String is an undeclared type?

In the example below, the extension function has an undeclared parameter type. String. Huh? How is that an undeclared type?

import UIKit

enum ColumnType: String {
    case StringArray
    , IntArray
    , DoubleArray
    , String = "text"
    , Int = "int"
    , Double = "double"
    , Unknown
}

extension ColumnType {
    func fromRaw(rawValue: String) -> ColumnType {
        if let valueType = ColumnType(rawValue: rawValue) {
            return valueType
        }

        switch rawValue {
        case "text", "Text":
            return .String
        case "int", "Int":
            return .Int
        case "double", "Double":
            return .Double
        default:
            return .Unknown
        }
    }
}