swift-mirror/stdlib/public/core/Codable.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

//===----------------------------------------------------------------------===//
// Codable
//===----------------------------------------------------------------------===//

/// A type that can encode itself to an external representation.
public protocol Encodable {
    /// Encodes this value into the given encoder.
    ///
    /// If the value fails to encode anything, `encoder` will encode an empty
    /// keyed container in its place.
    ///
    /// This function throws an error if any values are invalid for the given
    /// encoder's format.
    ///
    /// - Parameter encoder: The encoder to write data to.
    func encode(to encoder: Encoder) throws
}

/// A type that can decode itself from an external representation.
public protocol Decodable {
    /// Creates a new instance by decoding from the given decoder.
    ///
    /// This initializer throws an error if reading from the decoder fails, or
    /// if the data read is corrupted or otherwise invalid.
    ///
    /// - Parameter decoder: The decoder to read data from.
    init(from decoder: Decoder) throws
}

/// A type that can convert itself into and out of an external representation.
public typealias Codable = Encodable & Decodable

//===----------------------------------------------------------------------===//
// CodingKey
//===----------------------------------------------------------------------===//

/// A type that can be used as a key for encoding and decoding.
public protocol CodingKey: CustomStringConvertible, CustomDebugStringConvertible {
    /// The string to use in a named collection (e.g. a string-keyed dictionary).
    var stringValue: String { get }

    /// Initializes `self` from a string.
    ///
    /// - parameter stringValue: The string value of the desired key.
    /// - returns: An instance of `Self` from the given string, or `nil` if the given string does not correspond to any instance of `Self`.
    init?(stringValue: String)

    /// The int to use in an indexed collection (e.g. an int-keyed dictionary).
    var intValue: Int? { get }

    /// Initializes `self` from an integer.
    ///
    /// - parameter intValue: The integer value of the desired key.
    /// - returns: An instance of `Self` from the given integer, or `nil` if the given integer does not correspond to any instance of `Self`.
    init?(intValue: Int)
}

extension CodingKey {
    /// A textual representation of this key.
    public var description: String {
        let intValue = self.intValue?.description ?? "nil"
        return "\(type(of: self))(stringValue: \"\(stringValue)\", intValue: \(intValue))"
    }

    /// A textual representation of this key, suitable for debugging.
    public var debugDescription: String {
        return description
    }
}

//===----------------------------------------------------------------------===//
// Encoder & Decoder
//===----------------------------------------------------------------------===//

/// A type that can encode values into a native format for external representation.
public protocol Encoder {
    /// The path of coding keys taken to get to this point in encoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// Any contextual information set by the user for encoding.
    var userInfo: [CodingUserInfoKey : Any] { get }

    /// Returns an encoding container appropriate for holding multiple values keyed by the given key type.
    ///
    /// - parameter type: The key type to use for the container.
    /// - returns: A new keyed encoding container.
    /// - precondition: May not be called after a prior `self.unkeyedContainer()` call.
    /// - precondition: May not be called after a value has been encoded through a previous `self.singleValueContainer()` call.
    func container<Key>(keyedBy type: Key.Type) -> KeyedEncodingContainer<Key>

    /// Returns an encoding container appropriate for holding multiple unkeyed values.
    ///
    /// - returns: A new empty unkeyed container.
    /// - precondition: May not be called after a prior `self.container(keyedBy:)` call.
    /// - precondition: May not be called after a value has been encoded through a previous `self.singleValueContainer()` call.
    func unkeyedContainer() -> UnkeyedEncodingContainer

    /// Returns an encoding container appropriate for holding a single primitive value.
    ///
    /// - returns: A new empty single value container.
    /// - precondition: May not be called after a prior `self.container(keyedBy:)` call.
    /// - precondition: May not be called after a prior `self.unkeyedContainer()` call.
    /// - precondition: May not be called after a value has been encoded through a previous `self.singleValueContainer()` call.
    func singleValueContainer() -> SingleValueEncodingContainer
}

/// A type that can decode values from a native format into in-memory representations.
public protocol Decoder {
    /// The path of coding keys taken to get to this point in decoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// Any contextual information set by the user for decoding.
    var userInfo: [CodingUserInfoKey : Any] { get }

    /// Returns the data stored in `self` as represented in a container keyed by the given key type.
    ///
    /// - parameter type: The key type to use for the container.
    /// - returns: A keyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not a keyed container.
    func container<Key>(keyedBy type: Key.Type) throws -> KeyedDecodingContainer<Key>

    /// Returns the data stored in `self` as represented in a container appropriate for holding values with no keys.
    ///
    /// - returns: An unkeyed container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not an unkeyed container.
    func unkeyedContainer() throws -> UnkeyedDecodingContainer

    /// Returns the data stored in `self` as represented in a container appropriate for holding a single primitive value.
    ///
    /// - returns: A single value container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not a single value container.
    func singleValueContainer() throws -> SingleValueDecodingContainer
}

//===----------------------------------------------------------------------===//
// Keyed Encoding Containers
//===----------------------------------------------------------------------===//

/// A type that provides a view into an encoder's storage and is used to hold
/// the encoded properties of an encodable type in a keyed manner.
///
/// Encoders should provide types conforming to
/// `KeyedEncodingContainerProtocol` for their format.
public protocol KeyedEncodingContainerProtocol {
    associatedtype Key : CodingKey

    /// The path of coding keys taken to get to this point in encoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// Encodes a null value for the given key.
    ///
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if a null value is invalid in the current context for this format.
    mutating func encodeNil(forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Bool, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int8, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int16, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int32, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int64, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt8, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt16, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt32, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt64, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Float, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Double, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: String, forKey key: Key) throws

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode<T : Encodable>(_ value: T, forKey key: Key) throws

    /// Encodes a reference to the given object only if it is encoded unconditionally elsewhere in the payload (previously, or in the future).
    ///
    /// For `Encoder`s which don't support this feature, the default implementation encodes the given object unconditionally.
    ///
    /// - parameter object: The object to encode.
    /// - parameter key: The key to associate the object with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeConditional<T : AnyObject & Encodable>(_ object: T, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Bool?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Int?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Int8?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Int16?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Int32?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Int64?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: UInt?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: UInt8?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: UInt16?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: UInt32?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: UInt64?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Float?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: Double?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent(_ value: String?, forKey key: Key) throws

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeIfPresent<T : Encodable>(_ value: T?, forKey key: Key) throws

    /// Stores a keyed encoding container for the given key and returns it.
    ///
    /// - parameter keyType: The key type to use for the container.
    /// - parameter key: The key to encode the container for.
    /// - returns: A new keyed encoding container.
    mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey>

    /// Stores an unkeyed encoding container for the given key and returns it.
    ///
    /// - parameter key: The key to encode the container for.
    /// - returns: A new unkeyed encoding container.
    mutating func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer

    /// Stores a new nested container for the default `super` key and returns a new `Encoder` instance for encoding `super` into that container.
    ///
    /// Equivalent to calling `superEncoder(forKey:)` with `Key(stringValue: "super", intValue: 0)`.
    ///
    /// - returns: A new `Encoder` to pass to `super.encode(to:)`.
    mutating func superEncoder() -> Encoder

    /// Stores a new nested container for the given key and returns a new `Encoder` instance for encoding `super` into that container.
    ///
    /// - parameter key: The key to encode `super` for.
    /// - returns: A new `Encoder` to pass to `super.encode(to:)`.
    mutating func superEncoder(forKey key: Key) -> Encoder
}

// An implementation of _KeyedEncodingContainerBase and _KeyedEncodingContainerBox are given at the bottom of this file.

/// A concrete container that provides a view into an encoder's storage, making
/// the encoded properties of an encodable type accessible by keys.
public struct KeyedEncodingContainer<K : CodingKey> : KeyedEncodingContainerProtocol {
    public typealias Key = K

    /// The container for the concrete encoder. The type is _*Base so that it's generic on the key type.
    @_versioned
    internal var _box: _KeyedEncodingContainerBase<Key>

    /// Initializes `self` with the given container.
    ///
    /// - parameter container: The container to hold.
    public init<Container : KeyedEncodingContainerProtocol>(_ container: Container) where Container.Key == Key {
        _box = _KeyedEncodingContainerBox(container)
    }

    /// The path of coding keys taken to get to this point in encoding.
    /// A `nil` value indicates an unkeyed container.
    public var codingPath: [CodingKey] {
        return _box.codingPath
    }

    /// Encodes a null value for the given key.
    ///
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if a null value is invalid in the current context for this format.
    public mutating func encodeNil(forKey key: Key) throws {
        try _box.encodeNil(forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Bool, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Int, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Int8, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Int16, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Int32, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Int64, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: UInt, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: UInt8, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: UInt16, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: UInt32, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: UInt64, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Float, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: Double, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode(_ value: String, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes the given value for the given key.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encode<T : Encodable>(_ value: T, forKey key: Key) throws {
        try _box.encode(value, forKey: key)
    }

    /// Encodes a reference to the given object only if it is encoded unconditionally elsewhere in the payload (previously, or in the future).
    ///
    /// For `Encoder`s which don't support this feature, the default implementation encodes the given object unconditionally.
    ///
    /// - parameter object: The object to encode.
    /// - parameter key: The key to associate the object with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeConditional<T : AnyObject & Encodable>(_ object: T, forKey key: Key) throws {
        try _box.encodeConditional(object, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Bool?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Int?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Int8?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Int16?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Int32?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Int64?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: UInt?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: UInt8?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: UInt16?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: UInt32?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: UInt64?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Float?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: Double?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent(_ value: String?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Encodes the given value for the given key if it is not `nil`.
    ///
    /// - parameter value: The value to encode.
    /// - parameter key: The key to associate the value with.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    public mutating func encodeIfPresent<T : Encodable>(_ value: T?, forKey key: Key) throws {
        try _box.encodeIfPresent(value, forKey: key)
    }

    /// Stores a keyed encoding container for the given key and returns it.
    ///
    /// - parameter keyType: The key type to use for the container.
    /// - parameter key: The key to encode the container for.
    /// - returns: A new keyed encoding container.
    public mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
        return _box.nestedContainer(keyedBy: NestedKey.self, forKey: key)
    }

    /// Stores an unkeyed encoding container for the given key and returns it.
    ///
    /// - parameter key: The key to encode the container for.
    /// - returns: A new unkeyed encoding container.
    public mutating func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
        return _box.nestedUnkeyedContainer(forKey: key)
    }

    /// Stores a new nested container for the default `super` key and returns a new `Encoder` instance for encoding `super` into that container.
    ///
    /// Equivalent to calling `superEncoder(forKey:)` with `Key(stringValue: "super", intValue: 0)`.
    ///
    /// - returns: A new `Encoder` to pass to `super.encode(to:)`.
    public mutating func superEncoder() -> Encoder {
        return _box.superEncoder()
    }

    /// Stores a new nested container for the given key and returns a new `Encoder` instance for encoding `super` into that container.
    ///
    /// - parameter key: The key to encode `super` for.
    /// - returns: A new `Encoder` to pass to `super.encode(to:)`.
    public mutating func superEncoder(forKey key: Key) -> Encoder {
        return _box.superEncoder(forKey: key)
    }
}

/// A type that provides a view into a decoder's storage and is used to hold
/// the encoded properties of a decodable type in a keyed manner.
///
/// Decoders should provide types conforming to `UnkeyedDecodingContainer` for
/// their format.
public protocol KeyedDecodingContainerProtocol {
    associatedtype Key : CodingKey

    /// The path of coding keys taken to get to this point in decoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// All the keys the `Decoder` has for this container.
    ///
    /// Different keyed containers from the same `Decoder` may return different keys here; it is possible to encode with multiple key types which are not convertible to one another. This should report all keys present which are convertible to the requested type.
    var allKeys: [Key] { get }

    /// Returns whether the `Decoder` contains a value associated with the given key.
    ///
    /// The value associated with the given key may be a null value as appropriate for the data format.
    ///
    /// - parameter key: The key to search for.
    /// - returns: Whether the `Decoder` has an entry for the given key.
    func contains(_ key: Key) -> Bool

    /// Decodes a null value for the given key.
    ///
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: Whether the encountered value was null.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    func decodeNil(forKey key: Key) throws -> Bool

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Bool.Type, forKey key: Key) throws -> Bool

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Int.Type, forKey key: Key) throws -> Int

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Int8.Type, forKey key: Key) throws -> Int8

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Int16.Type, forKey key: Key) throws -> Int16

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Int32.Type, forKey key: Key) throws -> Int32

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Int64.Type, forKey key: Key) throws -> Int64

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: UInt.Type, forKey key: Key) throws -> UInt

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: UInt8.Type, forKey key: Key) throws -> UInt8

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: UInt16.Type, forKey key: Key) throws -> UInt16

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: UInt32.Type, forKey key: Key) throws -> UInt32

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: UInt64.Type, forKey key: Key) throws -> UInt64

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Float.Type, forKey key: Key) throws -> Float

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: Double.Type, forKey key: Key) throws -> Double

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode(_ type: String.Type, forKey key: Key) throws -> String

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func decode<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Bool.Type, forKey key: Key) throws -> Bool?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Int.Type, forKey key: Key) throws -> Int?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Int8.Type, forKey key: Key) throws -> Int8?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Int16.Type, forKey key: Key) throws -> Int16?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Int32.Type, forKey key: Key) throws -> Int32?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Int64.Type, forKey key: Key) throws -> Int64?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: UInt.Type, forKey key: Key) throws -> UInt?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: UInt8.Type, forKey key: Key) throws -> UInt8?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: UInt16.Type, forKey key: Key) throws -> UInt16?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: UInt32.Type, forKey key: Key) throws -> UInt32?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: UInt64.Type, forKey key: Key) throws -> UInt64?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Float.Type, forKey key: Key) throws -> Float?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: Double.Type, forKey key: Key) throws -> Double?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent(_ type: String.Type, forKey key: Key) throws -> String?

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    func decodeIfPresent<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T?

    /// Returns the data stored for the given key as represented in a container keyed by the given key type.
    ///
    /// - parameter type: The key type to use for the container.
    /// - parameter key: The key that the nested container is associated with.
    /// - returns: A keyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not a keyed container.
    func nestedContainer<NestedKey>(keyedBy type: NestedKey.Type, forKey key: Key) throws -> KeyedDecodingContainer<NestedKey>

    /// Returns the data stored for the given key as represented in an unkeyed container.
    ///
    /// - parameter key: The key that the nested container is associated with.
    /// - returns: An unkeyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not an unkeyed container.
    func nestedUnkeyedContainer(forKey key: Key) throws -> UnkeyedDecodingContainer

    /// Returns a `Decoder` instance for decoding `super` from the container associated with the default `super` key.
    ///
    /// Equivalent to calling `superDecoder(forKey:)` with `Key(stringValue: "super", intValue: 0)`.
    ///
    /// - returns: A new `Decoder` to pass to `super.init(from:)`.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the default `super` key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the default `super` key.
    func superDecoder() throws -> Decoder

    /// Returns a `Decoder` instance for decoding `super` from the container associated with the given key.
    ///
    /// - parameter key: The key to decode `super` for.
    /// - returns: A new `Decoder` to pass to `super.init(from:)`.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    func superDecoder(forKey key: Key) throws -> Decoder
}

// An implementation of _KeyedDecodingContainerBase and _KeyedDecodingContainerBox are given at the bottom of this file.

/// A concrete container that provides a view into an decoder's storage, making
/// the encoded properties of an decodable type accessible by keys.
public struct KeyedDecodingContainer<K : CodingKey> : KeyedDecodingContainerProtocol {
    public typealias Key = K

    /// The container for the concrete decoder. The type is _*Base so that it's generic on the key type.
    @_versioned
    internal var _box: _KeyedDecodingContainerBase<Key>

    /// Initializes `self` with the given container.
    ///
    /// - parameter container: The container to hold.
    public init<Container : KeyedDecodingContainerProtocol>(_ container: Container) where Container.Key == Key {
        _box = _KeyedDecodingContainerBox(container)
    }

    /// The path of coding keys taken to get to this point in decoding.
    /// A `nil` value indicates an unkeyed container.
    public var codingPath: [CodingKey] {
        return _box.codingPath
    }

    /// All the keys the `Decoder` has for this container.
    ///
    /// Different keyed containers from the same `Decoder` may return different keys here; it is possible to encode with multiple key types which are not convertible to one another. This should report all keys present which are convertible to the requested type.
    public var allKeys: [Key] {
        return _box.allKeys
    }

    /// Returns whether the `Decoder` contains a value associated with the given key.
    ///
    /// The value associated with the given key may be a null value as appropriate for the data format.
    ///
    /// - parameter key: The key to search for.
    /// - returns: Whether the `Decoder` has an entry for the given key.
    public func contains(_ key: Key) -> Bool {
        return _box.contains(key)
    }

    /// Decodes a null value for the given key.
    ///
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: Whether the encountered value was null.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    public func decodeNil(forKey key: Key) throws -> Bool {
        return try _box.decodeNil(forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Bool.Type, forKey key: Key) throws -> Bool {
        return try _box.decode(Bool.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Int.Type, forKey key: Key) throws -> Int {
        return try _box.decode(Int.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Int8.Type, forKey key: Key) throws -> Int8 {
        return try _box.decode(Int8.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Int16.Type, forKey key: Key) throws -> Int16 {
        return try _box.decode(Int16.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Int32.Type, forKey key: Key) throws -> Int32 {
        return try _box.decode(Int32.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Int64.Type, forKey key: Key) throws -> Int64 {
        return try _box.decode(Int64.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: UInt.Type, forKey key: Key) throws -> UInt {
        return try _box.decode(UInt.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: UInt8.Type, forKey key: Key) throws -> UInt8 {
        return try _box.decode(UInt8.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: UInt16.Type, forKey key: Key) throws -> UInt16 {
        return try _box.decode(UInt16.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: UInt32.Type, forKey key: Key) throws -> UInt32 {
        return try _box.decode(UInt32.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: UInt64.Type, forKey key: Key) throws -> UInt64 {
        return try _box.decode(UInt64.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Float.Type, forKey key: Key) throws -> Float {
        return try _box.decode(Float.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: Double.Type, forKey key: Key) throws -> Double {
        return try _box.decode(Double.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode(_ type: String.Type, forKey key: Key) throws -> String {
        return try _box.decode(String.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func decode<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T {
        return try _box.decode(T.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Bool.Type, forKey key: Key) throws -> Bool? {
        return try _box.decodeIfPresent(Bool.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Int.Type, forKey key: Key) throws -> Int? {
        return try _box.decodeIfPresent(Int.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Int8.Type, forKey key: Key) throws -> Int8? {
        return try _box.decodeIfPresent(Int8.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Int16.Type, forKey key: Key) throws -> Int16? {
        return try _box.decodeIfPresent(Int16.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Int32.Type, forKey key: Key) throws -> Int32? {
        return try _box.decodeIfPresent(Int32.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Int64.Type, forKey key: Key) throws -> Int64? {
        return try _box.decodeIfPresent(Int64.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: UInt.Type, forKey key: Key) throws -> UInt? {
        return try _box.decodeIfPresent(UInt.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: UInt8.Type, forKey key: Key) throws -> UInt8? {
        return try _box.decodeIfPresent(UInt8.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: UInt16.Type, forKey key: Key) throws -> UInt16? {
        return try _box.decodeIfPresent(UInt16.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: UInt32.Type, forKey key: Key) throws -> UInt32? {
        return try _box.decodeIfPresent(UInt32.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: UInt64.Type, forKey key: Key) throws -> UInt64? {
        return try _box.decodeIfPresent(UInt64.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Float.Type, forKey key: Key) throws -> Float? {
        return try _box.decodeIfPresent(Float.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: Double.Type, forKey key: Key) throws -> Double? {
        return try _box.decodeIfPresent(Double.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent(_ type: String.Type, forKey key: Key) throws -> String? {
        return try _box.decodeIfPresent(String.self, forKey: key)
    }

    /// Decodes a value of the given type for the given key, if present.
    ///
    /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call.
    ///
    /// - parameter type: The type of value to decode.
    /// - parameter key: The key that the decoded value is associated with.
    /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    public func decodeIfPresent<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T? {
        return try _box.decodeIfPresent(T.self, forKey: key)
    }

    /// Returns the data stored for the given key as represented in a container keyed by the given key type.
    ///
    /// - parameter type: The key type to use for the container.
    /// - parameter key: The key that the nested container is associated with.
    /// - returns: A keyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not a keyed container.
    public func nestedContainer<NestedKey>(keyedBy type: NestedKey.Type, forKey key: Key) throws -> KeyedDecodingContainer<NestedKey> {
        return try _box.nestedContainer(keyedBy: NestedKey.self, forKey: key)
    }

    /// Returns the data stored for the given key as represented in an unkeyed container.
    ///
    /// - parameter key: The key that the nested container is associated with.
    /// - returns: An unkeyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not an unkeyed container.
    public func nestedUnkeyedContainer(forKey key: Key) throws -> UnkeyedDecodingContainer {
        return try _box.nestedUnkeyedContainer(forKey: key)
    }

    /// Returns a `Decoder` instance for decoding `super` from the container associated with the default `super` key.
    ///
    /// Equivalent to calling `superDecoder(forKey:)` with `Key(stringValue: "super", intValue: 0)`.
    ///
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the default `super` key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the default `super` key.
    public func superDecoder() throws -> Decoder {
        return try _box.superDecoder()
    }

    /// Returns a `Decoder` instance for decoding `super` from the container associated with the given key.
    ///
    /// - parameter key: The key to decode `super` for.
    /// - returns: A new `Decoder` to pass to `super.init(from:)`.
    /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry for the given key.
    /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for the given key.
    public func superDecoder(forKey key: Key) throws -> Decoder {
        return try _box.superDecoder(forKey: key)
    }
}

//===----------------------------------------------------------------------===//
// Unkeyed Encoding Containers
//===----------------------------------------------------------------------===//

/// A type that provides a view into an encoder's storage and is used to hold
/// the encoded properties of an encodable type sequentially, without keys.
///
/// Encoders should provide types conforming to `UnkeyedEncodingContainer` for
/// their format.
public protocol UnkeyedEncodingContainer {
    /// The path of coding keys taken to get to this point in encoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// The number of elements encoded into the container.
    var count: Int { get }

    /// Encodes a null value.
    ///
    /// - throws: `EncodingError.invalidValue` if a null value is invalid in the current context for this format.
    mutating func encodeNil() throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Bool) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int8) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int16) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int32) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Int64) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt8) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt16) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt32) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: UInt64) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Float) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: Double) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode(_ value: String) throws

    /// Encodes the given value.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encode<T : Encodable>(_ value: T) throws

    /// Encodes a reference to the given object only if it is encoded unconditionally elsewhere in the payload (previously, or in the future).
    ///
    /// For `Encoder`s which don't support this feature, the default implementation encodes the given object unconditionally.
    ///
    /// For formats which don't support this feature, the default implementation encodes the given object unconditionally.
    ///
    /// - parameter object: The object to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    mutating func encodeConditional<T : AnyObject & Encodable>(_ object: T) throws

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Bool

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int8

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int16

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int32

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int64

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt8

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt16

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt32

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt64

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Float

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Double

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == String

    /// Encodes the elements of the given sequence.
    ///
    /// - parameter sequence: The sequences whose contents to encode.
    /// - throws: An error if any of the contained values throws an error.
    mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element : Encodable

    /// Encodes a nested container keyed by the given type and returns it.
    ///
    /// - parameter keyType: The key type to use for the container.
    /// - returns: A new keyed encoding container.
    mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type) -> KeyedEncodingContainer<NestedKey>

    /// Encodes an unkeyed encoding container and returns it.
    ///
    /// - returns: A new unkeyed encoding container.
    mutating func nestedUnkeyedContainer() -> UnkeyedEncodingContainer

    /// Encodes a nested container and returns an `Encoder` instance for encoding `super` into that container.
    ///
    /// - returns: A new `Encoder` to pass to `super.encode(to:)`.
    mutating func superEncoder() -> Encoder
}

/// A type that provides a view into a decoder's storage and is used to hold
/// the encoded properties of a decodable type sequentially, without keys.
///
/// Decoders should provide types conforming to `UnkeyedDecodingContainer` for
/// their format.
public protocol UnkeyedDecodingContainer {
    /// The path of coding keys taken to get to this point in decoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// Returns the number of elements (if known) contained within this container.
    var count: Int? { get }

    /// Returns whether there are no more elements left to be decoded in the container.
    var isAtEnd: Bool { get }

    /// The current decoding index of the container (i.e. the index of the next element to be decoded.)
    /// Incremented after every successful decode call.
    var currentIndex: Int { get }

    /// Decodes a null value.
    ///
    /// If the value is not null, does not increment currentIndex.
    ///
    /// - returns: Whether the encountered value was null.
    /// - throws: `DecodingError.valueNotFound` if there are no more values to decode.
    mutating func decodeNil() throws -> Bool

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Bool.Type) throws -> Bool

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Int.Type) throws -> Int

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Int8.Type) throws -> Int8

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Int16.Type) throws -> Int16

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Int32.Type) throws -> Int32

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Int64.Type) throws -> Int64

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: UInt.Type) throws -> UInt

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: UInt8.Type) throws -> UInt8

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: UInt16.Type) throws -> UInt16

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: UInt32.Type) throws -> UInt32

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: UInt64.Type) throws -> UInt64

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Float.Type) throws -> Float

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: Double.Type) throws -> Double

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode(_ type: String.Type) throws -> String

    /// Decodes a value of the given type.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A value of the requested type, if present for the given key and convertible to the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func decode<T : Decodable>(_ type: T.Type) throws -> T

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Bool.Type) throws -> Bool?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Int.Type) throws -> Int?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Int8.Type) throws -> Int8?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Int16.Type) throws -> Int16?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Int32.Type) throws -> Int32?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Int64.Type) throws -> Int64?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: UInt.Type) throws -> UInt?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: UInt8.Type) throws -> UInt8?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: UInt16.Type) throws -> UInt16?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: UInt32.Type) throws -> UInt32?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: UInt64.Type) throws -> UInt64?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Float.Type) throws -> Float?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: Double.Type) throws -> Double?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent(_ type: String.Type) throws -> String?

    /// Decodes a value of the given type, if present.
    ///
    /// This method returns `nil` if the container has no elements left to decode, or if the value is null. The difference between these states can be distinguished by checking `isAtEnd`.
    ///
    /// - parameter type: The type of value to decode.
    /// - returns: A decoded value of the requested type, or `nil` if the value is a null value, or if there are no more elements to decode.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type.
    mutating func decodeIfPresent<T : Decodable>(_ type: T.Type) throws -> T?

    /// Decodes a nested container keyed by the given type.
    ///
    /// - parameter type: The key type to use for the container.
    /// - returns: A keyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not a keyed container.
    mutating func nestedContainer<NestedKey>(keyedBy type: NestedKey.Type) throws -> KeyedDecodingContainer<NestedKey>

    /// Decodes an unkeyed nested container.
    ///
    /// - returns: An unkeyed decoding container view into `self`.
    /// - throws: `DecodingError.typeMismatch` if the encountered stored value is not an unkeyed container.
    mutating func nestedUnkeyedContainer() throws -> UnkeyedDecodingContainer

    /// Decodes a nested container and returns a `Decoder` instance for decoding `super` from that container.
    ///
    /// - returns: A new `Decoder` to pass to `super.init(from:)`.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null, or of there are no more values to decode.
    mutating func superDecoder() throws -> Decoder
}

//===----------------------------------------------------------------------===//
// Single Value Encoding Containers
//===----------------------------------------------------------------------===//

/// A container that can support the storage and direct encoding of a single
/// non-keyed value.
public protocol SingleValueEncodingContainer {
    /// The path of coding keys taken to get to this point in encoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// Encodes a null value.
    ///
    /// - throws: `EncodingError.invalidValue` if a null value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encodeNil() throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Bool) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Int) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Int8) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Int16) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Int32) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Int64) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: UInt) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: UInt8) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: UInt16) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: UInt32) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: UInt64) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Float) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: Double) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode(_ value: String) throws

    /// Encodes a single value of the given type.
    ///
    /// - parameter value: The value to encode.
    /// - throws: `EncodingError.invalidValue` if the given value is invalid in the current context for this format.
    /// - precondition: May not be called after a previous `self.encode(_:)` call.
    mutating func encode<T : Encodable>(_ value: T) throws
}

/// A `SingleValueDecodingContainer` is a container which can support the storage and direct decoding of a single non-keyed value.
public protocol SingleValueDecodingContainer {
    /// The path of coding keys taken to get to this point in encoding.
    /// A `nil` value indicates an unkeyed container.
    var codingPath: [CodingKey] { get }

    /// Decodes a null value.
    ///
    /// - returns: Whether the encountered value was null.
    func decodeNil() -> Bool

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Bool.Type) throws -> Bool

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Int.Type) throws -> Int

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Int8.Type) throws -> Int8

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Int16.Type) throws -> Int16

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Int32.Type) throws -> Int32

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Int64.Type) throws -> Int64

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: UInt.Type) throws -> UInt

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: UInt8.Type) throws -> UInt8

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: UInt16.Type) throws -> UInt16

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: UInt32.Type) throws -> UInt32

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: UInt64.Type) throws -> UInt64

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Float.Type) throws -> Float

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: Double.Type) throws -> Double

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode(_ type: String.Type) throws -> String

    /// Decodes a single value of the given type.
    ///
    /// - parameter type: The type to decode as.
    /// - returns: A value of the requested type.
    /// - throws: `DecodingError.typeMismatch` if the encountered encoded value cannot be converted to the requested type.
    /// - throws: `DecodingError.valueNotFound` if the encountered encoded value is null.
    func decode<T : Decodable>(_ type: T.Type) throws -> T
}

//===----------------------------------------------------------------------===//
// User Info
//===----------------------------------------------------------------------===//

/// A user-defined key for providing context during encoding and decoding.
public struct CodingUserInfoKey : RawRepresentable, Equatable, Hashable {
    public typealias RawValue = String

    /// The key's string value.
    public let rawValue: String

    /// Initializes `self` with the given raw value.
    ///
    /// - parameter rawValue: The value of the key.
    public init?(rawValue: String) {
        self.rawValue = rawValue
    }

    /// Returns whether the given keys are equal.
    ///
    /// - parameter lhs: The key to compare against.
    /// - parameter rhs: The key to compare with.
    public static func ==(_ lhs: CodingUserInfoKey, _ rhs: CodingUserInfoKey) -> Bool {
        return lhs.rawValue == rhs.rawValue
    }

    /// The key's hash value.
    public var hashValue: Int {
        return self.rawValue.hashValue
    }
}

//===----------------------------------------------------------------------===//
// Errors
//===----------------------------------------------------------------------===//

/// An error that occurs during the encoding of a value.
public enum EncodingError : Error {
    /// The context in which the error occurred.
    public struct Context {
        /// The path of `CodingKey`s taken to get to the point of the failing encode call.
        public let codingPath: [CodingKey]

        /// A description of what went wrong, for debugging purposes.
        public let debugDescription: String

        /// The underlying error which caused this error, if any.
        public let underlyingError: Error?

        /// Initializes `self` with the given path of `CodingKey`s and a description of what went wrong.
        ///
        /// - parameter codingPath: The path of `CodingKey`s taken to get to the point of the failing encode call.
        /// - parameter debugDescription: A description of what went wrong, for debugging purposes.
        /// - parameter underlyingError: The underlying error which caused this error, if any.
        public init(codingPath: [CodingKey], debugDescription: String, underlyingError: Error? = nil) {
            self.codingPath = codingPath
            self.debugDescription = debugDescription
            self.underlyingError = underlyingError
        }
    }

    /// `.invalidValue` indicates that an `Encoder` or its containers could not encode the given value.
    ///
    /// Contains the attempted value, along with context for debugging.
    case invalidValue(Any, Context)

    // MARK: - NSError Bridging

    // CustomNSError bridging applies only when the CustomNSError conformance is applied in the same module as the declared error type.
    // Since we cannot access CustomNSError (which is defined in Foundation) from here, we can use the "hidden" entry points.

    public var _domain: String {
        return "NSCocoaErrorDomain"
    }

    public var _code: Int {
        switch self {
        case .invalidValue(_, _): return 4866
        }
    }

    public var _userInfo: AnyObject? {
        // The error dictionary must be returned as an AnyObject. We can do this only on platforms with bridging, unfortunately.
        #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
            let context: Context
            switch self {
            case .invalidValue(_, let c): context = c
            }

            var userInfo: [String : Any] = [
                "NSCodingPath": context.codingPath,
                "NSDebugDescription": context.debugDescription
            ]

            if let underlyingError = context.underlyingError {
                userInfo["NSUnderlyingError"] = underlyingError
            }

            return userInfo as AnyObject
        #else
            return nil
        #endif
    }
}

/// An error that occurs during the decoding of a value.
public enum DecodingError : Error {
    /// The context in which the error occurred.
    public struct Context {
        /// The path of `CodingKey`s taken to get to the point of the failing decode call.
        public let codingPath: [CodingKey]

        /// A description of what went wrong, for debugging purposes.
        public let debugDescription: String

        /// The underlying error which caused this error, if any.
        public let underlyingError: Error?

        /// Initializes `self` with the given path of `CodingKey`s and a description of what went wrong.
        ///
        /// - parameter codingPath: The path of `CodingKey`s taken to get to the point of the failing decode call.
        /// - parameter debugDescription: A description of what went wrong, for debugging purposes.
        /// - parameter underlyingError: The underlying error which caused this error, if any.
        public init(codingPath: [CodingKey], debugDescription: String, underlyingError: Error? = nil) {
            self.codingPath = codingPath
            self.debugDescription = debugDescription
            self.underlyingError = underlyingError
        }
    }

    /// `.typeMismatch` indicates that a value of the given type could not be decoded because it did not match the type of what was found in the encoded payload.
    ///
    /// Contains the attempted type, along with context for debugging.
    case typeMismatch(Any.Type, Context)

    /// `.valueNotFound` indicates that a non-optional value of the given type was expected, but a null value was found.
    ///
    /// Contains the attempted type, along with context for debugging.
    case valueNotFound(Any.Type, Context)

    /// `.keyNotFound` indicates that a `KeyedDecodingContainer` was asked for an entry for the given key, but did not contain one.
    ///
    /// Contains the attempted key, along with context for debugging.
    case keyNotFound(CodingKey, Context)

    /// `.dataCorrupted` indicates that the data is corrupted or otherwise invalid.
    ///
    /// Contains context for debugging.
    case dataCorrupted(Context)

    // MARK: - NSError Bridging

    // CustomNSError bridging applies only when the CustomNSError conformance is applied in the same module as the declared error type.
    // Since we cannot access CustomNSError (which is defined in Foundation) from here, we can use the "hidden" entry points.

    public var _domain: String {
        return "NSCocoaErrorDomain"
    }

    public var _code: Int {
        switch self {
        case .keyNotFound(_, _):   fallthrough
        case .valueNotFound(_, _): return 4865
        case .typeMismatch(_, _):  fallthrough
        case .dataCorrupted(_):    return 4864
        }
    }

    public var _userInfo: AnyObject? {
        // The error dictionary must be returned as an AnyObject. We can do this only on platforms with bridging, unfortunately.
        #if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
            let context: Context
            switch self {
            case .keyNotFound(_,   let c): context = c
            case .valueNotFound(_, let c): context = c
            case .typeMismatch(_,  let c): context = c
            case .dataCorrupted(   let c): context = c
            }

            var userInfo: [String : Any] = [
                "NSCodingPath": context.codingPath,
                "NSDebugDescription": context.debugDescription
            ]

            if let underlyingError = context.underlyingError {
                userInfo["NSUnderlyingError"] = underlyingError
            }

            return userInfo as AnyObject
        #else
            return nil
        #endif
    }
}

// The following extensions allow for easier error construction.

internal struct _GenericIndexKey : CodingKey {
    var stringValue: String
    var intValue: Int?

    init?(stringValue: String) {
        return nil
    }

    init?(intValue: Int) {
        self.stringValue = "Index \(intValue)"
        self.intValue = intValue
    }
}

public extension DecodingError {
    /// A convenience method which creates a new .dataCorrupted error using a constructed coding path and the given debug description.
    ///
    /// Constructs a coding path by appending the given key to the given container's coding path.
    ///
    /// - param key: The key which caused the failure.
    /// - param container: The container in which the corrupted data was accessed.
    /// - param debugDescription: A description of the error to aid in debugging.
    static func dataCorruptedError<C : KeyedDecodingContainerProtocol>(forKey key: C.Key, in container: C, debugDescription: String) -> DecodingError {
        let context = DecodingError.Context(codingPath: container.codingPath + [key],
                                            debugDescription: debugDescription)
        return .dataCorrupted(context)
    }

    /// A convenience method which creates a new .dataCorrupted error using a constructed coding path and the given debug description.
    ///
    /// Constructs a coding path by appending a nil key to the given container's coding path.
    ///
    /// - param container: The container in which the corrupted data was accessed.
    /// - param debugDescription: A description of the error to aid in debugging.
    static func dataCorruptedError(in container: UnkeyedDecodingContainer, debugDescription: String) -> DecodingError {
        let context = DecodingError.Context(codingPath: container.codingPath + [_GenericIndexKey(intValue: container.currentIndex)!],
                                            debugDescription: debugDescription)
        return .dataCorrupted(context)
    }

    /// A convenience method which creates a new .dataCorrupted error using a constructed coding path and the given debug description.
    ///
    /// Uses the given container's coding path as the constructed path.
    ///
    /// - param container: The container in which the corrupted data was accessed.
    /// - param debugDescription: A description of the error to aid in debugging.
    static func dataCorruptedError(in container: SingleValueDecodingContainer, debugDescription: String) -> DecodingError {
        let context = DecodingError.Context(codingPath: container.codingPath,
                                            debugDescription: debugDescription)
        return .dataCorrupted(context)
    }
}

//===----------------------------------------------------------------------===//
// Keyed Encoding Container Implementations
//===----------------------------------------------------------------------===//

@_fixed_layout
@_versioned
internal class _KeyedEncodingContainerBase<Key : CodingKey> {
    // These must all be given a concrete implementation in _*Box.
    @_inlineable
    @_versioned
    internal var codingPath: [CodingKey] {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeNil(forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Bool, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Int, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Int8, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Int16, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Int32, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Int64, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: UInt, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: UInt8, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: UInt16, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: UInt32, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: UInt64, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Float, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: Double, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode(_ value: String, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encode<T : Encodable>(_ value: T, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeConditional<T : AnyObject & Encodable>(_ object: T, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Bool?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Int?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Int8?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Int16?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Int32?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Int64?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: UInt?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: UInt8?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: UInt16?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: UInt32?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: UInt64?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Float?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: Double?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent(_ value: String?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func encodeIfPresent<T : Encodable>(_ value: T?, forKey key: Key) throws {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func superEncoder() -> Encoder {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func superEncoder(forKey key: Key) -> Encoder {
        fatalError("_KeyedEncodingContainerBase cannot be used directly.")
    }
}

@_fixed_layout
@_versioned
internal final class _KeyedEncodingContainerBox<Concrete : KeyedEncodingContainerProtocol> : _KeyedEncodingContainerBase<Concrete.Key> {
    typealias Key = Concrete.Key

    @_versioned
    internal var concrete: Concrete

    @_inlineable
    @_versioned
    internal init(_ container: Concrete) {
        concrete = container
    }

    @_inlineable
    @_versioned
    override internal var codingPath: [CodingKey] {
        return concrete.codingPath
    }

    @_inlineable
    @_versioned
    override internal func encodeNil(forKey key: Key) throws {
        try concrete.encodeNil(forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Bool, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Int, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Int8, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Int16, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Int32, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Int64, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: UInt, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: UInt8, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: UInt16, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: UInt32, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: UInt64, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Float, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: Double, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode(_ value: String, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encode<T : Encodable>(_ value: T, forKey key: Key) throws {
        try concrete.encode(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeConditional<T : AnyObject & Encodable>(_ object: T, forKey key: Key) throws {
        try concrete.encodeConditional(object, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Bool?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Int?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Int8?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Int16?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Int32?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Int64?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: UInt?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: UInt8?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: UInt16?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: UInt32?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: UInt64?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Float?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: Double?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent(_ value: String?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func encodeIfPresent<T : Encodable>(_ value: T?, forKey key: Key) throws {
        try concrete.encodeIfPresent(value, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
        return concrete.nestedContainer(keyedBy: NestedKey.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
        return concrete.nestedUnkeyedContainer(forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func superEncoder() -> Encoder {
        return concrete.superEncoder()
    }

    @_inlineable
    @_versioned
    override internal func superEncoder(forKey key: Key) -> Encoder {
        return concrete.superEncoder(forKey: key)
    }
}

@_fixed_layout
@_versioned
internal class _KeyedDecodingContainerBase<Key : CodingKey> {
    @_inlineable
    @_versioned
    internal var codingPath: [CodingKey] {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal var allKeys: [Key] {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func contains(_ key: Key) -> Bool {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeNil(forKey key: Key) throws -> Bool {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Bool.Type, forKey key: Key) throws -> Bool {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Int.Type, forKey key: Key) throws -> Int {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Int8.Type, forKey key: Key) throws -> Int8 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Int16.Type, forKey key: Key) throws -> Int16 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Int32.Type, forKey key: Key) throws -> Int32 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Int64.Type, forKey key: Key) throws -> Int64 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: UInt.Type, forKey key: Key) throws -> UInt {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: UInt8.Type, forKey key: Key) throws -> UInt8 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: UInt16.Type, forKey key: Key) throws -> UInt16 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: UInt32.Type, forKey key: Key) throws -> UInt32 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: UInt64.Type, forKey key: Key) throws -> UInt64 {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Float.Type, forKey key: Key) throws -> Float {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: Double.Type, forKey key: Key) throws -> Double {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode(_ type: String.Type, forKey key: Key) throws -> String {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decode<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Bool.Type, forKey key: Key) throws -> Bool? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Int.Type, forKey key: Key) throws -> Int? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Int8.Type, forKey key: Key) throws -> Int8? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Int16.Type, forKey key: Key) throws -> Int16? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Int32.Type, forKey key: Key) throws -> Int32? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Int64.Type, forKey key: Key) throws -> Int64? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: UInt.Type, forKey key: Key) throws -> UInt? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: UInt8.Type, forKey key: Key) throws -> UInt8? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: UInt16.Type, forKey key: Key) throws -> UInt16? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: UInt32.Type, forKey key: Key) throws -> UInt32? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: UInt64.Type, forKey key: Key) throws -> UInt64? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Float.Type, forKey key: Key) throws -> Float? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: Double.Type, forKey key: Key) throws -> Double? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent(_ type: String.Type, forKey key: Key) throws -> String? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func decodeIfPresent<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T? {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func nestedContainer<NestedKey>(keyedBy type: NestedKey.Type, forKey key: Key) throws -> KeyedDecodingContainer<NestedKey> {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func nestedUnkeyedContainer(forKey key: Key) throws -> UnkeyedDecodingContainer {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func superDecoder() throws -> Decoder {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }

    @_inlineable
    @_versioned
    internal func superDecoder(forKey key: Key) throws -> Decoder {
        fatalError("_KeyedDecodingContainerBase cannot be used directly.")
    }
}

@_fixed_layout
@_versioned
internal final class _KeyedDecodingContainerBox<Concrete : KeyedDecodingContainerProtocol> : _KeyedDecodingContainerBase<Concrete.Key> {
    typealias Key = Concrete.Key

    @_versioned
    internal var concrete: Concrete

    @_inlineable
    @_versioned
    internal init(_ container: Concrete) {
        concrete = container
    }

    @_inlineable
    @_versioned
    override var codingPath: [CodingKey] {
        return concrete.codingPath
    }

    @_inlineable
    @_versioned
    override var allKeys: [Key] {
        return concrete.allKeys
    }

    @_inlineable
    @_versioned
    override internal func contains(_ key: Key) -> Bool {
        return concrete.contains(key)
    }

    @_inlineable
    @_versioned
    override internal func decodeNil(forKey key: Key) throws -> Bool {
        return try concrete.decodeNil(forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Bool.Type, forKey key: Key) throws -> Bool {
        return try concrete.decode(Bool.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Int.Type, forKey key: Key) throws -> Int {
        return try concrete.decode(Int.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Int8.Type, forKey key: Key) throws -> Int8 {
        return try concrete.decode(Int8.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Int16.Type, forKey key: Key) throws -> Int16 {
        return try concrete.decode(Int16.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Int32.Type, forKey key: Key) throws -> Int32 {
        return try concrete.decode(Int32.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Int64.Type, forKey key: Key) throws -> Int64 {
        return try concrete.decode(Int64.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: UInt.Type, forKey key: Key) throws -> UInt {
        return try concrete.decode(UInt.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: UInt8.Type, forKey key: Key) throws -> UInt8 {
        return try concrete.decode(UInt8.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: UInt16.Type, forKey key: Key) throws -> UInt16 {
        return try concrete.decode(UInt16.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: UInt32.Type, forKey key: Key) throws -> UInt32 {
        return try concrete.decode(UInt32.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: UInt64.Type, forKey key: Key) throws -> UInt64 {
        return try concrete.decode(UInt64.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Float.Type, forKey key: Key) throws -> Float {
        return try concrete.decode(Float.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: Double.Type, forKey key: Key) throws -> Double {
        return try concrete.decode(Double.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode(_ type: String.Type, forKey key: Key) throws -> String {
        return try concrete.decode(String.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decode<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T {
        return try concrete.decode(T.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Bool.Type, forKey key: Key) throws -> Bool? {
        return try concrete.decodeIfPresent(Bool.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Int.Type, forKey key: Key) throws -> Int? {
        return try concrete.decodeIfPresent(Int.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Int8.Type, forKey key: Key) throws -> Int8? {
        return try concrete.decodeIfPresent(Int8.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Int16.Type, forKey key: Key) throws -> Int16? {
        return try concrete.decodeIfPresent(Int16.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Int32.Type, forKey key: Key) throws -> Int32? {
        return try concrete.decodeIfPresent(Int32.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Int64.Type, forKey key: Key) throws -> Int64? {
        return try concrete.decodeIfPresent(Int64.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: UInt.Type, forKey key: Key) throws -> UInt? {
        return try concrete.decodeIfPresent(UInt.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: UInt8.Type, forKey key: Key) throws -> UInt8? {
        return try concrete.decodeIfPresent(UInt8.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: UInt16.Type, forKey key: Key) throws -> UInt16? {
        return try concrete.decodeIfPresent(UInt16.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: UInt32.Type, forKey key: Key) throws -> UInt32? {
        return try concrete.decodeIfPresent(UInt32.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: UInt64.Type, forKey key: Key) throws -> UInt64? {
        return try concrete.decodeIfPresent(UInt64.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Float.Type, forKey key: Key) throws -> Float? {
        return try concrete.decodeIfPresent(Float.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: Double.Type, forKey key: Key) throws -> Double? {
        return try concrete.decodeIfPresent(Double.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent(_ type: String.Type, forKey key: Key) throws -> String? {
        return try concrete.decodeIfPresent(String.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func decodeIfPresent<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T? {
        return try concrete.decodeIfPresent(T.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func nestedContainer<NestedKey>(keyedBy type: NestedKey.Type, forKey key: Key) throws -> KeyedDecodingContainer<NestedKey> {
        return try concrete.nestedContainer(keyedBy: NestedKey.self, forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func nestedUnkeyedContainer(forKey key: Key) throws -> UnkeyedDecodingContainer {
        return try concrete.nestedUnkeyedContainer(forKey: key)
    }

    @_inlineable
    @_versioned
    override internal func superDecoder() throws -> Decoder {
        return try concrete.superDecoder()
    }

    @_inlineable
    @_versioned
    override internal func superDecoder(forKey key: Key) throws -> Decoder {
        return try concrete.superDecoder(forKey: key)
    }
}

//===----------------------------------------------------------------------===//
// Primitive and RawRepresentable Extensions
//===----------------------------------------------------------------------===//

extension Bool : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Bool.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Int : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Int.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Int8 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Int8.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Int16 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Int16.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Int32 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Int32.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Int64 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Int64.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension UInt : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(UInt.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension UInt8 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(UInt8.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension UInt16 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(UInt16.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension UInt32 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(UInt32.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension UInt64 : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(UInt64.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Float : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Float.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension Double : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(Double.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

extension String : Codable {
    public init(from decoder: Decoder) throws {
        self = try decoder.singleValueContainer().decode(String.self)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self)
    }
}

public extension RawRepresentable where RawValue == Bool, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Bool, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Int, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Int, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Int8, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Int8, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Int16, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Int16, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Int32, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Int32, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Int64, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Int64, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == UInt, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == UInt, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == UInt8, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == UInt8, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == UInt16, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == UInt16, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == UInt32, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == UInt32, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == UInt64, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == UInt64, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Float, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Float, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == Double, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == Double, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

public extension RawRepresentable where RawValue == String, Self : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(self.rawValue)
    }
}

public extension RawRepresentable where RawValue == String, Self : Decodable {
    public init(from decoder: Decoder) throws {
      let decoded = try decoder.singleValueContainer().decode(RawValue.self)
      guard let value = Self(rawValue: decoded) else {
          throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
      }

      self = value
    }
}

//===----------------------------------------------------------------------===//
// Optional/Collection Type Conformances
//===----------------------------------------------------------------------===//

fileprivate func assertTypeIsEncodable<T>(_ type: T.Type, in wrappingType: Any.Type) {
    guard T.self is Encodable.Type else {
        if T.self == Encodable.self || T.self == Codable.self {
            preconditionFailure("\(wrappingType) does not conform to Encodable because Encodable does not conform to itself. You must use a concrete type to encode or decode.")
        } else {
            preconditionFailure("\(wrappingType) does not conform to Encodable because \(T.self) does not conform to Encodable.")
        }
    }
}

fileprivate func assertTypeIsDecodable<T>(_ type: T.Type, in wrappingType: Any.Type) {
    guard T.self is Decodable.Type else {
        if T.self == Decodable.self || T.self == Codable.self {
            preconditionFailure("\(wrappingType) does not conform to Decodable because Decodable does not conform to itself. You must use a concrete type to encode or decode.")
        } else {
            preconditionFailure("\(wrappingType) does not conform to Decodable because \(T.self) does not conform to Decodable.")
        }
    }
}

// FIXME: Uncomment when conditional conformance is available.
extension Optional : Encodable /* where Wrapped : Encodable */ {
    public func encode(to encoder: Encoder) throws {
        assertTypeIsEncodable(Wrapped.self, in: type(of: self))

        var container = encoder.singleValueContainer()
        switch self {
        case .none: try container.encodeNil()
        case .some(let wrapped): try (wrapped as! Encodable).encode(to: encoder)
        }
    }
}

extension Optional : Decodable /* where Wrapped : Decodable */ {
    public init(from decoder: Decoder) throws {
        // Initialize self here so we can get type(of: self).
        self = .none
        assertTypeIsDecodable(Wrapped.self, in: type(of: self))

        let container = try decoder.singleValueContainer()
        if !container.decodeNil() {
            let metaType = (Wrapped.self as! Decodable.Type)
            let element = try metaType.init(from: decoder)
            self = .some(element as! Wrapped)
        }
    }
}

// FIXME: Uncomment when conditional conformance is available.
extension Array : Encodable /* where Element : Encodable */ {
    public func encode(to encoder: Encoder) throws {
        assertTypeIsEncodable(Element.self, in: type(of: self))

        var container = encoder.unkeyedContainer()
        for element in self {
            // superEncoder appends an empty element and wraps an Encoder around it.
            // This is normally appropriate for encoding super, but this is really what we want to do.
            let subencoder = container.superEncoder()
            try (element as! Encodable).encode(to: subencoder)
        }
    }
}

extension Array : Decodable /* where Element : Decodable */ {
    public init(from decoder: Decoder) throws {
        // Initialize self here so we can get type(of: self).
        self.init()
        assertTypeIsDecodable(Element.self, in: type(of: self))

        let metaType = (Element.self as! Decodable.Type)
        var container = try decoder.unkeyedContainer()
        while !container.isAtEnd {
            // superDecoder fetches the next element as a container and wraps a Decoder around it.
            // This is normally appropriate for decoding super, but this is really what we want to do.
            let subdecoder = try container.superDecoder()
            let element = try metaType.init(from: subdecoder)
            self.append(element as! Element)
        }
    }
}

extension Set : Encodable /* where Element : Encodable */ {
    public func encode(to encoder: Encoder) throws {
        assertTypeIsEncodable(Element.self, in: type(of: self))

        var container = encoder.unkeyedContainer()
        for element in self {
            // superEncoder appends an empty element and wraps an Encoder around it.
            // This is normally appropriate for encoding super, but this is really what we want to do.
            let subencoder = container.superEncoder()
            try (element as! Encodable).encode(to: subencoder)
        }
    }
}

extension Set : Decodable /* where Element : Decodable */ {
    public init(from decoder: Decoder) throws {
        // Initialize self here so we can get type(of: self).
        self.init()
        assertTypeIsDecodable(Element.self, in: type(of: self))

        let metaType = (Element.self as! Decodable.Type)
        var container = try decoder.unkeyedContainer()
        while !container.isAtEnd {
            // superDecoder fetches the next element as a container and wraps a Decoder around it.
            // This is normally appropriate for decoding super, but this is really what we want to do.
            let subdecoder = try container.superDecoder()
            let element = try metaType.init(from: subdecoder)
            self.insert(element as! Element)
        }
    }
}

/// A wrapper for dictionary keys which are Strings or Ints.
internal struct _DictionaryCodingKey : CodingKey {
    let stringValue: String
    let intValue: Int?

    init?(stringValue: String) {
        self.stringValue = stringValue
        self.intValue = Int(stringValue)
    }

    init?(intValue: Int) {
        self.stringValue = "\(intValue)"
        self.intValue = intValue
    }
}

extension Dictionary : Encodable /* where Key : Encodable, Value : Encodable */ {
    public func encode(to encoder: Encoder) throws {
        assertTypeIsEncodable(Key.self, in: type(of: self))
        assertTypeIsEncodable(Value.self, in: type(of: self))

        if Key.self == String.self {
            // Since the keys are already Strings, we can use them as keys directly.
            var container = encoder.container(keyedBy: _DictionaryCodingKey.self)
            for (key, value) in self {
                let codingKey = _DictionaryCodingKey(stringValue: key as! String)!
                let valueEncoder = container.superEncoder(forKey: codingKey)
                try (value as! Encodable).encode(to: valueEncoder)
            }
        } else if Key.self == Int.self {
            // Since the keys are already Ints, we can use them as keys directly.
            var container = encoder.container(keyedBy: _DictionaryCodingKey.self)
            for (key, value) in self {
                let codingKey = _DictionaryCodingKey(intValue: key as! Int)!
                let valueEncoder = container.superEncoder(forKey: codingKey)
                try (value as! Encodable).encode(to: valueEncoder)
            }
        } else {
            // Keys are Encodable but not Strings or Ints, so we cannot arbitrarily convert to keys.
            // We can encode as an array of alternating key-value pairs, though.
            var container = encoder.unkeyedContainer()
            for (key, value) in self {
                // superEncoder appends an empty element and wraps an Encoder around it.
                // This is normally appropriate for encoding super, but this is really what we want to do.
                let keyEncoder = container.superEncoder()
                try (key as! Encodable).encode(to: keyEncoder)

                let valueEncoder = container.superEncoder()
                try (value as! Encodable).encode(to: valueEncoder)
            }
        }
    }
}

extension Dictionary : Decodable /* where Key : Decodable, Value : Decodable */ {
    public init(from decoder: Decoder) throws {
        // Initialize self here so we can print type(of: self).
        self.init()
        assertTypeIsDecodable(Key.self, in: type(of: self))
        assertTypeIsDecodable(Value.self, in: type(of: self))

        if Key.self == String.self {
            // The keys are Strings, so we should be able to expect a keyed container.
            let container = try decoder.container(keyedBy: _DictionaryCodingKey.self)
            let valueMetaType = Value.self as! Decodable.Type
            for key in container.allKeys {
                let valueDecoder = try container.superDecoder(forKey: key)
                let value = try valueMetaType.init(from: valueDecoder)
                self[key.stringValue as! Key] = (value as! Value)
            }
        } else if Key.self == Int.self {
            // The keys are Ints, so we should be able to expect a keyed container.
            let valueMetaType = Value.self as! Decodable.Type
            let container = try decoder.container(keyedBy: _DictionaryCodingKey.self)
            for key in container.allKeys {
                guard key.intValue != nil else {
                    // We provide stringValues for Int keys; if an encoder chooses not to use the actual intValues, we've encoded string keys.
                    // So on init, _DictionaryCodingKey tries to parse string keys as Ints. If that succeeds, then we would have had an intValue here.
                    // We don't, so this isn't a valid Int key.
                    var codingPath = decoder.codingPath
                    codingPath.append(key)
                    throw DecodingError.typeMismatch(Int.self,
                                                     DecodingError.Context(codingPath: codingPath,
                                                                           debugDescription: "Expected Int key but found String key instead."))
                }

                let valueDecoder = try container.superDecoder(forKey: key)
                let value = try valueMetaType.init(from: valueDecoder)
                self[key.intValue! as! Key] = (value as! Value)
            }
        } else {
            // We should have encoded as an array of alternating key-value pairs.
            var container = try decoder.unkeyedContainer()

            // We're expecting to get pairs. If the container has a known count, it had better be even; no point in doing work if not.
            if let count = container.count {
                guard count % 2 == 0 else {
                    throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath,
                                                                            debugDescription: "Expected collection of key-value pairs; encountered odd-length array instead."))
                }
            }

            let keyMetaType = (Key.self as! Decodable.Type)
            let valueMetaType = (Value.self as! Decodable.Type)
            while !container.isAtEnd {
                // superDecoder fetches the next element as a container and wraps a Decoder around it.
                // This is normally appropriate for decoding super, but this is really what we want to do.
                let keyDecoder = try container.superDecoder()
                let key = try keyMetaType.init(from: keyDecoder)

                guard !container.isAtEnd else {
                    throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath,
                                                                                 debugDescription: "Unkeyed container reached end before value in key-value pair."))
                }

                let valueDecoder = try container.superDecoder()
                let value = try valueMetaType.init(from: valueDecoder)

                self[key as! Key] = (value as! Value)
            }
        }
    }
}

//===----------------------------------------------------------------------===//
// Convenience Default Implementations
//===----------------------------------------------------------------------===//

// Default implementation of encodeConditional(_:forKey:) in terms of encode(_:forKey:)
public extension KeyedEncodingContainerProtocol {
    public mutating func encodeConditional<T : AnyObject & Encodable>(_ object: T, forKey key: Key) throws {
        try encode(object, forKey: key)
    }
}

// Default implementation of encodeIfPresent(_:forKey:) in terms of encode(_:forKey:)
public extension KeyedEncodingContainerProtocol {
    public mutating func encodeIfPresent(_ value: Bool?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Int?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Int8?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Int16?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Int32?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Int64?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: UInt?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: UInt8?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: UInt16?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: UInt32?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: UInt64?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Float?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: Double?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent(_ value: String?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }

    public mutating func encodeIfPresent<T : Encodable>(_ value: T?, forKey key: Key) throws {
        guard let value = value else { return }
        try encode(value, forKey: key)
    }
}

// Default implementation of decodeIfPresent(_:forKey:) in terms of decode(_:forKey:) and decodeNil(forKey:)
public extension KeyedDecodingContainerProtocol {
    public func decodeIfPresent(_ type: Bool.Type, forKey key: Key) throws -> Bool? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Bool.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Int.Type, forKey key: Key) throws -> Int? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Int.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Int8.Type, forKey key: Key) throws -> Int8? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Int8.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Int16.Type, forKey key: Key) throws -> Int16? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Int16.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Int32.Type, forKey key: Key) throws -> Int32? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Int32.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Int64.Type, forKey key: Key) throws -> Int64? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Int64.self, forKey: key)
    }

    public func decodeIfPresent(_ type: UInt.Type, forKey key: Key) throws -> UInt? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(UInt.self, forKey: key)
    }

    public func decodeIfPresent(_ type: UInt8.Type, forKey key: Key) throws -> UInt8? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(UInt8.self, forKey: key)
    }

    public func decodeIfPresent(_ type: UInt16.Type, forKey key: Key) throws -> UInt16? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(UInt16.self, forKey: key)
    }

    public func decodeIfPresent(_ type: UInt32.Type, forKey key: Key) throws -> UInt32? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(UInt32.self, forKey: key)
    }

    public func decodeIfPresent(_ type: UInt64.Type, forKey key: Key) throws -> UInt64? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(UInt64.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Float.Type, forKey key: Key) throws -> Float? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Float.self, forKey: key)
    }

    public func decodeIfPresent(_ type: Double.Type, forKey key: Key) throws -> Double? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(Double.self, forKey: key)
    }

    public func decodeIfPresent(_ type: String.Type, forKey key: Key) throws -> String? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(String.self, forKey: key)
    }

    public func decodeIfPresent<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T? {
        guard try self.contains(key) && !self.decodeNil(forKey: key) else { return nil }
        return try self.decode(T.self, forKey: key)
    }
}

// Default implementation of encodeConditional(_:) in terms of encode(_:), and encode(contentsOf:) in terms of encode(_:) loop.
public extension UnkeyedEncodingContainer {
    public mutating func encodeConditional<T : AnyObject & Encodable>(_ object: T) throws {
        try self.encode(object)
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Bool {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int8 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int16 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int32 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Int64 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt8 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt16 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt32 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == UInt64 {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Float {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == Double {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element == String {
        for element in sequence {
            try self.encode(element)
        }
    }

    public mutating func encode<T : Sequence>(contentsOf sequence: T) throws where T.Iterator.Element : Encodable {
        for element in sequence {
            try self.encode(element)
        }
    }
}

// Default implementation of decodeIfPresent(_:) in terms of decode(_:) and decodeNil()
public extension UnkeyedDecodingContainer {
    mutating func decodeIfPresent(_ type: Bool.Type) throws -> Bool? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Bool.self)
    }

    mutating func decodeIfPresent(_ type: Int.Type) throws -> Int? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Int.self)
    }

    mutating func decodeIfPresent(_ type: Int8.Type) throws -> Int8? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Int8.self)
    }

    mutating func decodeIfPresent(_ type: Int16.Type) throws -> Int16? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Int16.self)
    }

    mutating func decodeIfPresent(_ type: Int32.Type) throws -> Int32? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Int32.self)
    }

    mutating func decodeIfPresent(_ type: Int64.Type) throws -> Int64? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Int64.self)
    }

    mutating func decodeIfPresent(_ type: UInt.Type) throws -> UInt? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(UInt.self)
    }

    mutating func decodeIfPresent(_ type: UInt8.Type) throws -> UInt8? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(UInt8.self)
    }

    mutating func decodeIfPresent(_ type: UInt16.Type) throws -> UInt16? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(UInt16.self)
    }

    mutating func decodeIfPresent(_ type: UInt32.Type) throws -> UInt32? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(UInt32.self)
    }

    mutating func decodeIfPresent(_ type: UInt64.Type) throws -> UInt64? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(UInt64.self)
    }

    mutating func decodeIfPresent(_ type: Float.Type) throws -> Float? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Float.self)
    }

    mutating func decodeIfPresent(_ type: Double.Type) throws -> Double? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(Double.self)
    }

    mutating func decodeIfPresent(_ type: String.Type) throws -> String? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(String.self)
    }

    mutating func decodeIfPresent<T : Decodable>(_ type: T.Type) throws -> T? {
        guard try !self.isAtEnd && !self.decodeNil() else { return nil }
        return try self.decode(T.self)
    }
}