scala> class M[A]
defined class M

scala> implicit def ma0[A](a: A): M[A] = null
warning: there was one feature warning; re-run with -feature for details
ma0: [A](a: A)M[A]

scala> implicit def ma1[A](a: A): M[A] = null
warning: there was one feature warning; re-run with -feature for details
ma1: [A](a: A)M[A]

scala> def convert[F[X <: F[X]]](builder: F[_ <: F[_]]) = 0
warning: there was one feature warning; re-run with -feature for details
convert: [F[X <: F[X]]](builder: F[_ <: F[_]])Int

scala> convert(Some[Int](0))
<console>:15: error: no type parameters for method convert: (builder: F[_ <: F[_]])Int exist so that it can be applied to arguments (Some[Int])
 --- because ---
argument expression's type is not compatible with formal parameter type;
 found   : Some[Int]
 required: ?F[_$1] forSome { type _$1 <: ?F[_$2] forSome { type _$2 } }
       convert(Some[Int](0))
       ^
<console>:15: error: type mismatch;
 found   : Some[Int]
 required: F[_ <: F[_]]
       convert(Some[Int](0))
                        ^

scala> Range(1,2).toArray: Seq[_]
<console>:14: error: polymorphic expression cannot be instantiated to expected type;
 found   : [B >: Int]Array[B]
 required: Seq[_]
       Range(1,2).toArray: Seq[_]
                  ^

scala> 0
res2: Int = 0

scala> :quit