[SR-10781] Decode an encoded double optional fails

[swift-ci]

Previous ID	SR-10781
Radar	None
Original Reporter	ChaosCoder (JIRA User)
Type	Bug

Environment

Swift 5.0

Additional Detail from JIRA

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

md5: 076bfab4f40c06914f7da38d175cf6d4

Issue Description:

I'm trying to encode and decode a Codable struct that has a double optional (e.g. name: String??).

This way, encoding the struct with the JSONEncoder, I get a json with:

• the property and its value in it, if it is .some: { name: "123" }
• nothing, if it is .none: {}
• the property and null, if it is .some(.none): { name: null }

Trying to decode this json again does not handle the third case correctly, the decoded value is .none (like the second case):

import Foundation

struct S: Codable {
    let name: String??
}

let a = S(name: "123")
let jsonA = try! JSONEncoder.init().encode(a) // { name: "123" }
let a2 = try! JSONDecoder().decode(S.self, from: jsonA)
assert(a2.name == .some("123"))


let b = S(name: nil)
let jsonB = try! JSONEncoder.init().encode(b) // {}
let b2 = try! JSONDecoder().decode(S.self, from: jsonB)
assert(b2.name == .none)


let c = S(name: .some(nil)) // or .some(.none)
let jsonC = try! JSONEncoder.init().encode(c) // { name: null }
let c2 = try! JSONDecoder().decode(S.self, from: jsonC)
assert(c2.name == .some(.none)) // fails the assertion, because c2.name == .none

The problem is the last line, c2.name is .none and not .some(.none) as expected.

[belkadan]

I don't think that's what the default synthesized init(coder:) does. It seems like reasonable behavior for it to do so, though.