allOf with multiple $refs attempts to do multiple inheritance, yields "Class needs to be a trait to be mixed"

I am trying to generate a client for this API:
https://raw.githubusercontent.com/aeternity/aeternity/master/config/swagger.yaml

with the following build.sbt:

```
lazy val root = (project in file("."))
.settings(
name := "ae-client",
libraryDependencies ++= Seq(
akka, akkaHttp, circe, akkaStream, circeGeneric, jawn, circeJava8, circeJawn, circeParser,
catsEffect, http4sCirce, http4sClient, http4sCore, http4sDsl, http4sServer,
scalaTest % Test
)
)

guardrailTasks in Compile := List(
Client(file("/Users/aa/dev/git/Aethernity/ae-client/swagger.yaml"), framework="http4s"),
)

scalacOptions += "-Ypartial-unification"
```

Dependencies.scala
```
object Dependencies {
val circeVersion = "0.11.1"
val http4sVersion = "0.20.0-M7"

lazy val scalaTest = "org.scalatest" %% "scalatest" % "3.0.5"
lazy val akka = "com.typesafe.akka" %% "akka-actor" % "2.5.21"
lazy val akkaHttp = "com.typesafe.akka" %% "akka-http" % "10.1.7"
lazy val akkaStream = "com.typesafe.akka" %% "akka-stream" % "2.5.21"
lazy val circe = "io.circe" %% "circe-core" % circeVersion
lazy val circeGeneric = "io.circe" %% "circe-generic" % circeVersion
lazy val circeParser = "io.circe" %% "circe-parser" % circeVersion
lazy val circeJava8 = "io.circe" %% "circe-java8" % circeVersion
lazy val jawn = "org.typelevel" %% "jawn-parser" % "0.14.1"
lazy val circeJawn = "io.circe" %% "circe-jawn" % circeVersion
lazy val catsEffect = "org.typelevel" %% "cats-effect" % "1.2.0"
lazy val http4sCore = "org.http4s" %% "http4s-core" % http4sVersion
lazy val http4sServer = "org.http4s" %% "http4s-blaze-server" % http4sVersion
lazy val http4sCirce = "org.http4s" %% "http4s-circe" % http4sVersion
lazy val http4sDsl = "org.http4s" %% "http4s-dsl" % http4sVersion
lazy val http4sClient = "org.http4s" %% "http4s-blaze-client" % http4sVersion
}
```

I am not sure whether I have to specify all dependencies which are used by http4s template. It seems I have to.

In result, sbt plugin fails with lots of compilation errors. The first error is:
```
[error] /Users/aa/dev/git/Aethernity/ae-client/target/scala-2.12/src_managed/main/swagger/definitions/Channel.scala:5:187: not found: type EncodedHash
[error] case class Channel(id: String, initiatorId: String, responderId: String, channelAmount: Long, initiatorAmount: Long, responderAmount: Long, channelReserve: Long, delegateIds: IndexedSeq[EncodedHash] = IndexedSeq.empty, stateHash: String, round: Long, soloRound: Long, lockPeriod: Long, lockedUntil: Long)
[error] ^
[error] /Users/aa/dev/git/Aethernity/ae-client/target/scala-2.12/src_managed/main/swagger/definitions/ChannelCloseMutualTxJSON.scala:5:216: class ChannelCloseMutualTx needs to be a trait to be mixed in
[error] case class ChannelCloseMutualTxJSON(version: Long, channelId: String, fromId: String, initiatorAmountFinal: Long, responderAmountFinal: Long, ttl: Option[Long] = None, fee: Long, nonce: Long) extends GenericTx with ChannelCloseMutualTx

```

P.S. original issue: https://github.com/twilio/sbt-guardrail/issues/11