How do I subclass a struct in Swift?

It is not possible to subclass a struct in Swift, only classes can be subclassed. An extension is not a subclass, it's just adding additional functionality on to the existing struct, this is comparable to a category in Objective-C.

Please see the Apple Swift documentation here to read about the differences between struct and class.

Since Swift 5.1 it is now possible to do something similar to what you asked for: using composition, with KeyPath and dynamicMemberLookup. Have a look: https://medium.com/@acecilia/struct-composition-using-keypath-and-dynamicmemberlookup-kind-of-struct-subclassing-but-better-6ce561768612?source=friends_link&sk=da479578032c0b46c1c69111dfb6054e

You cannot subclass structures in Swift but you can, in some sense, mimic subclassing by creating a structure that simply vends out the structure that you wish to subclass. For example, if you wanted to subclass Calendar (a structure), you could create a structure that returns configured calendars.

struct WorldCalendar {
    let american1: Calendar = {
        var c = Calendar(identifier: .gregorian)
        c.timeZone = .current
        c.locale = Locale(identifier: "en_US_POSIX")
        return c
    }()
    
    static let american2: Calendar = {
        var c = Calendar(identifier: .gregorian)
        c.timeZone = .current
        c.locale = Locale(identifier: "en_US_POSIX")
        return c
    }()
    
    static func american3() -> Calendar {
        var c = Calendar(identifier: .gregorian)
        c.timeZone = .current
        c.locale = Locale(identifier: "en_US_POSIX")
        return c
    }
}

let worldCalendar = WorldCalendar()
let cal1 = worldCalendar.american1

let cal2 = WorldCalendar.american2

let cal3 = WorldCalendar.american3()

How you vend the configured structures (as an instance property, a static property, a static function, etc.) is just a matter of what better fits into your application and, frankly, personal preference.