``Optional[Type]`` ignores (implicit) None check, resulting in misstyping of ``Type`` to ``Optional[Type]``

Bug Report

It seems that when the check for None is implicit mypy misstype Type to Optional[Type]. I checked #9428 which look really similar, but think the problem is that the control flow in my example is a little convoluted and that the check is not really explicit. (This is pylint's https://github.com/PyCQA/pylint/blob/master/pylint/message/message_id_store.py#L79)

To Reproduce

class MessageIdStore:
    def __init__(self):
        self.__msgid_to_symbol: Dict[str, str] = {}
        self.__symbol_to_msgid: Dict[str, str] = {}

    def check_msgid_and_symbol(self, msgid: str, symbol: str) -> None:
        existing_msgid: Optional[str] = self.__symbol_to_msgid.get(symbol)
        existing_symbol: Optional[str] = self.__msgid_to_symbol.get(msgid)
        if existing_symbol is None and existing_msgid is None:
            return  # both symbol and msgid are usable
        if existing_msgid is not None:
            if existing_msgid != msgid:
                self._raise_duplicate_msgid(symbol, msgid, existing_msgid) 
        if existing_symbol != symbol:
            # This will emit: 'Argument 3 to "_raise_duplicate_symbol" of "MessageIdStore" 
            # has incompatible type "Optional[str]"; expected "str"'
            self._raise_duplicate_symbol(msgid, symbol, existing_symbol) 

    @staticmethod
    def _raise_duplicate_symbol(msgid: str, symbol: str, other_symbol: str) -> None:
        ...

    @staticmethod
    def _raise_duplicate_msgid(symbol: str, msgid: str, other_msgid: str) -> None:
        ...   

Actual Behavior

On line self._raise_duplicate_symbol(msgid, symbol, existing_symbol) we have an Argument 3 to "_raise_duplicate_symbol" of "MessageIdStore" has incompatible type "Optional[str]"; expected "str".

Expected Behavior

No error because if not(existing_symbol is None and existing_msgid is None) and not(existing_msgid is not None then we can deduce that existing_symbol is not None

Your Environment

• Mypy version used: v0.812
• Mypy command-line flags: : ["--ignore-missing-imports", "--scripts-are-modules"]

Start by running the provided reproducer with the stated mypy version and flags, then trace how its control-flow checks narrow existing_symbol and existing_msgid. Done means the final _raise_duplicate_symbol call accepts existing_symbol without an Optional[str] error while preserving the existing checks.