didSet on array crashes my playground

didSet_on_array.swift

// Playground - noun: a place where people can play

import UIKit

var str = "Hello, playground"


struct Thing {
    var arr: [Int] = [] {
    didSet {
        println("didSet{} called")
    }
    }
}

var t = Thing()
t.arr = [3,2]
t.arr.append(3)