Question

List(1,2) match {
  case List(1,_) => println("1 in postion 1")
  case _ => println("default")
}

compiles / works fine.
So do

List(1) match ...
List(3,4,5) match ...

but not

List() match ...

which results in the following error

found : Int(1)
required : Nothing
             case List(1,_) => println("1 in postion 1")

Why does List() try to match List(1,_)?

Answer 1

When you write List(), the type inferred is Nothing, which is subtype to everything.

What is happening is that Scala gives an error when you try impossible matches. For example, "abc" match { case 1 => } will result in a similar error. Likewise, because List(1, _) can be statically determined to never match List(), Scala gives an error.