パターンマッチの網羅性チェックとifガード節

pattaernMatch.scala

scala> def func(s:Option[String]) = s match{case Some(x) if x.size > 0 => x}
func: (s: Option[String])String

scala> def func(s:Option[String]) = s match{case Some(x) => x}
<console>:7: warning: match may not be exhaustive.
It would fail on the following input: None
       def func(s:Option[String]) = s match{case Some(x) => x}
                                    ^
func: (s: Option[String])String