Specifying a type member in an implicit search.

implicit-by-member.scala

scala> trait Foo { type X }
defined trait Foo

scala> implicit object foo extends Foo { type X = String }
defined module foo

scala> implicitly[Foo { type X = String }]
res0: Foo{type X = String} = foo$@6e1641c4

scala> implicitly[Foo]
res1: Foo = foo$@6e1641c4

scala> implicitly[Foo { type X = Char }]
<console>:10: error: could not find implicit value for parameter e: Foo{type X = Char}
              implicitly[Foo { type X = Char }]
                        ^

scala> import shapeless._
import shapeless._

scala> implicit def bar[H <: HList](h: H)(implicit len: Length[H] { type Out = Nat._2 }) = len
bar: [H <: shapeless.HList](h: H)(implicit len: shapeless.Length[H]{type Out = shapeless.Nat._2})shapeless.Length[H]{type Out = shapeless.Nat._2}

scala> bar(1 :: HNil)
<console>:14: error: could not find implicit value for parameter len: shapeless.Length[shapeless.::[Int,shapeless.HNil]]{type Out = shapeless.Nat._2}
              bar(1 :: HNil)
                 ^

scala> bar(1 :: 'a :: HNil)
res4: shapeless.Length[shapeless.::[Int,shapeless.::[Symbol,shapeless.HNil]]]{type Out = shapeless.Nat._2} = shapeless.Length$$anon$20@4b7cbfd