class Foo
class Test {
    def update[B](x : B, b : Int): Unit = {}
    def apply[B](x : B) = 1
}
class Test2 {
  type B = Foo
    def update(x : B, b : Int): Unit = {}
    def apply(x : B) = 1
}

object Test {
    def main(a : Array[String]): Unit = {
        val a = new Test
        val f = new Foo
        a(f) = 1 //works
        a(f) = a(f) + 1 //works
        a(f) += 1 //error: reassignment to val
    }
}
object Test2 {
    def main(args : Array[String]): Unit = {
                args(0) += "a"
        val a = new Test2
        val f = new Foo
        a(f) = 1 //works
        a(f) = a(f) + 1 //works
        a(f) += 1 //error: reassignment to val
    }
}