Type Inference

Demo.scala

import cats.data.EitherT
import cats.instances.future._
import cats.syntax.all._
import scala.concurrent.{ExecutionContext, Future}

/*
 * Compiled with:
 * Scala 2.12.4
 * cats-core 1.0.0-RC1
 * -Ypartial-unification
 */

object Demo {
  implicit val ec: ExecutionContext = ExecutionContext.global

  type Result[A] = EitherT[Future, String, A]

  val works: Result[Int] = 5.pure[Result]

  val fails: Result[Int] = 5.pure
  // [error] Demo.scala:13: type mismatch;
  // [error]  found   : scala.concurrent.Future[Int]
  // [error]  required: Demo.Result[Int]
  // [error]     (which expands to)  cats.data.EitherT[scala.concurrent.Future,String,Int]
  // [error]   val fails: Result[Int] = 5.pure
}