type is not recalculated when it was previously a List[Any] #8540

• Are you reporting a bug, or opening a feature request?
  A bug
• Please insert below the code you are checking with mypy,

from typing import Any

def get_any() -> Any:
    return {}

def fun(message_json) -> int:
    messages = [get_any()]
    messages = get_any()
    if not isinstance(messages, list):
        print(messages)
    return 2

• What is the actual behavior/output?

:12 error: Statement is unreachable  [unreachable]
            print(messages)

• What is the behavior/output you expect?
  I would expect one the following:
  a) Either I get an error because "messages" is reassigned to Any afetr it was a list in the previous line
  b) Or I don't get the "unreachable" error because before the "if", "messages" can indeed not be a list

Curiously with the code below, I don't get any error

from typing import Any

def get_any() -> Any:
    return {}

def fun(message_json) -> int:
    messages = [3] # <=== only this line changes
    messages = get_any()
    if not isinstance(messages, list):
        print(messages)
    return 2

• What are the versions of mypy and Python you are using?
  0.770 (with 0.761 I got the same error message but with "misc" code)

• What are the mypy flags you are using? (For example --strict-optional)
  --show-error-codes --warn-unreachable