False Positive when assigning to union type as dictionary key

Bug Report

When defining a dictionary where the values are of type Union[dict, None]/Optional[dict]/dict | None and then using a conditional to confirm that the value is a dictionary before index assignment, mypy incorrectly reports that the index assignment is not valid with the following error message:

Unsupported target for indexed assignment ("dict[str, int | float] | None")

To Reproduce

https://gist.github.com/mypy-play/89bae9639af70dd72064b748098e54c5

Expected Behavior

Mypy recognizes that, due to the conditional and the assignment to an empty dict within the conditional scope, that the value is not None and is in fact an empty dictionary and does not raise a type error.

Actual Behavior
Mypy is unable to recognize that the type of the value is not None and raises a type error.

Your Environment

• Mypy version used: 1.11.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12.0

Start by running the linked mypy-play reproduction with mypy 1.11.1 on Python 3.12 and confirm the reported indexed-assignment error. Trace the type-checking path for the conditional assignment to determine where the non-None dictionary type is lost; done means the reproduction no longer reports this error and related checks remain correct.