Unexpected result of IsInstanceOf

[Ben Jones via Digitalmars-d-learn]

The following result doesn't make sense to me... how does 
isInstanceOf return false?

```
import std.traits;
import std.stdio;
import std.typecons;
auto f(T)(T t){
        return Nullable!T(t);   
}
void main(){
    auto f3 = f(3);
    writeln(typeof(f3).stringof);
    writeln(isInstanceOf!(Nullable, f3));
}
```
outputs

```
Nullable!int
false
```