem remained.