false positive attribute error with dataclasses and metaclasses

[crazystick]

Given the following code:

import dataclasses
from enum import EnumMeta, Enum
from typing import Optional, Union


class MyEnumMeta(EnumMeta):
    """Metaclass for Enum classes"""


class MyEnum(Enum, metaclass=MyEnumMeta):
    FIRST = "thing1"


@dataclasses.dataclass
class Holder:
    thing: Optional[Union[MyEnum, str]] = dataclasses.field(default=None)

    def to_dto(self):
        return {
            "thing": self.thing.value if isinstance(self.thing, MyEnum) else self.thing
        }


def test_enums():
    none_thing = None
    enum_thing = MyEnum.FIRST
    string_thing = "thing2"

    my_value = none_thing.value if isinstance(none_thing, MyEnum) else none_thing
    assert my_value is None
    my_value = enum_thing.value if isinstance(enum_thing, MyEnum) else enum_thing
    assert my_value == "thing1"
    my_value = string_thing.value if isinstance(string_thing, MyEnum) else string_thing
    assert my_value == "thing2"

    holder = Holder(thing=None)
    assert holder.to_dto() == {"thing": None}
    holder = Holder(thing=enum_thing)
    assert holder.to_dto() == {"thing": "thing1"}
    holder = Holder(thing=string_thing)
    assert holder.to_dto() == {"thing": "thing2"}


if __name__ == "__main__":
    test_enums()

    print("Everything passed")

produces the following errors:

File "enum_error.py", line 20, in to_dto: No attribute 'value' on None [attribute-error]
  In Optional[Union[Any, str]]
Called from (traceback):
  line 45, in current file
  line 37, in test_enums
File "enum_error.py", line 20, in to_dto: No attribute 'value' on str [attribute-error]
  In Optional[Union[Any, str]]
Called from (traceback):
  line 45, in current file
  line 37, in test_enums

However no error is produced if you remove the dataclass (i.e. just assigning to my_value), and also no error is produced when removing the metaclass from MyEnum