Why I can't pass to datetime.benchmark function with parameters?

[Suliman via Digitalmars-d-learn]

Working:
void main()
{
  auto r = benchmark!(foo)(1);
}

void foo()
{

}


Do not working:
void main()
{
  auto r = benchmark!(foo())(1);
}

void foo()
{

}

>Error: expression foo() is void and has no value

Do not working:
void main()
{
  auto r = benchmark!(4)(1);
}

void foo(int i)
{
  writeln(i);
}


Why I second two variants do not compile?