struct S

{

    int n;

​

    void toString (Output) (ref Output output)

        if (isOutputRange!(Output, char))

    {

        import std.format : formattedWrite;

        output.formattedWrite!"%s"(2 * this.n);

    }

}

void main ()

{

    import std.range : empty, front, popFront, isOutputRange;

​

    auto s = S(2);    

    char[] output;

    static assert(isOutputRange!(typeof(output), char));

    s.toString(output);

}