Type constraint

[Joel via Digitalmars-d-learn]

I’ve got a struct that has a method that adds numbers together. I 
want to do something like this, static if (isInteger!T) … but it 
isn’t working. static if (is(T==int)) works for one integer type.

```d
struct List(T) {
     auto addUp()
         If (isInteger!T) {
             (Add numbers)
         }
    }
}
```