When did `guard let foo = foo` become legal?

Question

Back in November of 2016 I posted a question asking why I couldn\'t use guard to create an unwrapped version of a variable using the same name as the optional, like you can

Answer 1

TL;DR

guard let foo = foo is legal if foo was defined in another scope.

---

The example from your linked question:

func test()
{
  let a: Int? = 1

  guard let a = a else{
    return
  }
  print("a = \(a)")
}

still doesn't work because the guard statement is trying to create another variable a in the same scope.

This example:

//Test of using guard to create an unwrapped version of a var, like if let
func guardTest(_ viewController: UIViewController?) -> UIViewController? {
  // Check if the current viewController exists
  print(String(describing: viewController))
  guard let viewController = viewController else {
    return nil
  }
  print(String(describing: viewController))

  return viewController
}

works for the same reason that this does:

func test(a: Int)
{
    print(type(of: a))  // Int

    let a = 3.14

    print(type(of: a))  // Double
}

The parameter to the function is defined in a different scope, so Swift allows you to create a local variable with the same name.