1

When referencing an actor's method's type, the compiler issues the following error.

let g = ActorType.f (i:) // Error: Call to actor-isolated instance method 'f(i:)' in a synchronous main actor-isolated context

But, there is no such error if the type is class:

let f = ClassType.f (i:) // Okay
Details 
@main
enum TypeOfFunction {
    static func main () async throws {
        let f = ClassType.f (i:) // Okay
        print ("type of class function is", type (of: f))
        
        let g = ActorType.f (i:) // Error: Call to actor-isolated instance method 'f(i:)' in a synchronous main actor-isolated context
        print ("type of actor function is", type (of: g))
    }
}

actor ActorType {
    func f (i: Int) -> Int {i}
}

class ClassType {
    func f (i: Int) -> Int {i}
}