C++ :: Why Do Virtual Enums Compile When They Cannot Be Defined In Derived Class

Assume this class:

class GenericTrafficLight {
public:
virtual enum LightState;
void setLightState(LightState newState) {
currentState = newState;
}
private:
LightState currentState;
};

And this deriving class:

class FuturisticTrafficLight : public GenericTrafficLight {
public:
enum LightState {
LIGHT_STATE_RED = 0,
LIGHT_STATE_YELLOW = 1,
LIGHT_STATE_CYAN = 2,
LIGHT_STATE_GREEN = 3
};
};

This yields this error: "C2911 (...) cannot be declared or defined in the current scope" in the deriving class's enum definition.