Fix for pep585 with __future__ import

[cdce8p]

Description

Fix PR fixes an issue introduced with #7963. When using from __future__ import annotations with py37 and above, the following should pass:

from __future__ import annotations

t1: list[int]
T2 = list[int]  # This line caused an error

Test Plan

I've added some tests to verify that the change works as intended.

--
cc: @TH3CHARLie

[TH3CHARLie]

I don't think the case you presented is valid Python so mypy does the right thing about reporting error here.

[cdce8p]

If I'm not missing something, this should be a valid type alias. It might not be that useful for list[int] but image something a little more complex:

MyCustomType = dict[str, typing.Optional[tuple[int, float]]
my_var: MyCustomType = {...}

https://docs.python.org/3/library/typing.html#type-aliases

[TH3CHARLie]

If I'm not missing something, this should be a valid type alias. It might not be that useful for list[int] but image something a little more complex:

MyCustomType = dict[str, typing.Optional[tuple[int, float]]
my_var: MyCustomType = {...}

https://docs.python.org/3/library/typing.html#type-aliases

Sorry it's my miss that I didn't see that the PR is related to PEP585, yeah it should support alias.

[hauntsaninja]

It is not a valid type alias for Python 3.8 and lower, regardless of whether from __future__ import annotations is used.

While this is valid code in Python 3.9, we definitely should still complain about this for older Python versions.

[cdce8p]

@hauntsaninja I didn't follow the discussion exactly around PEP585 so I might be missing something. However as far as I understand it, the backwards compatibility (until 37) is only dependent on the typechecker understanding it [1]. Type aliases are already supported [2]. Again I might be missing something.

[1] https://www.python.org/dev/peps/pep-0585/#id5
[2] https://docs.python.org/3.7/library/typing.html#type-aliases

[hauntsaninja]

Maybe I'm missing something too! :-) What I'm trying to say is that mypy should mirror runtime behaviour by complaining about this snippet for Python versions where we get TypeErrors:

~/tmp λ cat test74.py
from __future__ import annotations

t1: list[int]
T2 = list[int]
print("okay!")

~/tmp λ python3.7 test74.py
Traceback (most recent call last):
  File "test74.py", line 4, in <module>
    T2 = list[int]
TypeError: 'type' object is not subscriptable

~/tmp λ python3.8 test74.py
Traceback (most recent call last):
  File "test74.py", line 4, in <module>
    T2 = list[int]
TypeError: 'type' object is not subscriptable

~/tmp λ python3.9 test74.py
okay!

There's a difference between a type annotation and a type alias. PEP 563 goes into more detail here.

[hauntsaninja]

Note that once mypy supports PEP 613 (#9404) we should probably also allow T2: TypeAlias = "list[str]" on Python 3.7 (note the quotes).

[cdce8p]

I think I understand your point now. You're right! I was testing with py39 which is why I didn't recognize the error 🙈
Thanks for pointing it out. I'll close this PR. #9564 should implement that case for py39.