gistfile1.txt

let attributes = [ NSForegroundColorAttributeName : self.view.tintColor ]
let attributedString = NSAttributedString(string: NSLocalizedString("Sign In", comment: ""), attributes: attributes)
warns: (Expression implicitly coerced from 'UIColor?' to Any)

right way:
let attributes: [ String : UIColor ]
if let maybeAttributes = [ NSForegroundColorAttributeName : self.view.tintColor ] {
  attributes = maybeAttributes
} else {
  assertionFailure("self.view.tintColor is nil")
}

how it will be:
let attributes = [ NSForegroundColorAttributeName : self.view.tintColor! ]
let attributedString = NSAttributedString(string: NSLocalizedString("Sign In", comment: ""), attributes: attributes)