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phischuon Dotty: a next generation compiler for Scala

The linked page links to a set of slides: http://www.slideshare.net/Odersky/compilers-are-databases

I have a question regarding slides 29 to 31.

The author says an untyped tree is represented as Tree[Nothing]. This leads to undesired variance behavior which is in turn overriden.

But there cannot be any instances of type Tree[Nothing] because there are no instances of type Nothing. Wouldn't an untyped tree be better represented as Tree[Unit]?