`Task { [weak self] in self?.prop = value }` causes non-isolated mutation error https://twitter.com/inamiy/status/1603267958209331200

task-weak-self-nonisolated-error.swift

actor A {
    var prop: Int = 0

    func foo() {
        // OK
        Task {
            self.prop = 1
        }

        // ERROR: Actor-isolated property 'prop' can not be mutated from a non-isolated context
        Task { [weak self] in
            self?.prop = 1
        }

        // ERROR: Actor-isolated property 'prop' can not be mutated from a non-isolated context
        Task { [weak self] in
            guard let self = self else { return }
            self.prop = 1
        }

        // Requires setter and `await`
        Task { [weak self] in
            await self?.setProp(1)
        }
    }

    func setProp(_ prop: Int) {
        self.prop = prop
    }
}

let a = A()
await a.foo()

Raised ticket: swiftlang/swift#62604