Broken backward compatibility in CatsInterop since v2.7.0

[Sergey Torgashov]

Consider a reproducer:

import caliban.interop.cats.implicits._
import caliban.schema.Schema.auto._
import caliban.{RootResolver, graphQL}
import cats.effect.std.{Console, Dispatcher}
import cats.effect.{IO, IOApp}
import zio.Runtime

case class Queries(one: IO[String])

object ExampleApp extends IOApp.Simple {
  implicit val zioRuntime: Runtime[Any] = Runtime.default

  override def run: IO[Unit] =
    Dispatcher.parallel[IO].use { implicit dispatcher =>
      val rendered = graphQL(RootResolver(Queries(IO.pure("")))).render

      Console[IO].println(rendered)
    }
}

With Caliban versions up to v2.6.0 (inclusive), the output is the following:

schema {
  query: Queries
}
type Queries {
  one: String!
}

Starting with Caliban v2.7.0 the output changes to this one:

schema {
  query: Queries
}
type Queries {
  one: String
}

I.e. The one query suddenly becomes nullable (even though no changes to the model was made).
Works the same for both Scala 2.13 and 3.3.

[Sergey Torgashov]

I believe the culprit is this line:

caliban/interop/cats/src/main/scala/caliban/interop/cats/CatsInterop.scala

Line 195 in c0ac37c

 override def canFail: Boolean  = true 

introduced in #2180

I.e. it forces canFail to true regardless of the underlying Schema and somehow makes the query rendered as nullable even if the corresponsing nullable property is set to false.

If I change canFail to

      override def canFail: Boolean  = ev.canFail

then the model above becomes rendering as is used to be prior to v2.7.0.