class BaseClass {}

protocol ClassConstraining where Self: BaseClass { // remove `where Self: BaseClass` to stop crashing
    var value: Int64? { get }
}

class DerivedClass: BaseClass, ClassConstraining {
    var value: Int64?
    
    init(value: Int64?) { 
        self.value = value
        super.init()
    }
}

let derived = DerivedClass(value: 3)

let base: BaseClass = derived

(base as? ClassConstraining)?.value