Incorrect scalac warnings on aliased java enum

scala_java_enums.scala

Welcome to Scala 2.12.3 (OpenJDK 64-Bit Server VM, Java 1.8.0_181).
Type in expressions for evaluation. Or try :help.

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

type              TU = java.util.concurrent.TimeUnit
val NANOSECONDS:  TU = java.util.concurrent.TimeUnit.NANOSECONDS
val MICROSECONDS: TU = java.util.concurrent.TimeUnit.MICROSECONDS
val MILLISECONDS: TU = java.util.concurrent.TimeUnit.MILLISECONDS
val SECONDS:      TU = java.util.concurrent.TimeUnit.SECONDS
val MINUTES:      TU = java.util.concurrent.TimeUnit.MINUTES
val HOURS:        TU = java.util.concurrent.TimeUnit.HOURS
val DAYS:         TU = java.util.concurrent.TimeUnit.DAYS


// Now define a match over all cases
def foo(x: TU): String = x match {
  case NANOSECONDS => "ns"
  case MICROSECONDS => "micros"
  case MILLISECONDS => "ms"
  case SECONDS => "s"
  case MINUTES => "mins"
  case HOURS => "hr"
  case DAYS => "days"
}

// Exiting paste mode, now interpreting.

<console>:20: warning: match may not be exhaustive.
It would fail on the following inputs: DAYS, HOURS, MICROSECONDS, MILLISECONDS, MINUTES, NANOSECONDS, SECONDS
       def foo(x: TU): String = x match {
                                ^
defined type alias TU
NANOSECONDS: TU = NANOSECONDS
MICROSECONDS: TU = MICROSECONDS
MILLISECONDS: TU = MILLISECONDS
SECONDS: TU = SECONDS
MINUTES: TU = MINUTES
HOURS: TU = HOURS
DAYS: TU = DAYS
foo: (x: TU)String

         
/* BUT! Non-aliased enums */         
         
Welcome to Scala 2.12.3 (OpenJDK 64-Bit Server VM, Java 1.8.0_181).
Type in expressions for evaluation. Or try :help.

scala> import java.util.concurrent.TimeUnit._
import java.util.concurrent.TimeUnit._

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

def foo(x: java.util.concurrent.TimeUnit): String = x match {
  case NANOSECONDS => "ns"
  case MICROSECONDS => "micros"
  case MILLISECONDS => "ms"
  case SECONDS => "s"
  case MINUTES => "mins"
  case HOURS => "hr"
  case DAYS => "days"
}

// Exiting paste mode, now interpreting.

foo: (x: java.util.concurrent.TimeUnit)String