message: function_ref: remove unnecessary defaulted copy constructors
diff: @@ -107,8 +107,6 @@ private:
  }
  public:
  function_ref() = default;
  - function_ref(function_ref &) = default;
  - function_ref(function_ref const &) = default;
  template<typename T>
  function_ref(T const &t)
  : data_(&t)
  