opCast'ing strings

[helxi]

struct Fraction
{
private:
	int numerator = 1, denominator = 1;
public:
	string opCast(T : string)() const
	{
		import std.conv : to;

		return numerator.to!string() ~ "/" ~ denominator.to!string();
	}
}

void main()
{
	import std.stdio, std.conv;

	Fraction n = Fraction(23, 11);
	writeln(n.to!string(), " ", n.opCast!string);
}



In this program, casting using to does not work as intended 
(returning 23/11) on the struct. However, calling opCast directly 
seems to do the job. Why is that?