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regression: type alias for NoReturn no longer works as a type annotation #11903

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#11912
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regression: type alias for NoReturn no longer works as a type annotation#11903
#11912
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bugmypy got something wrong

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@DetachHead
DetachHead
opened on Jan 5, 2022

this worked in 0.921

from typing import NoReturn

Never = NoReturn

a: Never # error: Variable "__main__.Never" is not valid as a type  [valid-type]

b: NoReturn # no error

https://mypy-play.net/?mypy=0.930&python=3.10&flags=show-error-codes%2Callow-redefinition%2Cstrict%2Ccheck-untyped-defs%2Cdisallow-any-decorated%2Cdisallow-any-expr%2Cdisallow-any-explicit%2Cdisallow-any-generics%2Cdisallow-any-unimported%2Cdisallow-incomplete-defs%2Cdisallow-subclassing-any%2Cdisallow-untyped-calls%2Cdisallow-untyped-decorators%2Cdisallow-untyped-defs%2Cno-implicit-optional%2Cno-implicit-reexport%2Cstrict-equality%2Cwarn-incomplete-stub%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable%2Cwarn-unused-configs%2Cwarn-unused-ignores&gist=899d68c2a1cd7316120ca883780527c7

inb4 "that's not what NoReturn is for". mypy treats NoReturn more like a Never type, meaning it's valid as a type annotation. see #11508 (comment)

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  1. DetachHead
    added
    bugmypy got something wrong
    on Jan 5, 2022
  2. DetachHead
    changed the title [-]type alias for `NoReturn` no longer works as a type annotation[/-] [+]regression: type alias for `NoReturn` no longer works as a type annotation[/+] on Jan 5, 2022

  3. hauntsaninja commented on Jan 5, 2022

    @hauntsaninja
    hauntsaninja
    on Jan 5, 2022
    Collaborator

    Thanks for the report. I think this is a side effect of python/typeshed#6290

    Not seriously suggesting this, but a fun workaround happens to be from mypy_extensions import NoReturn, since I did a slightly different thing in python/typeshed#6595

  4. hauntsaninja commented on Jan 5, 2022

    @hauntsaninja
    hauntsaninja
    on Jan 5, 2022
    Collaborator

    Btw, if you have a repo with code that contains this, let me know and I'll add it to mypy primer so these regressions don't happen in the first place :-)

  5. sobolevn commented on Jan 5, 2022

    @sobolevn
    sobolevn
    on Jan 5, 2022
    Member

    Because I have literally 100s other issues to fix, I am playing around with this one 🤔

  6. AlexWaygood commented on Jan 5, 2022

    @AlexWaygood
    AlexWaygood
    on Jan 5, 2022
    Member

    Here's another workaround that works fine on 0.930:

    from typing import NoReturn as Never
from typing import NoReturn

a: Never  # no error
b: NoReturn  # no error
  7. hauntsaninja
    closed this as completedin #11912on Jan 6, 2022
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