There should be an inverse of `Type`

Specifically, it would be nice if there was a keyword Instance such that Instance[Type[_T]] is equivalent to _T

I'm thinking about this mostly in the context of working with metaclasses.

Suppose I have a class with a class method that creates an instance of the class written so it can be reused by subclasses. This can be typed in the following way:

_ThingInstance = TypeVar('_ThingInstance', bound='Thing')

class Thing():

    @classmethod
    def make_new(cls: Type[_ThingInstance]) -> _ThingInstance:
        return cls()

And this is automatically usable by subclasses.

What I want is an Instance type such that I can correctly type the same logic defined within a metaclass like so:

_ThingClass = TypeVar('_ThingClass', bound='MetaThing')

class MetaThing(type):

  def make_new(cls: _ThingClass) -> Instance[_ThingClass]:
      return cls()

As far as I can tell, there's no way to correctly annotate this

提案されている Instance[_ThingClass] アノテーションから始め、示されている Type[_ThingInstance] パターンと比較します。メタクラスメソッドに対して逆方向がどのように振る舞うべきかを定義し、型チェックのカバレッジで例を検証します。要求されたアノテーションが意図した推論済みのインスタンスタイプで受け入れられれば完了です。