Weird UFC and opCall issues

[Jay Pinkman via Digitalmars-d-learn]

void main () {
    struct X {
        string x;

        void opCall (string y) {
            import std.stdio;
            writeln("%s %s!", x, y);
        }
    }
    auto x = X("hello");
    "world".x;
}

source/main.d(12): Error: need 'this' for 'opCall' of type 
'void(string y)'
source/main.d(13): Error: no property 'x' for type 'string'

but why?