Can't summon Generic instance for T within T's companion object: error super constructor cannot be passed a self reference

[neko-kai]

As an example, the following code breaks:

import shapeless._

case class Abc(x: Int)

object Abc extends WithGeneric[Abc]

abstract class WithGeneric[T: Generic] {
  val generic: Generic[T] = implicitly[Generic[T]]
}

with error:

Error:(9, 21) super constructor cannot be passed a self reference unless parameter is declared by-name
object Abc extends WithGeneric[Abc]
                   ^

Note that changing the name of the object makes it work:

object NotAbc extends WithGeneric[Abc]

Our use case for this is that we want to be able to define helper traits that provide circe & pureconfig codec instances for case classes:

abstract class WithCirce[A: DerivedEncoder: DerivedDecoder] {
  implicit val enc: Encoder[A] = implicitly[DerivedEncoder[A]]
  implicit val dec: Decoder[A] = implicitly[DerivedDecoder[A]]
}

Both circe and pureconfig are based on shapeless, so right now we can't write this and have to workaround by using Cached[Lazy[_]]. Non-shapeless based derivations work as expected however.

[milessabin]

The trouble is the materialized Generic[T], where T is instantiated at a case class, will embed a reference to the case class companion. Hence your construct object Abc extends WithGeneric[Abc] is self-referential in a way that's not easily fixed.

It's just conceivable that byname implicits in 2.13.x will make it possible to make some progress, but for now I don't think there's a lot that can be done.

[joroKr21]

It's just conceivable that byname implicits in 2.13.x will make it possible to make some progress, but for now I don't think there's a lot that can be done.

No progress unfortunately

[joroKr21]

This could be mitigated if case classes use constructors and field access instead of the companion's apply / unapply but it's tricky to do. I remember attempting that at some point and it didn't work.