Exhaustiveness failure with type aliases

foo.scala

➜  scala-2.13.0-M5 ./bin/scala                                                                                                                                 [I]
Welcome to Scala 2.13.0-M5 (OpenJDK 64-Bit Server VM, Java 11.0.1).
Type in expressions for evaluation. Or try :help.

scala> :paste
// Entering paste mode (ctrl-D to finish)

sealed trait Color
case object Red extends Color
case object Blue extends Color
case object Green extends Color

type RGB = Color
val R: RGB = Red
val B: RGB = Blue
val G: RGB = Green

def go(c: RGB): Int = c match {
  case R => 0
  case G => 1
  case B => 2
}

// Exiting paste mode, now interpreting.

       def go(c: RGB): Int = c match {
                             ^
On line 11: warning: match may not be exhaustive.
       It would fail on the following inputs: Blue, Green, Red
defined trait Color
defined object Red
defined object Blue
defined object Green
defined type alias RGB
R: RGB = Red
B: RGB = Blue
G: RGB = Green
go: (c: RGB)Int