https://stackoverflow.com/q/78999607/1271826

ComplexData.swift

final class ComplexData: @unchecked Sendable {
    private let lock = NSLock()

    private var _name: String
    var name: String {
        get { lock.withLock { _name } }
        set { lock.withLock { _name = newValue } }
    }

    init(name: String) {
        _name = name
    }
}

By the way, if you're worried about how hairy ComplexData might get with lots and lots of properties, you can abstract this synchronization logic out into a property wrapper:

@propertyWrapper
struct Synchronized<T>: @unchecked Sendable {
    private var _wrappedValue: T
    private let lock = NSLock()

    var wrappedValue: T {
        get { lock.withLock { _wrappedValue } }
        set { lock.withLock { _wrappedValue = newValue } }
    }

    init(wrappedValue: T) {
        _wrappedValue = wrappedValue
    }
}

final class ComplexData: @unchecked Sendable {
    @Synchronized var firstName: String
    @Synchronized var lastName: String

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
}