protocol A {
    var a: Double?
    var b: Double?
    var c: Double?
    var d: A?
}

class Model1: A {
    var a: Double?
    var b: Double?
    var c: Double?
    var d: A?

    // some other properties...
}

class Model2: A {
    var a: Double?
    var b: Double?
    var c: Double?
    var d: A?

    // some other properties...
}

class Model2: A {
    var a: Double?
    var b: Double?
    var c: Double?
    var d: Model1?

    // some other properties...
}

Type 'Model2' does not conform to protocol 'A'