datetimes `__sub__` is not symmetrical in subclass.

import datetime
from typing import Any

class MyDatetime(datetime.datetime):
    def __eq__(self, other: Any) -> bool:
        if isinstance(other, datetime.datetime):
            delta = self - other  # error: No overload variant of "__sub__" of "datetime" matches argument type "datetime"  [operator]
            delta = other - self  # OK
            return True
        else:
            return NotImplemented

python: 3.9
mypy: 0.950
playground: https://mypy-play.net/?mypy=0.950&python=3.9&gist=ac2ffa60be085e552cb67043afa59e07

The applicability of the superclass __sub__ method is only recognized for datetime - MyDatetime, but not for MyDatetime - datetime.

Looking into typeshed, there is this annotation:

_D = TypeVar("_D", bound=date)

class datetime(date):
    if sys.version_info >= (3, 8):
        def __sub__(self: _D, __other: _D) -> timedelta: ...
    else:
        def __sub__(self, __other: datetime) -> timedelta: ...

Telling mypy that the python version is 3.7 gets rid of the error, so there seems to be some issue, where the _Ds are not treated the same way depending on the order of operations.

The same error is not reported for mypy 0.921 and earlier: https://mypy-play.net/?mypy=0.921&python=3.9&gist=ac2ffa60be085e552cb67043afa59e07

Start by running the linked mypy-play reproducer with Python 3.9 and comparing behavior between mypy 0.921 and 0.950. Then inspect the typeshed datetime.sub annotations, focusing on the _D type variable and operand order; done means both subtraction directions type-check consistently for the subclass example.