ValueDiscard.scala

def lazyFun(aThing: => Unit) = {
  println("doing a thing")
  aThing
  println("done")
}

// correct
lazyFun(println("a thing"))
// incorrect - compiles thanks to value discard, but doesn't print
lazyFun(() => println("another thing"))

trait MyUnit
object MyUnit extends MyUnit

def lazyFun2(aThing: => MyUnit) = {
  println("doing a thing")
  aThing
  println("done")
}

// correct
lazyFun2({ println("a thing"); MyUnit })
// these won't compile
// lazyFun2(() => { println("another thing"); MyUnit })
// lazyFun2({ () => println("another thing"); MyUnit })