[SR-4852] Property modification doesn't take effect

[pushkarnk]

Previous ID	SR-4852
Radar	None
Original Reporter	@pushkarnk
Type	Bug
Status	Closed
Resolution	Cannot Reproduce

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: f7ac911269dbb72104e8be21707022f9

Issue Description:

Here's a simple test program that reproduces a apparently weird problem:

enum State {
    case ready(TState)
    case inProgress(TState)

    var isPaused: Bool {
        switch self {
        case .ready: return false
        case .inProgress: return false
        }
    }
}

enum Drain {
    case foo(String)
    case bar(Int, String)
}

struct TState {
    let drain: Drain
}

class Task {
    var state = State.ready(TState(drain: .foo("wow"))) { 
        willSet {
            if newValue.isPaused { }
        }
    }

    func resume() {
        if case .ready(let tState) = state {
            print("before => \(state)")
            state = .inProgress(tState) //this assignment does not happen
            print("after => \(state)")
        }
    }
}    

//main
Task().resume()

Expected output:

before => ready(Test.TState(drain: Test.Drain.foo("wow")))
after => inProgress(Test.TState(drain: Test.Drain.foo("wow")))

Actual output (with HEAD):

before => ready(Test.TState(drain: Test.Drain.foo("wow")))
after => ready(Test.TState(drain: Test.Drain.foo("wow")))

PS: This issue is blocking a bunch of PRs in Foundation. This problem is not seen in swift-DEVELOPMENT-SNAPSHOT-2017-04-24-a-ubuntu16.04, so it is a regression.

[pushkarnk]

After updating the repos today, I no longer see this problem.

Just for the record: I do see the problem with swift-DEVELOPMENT-SNAPSHOT-2017-05-09-a