Union passthrough does not work when target union uses a new-style type alias as element for passthrough

[jhominal]

Hello,

I have just found an issue with the configure_union_passthrough strategy, when using the 3.12 type alias as an element of a union to structure.

Here is my attempt at an MRE, tested on Python 3.12:

import dataclasses
import typing

import cattrs
import cattrs.strategies


converter = cattrs.Converter()
cattrs.strategies.configure_union_passthrough(str | int, converter)


@dataclasses.dataclass
class DataClass:
    field: str


type NewScalar = str
OldScalar: typing.TypeAlias = str

if __name__ == "__main__":
    data = {"field": "value"}

    v1 = converter.structure(data, OldScalar | DataClass)  # This one works
    print(v1)
    v2 = converter.structure(data, NewScalar | DataClass)  # This one does not work
    print(v2)

The line before last raises cattrs.errors.StructureHandlerNotFoundError: Unsupported type: NewScalar | __main__.DataClass. Register a structure hook for it.

[Tinche]

Got it. We should probably be smart here and support both type aliases and newtypes out of the box. I'll see what I can do in the next few days.

[jhominal]

Given the way that new-style type aliases can be nested, I wonder if that would basically require a function that "resolves" union by going through the layers of TypeAliasType values.

The thing that worries me here is the way that a bugfix interacts with the way that cattrs currently documents hooks that resolve specifically on a new-style type alias.