final class OptPlus[+A](val x: A) extends AnyVal { }

// ./a.scala:1: error: covariant type A occurs in contravariant position in type OptPlus[A] of value $this
// final class OptPlus[+A](val x: A) extends AnyVal { }
//             ^
// one error found

object OptPlus {
  final <synthetic> def extension$hashCode[A >: Nothing <: Any]($this: OptPlus[A])(): Int = $this.x.hashCode();
}

  // Not like this
  def f[A]($this: OptPlus[A])(args) = ...
  
  // Like this instead
  def f[A1 >: A]($this: OptPlus[A1])(args) = ...
  
  // In the case of hashCode and others which don't reference the
  // type parameter at all, they could be like this
  def f($this: OptPlus[_])(args) = ...
  // but I don't know if there's any meaningful advantage to that.