Encoder derivation fails for deeply nested classes

I encountered what looks like a bug in circe 0.14.0-M7 and Scala 3.0.0. Right now, I cannot come up what exactly breaks the derivation. My assumption is it breaks when a class structure is “deeply” nested - when more derivations need to be generated.

import io.circe.{Encoder, Json}
import io.circe.syntax._
import io.circe.generic.semiauto._

object Main {
  /* If Encoder[Foo] is not defined, compilation fails with
  [error] 7 |  given Encoder[Dto] = deriveEncoder[Dto]
  [error]   |                       ^^^^^^^^^^^^^^^^^^
  [error]   |  cannot reduce summonFrom with
  [error]   |   patterns :  case given encodeA @ _:io.circe.Encoder[Seq[Main.Foo]]
  [error]   |               case given evidence$1 @ _:deriving.Mirror.Of[Seq[Main.Foo]]
  [error]   | This location contains code that was inlined from Derivation.scala:14
  [error]   | This location contains code that was inlined from Derivation.scala:38
  [error]   | This location contains code that was inlined from Derivation.scala:12
  [error]   | This location contains code that was inlined from Derivation.scala:50
  [error]   | This location contains code that was inlined from semiauto.scala:27

  If Endoder[Foo] is defined, compilation fails with:
  [error] 18 |  given Encoder[Foo] = deriveEncoder[Foo]
  [error]    |                       ^^^^^^^^^^^^^^^^^^
  [error]    |                     Maximal number of successive inlines (32) exceeded,
  [error]    |                     Maybe this is caused by a recursive inline method?
  [error]    |                     You can use -Xmax-inlines to change the limit.
  [error]    | This location contains code that was inlined from Main.scala:18
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from Derivation.scala:16
  [error]    | This location contains code that was inlined from Derivation.scala:38
  [error]    | This location contains code that was inlined from Derivation.scala:12
  [error]    | This location contains code that was inlined from Derivation.scala:50
  [error]    | This location contains code that was inlined from semiauto.scala:27
  [error] one error found
  [error] one error found
  [error] (Compile / compileIncremental) Compilation failed
  */


  given Encoder[Foo] = deriveEncoder[Foo]
  given Encoder[Dto] = deriveEncoder[Dto]

  case class Dto(elements: Seq[Foo])
  case class Foo(bar: Option[Bar])
  case class Bar(baz: Option[Baz])
  case class Baz(qux: Qux)
  case class Qux(a: Int)
}

Derivation for Seq[Foo] is already mentioned here.

[xuwei-k]

https://github.com/argonaut-io/argonaut/commit/ae11983a34986b9d9273295aad9c09a7079f95f0 Scala 3 recursive case classes example in another json library

[travisbrown]

@FabioPinheiro As far as I know there’s no regression here—this is a known issue with circe-generic’s fully automatic derivation on Scala 3 (see e.g. https://github.com/circe/circe/releases/tag/v0.14.0: “fully-automatic derivation does not currently support recursive case classes”).

There’s no timeline for this to be fixed. A fix is likely to fall out naturally from other planned improvements to the Shapeless-free generic derivation mechanism on Scala 3, but generic.auto support on Scala 3 in particular is unlikely to be a priority for me personally (unless someone wants to sponsor that work).