Implicit conversion does not work for mix-in types

[Atry]

Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_152).
Type in expressions for evaluation. Or try :help.

scala> type A = {
     |   def self: Option[String]
     | }
defined type alias A

scala> type B = {
     |   def self: Option[AnyRef]
     | }
defined type alias B

scala> def convert(ab: A with B) = ab.ensuring { _.isInstanceOf[AnyRef] }
<console>:13: error: value ensuring is not a member of A with B
       def convert(ab: A with B) = ab.ensuring { _.isInstanceOf[AnyRef] }
                                      ^

Note that ensuring is an extension method defined in Predef