public protocol PropertyDescribing {
  associatedtype Content
  var name: String { get }
}

public struct PropertyDescriptor<Content>: PropertyDescribing {
  // ...
}

public struct UnsafePropertyDescriptor: PropertyDescribing {
  public typealias Content = Any
  // ...
}

@magic protocol Reflectable {
  public static func properties: [String]
  public static func unsafeDescriptor(forProperty name: String) throws -> UnsafePropertyDescriptor
  /// needn't be inferred from the return type if the specializing function proposal is accepted.
  public static func descriptor<Value>(forProperty name: String) throws -> PropertyDescriptor<Value>

  public func getValue<Descriptor: PropertyDescribing>(using descriptor: Descriptor) -> Descriptor.Content
  /// Throws an error if the property is read-only.
  public mutating func setValue<Descriptor: PropertyDescribing>(value: Descriptor.Content, using descriptor: Descriptor) throws
}

extension Reflectable {
  public func getValue(forProperty name: String) throws -> Any {
    return getValue(try Self.unsafeDescriptor(forProperty: name))
  }

  public mutating func setValue(value: Any, forProperty name: String) throws {
    try setValue(value, using: try Self.unsafeDescriptor(forProperty: name))
  }
}

func callSite() {
  let myObject = ReflectableClass()

  /// Unsafe shorthand
  let description: Any = try! myObject.getValue(forProperty: "description")

  /// Typed version.
  let descriptor = try! SomethingReflectable.descriptor<String>(forProperty: "descriptor")
  let description2: String = myObject.getValue(using: descriptor)

  let myValue = ReflectableStruct()

  /// BAMM!
  try! myValue.setValue("Testing", forProperty: "message") /// compiler warning: cannot mutate `myValue`.
}