This should be a "tribool"-like type.

enum Value : byte { Undef = 0, True = 1, False = -1 };

unittest
{
    with(Value) {
    assert(~Undef == Undef); // failure
    assert(~True  == False);
    assert(~False == True);
}

When I overload opUnary!("~") in this way, unittests fail.

Value opUnary(string op)(Value operand) if(op == "~")
out(result)
{
    with(Value) assert(result == Undef ||
                       result == False ||
                       result == True);
}
body
{
    return cast(Value)(operand * -1);
}

What is the correct way to do this?

This works:
assert(Value.Undef.opUnary!"~"() == Value.Undef);

Tobias Pankrath:

> When I overload opUnary!("~") in this way, unittests fail.

Currently in D you can only overload struct and class operators.

Bye,
bearophile