これ↓めっちゃ便利じゃないですか？

infix operator ..<?: RangeFormationPrecedence func ..<?<T>(lhs: T, rhs: T) -> Range<T>? where T: Comparable { guard lhs <= rhs else { return nil } return lhs ..< rhs } infix operator ...?: RangeFormationPrecedence func ...?<T>(lhs: T, rhs: T) -> ClosedRange<T>? where T: Comparable { guard lhs <= rhs else { return nil } return lhs ... rhs }