What is the difference between these template declaration forms?

[cc via Digitalmars-d-learn]

Are these identical?  Or is there a different usage for the (T : 
something) form?
```d
        auto opCast(T)() if (is(T == bool)) {
                return _obj !is null;
        }
```
```d
        auto opCast(T : bool)() {
                return _obj !is null;
        }
```