Open Radar

 
Summary:
Accessing a sub-property on a protocol-typed property of a class causes the `didSet` for that property to be called, leading to an EXC_BAD_ACCESS crash if anything in the `didSet` closure causes it to be accessed again.

Steps to Reproduce:
Simply use the following code in a playground:

protocol Bob {
	var name: String { get }
}

class Bobby: Bob {
	let name: String = "Bob"
}

class Fred {
	var viewModel: Bob {
		didSet {
			print("setter")
			self.bind()
		}
	}
	
	init(viewModel: Bob) {
		self.viewModel = viewModel
		self.bind()
	}
	
	func bind() -> Void {
		print("vm name is \(self.viewModel.name)")
	}
}

let vm: Bobby = Bobby()
let fred: Fred = Fred(viewModel: vm)

Expected Results:
The console should simply log "my name is Bob"

Actual Results:
The console goes into a never-ending loop eventually causing an EXC_BAD_ACCESS crash

Version:
Xcode 7A218 on OS X 10.10.5 (14F27)

Notes:
If, in the protocol defined above, we add the `class` restriction then this no longer causes a crash, which would seem to imply that accessing the property's sub-property is seen as causing the property to mutate.

Configuration:
MacBookPro11,1