All of unique_ptr::operator=(unique_ptr&&), shared_ptr(shared_ptr&&), and shared_ptr::operator=(shared_ptr&&) explicitly leave their source with .get()==nullptr. unique_ptr(unique_ptr&&)'s wording delegates that to the "transfer ownership" definition in [unique.ptr], without explicitly referring to the section of the definition, which caused me to miss that it was defined at all.

As far as I can see, that term is only used twice in the standard, for the unique_ptr move constructor and templated move constructor, and I believe its other 2 bullets are repeated by the existing postconditions for those operations. By adding u.get()==nullptr to those two sets of postconditions, we might be able to remove the whole definition of "transfer ownership".