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

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

Comment by Guillermo Ignacio Enriquez Gutierrez (JIRA)

BTW, UITableViewDatasource has a method:

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.