Why Future(Failure(new Exception)) returns Success instead of failure?

Future.failed can create a failed future, for example

Future.failed(new Exception)

or throw inside the future

Future(throw new Exception)

or call Future.fromTry

Future.fromTry(Failure(new Exception))

however

Future(Failure(new Exception))

does not represent a failed future because

Failure(new Exception)

is, despite possibly misleading names, just a regular value, for example,

val x = Failure(new Exception)
val y = 42
Future(x)
Future(y)

so Future(x) is a successful future for the same reason Future(y) is a successful future.

You can think of Future as a kind of async try-catch, so if you are not throwing inside the try

try {
  Failure(new Exception) // this is not a throw expression
} catch {
  case exception =>      // so exception handler does not get executed
}

then catch handler does not get executed.