trait A { def hi = "A" }
trait B { def hi = "B" }
class C extends A with B

<console>:9: error: overriding method hi in trait A of type => String;
 method hi in trait B of type => String needs `override' modifier
       class C extends A with B
             ^

scala> trait B { override def hi = "B" }
<console>:7: error: method hi overrides nothing
       trait B { override def hi = "B" }
                              ^

class C extends A with B {
  override def hi = "C"
}