extension A {
    struct S {
        var x = ""
    }
}

class B: A {

}

A.S().x

var c = B()

c.S().x

extension A {
    struct S {
        var x = ""
    }
    func getS() {
        // return all members of S (not just x, there can be multiple variables)
    }
}

class A {}

extension A {
    struct S {
        var x = ""
    }

    static var s = S()
}

var b = B()
type(of: b).s.x

struct S {
    var x = ""
    func getProperties() -> ??? {
        ???
    }
}