What's wrong with opCall ?

[Dmitry Olshansky]

Hello,

Considering my effort on rework of the range API for upcoming 
std.datetime (It's coming, Jonathan, but far slower then I thought).
I was in determined to see where can I get with function-like objects, 
but it seems I'm missing something.
The following program do not compile:
struct S{
    ubyte _d;
    this(ubyte d){
        _d = d;
    }
    int opCall(string a){
        return  _d;
    }
}
void main(){
    S s = S(22);
    assert(s.opCall("A") == 22);
//    s("A");//uncoment to get compile error
}

bug.d(14): Error: constructor bug.S.this (ubyte d) is not callable using 
argument types
bug.d(14): Error: cannot implicitly convert expression ("A") of type 
string to ubyte

So it appears that compiler views it as a call to constructor instead. 
What's the problem?

--
Dmitry Olshansky