Allow redefinition even if a closure function is created between definitions

Bug Report

I was trying to debug why I was getting no-redef errors when I had --allow-redefinition. I had seen https://github.com/python/mypy/issues/6963 so, I figured that because both the definition and use of the variable were in a loop it was not triggering that condition (variable use). This did not turn out to be the case, and I was looking through similar issues I noticed #7984 and the example given did exhibit what I was trying to do and what I expected to work. I was able to confirm that it was working on the version of mypy I was using and I set to reduce my test case to something that was easy to reproduce. I made sure everything was typed by stubbing in concrete definitions and started to pull pieces of my test case apart.

Quickly and surprisingly I found out that a closure function I had defined was breaking the redefinition, but the enclosed function was unrelated to the variable. Without further ado:

To Reproduce

# mypy: allow-redefinition
from random import random

def f() -> None:
    x = 0

    reveal_type(x)
    print(x)

    def f2() -> float:
        return random()

    x = 'x'
    reveal_type(x)

Expected Behavior

reveal_type 1: int
reveal_type: 2: 'str'

Actual Behavior

note: In function "f":
7: note: Revealed type is 'builtins.int'
13: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
        x = 'x'
            ^
t2.py:14: note: Revealed type is 'builtins.int'

Your Environment

• Mypy version used: 0.800
• Mypy command-line flags: --warn-unused-configs --allow-redefinition
• Mypy configuration options from mypy.ini (and other config files):

[mypy]                                                                         
allow_redefinition = true                                                                                                                  
pretty = true                                                                  
show_error_codes = true                                                        
show_error_context = true                                                      
warn_unused_ignores = true                                                     
warn_unreachable = true

• Python version used: 3.8.7

Start by running the issue's minimal Python reproduction with --allow-redefinition and compare the two reveal_type results. Trace the redefinition check around the nested f2 closure; done when the second assignment is accepted and the revealed types are int then str.