public class Product: Codable {

    public let name: String

    init(name: String) {
        self.name = name
    }

    public final class Executable: Product {

        public let targets: [String]

        init(name: String, targets: [String]) {
            self.targets = targets
            super.init(name: name)
        }

        private enum CodingKeys: CodingKey {
            case targets
        }

        public required init(from decoder: Decoder) throws {
            let values = try decoder.container(keyedBy: CodingKeys.self)
            self.targets = try values.decode([String].self, forKey: .targets)
            try super.init(from: decoder)
        }

        public override func encode(to encoder: Encoder) throws {
            var container = encoder.container(keyedBy: CodingKeys.self)
            try container.encode(targets, forKey: .targets)
            try super.encode(to: encoder)
        }
    }
}

Compile Swift Module 'codable' (1 sources)
/private/tmp/codable/Sources/codable/main.swift:23:57: error: ambiguous reference to member 'CodingKeys'
            let values = try decoder.container(keyedBy: CodingKeys.self)
                                                        ^~~~~~~~~~
/private/tmp/codable/Sources/codable/main.swift:18:22: note: found this candidate
        private enum CodingKeys: CodingKey {
                     ^
<unknown>:0: note: found this candidate
/private/tmp/codable/Sources/codable/main.swift:29:56: error: ambiguous reference to member 'CodingKeys'
            var container = encoder.container(keyedBy: CodingKeys.self)
                                                       ^~~~~~~~~~
/private/tmp/codable/Sources/codable/main.swift:18:22: note: found this candidate
        private enum CodingKeys: CodingKey {
                     ^
<unknown>:0: note: found this candidate
error: terminated(1): /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2017-12-30-a.xctoolchain/usr/bin/swift-build-tool -f /private/tmp/codable/.build/debug.yaml main