Generic[T_co] erroring when T_co = TypeVar("T_co", A, B, covariant=True) and passed subclass of A

I am reporting what I believe is a bug in mypy's interpretation of TypeVar when passed two constraining types and also covariant=True.

I think the below code sample is valid (per Liskov Substitution Principle), and is erroneously being reported as a type error by mypy. Here is my working example:

from typing import TypeVar, Generic, Union

class A:
    pass

class ASub(A):
    pass

class B:
    pass

# Case 1... Success: no issues found
# T_co = TypeVar("T_co", covariant=True)

# Case 2... error: Value of type variable "T_co" of "SomeGeneric" cannot be "ASub"  [type-var]
T_co = TypeVar("T_co", A, B, covariant=True)

# Case 3... Success: no issues found
# T_co = TypeVar("T_co", bound=Union[A, B], covariant=True)


class SomeGeneric(Generic[T_co]):
    pass

class SomeGenericASub(SomeGeneric[ASub]):
    pass

Potentially Related Issue

I may be re-asking the contents of https://github.com/python/mypy/issues/8611, but to be honest, I get confused by the core of the issue

non-empty covariant generic subclass of a contravariant base

Versions

python==3.6.5
mypy==0.770

Start by reproducing the reported Case 2 with Python 3.6.5 and mypy 0.770, then compare its constrained TypeVar behavior with the unconstrained and bound examples. Read the potentially related discussion in issue #8611; done when the intended behavior for SomeGeneric[ASub] is established and the erroneous diagnostic is resolved or explained.