SE-0320: Coding of non String / Int keyed Dictionary into a KeyedContainer

Evolution Proposal Reviews
6

Could a task-local flag and property wrapper be used for opting in?

public enum DictionaryTaskLocals {
  @TaskLocal
  public static var shouldUseKeyedContainer: Bool = false

  @propertyWrapper
  public struct ShouldUseKeyedContainer<Wrapped> {
    public var wrappedValue: Wrapped
    public init(wrappedValue: Wrapped) {
      self.wrappedValue = wrappedValue
    }
  }
}

They would only be available on OS versions that support the new behavior.

enum MyEnum: String, Codable { case one, two }

struct MyStruct: Codable {
  @DictionaryTaskLocals.ShouldUseKeyedContainer
  var array: [MyStruct]

  @DictionaryTaskLocals.ShouldUseKeyedContainer
  var dictionary: [MyEnum: MyStruct]
}

The property wrapper would bind to the task-local flag, and then forward calls to the dictionary.

extension DictionaryTaskLocals.ShouldUseKeyedContainer: Decodable
where Wrapped: Decodable {
  public init(from decoder: Decoder) throws {
    wrappedValue =
    try DictionaryTaskLocals.$shouldUseKeyedContainer.withValue(true) {
      try .init(from: decoder)
    }
  }
}

extension DictionaryTaskLocals.ShouldUseKeyedContainer: Encodable
where Wrapped: Encodable {
  public func encode(to encoder: Encoder) throws {
    try DictionaryTaskLocals.$shouldUseKeyedContainer.withValue(true) {
      try wrappedValue.encode(to: encoder)
    }
  }
}

The dictionary would only attempt the new behavior if the task-local flag is set.
It would fallback to the old behavior when a keyed container isn't available.

(EDIT: Generalized the property wrapper.)