Demonstration of strange need for Lazy

gistfile1.scala

import shapeless.{::, Generic, HNil, Lazy}

case class StringWrapper(s: String)

trait F[W, V]

object F {
  implicit def genWV[W, V](implicit
                           gen: Lazy[Generic.Aux[W, V :: HNil]]
                           // Remove the Lazy and the Test object below won't compile
                           // gen: Generic.Aux[W, V :: HNil]
                          ): F[W, V] = new F[W, V] {}
}

trait G[V]

object G {
  implicit def genG[W, V](implicit
                          f: F[W, V],
                          g: G[V]
                         ): G[W] = new G[W] {}

  implicit val fString = new G[String] {}
}

object Test {
  val g = implicitly[G[StringWrapper]]
}