Here is the problem:

enum MyEnum { ONE, TWO }

int main(string[] args)
{
    MyEnum a = MyEnum.ONE;
    MyEnum? b = MyEnum.ONE;

    if (a == b)
        stdout.printf("OK\n");
    else
        stdout.printf("KO\n");

    return 0;
}

If 'a' and 'b' are of type "int" and "int?", respectively, the comparison works as expected and the program prints "OK".

However, the above program prints "KO". During the compilation, gcc reports this warning:
> warning: comparison between pointer and integer

Here is the relevant C code generated:

MyEnum a;
MyEnum* b;
MyEnum _tmp2_;
MyEnum* _tmp3_;
// ...
_tmp2_ = a;
_tmp3_ = b;
if ((&_tmp2_) == (*_tmp3_)) {
// ...