mypy

fdae42fe - Fix crash involving explicit reexport, import cycle, wildcard (#11632)

Fix crash involving explicit reexport, import cycle, wildcard (#11632) Fixes #8481 Fixes #9941 Fixes #11025 Fixes #11038 This is the unresolved crash that's been reported the most times, e.g., it's as easy to repro as `mypy --no-implicit-reexport -c 'import pytorch_lightning'` (or even `import torch`, with the right PyTorch version). I know of multiple popular Python packages that have made changes just to work around this crash. I would love to see this released, potentially along with #11630. Thanks to @rraval for making a minimal repro! The fix ended up being a one-liner, but it took me a bit to track down :-) Since things worked with implicit reexport, I differentially patched in explicit reexport logic to narrow things down. This let me observe that if I hardcoded pkg.a.B to get reexported, we hit this branch, which clobbers the PlaceholderNode for pkg.c.B, which fixes things: https://github.com/python/mypy/blob/f79e7afec2c863c34d7a9b41ebb732dc26128fff/mypy/semanal.py#L2028 Which is a little weird — we shouldn't have a PlaceholderNode for pkg.c.B at all. But with a breakpoint in that branch, it was easy to notice that with `--no-implicit-reexport` pkg.a.B was first created with `module_public=True` (resulting in creation of a PlaceholderNode in pkg.c.B) and only on a later pass acquired `module_public=False`. So tracking down where pkg.a.B symbol was first created with `module_public=True` led me to this "obvious" bug.