Cannot resolve recursive/cyclic definitions (v0.991, Python 3.8/3.9)

[bartfeenstra]

Bug Report

Mypy fails to check recursive/cyclic type aliases, even with #731 being part of v0.991. However, this happens on Python 3.8 and 3.9 only (and possibly below), but not on 3.10 or 3.11.
Example:

RecursiveTypeAlias: TypeAlias = Union[bool, int, float, str, Sequence['RecursiveTypeAlias']]

for which mypy raises error: Cannot resolve name "RecursiveTypeAlias" (possible cyclic definition) [misc].

To Reproduce

• Create an environment with:
  • Mypy v0.991
  • Python 3.8 or 3.9
  • typing_extensions 4.4.0
• Create a Python file with the offending type alias. See https://github.com/bartfeenstra/mypy-type-alias/blob/main/mypy-type-alias/main.py for additional imports for cross-Python-version compatibility.
• Run mypy without any additional options or arguments

Alternatively, clone https://github.com/bartfeenstra/mypy-type-alias and run tox.

Expected Behavior
Mypy not raising any errors for recursive/cyclig type aliases and correctly parsing them as it does on Python 3.10 and 3.11.

Actual Behavior
Mypy fails on recursive/cyclic type aliases with the error error: Cannot resolve name "RecursiveTypeAlias" (possible cyclic definition) [misc].

[JukkaL]

On a recent master this works on Python 3.8:

from typing_extensions import TypeAlias
from typing import Union, Sequence

RecursiveTypeAlias: TypeAlias = Union[bool, int, float, str, Sequence['RecursiveTypeAlias']]

The issue is with imports. Mypy doesn't support this on Python 3.8:

try:
    from typing import TypeAlias  # type: ignore
except ImportError:
    from typing_extensions import TypeAlias

As a workaround, this should work:

try:
    from typing_extensions import TypeAlias
except ImportError:
    from typing import TypeAlias  # type: ignore

[bartfeenstra]

I can confirm this works with mypy commit 3a3cf412b278ee7bf710742b168beed41c1c02f2 in combination with the imports turned around. Thanks!

[ssbarnea]

For reference, this is the full solution that seems to currently work with py39+ and also pass mypy and ruff:

from __future__ import annotations
from collections.abc import Mapping, Sequence
from typing import Any, Union

try:
    from typing_extensions import TypeAlias
except ImportError:
    from typing import TypeAlias  # type: ignore[no-redef,attr-defined]

JSON: TypeAlias = Union[dict[str, "JSON"], list["JSON"], str, int, float, bool, None]
JSON_ro: TypeAlias = Union[
    Mapping[str, "JSON_ro"],
    Sequence["JSON_ro"],
    str,
    int,
    float,
    bool,
    None,
]

Note the awkward import fallback is needed because doing it the opposite does not work py39 or py38 either, only importing first from typing_extensions seems to be working well.