Misleading error for type class instances related to Future

[lambdista]

When using Future-related type classes you get a misleading error: could not find implicit value for parameter instance.... It's as if it cannot find an instance of the given type class for Future in scope when it actually cannot just find an ExecutionContext. Example, with a REPL session, for Functor:

scala> import scala.concurrent.Future
import scala.concurrent.Future

scala> import cats.Functor
import cats.Functor

scala> import cats.std.future._
import cats.std.future._

scala> Functor[Future].map(Future.successful(42))(_ + 1)
<console>:16: error: could not find implicit value for parameter instance: cats.Functor[scala.concurrent.Future]
       Functor[Future].map(Future.successful(42))(_ + 1)
              ^

but after importing the default ExecutionContext it works:

scala> import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.ExecutionContext.Implicits.global

scala> Functor[Future].map(Future.successful(42))(_ + 1)
res1: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@6335ab80

[ceedubs]

For reference, here is a Gitter conversation that happened about this.

[armanbilge]

Future instances are deprecated now via #4230.