[red-knot] False-positive errors for a `sys.version_info`-guarded import

[AlexWaygood]

Summary

If python-version is set to 3.9, red-knot emits an error on the following code:

import sys

if sys.version_info >= (3, 11):
    from typing import Self  # error: Module `typing` has no member `Self`

Red-knot correctly understands that the code is okay if python-version is set to 3.11 or higher. However, this kind of sys.version_info-guarded import is reasonably common in code that aims to support old Python versions -- e.g. lots of libraries do things like this, so that they can have typing_extensions only as a dependency on old Python versions:

import sys

if sys.version_info >= (3, 11):
    from typing import Self
else:
    from typing_extensions import Self

Mypy and pyright both support this pattern, so I think it's quite important for red-knot to also support it (though it's perhaps not something that's essential to do before the alpha release).

• https://playknot.ruff.rs/a4955e44-b332-43c1-afc1-2063e4d6fc70
• https://mypy-play.net/?mypy=latest&python=3.9&gist=3077724b9d32730ed4970d8feda7f3bc
• https://pyright-play.net/?pythonVersion=3.9&code=JYWwDg9gTgLgBAZwJ4IFCuAM0SgdANwFMoFgIA7AfWHMwjgD4BeOACgGYAaOARh4EoAXKjii4mKBBBwYSMDQDmcUJFhwAyoQA2mVEA

[sharkdp]

This looks like a duplicate of #15797 or astral-sh/ty#160?

[AlexWaygood]

Hmm, yes, it appears as though it's a combination of both issues. Mypy seems to view the code under the if sys.version_info >= (3, 11) branch as unreachable -- this can be seen from the fact that it does not emit any errors on this code:

import sys

if sys.version_info >= (3, 11):
    from typing import Self
    "foo" + 42

but note that mypy does not emit an [unreachable] error on that snippet even if you enable its (off-by-default) --warn-unreachable option. @sharkdp and I surmise that the [unreachable] logic in mypy applies some special handling for sys.version_info branches, since these are in some sense "deliberately unreachable" (a diagnostic telling you off about the unreachability would only be annoying).

For something more complex such as this, mypy does emit warnings, since it no longer understands the condition to be always true (although red-knot does):

import sys

if 42 and sys.version_info >= (3, 11):
    from typing import Self  # mypy error: Module "typing" has no attribute "Self"  [attr-defined]