IntelliJFalsePositive.scala

case class Operand[A](key: String, value: A)

case class Foo(id: Long, bar: String)

trait Operation[A] {
  def operator: String

  def operand: Option[Operand[A]]
}

object Operation {
  case class FooOperation(foo: Foo) extends Operation[Foo] {
    override def operator: String = "foo"


    // IntelliJ Community Edition 15.0,4 does not highlight the compilation error for this:
    override def operand: Option[Foo] = Some(foo)

    // The compilable version is something like the following:
    // override def operand: Option[Operand[Foo]] = Some(Operand("someKey", foo))
  }
}