(🎁) Support typing.assert_never (and the one from typing_extensions)

[KotlinIsland]

def assert_never(__arg: Never) -> Never: 
    """Custom impl for demonstration purposes only. (mypy doesn't have stubs for the real one)"""
    ...

assert_never()  # no error

Mypy should report an error when assert_never is used in a position that is reachable.

Related: #12253

[JelleZijlstra]

Your example gives two straightforward errors (Never is not defined and assert_never() takes one argument).

This fixed version behaves as expected:

from typing import NoReturn as Never

def assert_never(__arg: Never) -> Never: 
    """Custom impl for demonstration purposes only. (mypy doesn't have stubs for the real one)"""
    ...

def f(x: bool):
    assert_never(x)  # error
    if x is True:
        pass
    elif x is False:
        pass
    else:
        assert_never(x)  # no error

Can you clarify what you think is wrong?

typing.assert_never shouldn't need any specific support in mypy because the stubs are sufficient.

[KotlinIsland]

😳

True, sorry about that.

[KotlinIsland]

@JelleZijlstra Why does Never/raise suppress unreachable errors? #10916