Here's a minimized reproduction based on scala/collection-strawman#23:

trait X[A] {
  def map[B](f: A => B): Unit
}

trait Y[A] extends X[A] {
  def map[B](f: A => B)(implicit ordering: Ordering[B]): Unit
}

trait Z[A] extends Y[A]

class StrawmanTest {
  (null: Y[Int]).map(x => x.toString) // compiles
  (null: Z[Int]).map(x => x.toString) // doesn't compile
}

This seems to hit a corner case or bug in scala/scala#5307. In the second case functionProto in handleOverloaded computes a WildcardType because the abstract type A is not instantiated as expected by tp.asSeenFrom in funArgTypes. In the first case it gets instantiated as Int.