Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions for evaluation. Or try :help.

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

trait Converter {
  type To
  type From
}
object Converter {
  type Aux[-From0, +To0] = Converter {
    type To <: To0
    type From >: From0
  }
}
trait Outer {
  trait InnerLike { this: Inner =>
    type To
    type From
    type F = Converter.Aux[From, To]
  }
  type Inner <: InnerLike
  def cast(inner: Inner)(f: inner.F): Inner#F = f
}

// Exiting paste mode, now interpreting.

<console>:28: error: type mismatch;
 found   : inner.F
    (which expands to)  Converter{type To <: inner.To; type From >: inner.From}
 required: Converter{type To <: _2.To; type From >: _2.From} forSome { val _2: Outer.this.Inner }
         def cast(inner: Inner)(f: inner.F): Inner#F = f
                                                       ^