Not importing implicitConversions does not prevent compilation

gistfile1.txt

### With import, without -feature flag: Compiles: YES

$ cat D.scala 
import language.implicitConversions

object D {
  implicit def i2s(i: Int): String = (i - 1).toString

  val num: String = 44

  def main(args: Array[String]): Unit = println(num)
}

$ rm *class

$ ~/lib/scala-2.11.8/bin/scalac D.scala 

$ ~/lib/scala-2.11.8/bin/scala D
43

### With import, with -feature flag: Compiles: YES

$ rm *class

$ ~/lib/scala-2.11.8/bin/scalac -feature D.scala 

$ ~/lib/scala-2.11.8/bin/scala D
43

### Without import, without -feature flag: Compiles: YES

$ cat D.scala 
//import language.implicitConversions

object D {
  implicit def i2s(i: Int): String = (i - 1).toString

  val num: String = 44

  def main(args: Array[String]): Unit = println(num)
}

$ rm *class

$ ~/lib/scala-2.11.8/bin/scalac D.scala 
warning: there was one feature warning; re-run with -feature for details
one warning found

$ ~/lib/scala-2.11.8/bin/scala D
43

### Without import, with -feature flag: Compiles: YES

$ rm *class

$ ~/lib/scala-2.11.8/bin/scalac -feature D.scala 
D.scala:4: warning: implicit conversion method i2s should be enabled
by making the implicit value scala.language.implicitConversions visible.
This can be achieved by adding the import clause 'import scala.language.implicitConversions'
or by setting the compiler option -language:implicitConversions.
See the Scaladoc for value scala.language.implicitConversions for a discussion
why the feature should be explicitly enabled.
  implicit def i2s(i: Int): String = (i - 1).toString
               ^
one warning found

$ ~/lib/scala-2.11.8/bin/scala D
43