Can't use disambiguation strategy together with `Optional`

• cattrs version: 23.1.0.dev0
• Python version: 3.11.1
• Operating System: Manjaro KDE

Description

An optional tagged union doesn't work.

I believe the Optional (or | None) portion could be parsed beforehand, like it is on most fields, and the remaining type (Foo | Bar) then parsed separately, thus recognizing it as a tagged union.

What I Did

import attrs
import cattrs
from cattrs.strategies import configure_tagged_union


@attrs.define
class Foo:
    n: int = 42


@attrs.define
class Bar:
    n: int = 0


@attrs.define
class Baz:
    c: Foo | Bar


@attrs.define
class BazNone:
    c: Foo | Bar | None  # Could also have `= None` as a default value


converter = cattrs.Converter()
configure_tagged_union(Foo | Bar, converter=converter)
print(converter.unstructure(Baz(Bar(123))))  # {'c': {'n': 123, '_type': 'Bar'}}
print(converter.unstructure(BazNone(Bar(123))))  # {'c': {'n': 123}}

Start by reproducing the example with configure_tagged_union and the BazNone annotation, then inspect the tagged-union handling around that entry point. Done means an Optional or | None wrapper still uses the configured Foo | Bar disambiguation and preserves the tagged output shown for Baz.