is operator and SortedRange

[RazvanN via Digitalmars-d-learn]

I am a bit confused about how the is operator works. I have a 
function which receives an InputRange and a predicate. Now I need 
to be able to test if the InputRange is actually a SortedRange. I 
don't care about how the datatypes behind the SortedRange or the 
predicate, I just need to see if the object is a SortedRange. I 
have tried the following test:

static if(is(typeof(haystack) == SortedRange!(T, _pred), T, 
_pred))

where haystack is the InputRange, but the test fails. Is there a 
way to test if the InputRange is a SortedRange without having to 
explicitly pass the primitive tupe on top of which the 
SortedRange is built?