How to convince the compile that the variance is safe

FixingVarianceErrors.scala

object FixingVarianceErrors {

  trait InvariantThing[T]

  trait CovariantThing[+T] {

    // Fails to compile with error:
    // Error:(12, 9) covariant type T occurs in invariant position in
    // type => FixingVarianceErrors.InvariantThing[T] of method thing1
    def thing1: InvariantThing[T]

    protected[this] def thing2: InvariantThing[T]
  }
}

Neat little trick I noticed being used in Scala collections. I had no idea that this was even possible.