Uninitialized AnyVals sneaking past your validation

gistfile1.scala

final class Foo private (val value: Int) extends AnyVal {
  override def toString = value.toString
}
object Foo {
  def apply(value: Int) = {
    assert(value > 0)
    new Foo(value)
  }
}

val x: Foo = _
// scala> x = 0

case class FooBar(foo: Foo)
class UninitializedAnyValsInConstructor {
  val bar: FooBar = FooBar(x)
  val x: Foo = Foo(1)
}

val y = new UninitializedAnyValsInConstructor
// y.bar == FooBar(Foo(0))