Generating and interpreting tags for union types

In the following code the __tag__ field is not generated for a member of a union type:

from dataclasses import dataclass

import dataclass_wizard as dw
from dataclass_wizard import LoadMeta


@dataclass
class ResErr:
    item_id: str
    msg: str


@dataclass
class ResOk:
    item_id: str
    item: str


Res = ResErr | ResOk

LoadMeta(tag='ResErr').bind_to(ResErr)
LoadMeta(tag='ResOk').bind_to(ResOk)

err = ResErr(item_id='Q0001', msg='err')
print(dw.asdict(err))
# {'item_id': 'Q0001', 'msg': 'err'}

When specifying the union type explicitely in asdict:

res = dw.asdict(err, cls=Res)

The following error occurs:

Traceback (most recent call last):
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/_dumpers.py", line 1288, in asdict
    return cls.__dataclass_wizard_to_dict__(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'types.UnionType' object has no attribute '__dataclass_wizard_to_dict__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/foo/Code/dw-test/src/kgqa_eval/scripts/dw_tag_1.py", line 31, in <module>
    print(dw.asdict(err, cls=Res))
          ~~~~~~~~~^^^^^^^^^^^^^^
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/_dumpers.py", line 1292, in asdict
    fn = dump_func_for_dataclass(cls)
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/_dumpers.py", line 831, in dump_func_for_dataclass
    cls_fields = dataclass_fields(cls)
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/utils/_dataclass_compat.py", line 113, in dataclass_fields
    return FIELDS[cls]
           ~~~~~~^^^^^
  File "/home/foo/.local/share/uv/python/cpython-3.13.13-linux-x86_64-gnu/lib/python3.13/weakref.py", line 415, in __getitem__
    return self.data[ref(key)]
                     ~~~^^^^^
TypeError: cannot create weak reference to 'types.UnionType' object

When a container type is defined, then the __tag__ field is generated:

@dataclass
class Container:
    res: Res


err = Container(res=ResErr(item_id='Q0001', msg='err'))
print(dw.asdict(err))
# {'res': {'item_id': 'Q0001', 'msg': 'err', '__tag__': 'ResErr'}}

Is it possible to generate the __tag__ field in the first example as well?

It also would be nice to be able to parse a dict to the Res union type:

res = dw.fromdict(Res, {'item_id': 'Q0001', 'msg': 'err', '__tag__': 'ResErr'})
print(res)

Currenty this results in the following error:

Traceback (most recent call last):
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/_loaders.py", line 1671, in fromdict
    return cls.__dataclass_wizard_from_dict__(d)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'types.UnionType' object has no attribute '__dataclass_wizard_from_dict__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/foo/Code/dw-test/src/kgqa_eval/scripts/dw_tag_1.py", line 34, in <module>
    err = dw.fromdict(Res, {'item_id': 'Q0001', 'msg': 'err', '__tag__': 'ResErr'})
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/_loaders.py", line 1674, in fromdict
    fn = load_func_for_dataclass(cls)
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/_loaders.py", line 1142, in load_func_for_dataclass
    fields = dataclass_fields(cls)
  File "/home/foo/Code/dw-test/.venv/lib/python3.13/site-packages/dataclass_wizard/utils/_dataclass_compat.py", line 113, in dataclass_fields
    return FIELDS[cls]
           ~~~~~~^^^^^
  File "/home/foo/.local/share/uv/python/cpython-3.13.13-linux-x86_64-gnu/lib/python3.13/weakref.py", line 415, in __getitem__
    return self.data[ref(key)]
                     ~~~^^^^^
TypeError: cannot create weak reference to 'types.UnionType' object

The used version of dataclass-wizard is 1.0.0.

Start with the asdict and fromdict entry points in _dumpers.py and _loaders.py, then trace the union handling behind the reported errors. Use the ResErr/ResOk examples as regression cases. Done means direct serialization adds the expected tag and fromdict can resolve the tagged Res union, while the existing Container behavior remains intact.