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

Start with the linked mypy-play reproducer and the shown generic inheritance example, then inspect how mypy checks the FooNumber.init() upcall. Done means the partial specialization no longer produces the reported incompatible-type errors while FooInt and FooFloat can specialize FooNumber as intended.