{n => n + 1} and {case n => n +1} are almost but not quite the same thing

function_vs_partial_function.scala

scala> val f: Function[Int, Int] = { n => n + 1 }
f: Function[Int,Int] = <function1>

scala> val f: Function[Int, Int] = { case n => n + 1 }
f: Function[Int,Int] = <function1>

scala> val pf: PartialFunction[Int, Int] = { n => n + 1 }
<console>:10: error: type mismatch;
 found   : Int => Int
 required: PartialFunction[Int,Int]
       val pf: PartialFunction[Int, Int] = { n => n + 1 }
                                               ^

scala> val pf: PartialFunction[Int, Int] = { case n => n + 1 }
pf: PartialFunction[Int,Int] = <function1>

// {n => n + 1} is a function
// {case n => n + 1} is a partial function, even though it only has one case that always matches