deleting a class-scoped variable that shadows a global variable causes mypy to think it's undefined

To Reproduce

from typing import Callable, TypeVar

T = TypeVar("T")

x = 1


def decorate(num: int) -> Callable[[T], T]:
    def thru(it: T) -> T:
        print("decorating", it, "with", num)
        return it
    return thru


class y(object):
    @decorate(x)
    def a(self) -> None:
        ...

    x = 2

    @decorate(x)
    def b(self) -> None:
        ...

    del x

    @decorate(x)
    def c(self) -> None:
        ...

Expected Behavior

This is perfectly valid, it should not cause an error! It works at runtime.

Actual Behavior

$ mypy class_scope.py 
class_scope.py:28: error: Trying to read deleted variable 'x'
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: all defaults
• Mypy configuration options from mypy.ini (and other config files): no mypy.ini
• Python version used: Python 3.8.5
• Operating system and version:

ProductName:	Mac OS X
ProductVersion:	10.15.7
BuildVersion:	19H2

Reproduce the issue from the class_scope.py example with mypy, using the reported Python and mypy versions as a baseline. Trace how the checker handles a class-scoped name after del x, then add a regression test showing that the later decorator use does not report an undefined variable.