Denoting un-special-ness with observer_ptr<T>

We've now seen two or three different smart pointer types (depending on whether you count weak_ptr as a pointer type in its own right, or more like a ticket for a shared_ptr). Each of these types carries with it some useful source-level information about lifetime management. For example, just from the function signatures of these two C++ functions, what can we say about their semantics?

    void remusnoc(std::unique_ptr<Widget> p);

    std::unique_ptr<Widget> recudorp();

We see that remusnoc takes a unique_ptr by value, which means that ownership of the controlled object is transferred to remusnoc. When we call this function, we must have unique ownership of a Widget, and after we call this function, we will no longer be able to access that Widget. We don't know whether remusnoc is going to destroy the Widget, keep it around, or attach it to some other object or thread; but it's explicitly no longer our concern. The remusnoc...