annotations ignored?

[NeQuissimus]

Are the annotations not supposed to mark code as "Don't worry, Shield"?

I have this code

  import Scalaz._

  implicit class MatcherOps[A: Equal](lhs: A) {
    @impure
    def <=>(rhs: A): Unit = Predef.assert(lhs === rhs, s"${lhs} != ${rhs}")
  }

Despite the annotation it will report error: possibly impure