`sealed` and `final` classes can be extended using SAM

[guyde2011]

This is pretty straight forward.

For example:

  val x: String <:< Int = _.toInt
  println(x("3") - 2)

Compiles and prints 1.

[allanrenucci]

Same for final abstract class:

scala> abstract final class Foo { def f(x: Int): Int }
// defined class Foo

scala> val foo: Foo = x => x
val foo: Foo = anon$1@42fc4ac4