A call like this:
```
from attr import attrib
from attr.validators import optional, instance_of
c = attrib(default=None, validator=optional(instance_of(bytes)))
```
Was returning no return value, because the first overload was T -> T.
The solution was to change the ordering so that the first overload is
None -> T and infer the type from the other arguments.
David Euresti
097e9f5e71