[Aschenbath]

With forbid_extra_keys=True, configure_tagged_union popped the tag out of the payload before dispatching to the member's structure hook. That pop is needed so members that don't declare the tag don't trip the extra-keys check — but when a member class does declare a field with the tag's name, the pop removed data its hook requires:

@define
class SpecialDevice:
    hostname: str
    port: int
    type: str

@define
class StandardDevice:
    hostname: str
    type: str

c = Converter(forbid_extra_keys=True)
configure_tagged_union(SpecialDevice | StandardDevice, c, tag_name="type", ...)
c.structure({"type": "special", "hostname": "h", "port": 1234}, ...)
# ClassValidationError caused by KeyError: 'type'

This moves the pop from the dispatchers into the per-member wrapper: the dispatchers now pass the payload through, and each member hook receives a tag-stripped copy only if that member does not consume the tag as one of its own fields (checked against its init fields, accounting for cattrs' leading-underscore key renaming). The default= fallback for unknown tags keeps the previous behavior (tag stripped before the default handler runs). Non-forbid_extra_keys converters are untouched.

Tests: new cases in tests/strategies/test_tagged_unions.py cover the reported scenario with and without default= (structure succeeds, the member's type field is populated from the tag, unstructure round-trips) and confirm members that don't declare the tag still structure fine under forbid_extra_keys=True. The strategies suite plus tests/test_unions.py pass: 285 passed, 15 xfailed. A changelog entry is added under NEXT in HISTORY.md.

Fixes #483

[Aschenbath]

Closing this one myself: I opened too many PRs here in a short time, which isn't fair to your review queue — sorry about that. I'm leaving #827 and #822 open. If this fix is something you'd want, I'm happy to reopen or rebase it later.