You just... Blew my mind.
So, but wait... Is the genitive "of water" functioning like an assertion? "All of the contents of the cup are water."
And wouldn't that assertion, in the case where the cup has no contents, be a vacuous truth?[0]
In other words, a cup of water, if it actually has water in it, cannot also be a cup of magma. But there's nothing stopping an cup with no contents at all being "of" both.