protobuf-c/t/issue220/issue220.proto

message TopLevel {
    oneof submessages {
        MessageType1 type1 = 1;
        MessageType2 type2 = 2;
    }
}

message MessageType1 {
    enum Flag {
        OK = 1;
    }
    optional Flag flag = 1;
}

message MessageType2 {
    enum AnotherFlag {
        OK = 1;
    }
    optional AnotherFlag flag = 1;
}