`do` returns T

[zuccoi]

I want do to return arbitrary value. When you don't return anything in block of do, it's usage is same as before.

[devxoul]

Use then instead if you'd like to use such like that :-)

[zuccoi]

then's primary use case is to customize newly created instance. So, then returns the instance of type Self.

On the other hand, do is not restricted to Self. do can return nothing, and instance with type of T (arbitrary type).

If you allow do to return T, you can do something like below

let firstDateOfNextMonth: Date = Calendar(identifier: .gregorian).do {
  var date = $0.date(bySetting: .day, value: 1, of: Date())!
  date = $0.date(byAdding: .month, value: 1, to: date)!
  return date
}

[RomanPodymov]

Hello @zuccoi Do you still want to merge this pull request?