Allow variables in pattern alternatives using union types

[Lasering]

For the following ADT hierarchy:

sealed trait Foo
case object Empty extends Foo
case class JustList(list: List[Int]) extends Foo
case class PathAndList(path: String, list: List[Int]) extends Foo

The following code could compile:

val a: Foo = new JustList(List(1, 2, 3))
a match {
 case JustList(list) | PathAndList(_, list) => println(list)
 case Empty => println("Empty")
}

Where the variable list would type check to List[Int] | List[Int] which could be further reduced to List[Int].
Currently the compiler issues the error Illegal variable list in pattern alternative.

The remaining scenarios could be type checked as follows:

• case JustList(list) | PathAndList(list, _) => would typecheck to List[Int] | String
• case JustList(list) | PathAndList(_, _) => would typecheck to List[Int] | Null

[smarter]

Duplicate of #12