scala> val x = null
x: Null = null

scala> val y: AnyVal = x
<console>:8: error: type mismatch;
 found   : Null
 required: AnyVal
Note that implicit conversions are not applicable because they are ambiguous:
 both method RichException in object Predef of type (self: Throwable)RichException
 and method augmentString in object Predef of type (x: String)scala.collection.immutable.StringOps
 are possible conversion functions from Null to AnyVal
       val y: AnyVal = x
                       ^

scala> val n = ???
scala.NotImplementedError: an implementation is missing
  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:225)
  ... 40 elided

scala> def n = ???
n: Nothing

scala> def y: AnyVal = n
y: AnyVal