assert_never() throws error on Python 3.10

[johnthagen]

Bug Report

assert_never() does not seem to work correctly on Python 3.10 with the latest mypy and typing-extensions.

To Reproduce

from typing_extensions import assert_never


def int_or_str(arg: int | str) -> None:
    match arg:
        case int():
            print("It's an int")
        case str():
            print("It's a str")
        case _ as unreachable:
            assert_never(unreachable)

But get an unexpected error:

$ mypy main.py
main.py:1: error: Module "typing_extensions" has no attribute "assert_never"
Found 1 error in 1 file (checked 1 source file)

The definition I see within typing-extensions has a conditional, but is none-the-less there:

if hasattr(typing, "assert_never"):
    assert_never = typing.assert_never
else:
    def assert_never(__arg: Never) -> Never:
        ...

Expected Behavior

No error and for type checking to be successful.

Your Environment

• Python 3.10.3
• Windows 10

mypy              0.942
mypy-extensions   0.4.3
typing_extensions 4.2.0

[tool.mypy]
ignore_missing_imports = true
strict = true

[JelleZijlstra]

Mypy doesn't look at the runtime code in typing-extensions, but at the stubs in https://github.com/python/typeshed/blob/master/stdlib/typing_extensions.pyi. The stubs update adding assert_never() must not have made it into the last mypy release yet. It will be in the next one.

For assert_never specifically, you can just define it in your own code directly with the same signature as in typing-extensions, since there's no magic around it.

[johnthagen]

I can confirm this is fixed in Mypy 0.950.