A opIndexUnary quiz

[bearophile]

A small quiz (it's a reprise of an older discussion). This code 
compiles with no errors, but do you see anything wrong here?

// -----------------
struct Foo {
    int x;
    alias this = x;
}

class Bar {
    Foo[] data;

    this() {
        data.length = 10;
    }

    Foo opIndex(uint i) {
        return data[i];
    }

    void opIndexUnary(string op)(uint i) if (op == "++") {
        data[i]++;
    }

    void opIndexUnaryRight(string op)(uint i) if (op == "++") {
        data[i]++;
    }
}

void main() {
    auto barfoo = new Bar;
    ++barfoo[3];
    assert(barfoo.data[3] == 1);
    barfoo[3]++;
    assert(barfoo.data[3] == 2);
}
// -----------------


Do you think this code should produce some compilation errors?

Bye,
bearophile