Enum conversion undefined behavior in C++

README.md

$ clang++ main.cpp -std=c++11 -o main && ./main
Converting red to string: red
Converting 2 to enum to string: yellow
Converting 20 to enum to string: undefined
Converting 20 to enum to int: 20

main.cpp

#import <iostream>

enum Color : int { red = 1, yellow = 2, green = 4, grey = 17, ranch_dressing = 42 };

std::string colorToString (Color x) {
    switch (x) {
        case red: return "red";
        case yellow: return "yellow";
        case green: return "green";
        case grey: return "grey";
        case ranch_dressing: return "ranch_dressing";
        default: return "undefined";
    }
}

int main (int argc, char** argv) {
    Color x = red;
    std::cout << "Converting red to string: " << colorToString(x) << std::endl;
    Color y = static_cast<Color>(2);
    std::cout << "Converting 2 to enum to string: " << colorToString(y) << std::endl;
    Color z = static_cast<Color>(20);
    std::cout << "Converting 20 to enum to string: " << colorToString(z) << std::endl;
    std::cout << "Converting 20 to enum to int: " << (int)(z) << std::endl;
}