std::shared_future::operator=

From cppreference.com
shared_future& operator=(const shared_future& rhs);
(1) (since C++11)
shared_future& operator=(shared_future&& rhs) noexcept;
(2) (since C++11)
shared_future& operator=(future<R>&& other);
(3) (since C++11)
shared_future& operator=(shared_future&& other);
(4) (since C++11)