Partial specialization of a generic type

I have have a generic class hierarchy, mirroring the hierarchy of the type parameter. I am trying to add proper type annotations to these classes.

In the example below (gist here) I'm attempting to partially specialize the Root class:

from typing import Generic, TypeVar

_T = TypeVar("_T")

class Root(Generic[_T]):
    def __init__(self, value: _T):
        self.value = value

_Number = TypeVar("_Number", int, float)

class FooNumber(Root[_Number]):
    def __init__(self, value: _Number):
        super().__init__(value)

... mypy reports on the __init__ upcall from FooNumber.__init__():

test-mypy-typevar-inherit.py:13: error: Argument 1 to "__init__" of "Root" has incompatible type "int"; expected "_Number"  [arg-type]
test-mypy-typevar-inherit.py:13: error: Argument 1 to "__init__" of "Root" has incompatible type "float"; expected "_Number"  [arg-type]

The goal is to then declare FooInt deriving from FooNumer[int] (and similarly for FooFloat).
A direct specialization of FooInt from Root[int] shows (as expected) no issue, and I don't feel it's very clear why this FooNumber definition causes any problem.

It seems there would be no reason for those errors.

Note: this point was first raised on Gitter and on stackoverflow

Beginne mit dem verknüpften mypy-play-Reproduzierer und dem gezeigten Beispiel für generische Vererbung und untersuche anschließend, wie mypy den FooNumber.init()-Upcall prüft. Erledigt ist die Aufgabe, wenn die partielle Spezialisierung nicht mehr die gemeldeten incompatible-type-Fehler erzeugt, während FooInt und FooFloat FooNumber wie beabsichtigt spezialisieren können.