Swift 2: Unexpectedly writing to a property when trying to read it - swift2

Question

The problem I'm having is that my property's willSet and didSet are being called even though I'm only reading the property, and this breaks my app.
I condensed my problem into a playground. Uncomment #1 to see the problem, or #2 to see the expected behavior.
What's going on here?
protocol Departure
{
var line: String? { get }
}
class MyDeparture : Departure
{
var line: String? = "SomeString"
}
#if true
// #1: this causes a write to tableContet later (!)
typealias TableSection = (title: String, rows: [Departure])
#else
// #2: this doesn't cause a write to tableContent later
typealias TableSection = (title: String, rows: [MyDeparture])
#endif
var tableContent: [TableSection] = [ TableSection(title: "MySectionTitle", rows: [ MyDeparture() ]) ]
{
willSet { print("willSet tableContent") }
didSet { print("didSet tableContent") }
}
func getDepartureDescription() -> String? {
print("start getDepartureDescription")
defer { print("end getDepartureDescription") }
#if true
// writes to tableContent in case #1
let lineNumber = tableContent[0].rows[0].line
#else
// never writes to table content
let row = tableContent[0].rows[0]
let lineNumber = row.line
#endif
return "Your line is \(lineNumber)"
}
getDepartureDescription()
This prints
start getDepartureDescription
willSet tableContent
didSet tableContent
end getDepartureDescription
I'm using Xcode 7 (7A218) GM seed. Everything worked as expected in Xcode 6.4 and Swift 1.2.
Side note:
At first I thought that the runtime was--on reading TableSection.rows--creating a new [Departure] array from the [MyDeparture] array that was assigned to it. But even correcting for that in the most explicit way I could think of didn't get rid of the problem:
// More correct types makes no difference:
var departures: [Departure] {
var result = Array<Departure>()
result.append(MyDeparture())
return result
}
var tableContent: [TableSection] = [ TableSection(title: "MyTitle", rows: departures ) ]

Answer 1

There's some sort of lazy initialisation going on. It's only when it gets to the line
let lineNumber = tableContent[0].rows[0].line
that the run time seems to be filling in the contents of the array.
If you declared the array as containing elements that conform to the Departure protocol, the runtime does not know how big the elements of tableContent actually are because both classes and structs can conform to Departure. Hence, I think it is recreating the array and triggering didSet and willSet erroneously.
I tried limiting your protocol to classes like so:
protocol Departure: class
{
var line: String? { get }
}
and the problem goes away because now the runtime knows the array can only contain class references.
I think this is a bug and you should raise it with Apple.