mypy ignores isinstance

[ghost]

After passing isinstance(obj, dict) successfully, obj is certainly of type dict.
mypy gives warnings as if there was no check.

To Reproduce

from typing import Callable

b: list[Callable[[str], str] | str] = []

def myfun(a: dict[str, str] | str) -> None:
    if isinstance(a, dict):
        b.append(lambda key: a[key])
    else:
        b.append(a)

myfun({"key": "hello"})
myfun("world")
print(b[0]("key"), b[1])  # hello world

Expected Behavior

There are no warnings.

Actual Behavior

1.py:7: error: Invalid index type "str" for "Union[Dict[str, str], str]"; expected type "Union[SupportsIndex, slice]"  [index]
1.py:13: error: "str" not callable  [operator]
Found 2 errors in 1 file (checked 1 source file)

Environment

• Mypy version used: mypy 0.991 (compiled: no)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: Python 3.11.0

[hauntsaninja]

https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions / #2608