While working on Scala, I noticed a definition:

type ??? = Nothing

as seems it’s used this way

def myMethod = ??? //TODO

But, if I change this to

type ??? = Nothing
type ^^^ = Nothing
def myMethod = ^^^ //TODO

It actually fails compilation right on this line. Why?

Test 1

Remove type ??? = Nothing, then many other places fail compilation. The line is effective

Test 2

Try to trace impl of ??? present at def myMethod = ??? //TODO, found in predef:

  /** `???` can be used for marking methods that remain to be implemented.
   *  @throws NotImplementedError
   */
  def ??? : Nothing = throw new NotImplementedError

Now it’s clear. The line I’ve been seeing defines a type ??? aliasing Nothing type. The val ??? is defined in predef and therefore auto-imported. If I look closer, I would’ve noticed that the errors from removing type ??? = Nothing doesn’t include def myMethod = ??? //TODO

Scala compiler is smart enough to tell, always, whether a type or a value is used