Treat a single integer value as a range in Swift

I need to validate the length of a string. The allowed values for the character count are:

• 6 – 9 characters
• 12 characters
• 15 characters

All strings with a different character count are invalid. Thus, I would like to create a Swift function that accepts a number of ranges and evaluates the string:

extension String {

    func evaluateLength(validCharacterCounts: Range<Int>...) -> Bool {
        // Implementation
    }

}

Now I can call the function for a single Int range:

"Live long and prosper".evaluateLength(validCharacterCounts: 6..<10)

and multiple Int ranges:

"Live long and prosper".evaluateLength(validCharacterCounts: 6..<10, 15..<20)

But I cannot call the function with single, isolated integer values:

"Live long and prosper".evaluateLength(validCharacterCounts: 6..<10, 12, 15)

because 12 and 15 are typed as Int and not as Range<Int>.

Swift compile error: Cannot convert value of type 'Int' to expected argument type 'Range'

Is there a way to treat a single Integer as a Range in Swift, like casting it automatically to Range<Int>?

(After all 5 is equivalent to 5..<6, so mathematically speaking 5 is a range as well.)