trait Type[T]: type Out type varchar given Type[varchar] with type Out = String class Placeholder[T, U] object Placeholder: def apply[T](using t: Type[T]): Placeholder[T, t.Out] = new Placeholder trait Encoder[P, X]: def encode(x: X): String object Encoder: def apply[P, X](placeholder: P)(using e: Encoder[P, X]): X => String = e.encode given [T, X]: Encoder[Placeholder[T, X], X] with def encode(x: X): String = ??? // the following compiles just fine Encoder(new Placeholder[varchar, String])("hello") // the following fails Encoder(Placeholder[varchar])("hello")