[SR-10859] Private(set) property can be set from outside via Binding

[swift-ci]

Previous ID	SR-10859
Radar	None
Original Reporter	ngocluu (JIRA User)
Type	Bug

Environment

Swift 5.1. Xcode 11 Beta (11M336w).

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, PropertyWrappers
Assignee	None
Priority	Medium

md5: c9c1759159a05fd13bb298b4803f1838

Issue Description:

BindableObject class:

import SwiftUI
import Combine

final class UserData: BindableObject  {
    let didChange = PassthroughSubject<Void, Never>()
    
    // PRIVATE SET
    private(set) var showFavoritesOnly = false {
        didSet {
            print("CHANGED")
            didChange.send(())
        }
    }
}

SwiftUI view:

struct ContentView : View {
    @EnvironmentObject var userData: UserData
    
    var body: some View {
        VStack {
            Toggle(isOn: $userData.showFavoritesOnly) {
                Text("Favorites only")
            }
            Button(action: toggle) {
                Text("Change")
            }
        }
    }
    
    func toggle() {
        $userData.showFavoritesOnly.value.toggle()
    }
}

The property of BindableObject is private(set), but it can be set from the view via Binding.

[belkadan]

I think this is expected behavior right now—what would the rules be to protect the binding?—but @DougGregor is taking property wrappers through the Swift Evolution Process and they're discussing how access control should work.