TwelveOrLess.swift

import Foundation

// ref: https://docs.swift.org/swift-book/documentation/the-swift-programming-language/properties/#Property-Wrappers
@propertyWrapper
struct TwelveOrLess {
    private var number = 0
    
    @MainActor
    var wrappedValue: Int {
        get { return number }
        set { number = min(newValue, 12) }
    }
}

struct SmallRectangle {
    @TwelveOrLess var height: Int
    @TwelveOrLess var width: Int
    
    func f() {
        print(Thread.isMainThread)  // true or false?
    }
}

Task.detached {
    let rectangle = await SmallRectangle()
    await rectangle.f()
}