Why idea scala plug-in work totally on the contrary with Scala compiler Follow

import scala.{Option => _, Either => _, _}

sealed trait Option[+A] {

  def map[B](f: A => B): Option[B] =  this match{
    case None => None
    case Some(a) => Some(f(a))
  }

  def flatMap[B] (f:A=> Option[B]): Option[B] = {
    case None => None
    case Some(_) => f(_)
  }


code above
method map() will pass in Scala compiler, but error in Idea.
method flatMap() error in Scala compiler, but no error showed in Idea

I use Idea 15.0.2, Scala plug-in 2.2.0, 
Scala SDK 11.7.2