ViewController.swift

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        Task.detached {
            let y = Y()
            await y.f()
        }
    }
}

protocol P {
    @MainActor func f()
}

struct Y: P {
    func f() {
        print(Thread.isMainThread)  // true
    }
}