Can not decode an entity which has Optional<URL> property.

URLCodableTest.playground

//: Playground - noun: a place where people can play

import UIKit

struct URLCodableTest: Codable {
    let ou: URL?
    let u: URL

    enum CodingKeys: String, CodingKey {
        case ou
        case u
    }

    init(ou: URL? = URL(string: "https://www.apple.com"), u: URL = URL(string: "https://www.apple.com")!) {
        self.ou = ou
        self.u = u
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        self.ou = try values.decodeIfPresent(URL.self, forKey: .ou)
        self.u = try values.decode(URL.self, forKey: .u)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(ou, forKey: .ou)
        try container.encode(u, forKey: .u)
    }
}

let t = URLCodableTest()

do {
    let d = try JSONEncoder().encode(t)
    print(d)

    let j = try? JSONSerialization.jsonObject(with: d, options: .allowFragments)
    print(j)
    /*
     Optional({
     ou =     {
     relative = "https://www.apple.com";
     };
     u = "https://www.apple.com";
     })
     */

    do {
        let tt = try JSONDecoder().decode(URLCodableTest.self, from: d)
        print(tt)
    } catch let e as DecodingError {
        switch e {
        case let .dataCorrupted(c):
            print(c)
        case let .keyNotFound(k, c):
            print(k, c)
        case let .typeMismatch(t, c):
            print(t, c.debugDescription)
            // -> String Expected to decode String but found a dictionary instead.
        case let .valueNotFound(t, c):
            print(t, c)
        }
    } catch let e {
        print(e.localizedDescription)
    }
} catch let e as EncodingError {
    switch e {
    case let .invalidValue(v, c):
        print(v, c)
    }
} catch let e {
    print(e.localizedDescription)
}