This should be added on top of #3883. We should add Unit to [quoted.Liftable](https://github.com/dotty-staging/dotty/blob/9057b92e62c398e14dfa394f8b4451c8fb7b6944/library/src/scala/quoted/Liftable.scala). I should be encoded as a [ValueExpr](https://github.com/dotty-staging/dotty/blob/9057b92e62c398e14dfa394f8b4451c8fb7b6944/library/src/scala/quoted/Expr.scala#L34), which would require a modification to its [unpickling](https://github.com/dotty-staging/dotty/blob/9057b92e62c398e14dfa394f8b4451c8fb7b6944/compiler/src/dotty/tools/dotc/core/quoted/PickledQuotes.scala#L40). With this change, the following code should work ```scala val u: Expr[Unit] = () ```