[SR-14975] `LocalizedError` properties get mangled by Objective-C bridge

[CharlesJS]

Previous ID	SR-14975
Radar	rdar://problem/81115919
Original Reporter	@CharlesJS
Type	Bug

Environment

macOS 21A5284e

Xcode Version 13.0 beta 3 (13A5192i)

Swift version 5.5 (swiftlang-1300.0.24.13 clang-1300.0.25.10)

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: f34c7dac79752ed46ed183110bcd487a

Issue Description:

So, let's say we have an enum that conforms to `LocalizedError`, which contains logic to bridge nicely to `NSError` so that Cocoa UIs will present our errors nicely:

import Foundation

enum Plague: LocalizedError {
    case fires
    case floods
    case frogs
    case flies
    
    var failureReason: String? {
        switch self {
        case .fires:
            return "We didn't start the fire! But this error did"
        case .floods:
            return "Ya got trouble, right here in River City"
        case .frogs:
            return "Ribbit Ribbit"
        case .flies:
            return "Shoo fly, don't bother me"
        }
    }
}

let err = Plague.frogs

print(
    "Before encoding/decoding: localizedDescription is '\((err as NSError).localizedDescription)', ",
    "userInfo is \((err as NSError).userInfo)"
)

Running that, we get:

Before encoding/decoding: localizedDescription is 'The operation couldn’t be completed. Ribbit Ribbit', userInfo is [:]

The `userInfo` dictionary is empty, which is odd, but the `localizedDescription` does get displayed correctly after we bridge to `NSError`. So far so good.

But what if it goes through an `NSKeyedArchiver` (for example, if it gets sent over an XPC connection)?

let encoded = try! NSKeyedArchiver.archivedData(withRootObject: err, requiringSecureCoding: true)

let decoded = try! NSKeyedUnarchiver.unarchivedObject(ofClass: NSError.self, from: encoded)!

print("After: localizedDescription is '\(decoded.localizedDescription)', userInfo is \(decoded.userInfo)")

Suddenly, the `failureReason` string is lost:

After: localizedDescription is 'The operation couldn’t be completed. (main.Plague error 2.)', userInfo is [:]

However, since `Error` contains a `_userInfo` property, we can add an implemention for that ourselves:

var _userInfo: AnyObject? { [NSLocalizedFailureReasonErrorKey : self.failureReason!] as NSDictionary }

And that fixes it:

After: localizedDescription is 'The operation couldn’t be completed. Ribbit Ribbit', userInfo is ["NSLocalizedFailureReason": Ribbit Ribbit]

However: this only seems to work on error classes defined in the current module. If I try to add `_userInfo` to something defined in another module via an extension, it doesn't work:

extension POSIXError {
    var _userInfo: AnyObject? { [NSLocalizedFailureReasonErrorKey : "Hello"] as NSDictionary }
}

let err = POSIXError(.EINVAL)

print(
    "Before encoding/decoding: localizedDescription is '\((err as NSError).localizedDescription)', ",
    "userInfo is \((err as NSError).userInfo)"
)

let encoded = try! NSKeyedArchiver.archivedData(withRootObject: err, requiringSecureCoding: true)
let decoded = try! NSKeyedUnarchiver.unarchivedObject(ofClass: NSError.self, from: encoded)!

print("After: localizedDescription is '\(decoded.localizedDescription)', userInfo is \(decoded.userInfo)")

yields:

Before encoding/decoding: localizedDescription is 'The operation couldn’t be completed. Invalid argument',  userInfo is [:]
After: localizedDescription is 'The operation couldn’t be completed. Invalid argument', userInfo is [:]

This is not exclusive to Foundation-defined things like `POSIXError`; I've tried it with errors I've defined myself in other modules, and the behavior is the same. I can put a log in the `_userInfo` accessor, and it never gets called.

It seems to me that the expected behavior should be:

1. `LocalizedString` includes a default implementation for `_userInfo` that provides the various strings corresponding to the properties it supports.

2. We have logic in place to make sure `_userInfo` always gets propagated across when bridging to `NSError`, or at least when we're running it through an encoder.

Otherwise, the `userInfo` field is always unpopulated for bridged Swift errors, and in the end, all I have is an arbitrary integer and I don't know whether I need to get an extinguisher, an umbrella, some bug spray, or... whatever you do for frogs (a cat maybe?).

[typesanitizer]

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