mapping futures succeeds typecheck when it shouldn't

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import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

  def future1: Future[Unit] = Future.successful(())
  def future2: Future[Unit] = Future.successful(())

  def test: Future[Unit] = future1.map { _ => future2 }

the body of the test function returns Future[Future[Unit]], however, the compiler is happy to say that it is simply a Future[Unit], which leads to errors in code

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Imported From: https://issues.scala-lang.org/browse/SI-9822?orig=1
Reporter: Sarunas Valaskevicius (rakatan)
Affected Versions: 2.11.7

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@som-snytt said:
There's an option for that:

$ scala -Ywarn-value-discard
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_60).
Type in expressions for evaluation. Or try :help.

scala> import concurrent._, ExecutionContext.Implicits._
import concurrent._
import ExecutionContext.Implicits._

scala> def f1 = Future.successful(())
f1: scala.concurrent.Future[Unit]

scala> def f2 = Future.successful(())
f2: scala.concurrent.Future[Unit]

scala> f1 map (_ => f2)
res0: scala.concurrent.Future[scala.concurrent.Future[Unit]] = Success(scala.concurrent.impl.Promise$KeptPromise@258e2e41)

scala> def f3: Future[Unit] = f1 map (_ => f2)
<console>:19: warning: discarded non-Unit value
       def f3: Future[Unit] = f1 map (_ => f2)
                                           ^
f3: scala.concurrent.Future[Unit]

Include -Xfatal-warnings to lock it down.

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Sarunas Valaskevicius (rakatan) said:
Ah right, will add it to the set of my default options, thanks!