Type hinting doesn't make sense for a use clause. In there is supposed to be a list of variables that exist in the current scope that get copied into the closure*. As such the code should know the type (likely even the actual value) of each variable. Compare that with a parameter list where the function doesn't know what values someone may try to pass into it, thus typing becomes useful.

On that note, your example. It doesn't make sense either because $b does not exist, and if you run the code (without the stdClass hint) then you'll get a warning about that. If you then resolve the warning by defining $b then, well, you know exactly what $b is.

* References are a little different in that the function gets a copy of the reference, not of the underlying value. And like in other places, a by-ref variable doesn't have to exist at the time of the call.