HList foldLeft with tuple as zero

You want implicits to do too much work in a single step.

Try to add one more type parameter Out

implicit def genericInstance[T, HL <: HList, Out, LL <: HList](
  gen: Generic.Aux[T, HL],
  myTypeclass: Lazy[MyTypeclass[HL]],
  //folder: LeftFolder.Aux[HL, (HNil, Int), leftFolder.type, (LL, Int)],
  folder: LeftFolder.Aux[HL, (HNil, Int), leftFolder.type, Out],
  ev: Out <:< (LL, Int), // added
  reverse: Reverse.Aux[LL, HL]
): MyTypeclass[T] = (t: T) => {
  val generic = gen.to(t)
  val (transformed, idx) = myTypeclass.value.doSomething(generic)
  //val (ll, _) = transformed.foldLeft((HNil: HNil, 0))(leftFolder)
  val (ll, _) = ev(transformed.foldLeft((HNil: HNil, 0))(leftFolder))
  val reversed = reverse(ll)
  (gen.from(reversed), idx)

Read about over-constrained implicits:

https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:type-level-programming:chaining (4.3 Chaining dependent functions)

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