Circe auto generic derivation causes diverging implicit expansion of `HlistToFunc`

[zarthross]

I'll try and put in an example next week, but it appears the fix for #216 does break using circe's fully automatic derivation. In the mean time, the work around is just using the semiauto derivation instead.

[Igosuki]

The fix I currently have (which is of course unacceptable) :

  import authContext._
  import swaggerSyntax._
  import io.circe.generic.auto._
  import org.http4s.circe._
  import org.http4s.rho.bits._

Importing in that order enables circe macros to work

[Igosuki]

Ok, actually I just got rid of all the errors by just extending CirceInstances at package level so I can now remove all these imports and just keep

import authContext._
import swaggerSyntax._

[danxmoran]

I see the same error (or a very similar one) when using semiauto derivation with circe-derivation:

// build.sbt
scalaVersion := "2.12.8"

scalacOptions += "-Ypartial-unification"

libraryDependencies := Seq(
  "io.circe" %% "circe-core" % "0.11.1",
  "io.circe" %% "circe-derivation" % "0.11.0-M1",
  "org.http4s" %% "http4s-circe" % "0.20.0-M6",
  "org.http4s" %% "http4s-dsl" % "0.20.0-M6",
  "org.http4s" %% "rho-swagger" % "0.19.0-M6",
  "org.typelevel" %% "cats-effect" % "1.2.0"
)

// src/main/scala/Example.scala
import cats.effect.IO
import io.circe.Encoder
import io.circe.derivation.deriveEncoder
import org.http4s.circe.CirceEntityEncoder
import org.http4s.rho.RhoRoutes

object Example extends CirceEntityEncoder {

  case class Response(a: String)
  object Response {
    implicit val encoder: Encoder[Response] = deriveEncoder
  }

  val routes = new RhoRoutes[IO] {
    GET / "example" |>> { () =>
      if (true) {
        Ok(Response("foo"))
      } else {
        InternalServerError(Response("bar"))
      }
    }
  }
}

As mentioned in #216 (comment), turning off partial unification gets things to compile, but that's not feasible in my full project. The two other workarounds I've found are:

1. Explicitly call .asJson on each response. This fixes compilation, but leaves the model's info out of the eventually-generated Swagger documentation.
2. Copy-paste the derivation rule provided by CirceEntityEncoder, but with pinned type parameters:

   // This works
   import cats.effect.IO
   import io.circe.Encoder
   import io.circe.derivation.deriveEncoder
   import org.http4s.EntityEncoder
   import org.http4s.circe.CirceInstances
   import org.http4s.rho.RhoRoutes
   
   object Example extends CirceInstances {
   
     case class Response(a: String)
     object Response {
       implicit val encoder: Encoder[Response] = deriveEncoder
     }
   
     val routes = new RhoRoutes[IO] {
       GET / "example" |>> { () =>
         implicit val e: EntityEncoder[IO, Response] =
           jsonEncoderOf[IO, Response]
   
         if (true) {
           Ok(Response("foo"))
         } else {
           InternalServerError(Response("bar"))
         }
       }
     }
   }

[PawelJ-PL]

Currently I'm using solution 2. provided by @danxmoran and model is generating properly