false positive unreachable code warning

[Atry]

Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions for evaluation. Or try :help.

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

sealed trait Base {
  type A
  type B
}
object Base {
  type Aux[A0, B0] = Base {
    type A = A0
    type B = B0
  }
}

trait Child0 extends Base
trait Child1 extends Base

val base: Base.Aux[Float, Float] = new Child1 {
  override type A = Float
  override type B = Float
}
base match {
  case child0: Child0 =>
    throw new AssertionError()
  case child1: Child1 =>
    child1.ensuring(_ == base)
}

// Exiting paste mode, now interpreting.

<console>:35: warning: unreachable code
           child1.ensuring(_ == base)
                          ^
defined trait Base
defined object Base
defined trait Child0
defined trait Child1
base: Base.Aux[Float,Float] = $anon$1@71ad3d8a
res0: Base{type A = Float; type B = Float} with Child1 = $anon$1@71ad3d8a

The unreachable code mentioned by the warning message is actually reachable.

[Atry]

A workaround:

(base: { type A = Float; type B = Float}) match {
  case child0: Child0 =>
    throw new AssertionError()
  case child1: Child1 =>
    child1.ensuring(_ == base)
}

[SethTisue]

consolidating at #11457