Do not display import-related errors after module-level always false assert

[sterliakov]

Fixes #19346

[A5rocks]

One reason I'm surprised about this is that my impression was that after a module-level assert that is unreachable, mypy removes all nodes. Am I misremembering? Or is this an earlier pass?

[sterliakov]

@A5rocks Great question. Imports are collected during parsing, and "definitely unreachable" truncation step follows (SemanticAnalyzerPreAnalysis). It does not modify MypyFile.imports collected at the previous step, so we still treat such modules as dependencies even if the corresponding import statement no longer exists, and "missing module" errors are produced by build manager when any dependency can't be resolved. Those imports aren't marked unreachable either. That probably should be fixed instead, thanks! That will be a better approach.

[A5rocks]

This makes sense to me!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

PyWinCtl (https://github.com/Kalmat/PyWinCtl)
- src/pywinctl/_pywinctl_win.py:28: error: Cannot find implementation or library stub for module named "pywinbox"  [import-not-found]
- src/pywinctl/_pywinctl_macos.py:25: error: Cannot find implementation or library stub for module named "pywinbox"  [import-not-found]

[petamasS3D]

@A5rocks , @JukkaL
Last meaningful change is 2 weeks old on this PR - how can this be unblocked?

[sterliakov]

@petamasS3D just wait - I usually follow up on my PRs monthly if they lack attention. Things just aren't quickly merged here except for trivial fixes/emergencies, and this one certainly isn't a trivial fix (and is a bit hacky), so it's normal to wait a while and gather feedback from other maintainers. If this issue is blocking you, you can safely add an ignore comment - typechecker false positive is one of perfectly legitimate reasons to use them.

[petamasS3D]

@sterliakov: thanks for the insight! I saw that there are a LOT of newer PRs already merged, and was afraid this one somehow got lost. It's not blocking me at the moment (I've worked around the issue using an if around the imports, and asserting afterwards), just hoped for a quick resolution after you immediately jumped on my bug report. (Thank you for that!) I'll wait patiently until the process completes. :)