Wrong version of typing-extensions used.

[ngnpope]

Bug Report

I was attempt to use assert_never() from typing-extensions>=4.1.0 but I get the following when running mypy:

example.py:1: error: Module "typing_extensions" has no attribute "assert_never"  [attr-defined]

I noticed that the following file exists in typeshed/stdlib even though typing-extensions is not part of the standard library:

https://github.com/python/mypy/blob/master/mypy/typeshed/stdlib/typing_extensions.pyi

This prevents use of a newer version of typing_extensions with mypy because the types are always fixed at the version distributed with mypy. This is somewhat surprising and the error is very obscure.

To Reproduce

(Write your steps here:)

1. Install typing-extensions>=4.1.0
2. Add from typing_extensions import assert_never
3. Run mypy

Expected Behavior

mypy recognises imports of new things from newer versions of typing-extensions.

Actual Behavior

mypy complains that assert_never() doesn't exist in typing-extensions.

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): pretty, strict, show-error-codes, warn-unreachable
• Python version used: 3.9
• Operating system and version: Linux

[JelleZijlstra]

Mypy bundles its own copy of typeshed, and the most recent release does not yet include typing_extensions.assert_never. The next release will.

In the meantime, you can use --custom-typeshed-dir to point to an updated copy of typeshed.