Hi! Tried this piece of code:

from prefect import flow, task
from prefect.states import Completed

@task
def task_a():
    return Completed(message='A', data={'A': 1})

@flow
def some_flow():
    a = task_a.submit().result()
    return

if __name__ == '__main__':
    some_flow()

It works on Prefect2.20.0, but doesn't on Prefect3.0.0rc16. On Prefect3 it produces such error message:

16:44:46.929 | ERROR   | Task run 'task_a-0' - Task run failed with exception: TypeError("Can't instantiate abstract class BaseResult with abstract methods create, get") - Retries are exhausted
Traceback (most recent call last):
  File "My_Path\venv\lib\site-packages\prefect\task_engine.py", line 804, in run_context
    yield self
  File "My_Path\venv\lib\site-packages\prefect\task_engine.py", line 1420, in run_task_sync
    engine.call_task_fn(txn)
  File "My_Path\venv\lib\site-packages\prefect\task_engine.py", line 832, in call_task_fn
    result = call_with_parameters(self.task.fn, parameters)
  File "My_Path\venv\lib\site-packages\prefect\utilities\callables.py", line 208, in call_with_parameters
    return fn(*args, **kwargs)
  File "My_Path\test_flow4.py", line 6, in task_a
    return Completed(message='A', data={"A": 1})
  File "My_Path\venv\lib\site-packages\prefect\states.py", line 556, in Completed
    return cls(type=StateType.COMPLETED, **kwargs)
  File "My_Path\venv\lib\site-packages\pydantic\main.py", line 176, in __init__
    self.__pydantic_validator__.validate_python(data, self_instance=self)
  File "My_Path\venv\lib\site-packages\prefect\results.py", line 405, in __new__
    return super().__new__(cls)
TypeError: Can't instantiate abstract class BaseResult with abstract methods create, get
16:44:47.005 | ERROR   | Task run 'task_a-0' - Finished in state Failed("Task run encountered an exception TypeError: Can't instantiate abstract class BaseResult with abstract methods create, get")

Seems that I cannot pass a dict as data on Prefect3, because the code works for

data='A'

,

data=1

, and

data={1}

. Is there something that should be done differently (if I want to pass dict as a parameter) or it's unexpected type as of now?

Thank you for opening an issue for this!! It is indeed a bug. For anyone else who finds this, the relevant issue is here: https://github.com/PrefectHQ/prefect/issues/14927