scala error self type

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait A { def x = "foo" }
object A { self: A => println(self.x) }

// Exiting paste mode, now interpreting.

<console>:8: error: type mismatch;
 found   : A.type
 required: ?{def x: ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method any2Ensuring in object Predef of type [A](x: A)Ensuring[A]
 and method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A]
 are possible conversion functions from A.type to ?{def x: ?}
       object A { self: A => println(self.x) }
                                     ^