Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> object X { implicit class Y(self: String) } // or object X { def Y(s: String): Int = 42 }
defined object X

scala> (null: Any) match { case X.Y.Z() => }
java.lang.NullPointerException
    at scala.tools.nsc.transform.patmat.ScalacPatternExpanders$ScalacPatternExpander$class.applyMethodTypes(ScalacPatternExpanders.scala:65)
    at scala.tools.nsc.transform.patmat.ScalacPatternExpanders$alignPatterns$.applyMethodTypes(ScalacPatternExpanders.scala:97)
    at scala.tools.nsc.transform.patmat.ScalacPatternExpanders$alignPatterns$.apply(ScalacPatternExpanders.scala:136)
    at scala.tools.nsc.transform.patmat.ScalacPatternExpanders$alignPatterns$.apply(ScalacPatternExpanders.scala:159)
    at scala.tools.nsc.transform.patmat.MatchTranslation$MatchTranslator$ExtractorCall$.apply(MatchTranslation.scala:384)

or without parens a different crash:

scala> object X { implicit class Y(self: String) }
defined object X

scala> (null: Any) match { case X.Y.Z => }
java.lang.NullPointerException
    at scala.tools.nsc.ast.TreeInfo$ValueClass$$anonfun$valueUnbox$1.apply(TreeInfo.scala:45)
    at scala.tools.nsc.ast.TreeInfo$ValueClass$$anonfun$valueUnbox$1.apply(TreeInfo.scala:45)
    at scala.reflect.internal.SymbolTable.enteringPhase(SymbolTable.scala:235)
    at scala.tools.nsc.ast.TreeInfo$ValueClass$.valueUnbox(TreeInfo.scala:45)
    at scala.tools.nsc.ast.TreeInfo$ValueClass$Unbox$.unapply(TreeInfo.scala:50)
    at scala.tools.nsc.ast.TreeInfo$ValueClass$BoxAndUnbox$.unapply(TreeInfo.scala:63)
    at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:37)
    at scala.tools.nsc.transform.PostErasure$PostErasureTransformer.transform(PostErasure.scala:22)