Check if template has been passed a reference type or value type?

[1967 via Digitalmars-d-learn]

I've got a template that takes in a type. Sometimes the type is a 
class, sometimes a struct, sometimes just an int. It doesn't much 
matter what it is, but if it's a reference type I need to check 
if it's null so I can make it not null before using it. I get an 
error if I try to check if a value type is null so I'd like to 
put a "static if" to check if the type passed to the template is 
a nullable one so I can avoid checking the value types passed in. 
What do I have to do to check for nullability?