typing_extensions TypeVar seems to be unsupported

[evtn]

Bug Report

Mypy doesn't support typing_extensions.TypeVar at all

To Reproduce

Playground

from typing import Generic
from typing_extensions import (
    TypeVar,
)

T = TypeVar("T")

class A(Generic[T]):
    def __init__(self, value: T):
        self.value = value

a: A = A(8)
b: A[str] = A("")

Expected Behavior

This code should work fine (importing TypeVar from typing works, but my actual use case depends on default argument of TypeVar, so I can't use typing.TypeVar)

Actual Behavior

main.py:8: error: Free type variable expected in Generic[...]  [misc]
main.py:9: error: Variable "__main__.T" is not valid as a type  [valid-type]
main.py:9: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
main.py:13: error: "A" expects no type arguments, but 1 given  [type-arg]
Found 3 errors in 1 file (checked 1 source file)

Your Environment

mypy Playground, latest mypy (0.991)

[JelleZijlstra]

This should be an easy fix if you're up for it. For inspiration, you can look at the commit adding support for typing_extensions.overload: 0df8cf5.

[evtn]

I would be happy to contribute, but it seems that this approach would just alias typing.TypeVar.

My actual usage (not from example) relies on default argument, which is absent from typing version (because it's from PEP 696 which is a draft).

[JelleZijlstra]

Yes, that would be much harder. :) I think @Gobot1234 already has a draft implementation of PEP 696 for mypy.

[Gobot1234]

https://github.com/Gobot1234/mypy/tree/TypeVar-defaults is my draft implementation of PEP 696, its not bug free and is more lax than the current specification but it should work for most basic things.

[cdce8p]

I opened #14313 to add basic support for the typing_extensions variant. This doesn't include default just yet though.

@Gobot1234 Would you be able to rebase your TypeVar-defaults branch? A lot has happened in the last 2 months and it got a bit out of sync. That would make it much easier to see how we can add it to mypy.

[Gobot1234]

@cdce8p should be done now, not sure how out of sync everything is, it sure does look like a lot changed