'subscript' is unavailable: cannot subscript String with a CountableClosedRange<Int>, see the documentation comment for discussion

In Swift 4, I'm getting this error when I try to take a Substring of a String using subscript syntax.

'subscript' is unavailable: cannot subscript String with a CountableClosedRange, see the documentation comment for discussion

For example:

let myString: String = "foobar"
let mySubstring: Substring = myString[1..<3]

Two questions:

1. How can I resolve this error?
2. Where is "the documentation comment for discussion" that was referred to in the error?

1. If you want to use subscripts on Strings like "palindrome"[1..<3] and "palindrome"[1...3], use these extensions.

Swift 4

extension String {
    subscript (bounds: CountableClosedRange<Int>) -> String {
        let start = index(startIndex, offsetBy: bounds.lowerBound)
        let end = index(startIndex, offsetBy: bounds.upperBound)
        return String(self[start...end])
    }

    subscript (bounds: CountableRange<Int>) -> String {
        let start = index(startIndex, offsetBy: bounds.lowerBound)
        let end = index(startIndex, offsetBy: bounds.upperBound)
        return String(self[start..<end])
    }
}

Swift 3

For Swift 3 replace with return self[start...end] and return self[start..<end].

2. Apple didn't build this into the Swift language because the definition of a 'character' depends on how the String is encoded. A character can be 8 to 64 bits, and the default is usually UTF-16. You can specify other String encodings in String.Index.

This is the documentation that Xcode error refers to.

More on String encodings like UTF-8 and UTF-16