If one calls a Predictor or Transformer with a wrong input type, then the Scala compiler tries to apply the fallback implicit value for this operation type. However, since the return type of the operations is parameterized, it will infer it to be Nothing. The problem is then that the implicit value Operation[Self, Input, Nothing] cannot be unified with the implicit parameter Operation[Self, Input, Output]. This seems to be a known Scala issue https://issues.scala-lang.org/browse/SI-1570.
I propose to fix the output type of the implicit values to Any which will avoid that Nothing is inferred. This should solve the problem.