get_type_hints change in behavior for Annotated with from future import annotations 3.9+

列挙されている Python バージョン全体で issue の Annotated と forward-reference の例を再現し、その後、報告で言及されている _eval_type パスを調査します。Annotated がある場合とない場合の動作を比較し、意図された get_type_hints の結果を判断します。動作に一貫性があるか、または回帰テストによってサポート対象のセマンティクスが明示されれば完了です。

Bug report

from __future__ import annotations

from typing import Sequence, Union
from typing_extensions import Annotated, get_type_hints

Config = Union[str, Sequence["Config"]]
class Foo:
 x: Annotated[Config, "hi"]

print(Config)
print(get_type_hints(Foo)["x"])

The output prints are different in 3.8.13 vs 3.9.13/3.10.4/3.11.0b1. In 3.8.7 the output is,

typing.Union[str, typing.Sequence[ForwardRef('Config')]]
typing.Union[str, typing.Sequence[ForwardRef('Config')]]

In 3.9.13/3.10.4/3.11.0b1 the output is,

typing.Union[str, typing.Sequence[ForwardRef('Config')]]
typing.Union[str, typing.Sequence[typing.Union[str, typing.Sequence[ForwardRef('Config')]]]]

get_type_hints is doing 1 type expansion on forward reference. There's second inconsistency here which is that Annotated changes output in 3.9+. I'd expect get_type_hints(Foo, include_extras=False) to have same type with or without Annotated. If I drop Annotated and instead test

Config = Union[str, Sequence["Config"]]
class Foo:
 x: Config

print(Config)
print(get_type_hints(Foo)["x"])

the 1 type expansion goes away and output is same as 3.8.7 output.

The from __future__ import annotations is important or you can also manually quote and do x: "Config".

The exact behavior is niche and would be understandable if this was documented as undefined implementation detail similar to how cross module aliases are awkward to work with at runtime.

Expansion affected runtime internal serialization tool I was using and I ended up needing to do some workarounds to fix tests. I haven't tried testing if pydantic/similar libraries handle this case well or not. Expansion also affects documentation produced by a tool like sphinx-autodoc-typehints.

Your environment
I'm on mac, although hope this isn't platform specific. I also tried testing from typing import Annotated instead of typing_extensions for 3.9+ and it didn't resolve inconsistency. My first guess is _eval_type is where this behavior change comes from. Newest version I tested up to was 3.11.0b1.