abstractTypeClass.scala

trait A[T] {
  type F[_]
}

trait B1[T] extends A[T]

object B1 {
  type Aux[T, Fʹ[_]] = B1[T] { type F[A] = Fʹ[A] }
}

trait B2[T] extends A[T]

object B2 {
  type Aux[T, Fʹ[_]] = B2[T] { type F[A] = Fʹ[A] }
}

trait C[T] extends B1[T] with B2[T]

object C {
  type Aux[T, Fʹ[_]] = C[T] { type F[A] = Fʹ[A] }
}

object Test {
  def b1[T, F[_]](implicit ev: B1.Aux[T, F]): Unit = ()

  def b2[T, F[_]](implicit ev: B2.Aux[T, F]): Unit = ()

  def c[T, F[_]](implicit ev: C.Aux[T, F]): (Unit, Unit) = (b1, b2)
}

// [error] abstractTypeClass.scala:28: could not find implicit value for parameter ev: B1.Aux[T,F]
// [error]   def c[T, F[_]](implicit ev: C.Aux[T, F]): (Unit, Unit) = (b1, b1)
// [error]                                                             ^
// [error] abstractTypeClass.scala:28: could not find implicit value for parameter ev: B2.Aux[T,F]
// [error]   def c[T, F[_]](implicit ev: C.Aux[T, F]): (Unit, Unit) = (b1, b2)
// [error]                                                                 ^

If I try to pass ev explicitly, I get the following:

[error] /Users/greg/Documents/SlamData/quasar/frontend/src/main/scala/quasar/abstractTypeClass.scala:46: inferred kinds of the type arguments (T,this.F[A]) do not conform to the expected kinds of the type parameters (type T,type F).
[error] this.F[A]'s type parameters do not match type F's expected parameters:
[error] type F has one type parameter, but type F has one
[error]   def c[T, F[_]](implicit ev: C.Aux[T, F]): (Unit, Unit) = (b1(ev), b2(ev))
[error]                                                             ^

“type F has one type parameter, but type F has one” is an interesting error …