Scala, pattern matching and Seq <-> List variance.

gistfile1.txt

scala> def withList[T](x: List[T]) = x match { case Seq(1, 2, 3) => 1 }                         
withList: [T](x: List[T])Int                                                                    
                                                                                                
scala> withList(List(1, 2, 3))                                                                  
res6: Int = 1                                                                                   
                                                                                                
scala> withList(Seq(1, 2, 3))                                                                   
<console>:9: error: type mismatch;                                                              
 found   : Seq[Int]                                                                             
 required: List[?]                                                                              
              withList(Seq(1, 2, 3))                                                            
                          ^                                                                     
                                                                                                
scala> def withSeq[T](x: Seq[T]) = x match { case List(1, 2, 3) => 1 }                          
<console>:7: warning: match is not exhaustive!                                                  
missing combination              *           Nil             *             *                    
                                                                                                
       def withSeq[T](x: Seq[T]) = x match { case List(1, 2, 3) => 1 }                          
                                   ^                                                            
warning: there were 1 unchecked warnings; re-run with -unchecked for details                    
withSeq: [T](x: Seq[T])Int                                                                      
                                                                                                
scala> withSeq(List(1, 2, 3))                                                                   
res8: Int = 1                                                                                   
                                                                                                
scala> withSeq(Seq(1, 2, 3))                                                                    
res9: Int = 1