[SR-5858] Incorrectly implementing LocalizedError protocols does not result in an error at compile time but an incorrect error localized description

[swift-ci]

Previous ID	SR-5858
Radar	rdar://problem/41721949
Original Reporter	nacho4d (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Environment

Reproducable everywhere I have tried:

Stable Xcode 8.3
Latests Xcode 9 beta 6
Swift Dev. 4.0 (Sep 5, 2017) Platform: Linux (x86_64)

Additional Detail from JIRA

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

md5: b7f836ae6e1516ca27059acef1da0130

relates to:

• SR-522 Protocol funcs cannot have covariant returns

Issue Description:

As in the summary, incorrectly implementing LocalizedError protocol leads to an incorrect localizedDescription string. Instead of an incorrect "The operation couldn’t be completed" when I call localizedDescription I would expect the compiler to complain because my implementation is incorrect.

See below two implementations:

import Foundation

// This is the bad error.
enum BadError: LocalizedError {
    case myFailure
    // Note the return type is not Optional<String> but String (Due to a typo, bad copy-pasting, whatever)
    var errorDescription: String {
        switch self {
        case .myFailure: return "BadErrorMyFailure"
        }
    }
}

// This is the good error
enum GoodError: LocalizedError {
    case myFailure
    var errorDescription: String? {
        switch self {
        case .myFailure: return "GoodErrorMyFailure"
        }
    }
}

// No compiler error nothing. This compiles without problem in Swift3 and latest 4 too 
print(BadError.myFailure.localizedDescription)
// "The operation couldn’t be completed. (__lldb_expr_32.BadError error 0.)"
// I expect a error at compile time since `errorDescription:String? { get }` is expected but I am writing `errorDescription:String { get }` (without the question mark)

print(GoodError.myFailure.localizedDescription)
// "GoodErrorMyFailure"

Same example in swift sandbox

[belkadan]

@DougGregor, do you remember why LocalizedError is designed this way?

[belkadan]

(In general, we should also consider non-optional properties to satisfy optional ones. We have a separate bug for that.)

[swift-ci]

Comment by Guillermo Ignacio Enriquez Gutierrez (JIRA)

BTW, UITableViewDatasource has a method:

[DougGregor]

This has been addressed by the “near-miss” diagnostics from Swift 4.2. If you separate out your protocol conformances into separate extensions, which we generally consider good style, you will get a warning:

enum BadError {
case myFailure
}

extension BadError: LocalizedError {
// Note the return type is not Optional<String> but String (Due to a typo, bad copy-pasting, whatever)
var errorDescription: String { // warning: property 'errorDescription' nearly matches defaulted requirement 'errorDescription' of protocol 'LocalizedError'
switch self {
case .myFailure: return "BadErrorMyFailure"
}
}
}

Your original example, however, will not produce the warning. We initially did produce a warning for your example (in the earlier versions), but it was too noisy. So, we produce the warning when the extension declares the conformance and has a near-meatch with that protocol.