lower bounds make the wheels come off

[scabug]

"inferred type arguments do not conform", you don't say. Hey scalac, that's what you get when you go around inferring polytypes!

scala> def f[T, CC[X] <: Traversable[X], DD[X] >: CC[X]](xs: CC[T])(implicit cbf: collection.generic.CanBuildFrom[Traversable[T], T, DD[T]]): DD[T] = xs.map(x => x)
warning: there were 2 feature warnings; re-run with -feature for details
f: [T, CC[X] <: Traversable[X], DD[X] >: CC[X]](xs: CC[T])(implicit cbf: scala.collection.generic.CanBuildFrom[Traversable[T],T,DD[T]])DD[T]

scala> f(1 to 10 toStream)
<console>:9: error: inferred type arguments [Int,scala.collection.immutable.Stream,[X]scala.collection.immutable.Stream] do not conform to method f's type parameter bounds [T,CC[X] <: Traversable[X],DD[X] >: CC[X]]
              f(1 to 10 toStream)
              ^
<console>:9: error: type mismatch;
 found   : scala.collection.immutable.Stream[Int]
 required: CC[T]
              f(1 to 10 toStream)
                        ^
<console>:9: error: Cannot construct a collection of type DD[T] with elements of type T based on a collection of type Traversable[T].
              f(1 to 10 toStream)
               ^

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6717?orig=1 Reporter: @paulp