std::promise::operator=

From cppreference.com
promise& operator=( promise&& other );
(1) (since C++11)
promise& operator=( const promise& rhs ) = delete;
(2) (since C++11)

Assigns the contents.

1) Move assignment operator. Assigns the shared state of other using move semantics. other is in valid, but undefined state afterwards.
2) promise is not copy-assignable.

[edit] Parameters

other - another promise to acquire the state from

[edit] Return value

*this

[edit] Exceptions

1)
noexcept specification:  
noexcept
  (since C++11)