[SR-10464] Optional URL value with Decodable protocol failing

[swift-ci]

Previous ID	SR-10464
Radar	None
Original Reporter	szuniverse (JIRA User)
Type	Bug

Environment

Swift 4.2
Xcode 10.1 (10B61)

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug, Codable
Assignee	bendjones (JIRA)
Priority	Medium

md5: d568d9a734aa22a2960bae6ad83bfe20

Issue Description:

struct MyStruct: Decodable {
    let id: String
    let homepageUrl: URL?

    //    private enum CodingKeys: String, CodingKey {
    //        case id, homepageUrl = "homepage_url"
    //    }
}

let dict = [
    "id": "1",
    "homepage_url": ""
]

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase

let data = try JSONSerialization.data(withJSONObject: dict)
let myStruct = try decoder.decode(MyStruct.self, from: data)
print("m: \(myStruct.id) - \(String(describing: myStruct.homepageUrl))")

In this case the error message is `Invalid URL string.` which is correct because it's an empty string so its not a valid URL, but the `homepageUrl` is optional, so it should produce a nil value instead of failing.

If the `MyStruct` is using `CodingKeys` (uncomment that part of the code), then its working fine with empty string in dictionary.

I think its a swift bug because: If `keyDecodingStrategy` is `convertFromSnakeCase` then it should produce nil without using `CodingKeys`.

[belkadan]

I don't think the empty string is considered the same as a null value, and Optional does not mean "ignore errors" for Codable's purposes. cc bendjones (JIRA User)

[swift-ci]

Comment by Szurma David (JIRA)

yeah thats true, but the weird thing is that if the struct using CodingKeys the behaviour is not the same. I don't think that the CodingKeys usage should modify the result if the keyDecodingStrategy is convertFromSnakeCase

[belkadan]

Ah, I see. Sorry for not reading closely!

[swift-ci]

Comment by Szurma David (JIRA)

Any update?

[poya]

I guess the reason it's "working fine with empty string in dictionary" when you add CodingKeys is because the decoder then no longer matches "homepage_url" key to your homepageUrl property. That can be validated by putting a valid URL string in your json and your homepageUrl would still be nil. With the decoder set to .convertFromSnakeCase snake-case keys are converted to camel-case keys, and that means that the manual snake-case mapping you do in your CodingKeys results in a mismatch and hence homepageUrl will never be set. Keeping the CodingKeys and removing the .convertFromSnakeCase you match again, and the `Invalid URL String` error would return.