move_only_function& operator=( move_only_function&& other );

move_only_function& operator=( const move_only_function& ) = delete;

move_only_function& operator=( std::nullptr_t ) noexcept;

template< class F > 
move_only_function& operator=( F&& f );

1) Moves the target of other to *this or destroys the target of *this (if any) if other is empty, by auto(std::move(other)).swap(*this). other is in a valid state with an unspecified value after move assignment.

2) Copy assignment operator is deleted. std::move_only_function does not satisfy CopyAssignable.

3) Destroys the current target if it exists. *this is empty after the call.

4) Sets the target of *this to the callable f, or destroys the current target if f is a null function pointer, a null pointer to member function, or an empty std::move_only_function, as if by executing move_only_function(std::forward<F>(f)).swap(*this);. This overload participates in overload resolution only if the constructor of move_only_function from F participates in overload resolution. The program is ill-formed or has undefined behavior if the selected constructor call is ill-formed or has undefined behavior.