Why does a skalak cause a "divergent implicit expansion" error?

Question

In the following code, I am trying to get typeclass instances with formless. However, in the case of a more complex case class (which translates to a more complex HList), the compiler gives me a "divergent implicit extension", although it does not seem to allow the same type of implicit type twice. Maybe I am missing some other compiler rule?

(Fiddle: https://scalafiddle.io/sf/WEpnAXN/0 )

import shapeless._

trait TC[T]

sealed trait Trait1
case class SimpleClass(a: String) extends Trait1

sealed trait Trait2
case class ComplexClass(a: String, b: String) extends Trait2

object Serialization extends App {

    //Instances for HList
    implicit val hnilInstance: TC[HNil] = ???
    implicit def hconsInstance[H, T <: HList] (implicit t: TC[T]): TC[H :: T] = ???

    //Instances for CoProduct
    implicit val cnilInstance: TC[CNil] = ???
    implicit def cconsInstance[H, T <: Coproduct] (implicit h: TC[H], t: TC[T]): TC[H :+: T] = ???

    //Instances for Generic, relying on HNil & HCons
    implicit def genericInstance[T, H] (implicit g: Generic.Aux[T, H], t: TC[H]): TC[T] = ???

    the[TC[SimpleClass :+: CNil]]  //Works
    the[TC[Trait1]]                //Works
    the[TC[ComplexClass :+: CNil]] //Works
    the[TC[Trait2]]                //Fails with diverging implicit expansion
}

When trying to solve, the the[TC[Trait1]]compiler should do something like this:

TC[Trait1]
    Generic[Trait1]
    TC[SimpleClass :+: CNil]
        TC[SimpleClass]
            Generic[SimpleClass]
            TC[String :: HNil]
        TC[CNil]

, , . , case 2-, - - : Lazy, ?

TC[Trait2]
    Generic[Trait2]
    TC[ComplexClass :+: CNil]
        TC[ComplexClass]
            Generic[ComplexClass]
            TC[String :: String :: HNil]
        TC[CNil]

, directy.

Answer 1

, , , , , , println . 2.12 - dominates , :

overlaps(dtor1, dted1) && (dtor1 =:= dted1 || {
  val dtorC = complexity(dtor1)
  val dtedC = complexity(dted1)
  val result = dtorC > dtedC

  println(if (result) "Dominates:" else "Does not dominate:")
  println(s"$dtor (complexity: $dtorC)")
  println(s"$dted (complexity: $dtedC)")
  println("===========================")
  result
})

sbt publishLocal scalac :

Dominates:
TC[shapeless.::[String,shapeless.::[String,shapeless.HNil]]] (complexity: 7)
TC[shapeless.:+:[ComplexClass,shapeless.CNil]] (complexity: 6)
===========================

, TC String :: String :: HNil ( node ), ComplexClass :+: CNil ( ). , String :: String :: HNil ComplexClass :+: CNil, - , .

, , , , . :

case class Foo(i: Int) extends Trait2

, :

Does not dominate:
TC[shapeless.::[String,shapeless.::[String,shapeless.HNil]]] (complexity: 7)
TC[shapeless.:+:[ComplexClass,shapeless.:+:[Foo,shapeless.CNil]]] (complexity: 9)

, ComplexClass hlist Trait2, , Trait2 ( TC, ) .

, ,