Scala variance sample

0_.md

class Foo[+A]
class Bar[-A]

class Test1[A] {
  def piyo(a: A): Unit = ???      // OK
  def poyo(a: Foo[A]): Unit = ??? // OK
  def puyo(a: Bar[A]): Unit = ??? // OK

  def paya(): A = ???             // OK
  def poya(): Foo[A] = ???        // OK
  def puya(): Bar[A] = ???        // OK
}
class Test2[+A] {
  def piyo(a: A): Unit = ???      // NG
  def poyo(a: Foo[A]): Unit = ??? // NG
  def puyo(a: Bar[A]): Unit = ??? // OK

  def paya(): A = ???             // OK
  def poya(): Foo[A] = ???        // OK
  def puya(): Bar[A] = ???        // NG
}
class Test3[-A] {
  def piyo(a: A): Unit = ???      // OK
  def poyo(a: Foo[A]): Unit = ??? // OK
  def puyo(a: Bar[A]): Unit = ??? // NG

  def paya(): A = ???             // NG
  def poya(): Foo[A] = ???        // NG
  def puya(): Bar[A] = ???        // OK
}