[1.20 regression] _Environ.get(key, default) returns str | None when default is an alias to Any

[pablogsal]

Since mypy 1.20, passing a value whose type is an alias to Any (e.g. Incomplete: TypeAlias = Any) as the default argument of os.environ.get causes mypy to infer str | None instead of Any, producing a spurious [arg-type] error at the use site.

Reproducer: https://mypy-play.net/?gist=7a79f64ac9733542b571cc358f6d3165

import os
from pathlib import Path
from typing import Any
from typing_extensions import TypeAlias

Incomplete: TypeAlias = Any

A: Incomplete = Path()
reveal_type(A)               # Any  (good)

B = os.environ.get('TEST', A)
reveal_type(B)               # str | None  (bad)  (expected: Any)

C = Path(B)                  # error: Argument 1 to "Path" has incompatible
                             # type "str | None"; expected "str | PathLike[str]"

Bisected to 30260ca57 which added explicit _Environ.get/pop overloads where the first overload is def get(self, key, default: None = None) -> AnyStr | None. Before that sync, _Environ inherited Mapping.get, whose first overload took no default argument, so 2-arg calls couldn't match it.

Versions: broken on mypy ≥ 1.20.0; works on mypy ≤ 1.19.1.

[hauntsaninja]

Bisects to this typeshed sync #20505

[hauntsaninja]

Looks like it'll be python/typeshed#15194

[hauntsaninja]

So either typeshed needs to be changed, or the following behaviour of mypy needs to be changed:

from typing import Any, Generic, TypeVar, overload
from typing_extensions import TypeAlias

_T = TypeVar("_T")

class Env:
    @overload
    def get(self, key: str, default: None = None) -> str | None: ...
    @overload
    def get(self, key: str, default: str) -> str: ...
    @overload
    def get(self, key: str, default: _T) -> str | _T: ...

Incomplete: TypeAlias = Any

def main(unknown: Incomplete, env: Env):
    reveal_type(env.get('key', unknown))

This is similar to e.g. pyright

[hauntsaninja]

Reminds me of python/typeshed#13222

[pablogsal]

This is the reason i opened #21345 (sorry for not linking it here)