sample.swift

protocol Permission {}
enum Default: Permission {}
enum Strict: Permission {}

protocol P {
    associatedtype PermissionMode: Permission = Default
    func f() -> String
}

extension P {
    func callF() -> String {
        return "default " + #function + " - " + f()
    }
    
    func f() -> String {
        return "default " + #function + " - " + v
    }
    
    var v: String { return "default " + #function }
}

extension P where Self.PermissionMode == Strict {
    func f() -> String {
        return "Self.PermissionMode == Strict " + #function + " - " + v
    }
}

struct A: P {}
struct B: P {
    typealias PermissionMode = Strict
}


A().callF() // "default callF() - default f() - default v"
B().callF() // "default callF() - Self.PermissionMode == Strict f() - default v"