StringRepresentable casting with generics

StringRepresentable.swift

enum StringRepresentableType: String {
    case foo
    case bar
}

typealias JSONDictionary = [String: Any]

extension JSONDictionary {
    func value<T, R>(for key: String) -> T? where T: RawRepresentable, T.RawValue == R {

        guard let rawValue = self[key] as? R,
            let value = T(rawValue: rawValue) else {
            return nil
        }

        return value
    }
}

let barValue = StringRepresentableType(rawValue: "bar")!

let dict: JSONDictionary = ["fooKey": "foo", "barKey": "bar"]
if let value: StringRepresentableType = dict.value(for: "barKey") {
    print(value)
}