Why does `Some(123).isInstanceOf[Option[List[String]]]` *not* give an unchecked warning?

When using .isInstanceOf[GenericType[SomeOtherType]], where GenericType and SomeOtherType are arbitrary types (of suitable kind), the Scala compiler gives an unchecked warning due to type erasure:

scala> Some(123).isInstanceOf[Option[Int]]
<console>:8: warning: non variable type-argument Int in type Option[Int] is unchecked since it is eliminated by erasure
              Some(123).isInstanceOf[Option[Int]]
                                    ^
res0: Boolean = true

scala> Some(123).isInstanceOf[Option[String]]
<console>:8: warning: non variable type-argument String in type Option[String] is unchecked since it is eliminated by erasure
              Some(123).isInstanceOf[Option[String]]
                                    ^
res1: Boolean = true

However, if SomeOtherType is itself a generic type (e.g. List[String]), no warning is emitted:

scala> Some(123).isInstanceOf[Option[List[String]]]
res2: Boolean = true

scala> Some(123).isInstanceOf[Option[Option[Int]]]
res3: Boolean = true

scala> Some(123).isInstanceOf[Option[List[Int => String]]]
res4: Boolean = true

scala> Some(123).isInstanceOf[Option[(String, Double)]]
res5: Boolean = true

scala> Some(123).isInstanceOf[Option[String => Double]]
res6: Boolean = true

(recall that tuples and => are syntactic sugar for Tuple2[] and Function2[] generic types)

Why is no warning emitted? (All these are in the Scala REPL 2.9.1, with the -unchecked option.)