Mypy doesn't seem to use types from @overload-ed __sub__

Bug Report

The __sub__ method on the Arrow class from the arrow package has several @overloads:

@overload
def __sub__(self, other: Union[timedelta, relativedelta]) -> "Arrow":
    pass  # pragma: no cover

@overload
def __sub__(self, other: Union[dt_datetime, "Arrow"]) -> timedelta:
    pass  # pragma: no cover

def __sub__(self, other: Any) -> Union[timedelta, "Arrow"]:

    if isinstance(other, (timedelta, relativedelta)):
        return self.fromdatetime(self._datetime - other, self._datetime.tzinfo)

    elif isinstance(other, dt_datetime):
        return self._datetime - other

    elif isinstance(other, Arrow):
        return self._datetime - other._datetime

    return NotImplemented

In particular, one Arrow object minus another Arrow object will always yield a timedelta. However, it seems mypy does not see this, and mistakes the type for Arrow.

To Reproduce

I discovered the issue when finding the time diff between two arrow timestamps:

import arrow

a1 = arrow.get()
a2 = arrow.get()

diff = a2 - a1
print(diff.total_seconds())

Which, when run with mypy yields:

$ mypy test.py
test.py:6: error: "int" not callable
Found 1 error in 1 file (checked 1 source file)

Setting the type explicitly reviles that mypy thinks the type is Arrow, and not timedelta:

from datetime import timedelta
import arrow

a1 = arrow.get()
a2 = arrow.get()

diff: timedelta = a2 - a1
print(diff.total_seconds())

Running mypy:

$ mypy test.py
test.py:7: error: Incompatible types in assignment (expression has type "Arrow", variable has type "timedelta")
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

The second @overload on Arrow's __sub__ method specifies that the diff of two Arrow objects is a timedelta. There should be no type error assigning that to a timedelta variable.

Actual Behavior

Mypy mistakenly thinks the diff is Arrow, which creates a false positive.

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9.6
• Operating system and version: Mac OS 11.6 (Big Sur)

首先，使用 Python 代码片段和所述的 mypy 0.910 环境重现报告中的推断错误。阅读 arrow/arrow.py 中的重载声明，然后跟踪 mypy 对重载 sub 调用的处理；当 Arrow 减去 Arrow 被推断为 timedelta，且所示赋值和 total_seconds() 调用通过类型检查时，即表示完成。