Errors on multi-variable assignments of types

Bug Report

MyPy seems to have issues when multi-variable assignments of types to their aliases are involved.

To Reproduce
Run mypy on the following two sources:

from typing import Any

typ_a = Any
typ_b = Any
def test1(x: typ_a, y: typ_b): pass

from typing import Any

typ_a, typ_b = Any, Any
def test2(x: typ_a, y: typ_b): pass

typ_a = typ_b = Any
def test3(x: typ_a, y: typ_b): pass

Expected Behavior
MyPy does not show any errors for any of the function's signatures, since they are all essentially the same.

Actual Behavior
MyPy shows the following errors for test2 and test3:

repro2.py:4: error: Variable "repro2.typ_a" is not valid as a type
repro2.py:4: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
repro2.py:4: error: Variable "repro2.typ_b" is not valid as a type
repro2.py:7: error: Variable "repro2.typ_a" is not valid as a type
repro2.py:7: note: See https://mypy.readthedocs.io/en/latest/common_issues.html#variables-vs-type-aliases
repro2.py:7: error: Variable "repro2.typ_b" is not valid as a type
Found 4 errors in 1 file (checked 1 source file)

Confusingly, if one merges the two source files, no errors are shown.

Your Environment

• Mypy version used: 0.800
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9.1
• Operating system and version: Ubuntu 20.04

まず repro2.py の3つの再現例に対して mypy を実行し、単一代入、タプル代入、連鎖代入が型エイリアスとしてどのように記録されるかを比較します。これらの代入と関数アノテーションに対する型チェックの経路を追跡します。完了条件は、3つすべてのシグネチャでエラーが発生せず、既存のエイリアスおよび変数のチェックが変更されないことです。