Why am I not being warned about a non-exhaustive match? - scala

Question

Compiling this code does not result in any warnings about exhaustive pattern matches.
package example
object Hello extends App {
class Problem extends Exception {
override def getCause() : Throwable = new NotImplementedError("I'm not done")
}
handle(new Problem)
def handle(ex: Exception): Unit = {
println("handler called: %s".format(ex))
val result = ex.getCause match {
case null => println("null")
case e : Exception => println("except")
// no warning about the match not being exhaustive
// case t : Throwable => println("throw")
}
}
}
When I run the code I get this stack trace:
scala.MatchError: scala.NotImplementedError: I'm not done (of class scala.NotImplementedError)
at example.Hello$.handle(Hello.scala:13)
at example.Hello$.delayedEndpoint$example$Hello$1(Hello.scala:9)
at example.Hello$delayedInit$body.apply(Hello.scala:3)
at scala.Function0.apply$mcV$sp(Function0.scala:39)
at scala.Function0.apply$mcV$sp$(Function0.scala:39)
at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
at scala.App.$anonfun$main$1(App.scala:76)
at scala.App.$anonfun$main$1$adapted(App.scala:76)
at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:563)
at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:561)
at scala.collection.AbstractIterable.foreach(Iterable.scala:926)
at scala.App.main(App.scala:76)
at scala.App.main$(App.scala:74)
at example.Hello$.main(Hello.scala:3)
at example.Hello.main(Hello.scala)
If I uncomment out the line case t : Throwable => println("throw"), the program prints throw.
Why isn't a warning printed to the console when I compile this code? Thanks!