It's safe to use lazy val rather than val

lazy-val-works-with-override.scala

class Bar1 {
  lazy val a = "a"
  lazy val b = a + "b"
}

class Bar2 extends Bar1 {
  override lazy val a = "A"
}

(new Bar1).b  // ab                                                         
(new Bar2).b  // Ab                                                         

val-fails-in-override.scala

class Foo1 {
  val a = "a"
  val b = a + "b"
}

class Foo2 extends Foo1 {
  override val a = "A"
}

(new Foo1).b  // ab                                                         
(new Foo2).b  // nullb