infer.scala

// Thanks for inferring types which I can't even access, scala.
scala> def f[A, CC[X]](xs: CC[A]) = xs
warning: there were 1 feature warning(s); re-run with -feature for details
f: [A, CC[X]](xs: CC[A])CC[A]

scala> f(scala.collection.immutable.BitSet(1))
res0: scala.collection.AbstractSet[Int] = BitSet(1)

scala> var x: scala.collection.AbstractSet[Int] = res0
<console>:9: error: class AbstractSet in package collection cannot be accessed in package collection
       var x: scala.collection.AbstractSet[Int] = res0
                               ^