std::packaged_task<R(Args...)>::operator= (3) - Linux Manuals
packaged_task& operator=( const packaged_task& ) = delete; (1) (since C++11)
packaged_task& operator=( packaged_task&& rhs ) noexcept; (2) (since C++11)
1) Copy assignment operator is deleted, std::packaged_task is move-only.
2) Releases the shared state, if any, destroys the previously-held task, and moves the shared state and the task owned by rhs into *this. rhs is left without a shared state and with a moved-from task.
rhs - the std::packaged_task to move from
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
LWG_2067 C++11 the deleted copy assignment operator took reference to non-const made const
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Reason: no example