This is required by the standard. unique_ptr<Gadget> in your example uses default_delete<Gadget> deleter, which in turn requires that type Gadget is complete at the point where this deleter is used, otherwise program is ill-formed.

Intention is to avoid undefined behaviour in situation where you delete an object with incomplete class type at the point of deletion, that has a non-trivial destructor or a deallocation function.

Edit: An deleter is used because of default generated copy constructor.