func whatever(foo: @autoclosure () -> Int) {
  let x = foo()
  print(x)
}

whatever(foo: 5)
// prints: 5

whatever(foo: { 5 })
// Error: Function produces expected type 'Int'; did you mean to call it with '()'?

struct Defaults {

  static var dispatcher: Defaults = ...

  subscript<T>(setting: Setting<T>) -> T { ... }

  struct Setting<T> {
    let key: String
    let defaultValue: () -> T

    init(key: String, defaultValue: @escaping @autoclosure () -> T) {
      self.key = key
      self.defaultValue = defaultValue
    }
  }
}

extension Defaults.Setting {
  static var nickname: Defaults.Setting<String> {
    return Defaults.Setting(key: "__nickname", defaultValue: "Angela Merkel")
  }
}

//  Usage:
Defaults.dispatcher[.nickname] = "Emmanuel Macron"

extension Defaults.Setting {
  var withHashedKey: Defaults.Setting<T> {
    return Defaults.Setting(key: key.md5(), defaultValue: defaultValue)
    // Error: Cannot convert return expression of type 'Defaults.Setting<() -> T>' to return type 'Defaults.Setting<T>'
  }
}

extension Defaults.Setting {
  init(key: String, defaultValue: @escaping () -> T) {
    self.init(key: key, defaultValue: defaultValue)
  }
}