N3797 20.8.1.2.5 [unique.ptr.single.modifiers]/4 says: "assigns p to the stored pointer, and then if the old value of the stored pointer, old_p, was not equal to nullptr, calls get_deleter()(old_p)."

It should say "if and only if" in order to be absolutely clear that the deleter won't be called with a null pointer.

(I believe that the intention was obvious, so this is editorial. Note that 20.8.1.2.2 [unique.ptr.single.dtor]/2's phrasing is unambiguous: "If get() == nullptr there are no effects. Otherwise get_deleter()(get()).")