SyntheticMembersInjector#injectInners does not get called though the class gets loaded

I have an injector implementation like this below, as per the logs the class gets loaded but injectInners does not get fired.

How can I get this to work?

final class Injector extends SyntheticMembersInjector {
  private val Log = Logger.getInstance(classOf[Injector])
  Log.setLevel(Level.DEBUG)
  Log.debug(s"Injector loaded")

  override def injectInners(source: ScTypeDefinition): Seq[String] = {
    Log.debug(s"from InjectInners")
    source.extendsBlock.members.flatMap {
      case c: ScClass if c.name == "A" => {
        Seq("""def hello(number: Int) = { println("hi" + number + "!") }""")
      }
      case _ => Seq.empty
    }
  }

}