Allow Singleton types in Union types

[odersky]

#1550 disallows singleton types in union types to avoid the problems uncovered by #829. But it would be good to allow them, if we can find a solid solution.

The current scheme is: (1) singleton types in unions are not allowed. (2) Therefore, when taking the lub, we widen every singleton type. Thats simple and consistent, but also restrictive.

[smarter]

Note that singleton types in unions can still slip through the cracks:

scala> def or[T](x: T | Int) = x 
or: [T](x: T | Int)T | Int
scala or["foo"]("bar") 
res13: String | Int = bar

It's probably not worth trying to disallow this case before we try to make singleton types in unions actually work.

[propensive]

I uncovered another valid -- I think -- use-case while experimentitng with enums, which doesn't involve singleton literal types.

This is intended to be a linked-list-like datastructure, but with alternating types, and it might make a good test case, but currently doesn't compile due to the restriction.

enum Fence[+T, +S] {
  case End
  case Post(value: T, next: Panel[T, S] | End.type)
  case Panel(value: S, next: Post[T, S])
}

import Fence._
val fence = Post(1, Panel("two", Post(3, End)))

[soronpo]

Would have been nice to do:
type Binary = 0 | 1
There are other ways to achieve this constraint, though.

[panacekcz]

Just a curiosity, the or["foo"]("bar") example doesn't compile anymore, but it is still possible to get unsoundness with literal types in a union.

object Get2As1 {
  class OrIntroFn[T, U, TU >: T|U]{
    type V = TU
    def tuToV(): (TU => V) = p => p
  }
  class Or11X[X] extends OrIntroFn[1&1, X, (1&1|X)]{
    def get2as11X:V = tuToV()(2)
  }
  class Or11Nothing extends Or11X[Nothing]
  def get2as1:1 = new Or11Nothing().get2as11X

  def main(a:Array[String]) = {
    println(get2as1) // prints 2
    val one:1 = get2as1
    println(one) // prints 1
  }
}