import Foundation

extension Optional {
    func defaultValue<T>(_ defaultValue: T) -> T {
        switch self {

        case .none:
            return defaultValue
        case .some(let value):

            guard let valueT = value as? T else {
                return defaultValue
            }
            return valueT
        }
    }
}

func defaultDate() -> Date {
    print("Called")
    return Date()
}

let myDate: Date? = Date()

let _ = myDate.defaultValue(defaultDate()) // This will print "Called", even when myDate is clearly not nil.