a Type is not recognized properly for subclassing

Bug Report

I'm explicitly typing a variable with Type (or type) and mypy thinks I should not subclass it.

To Reproduce

from typing import Type

class Base:
    ...

def get_base() -> Type[Base]:
    return Base

BaseAlias: Type[Base] = get_base()

class Derived(BaseAlias):
    ...

Expected Behavior

There should be no error. I'm subclassing a type, which is ok. Furthermore, mypy has the information that it's a type so subclassing should not be questioned.

Actual Behavior

footypes.py:11: error: Variable "footypes.BaseAlias" is not valid as a type
footypes.py:11: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
footypes.py:11: error: Invalid base class "BaseAlias"
Found 2 errors in 1 file (checked 1 source file)

Same behavior if specifying BaseAlias: type = get_base() instead.

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.0
• Operating system and version: python:3.9 docker image

footypes.py に示されている最小再現例から始め、それに対して mypy を実行して BaseAlias に関する 2 つの診断を確認します。変数アノテーションと基底クラスがどのように解析されるかを追跡し、その後、例がエラーを生成しないことを示すリグレッションテストのカバレッジを追加します。