howto make subclassing with value parameters compatible with mypy generics?

I'm looking for an easy way to pass a parameter to a subclass. https://www.python.org/dev/peps/pep-0487/ provides such a notation: class C(Base, arg=value). This works perfecly and typechecks correctly via mypy for a normal class, but it fails with a generic class inheriting some Generic[T] (see code below). What did I miss ?

I'm using Python 3.6.8 (stock Python3 of Ubuntu 18.04) and mypy 0.740.

Here are two minimal standalone examples: one can put them in two .py files, typecheck them and run them.

this code typechecks and runs fine:

# ok.py
# PEP 487, adapted from QuestBase example

class Foo(object):
    variable = "???"
    def __init_subclass__(cls, arg: str, **kwargs) -> None:
        cls.variable = arg
        super().__init_subclass__(**kwargs) # type: ignore

class Bar(Foo, arg="value"): ...

print(Bar.variable)

This code typechecks but fails at runtime with TypeError: __init_subclass__() missing 1 required positional argument: 'arg':

# problem.py
from typing import Generic, TypeVar
T = TypeVar('T')

# PEP 487, adapted from QuestBase example

class Foo(Generic[T]):
    variable = "???"
    def __init_subclass__(cls, arg: str, **kwargs) -> None:
        cls.variable = arg
        super().__init_subclass__(**kwargs) # type: ignore

class Bar(Foo[T], arg="value"): ... # crash

Crash log:

Traceback (most recent call last):
  File "problem.py", line 12, in <module>
    class Bar(Foo[T], arg="value"): ... # crash
  File "/usr/lib/python3.6/typing.py", line 682, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3.6/typing.py", line 1143, in __getitem__
    orig_bases=self.__orig_bases__)
  File "/usr/lib/python3.6/typing.py", line 978, in __new__
    self = super().__new__(cls, name, bases, namespace, _root=True)
  File "/usr/lib/python3.6/typing.py", line 137, in __new__
    return super().__new__(cls, name, bases, namespace)
  File "/usr/lib/python3.6/abc.py", line 133, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
TypeError: __init_subclass__() missing 1 required positional argument: 'arg'

Obviously the crash in Python internal files reveals a problem in my file. Again, what did I miss ?

Is there a really typechecked solution or workaround ?

The following example is not a nice solution (wrong but typechecks):

# fake.py
from typing import Generic, TypeVar
T = TypeVar('T')


class Foo(Generic[T]):
    variable = "???"

class Bar(Foo[T]):
    variable = "value"

class KO(Foo[T]):
    ... # forgot assignment but still typechecks


print(KO.variable) # "???"

The following example, using a function which builds a class, is fine at runtime but doesn't typecheck: mypy doesn't recognize the result of a function as a derivable base class:

# param.py
from typing import Generic, TypeVar, Type
T = TypeVar('T')

class Foo(Generic[T]):
    variable = "???"

def bar(arg: str) -> Type[Foo[T]]:
    class C(Foo[T]):
        variable = arg
    return C

Bar: Type[Foo[float]] = bar("value")
print(Bar.variable)

class Baz(Bar): ... # doesn't typecheck
print(Baz.variable)

Error log:

param.py:16: error: Variable "param.Bar" is not valid as a type
param.py:16: error: Invalid base class "Bar"