@omochimetaru
The downside of having two types is the inconvenience of sometimes having a Substring when you need a String, and vice-versa. It is likely this would be a significantly bigger problem than with Array and ArraySlice, as slicing of String is such a common operation. It is especially relevant to existing code that assumes String is the currency type -- that is, the default string type used for everyday exchange between APIs. To ease the pain of type mismatches, Substring should be a subtype of String in the same way that Int is a subtype of Optional<Int>. This would give users an implicit conversion from Substring to String, as well as the usual implicit conversions such as [Substring] to [String] that other subtype relationships receive.