Protocol Buffers 3: Enums as keys in a map
<p>Enums are not allowed to be used as keys in map. PaxType here is an enum and not allowed to be used as key.</p>

<pre><code>enum PaxType {
    ADULT = 0 ;
    CHILD = 1 ;
    INFANT = 2 ;
}

message FlightData {
    map&lt;PaxType, FareType&gt; fareType = 1;
}
</code></pre>