Dict update typing error, update Dict[Union[int, str], str] from Dict[int, str]

Bug Report

To Reproduce

from typing import Dict, Union

dict_a: Dict[Union[int, str], int] = {}
dict_b: Dict[int, int] = {}
dict_a.update(dict_b)
print(dict_a)

Expected Behavior

No mypy error. dict_b has stricter typing than dict_a, this update operation should be fine.
Actual Behavior

error: Argument 1 to "update" of "MutableMapping" has incompatible type "dict[int, int]"; expected "SupportsKeysAndGetItem[Union[int, str], int]"  [arg-type]

Your Environment

• Mypy version used: 1.5.1 (compiled: yes)
• Python version used: 3.9.16

Other Notes
updating items iteratively apparently has no issue.

from typing import Dict, Union

dict_a: Dict[Union[int, str], int] = {}
dict_b: Dict[int, int] = {}
for k, v in dict_b.items():
    dict_b[k] = v
print(dict_a)

Using Any of course has no issue too.

from typing import Any, Dict

dict_a: Dict[Any, int] = {}
dict_b: Dict[int, int] = {}
dict_a.update(dict_b)
print(dict_a)

Start by running the Python reproduction with mypy 1.5.1 and compare the reported update error with the iterative update example. Trace the typing used for MutableMapping.update and add a regression test showing that dict[int, int] can update Dict[Union[int, str], int] without an error.