When enum variable is marked as nullable by "?", it stops working.
Here is a small piece of code demonstrating the problem:

enum MyEnum { ONE, TWO }
int main(string[] args)
{
    MyEnum a = MyEnum.ONE;
    if (a == MyEnum.ONE)
        stdout.printf("a is one\n");

    MyEnum? b = MyEnum.ONE;
    if (b == MyEnum.ONE)
        stdout.printf("b is one\n");
    return 0;
}

It should print:
a is one
b is one

but it prints only the first line instead.

If "MyEnum? b" is replaced by "MyEnum b" everything works.