Generate a warning/error on assigning values of type NoReturn

[Wilfred]

import sys
from typing import List
from mypy_extensions import NoReturn

def never_returns() -> NoReturn:
    sys.exit(1)

def foo() -> None:
    x = never_returns()

It would be nice if mypy warned in this situation.

[ilevkivskyi]

This example actually generates an error on master:

__tmp__.py:9: error: Need type annotation for variable

For me this is OK. We can keep this open if you want a dedicated error message for such cases, but I think this is quite low priority.

[sobolevn]

Here's another example of how NoReturn behaves incorrectly:

from typing import NoReturn, Union


def one() -> NoReturn:
    raise ValueError()

def two() -> int:
    return 1

def three(arg: int) -> Union[int, NoReturn]:
    if arg > 0:
        return two()
    return one()

reveal_type(three(1))  # types.py:15: error: Revealed type is 'Union[builtins.int, <nothing>]'
reveal_type(three(1) + 1)  # types.py:16: error: Revealed type is 'builtins.int'

But, it surely should not do this.

[ilevkivskyi]

But, it surely should not do this.

Why is this incorrect? I don't see any problem with simplifying NoReturn out of the union, it is a subtype of all types. In Python almost every function can raise an exception, so there is no point in annotating anything as -> Union[int, NoReturn], it is just -> int.

[Dambre]

@ilevkivskyi Year later lol. What if the exception is raised explicitly? In my opinion it would make sense to note that this is expected behaviour, isn't it?

def somefunc(sth: int) -> Union[bool, NoReturn]:
    if sth:
        return True
    raise Exception

[ikeyan]

@Dambre
Then you are going to annotate almost every function with Union[..., NoReturn].
Read this: #4116 (comment)

[A5rocks]

Surely --warn-unreachable works well enough for this, right? If you have any functionality after the assignment, then it will warn. Better yet it even works transitively! str(never_returns()) will still warn!