Unbox Error for override property in Swift?

import Foundation

enum OperationError: Error {
    case ioError(_ message: String)

    var localizedDescription: String {
        switch self {
        case .ioError(let message):
            return message
        }
    }
}

func test1() throws {
    throw OperationError.ioError("io error")
}

do {
    try test1()
} catch {
    print("\(error) type: \(type(of: error))")
    print("\(error.localizedDescription)")
    print("\((error as? OperationError)?.localizedDescription ?? "")")
}

Output:

ioError("io error") type: OperationError
The operation couldn’t be completed. (SwiftErrorDemo.OperationError error 0.)
io error

It seems the custom error couldn't be invoked to the correct property, I know the localizedDescription property is from Foundation framework, but for real type of the error is OperationError, why does the override localizedDescription get called?

If you want to supply your own localizedDescription, adopt LocalizedError.

enum OperationError: LocalizedError {
    case ioError(_ message: String)

    var errorDescription: String? {
        switch self {
        case .ioError(let message): message
        }
    }
}

Your test code stays the same:

func test1() throws {
    throw OperationError.ioError("io error")
}

do {
    try test1()
} catch {
    print("\(error) type: \(type(of: error))")
    print("\(error.localizedDescription)")
    print("\((error as? OperationError)?.localizedDescription ?? "")")
}

Output from your test code:

ioError("io error") type: OperationError
io error
io error