Conversion to opaque alias of singleton type not allowed in the companion

[panacekcz]

When an opaque type is an alias of a singleton type (literal or term), then in the companion object, the conversion from the singleton type to the alias type does not work. The conversion in the other direction works.

Test code 1:

object A{
  opaque type T = 3
  object T {
    val x: T = 3
  }
}

Output

-- [E007] Type Mismatch Error: opaque-singleton.scala:4:15 ---------
4 |    val x: T = 3
  |               ^
  |               Found:    Int(3)
  |               Required: A.T.T'
  |
  |               where:    T  is a object in object A
  |                         T' is a type in object T
one error found

Test code 2:

object O{
  val x = 3
  opaque type T = x.type
  object T {
    def wrap(a: x.type): T = a   // Error
    def unwrap(a: T): x.type = a // OK
  }
}

[odersky]

This has to do with the fact that we don't allow yet singleton types in unions. Let's try to change that.