Using -XEmptyDataDecls I can declare an (almost) empty data type.

  data Void

However, I can't use an empty case to discriminate values of the empty type.

This does not work:

  get :: Void -> a

Only the type signature, but no body. [This would be useful also in other circumstances.]

  get :: Void -> a
  get v = case v of { }

An empty case in the body.