Inconsistent MRO error when `__new__` returns a subclass and `--warn-unreachable` is turned on

[tmke8]

Bug Report

The problem appeared in the pandas stubs in this issue: pandas-dev/pandas-stubs#269

There, I was told to report it here.

To Reproduce
I reduced the problem to a standalone example, but it might be possible to reduce it further:

Playground URL: https://mypy-play.net/?mypy=master&python=3.10&flags=warn-unreachable&gist=18e35c8047f69e2418498f85cc8f02d0

from __future__ import annotations
import datetime
from typing import Any, Generic, TypeVar, Union, overload

S1 = TypeVar("S1", int, datetime.datetime)

class Series(Generic[S1]):
    @overload
    def __new__(cls, data: datetime.datetime) -> TimestampSeries: ...

    @overload
    def __new__(cls, data: dict[int, S1]) -> Series[S1]: ...

    def __new__(cls, data: Union[datetime.datetime, dict[int, S1]]) -> Any:
        return

class TimestampSeries(Series[datetime.datetime]): ...

x = Series({0: 0, 1: 1})
reveal_type(x)
assert isinstance(x, Series)

Note that the error only appears with --warn-unreachable.

Expected Behavior

Given that the reveal_type() shows that the type is Series[int], I would expect the subsequent isinstance() call to succeed.

Actual Behavior

main.py:20: note: Revealed type is "__main__.Series[builtins.int]"
main.py:21: error: Subclass of "Series[int]" and "TimestampSeries" cannot exist: would have inconsistent method resolution order  [unreachable]
Found 1 error in 1 file (checked 1 source file)

Your Environment

See the mypy playground link.