Typeshed import cycle causes mypy to start claiming a fixed-length tuple type alias declared with `TypeAlias` is "not valid as a type"

检出 mypy-import-cycle-repro 分支并运行 python tests/mypy_test.py stdlib -p3.12，以重现 stdlib/zipfile.pyi 中的错误。沿着报告中列出的 stub 跟踪导入循环，尤其是 stdlib/importlib/machinery.pyi、stdlib/importlib/resources/readers.pyi、stdlib/zipfile.pyi、stdlib/typing_extensions.pyi 和 stdlib/types.pyi；当 mypy 能够处理该循环或以易于理解的方式报告该循环时，即表示完成。

Bug Report

We're having a bit of trouble over in https://github.com/python/typeshed/pull/11074. It seems like it's impossible to import importlib.readers inside stdlib/importlib/machinery.pyi and also use typing_extensions.TypeAlias to explicitly demarcate type aliases inside stdlib/zipfile.pyi. The cause appears to be a huge import cycle in our stubs for the standard library:

• importlib.machinery imports NamespaceReader from importlib.resources.readers
• importlib.reasources.readers imports zipfile.Path from zipfile
• zipfile imports TypeAlias from typing_extensions
• typing_extensions imports get_original_bases from types
• types imports ModuleSpec from importlib.machinery... and we're back at the beginning.

As a workaround, things seem to work fine for us if we just don't use typing_extensions.TypeAlias for the problematic alias in zipfile.pyi. But this behaviour from mypy seems buggy, so I figured it would be good to file a bug.

To Reproduce

I haven't been able to reproduce this issue outside of the typeshed context. However, I have reduced typeshed down to a "minimum viable typeshed" required to reproduce the bug (https://github.com/AlexWaygood/typeshed/tree/mypy-import-cycle-repro/stdlib). Here are the repro steps:

1. Clone https://github.com/AlexWaygood/typeshed
2. Checkout the mypy-import-cycle-repro branch of the repo
3. Create and activate a venv; run pip install -r requirements-tests.txt
4. Run python tests/mypy_test.py stdlib -p3.12

Expected Behavior

Mypy handles the import cycle -- or at least gives an intelligible error reporting what the problem is.

Actual Behavior

stdlib\zipfile.pyi:4: error: Variable "zipfile._DateTuple" is not valid as a type  [valid-type]
stdlib\zipfile.pyi:4: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases

Your Environment

• Mypy version used: 1.7.1
• Mypy command-line flags: https://github.com/AlexWaygood/typeshed/blob/4235e8c82a0378b7ce5df878659ed0e909692f13/tests/mypy_test.py#L252-L275
• Python version used: 3.12