How do i use std.functional.binaryFun?

[Gary Willoughby via Digitalmars-d-learn]

How do i successfully use std.functional.binaryFun in the 
following example?

import std.stdio;
import std.functional;

class Foo(T, alias greater = "a > b") if 
(is(typeof(binaryFun!(greater)(T.init, T.init)) == bool))
{
	private alias compare = binaryFun!(greater);

	public this()
	{
		writefln("%s", this.compare(2, 1));
	}
}

void main(string[] args)
{
	auto foo = new Foo!(int, "a < b"); // Works

	auto bar = new Foo!(int, delegate(int a, int b){ return a > b; 
}); // Linker error.
}

The first instantiation works when using a string but I get a 
linker error when i try and use a delegate as the compare 
function. Why is this? and what do i need to do to correct this?