Codable doesn't play nicely with subclasses

Codable.swift

import Foundation

class C: Codable {
    var p: String?
}

class D: C {
    var q: String?
}

let c = C()
c.p = "P in C"

let d = D()
d.p = "P in D"
d.q = "Q in D"

let cData = try! JSONEncoder().encode(c)
let dData = try! JSONEncoder().encode(d)

let cDecoded = try! JSONDecoder().decode(C.self, from: cData)
print(cDecoded.p ?? "Nope")

let dDecoded = try! JSONDecoder().decode(D.self, from: dData)
print(dDecoded.p ?? "Nope")
print(dDecoded.q ?? "Nope")

let subclassDecoded = try! JSONDecoder().decode(C.self, from: dData)
print(subclassDecoded.p ?? "Nope")
// print(subclassDecoded.q ?? "Nope") // won't compile