promiscuous.scala

scala> class Bippy
defined class Bippy

scala> val x = new Bippy
x: Bippy = Bippy@4919b727

scala> show"$x"
<console>:15: error: type mismatch;
 found   : Bippy
 required: psp.std.Shown[_]
              show"$x"
                    ^

scala> implicit val BippyShow = Show[Bippy](_ => "I'm Bippy!")
BippyShow: psp.std.Show[Bippy] = psp.std.Show$$anon$1@78146d52

scala> show"$x"
res1: String = I'm Bippy!