[SR-11638] willSet Property Observer Causes Infinite Recursion When Used With @Published

[swift-ci]

Previous ID	SR-11638
Radar	None
Original Reporter	dbianchi (JIRA User)
Type	Bug

Environment

Xcode 11.1 SwiftUI

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Bug, PropertyWrappers | |Assignee | None | |Priority | Medium | md5: 0c530cc423fc2d327dd8653d13b23958

Issue Description:

If a property is using the `@Published` wrapper and the `willSet` property observer to set the value, invite recursion will occur as a result of the `willSet` observer being called indefinitely. If I remove the `@Published` wrapper, the bug does not exist.

@Published var someVariable: String = "" {
    willSet {
        someVariable = oldValue
    }
}

[theblixguy]

Reproducer that doesn't involve Combine:

@propertyWrapper
struct Foo {
  var wrappedValue: Int
}

struct Bar {
  @Foo var prop: Int = 0 { 
    willSet { prop = newValue }
  }
}

var bar = Bar()
bar.prop = 1 // Crash

If you replace willSet with didSet then you have the same issue as well.

[theblixguy]

As a workaround, you can do _someVariable.wrappedValue = <value> in the willSet/didSet and this will not cause an infinite recursion.