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

首先对 repro2.py 中的三个复现用例运行 mypy，并比较单一赋值、元组赋值和链式赋值是如何被记录为类型别名的。跟踪这些赋值和函数注解的类型检查路径；完成的标准是三个签名都不产生错误，同时现有的别名和变量检查保持不变。