Output Range Problem? How to make it work?

[apz28 via Digitalmars-d-learn]

import std.range.primitives: isOutputRange;

    @safe:
    struct Buffer
    {
    @safe:

        void put(char c)
        {}
        void put(scope const(char)[] s)
        {}
    }

    struct Foo
    {
    @safe:

        string toString()
        {return null;}

        ref Writer toString(Writer, Char)(return ref Writer sink)
        if (isOutputRange!(Writer, Char) && isSomeChar!Char)
        {return sink;}

        string toString(const(char)[] fmt)
        {return null;}
    }

    void main()
    {   
        Buffer buffer;
        Foo foo;
        foo.toString(buffer);
    }


/* Getting this error
onlineapp.d(34): Error: none of the overloads of `toString` are 
callable using argument types `(Buffer)`, candidates are:
onlineapp.d(19):        `onlineapp.Foo.toString()`
onlineapp.d(26):        `onlineapp.Foo.toString(const(char)[] 
fmt)`
onlineapp.d(22):        `toString(Writer, Char)(return ref Writer 
sink)`
*/