surprising id-like behavior of AnyObject

gistfile1.swift

import Foundation

class Hrm {
    @objc func objcFunc() {
    }
    
    dynamic func dynamicFunc() {
    }
    
    func normalFunc() {
    }
}

let obj : AnyObject = NSString()
obj.objcFunc()
obj.dynamicFunc()
//obj.normalFunc() // this one does not compile