From cee5d3e9a29f43a10a8eeca760976657bf1689c9 Mon Sep 17 00:00:00 2001
From: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Date: Fri, 3 Dec 2021 06:11:44 -0800
Subject: [PATCH] Fix crash involving explicit reexport, import cycle, wildcard
 (#11632)
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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.
---
 mypy/semanal.py                       |  4 +++-
 test-data/unit/check-incremental.test | 25 +++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/mypy/semanal.py b/mypy/semanal.py
index e55fada5689c..21741f90a528 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -1933,7 +1933,9 @@ def report_missing_module_attribute(
         if self.is_incomplete_namespace(import_id):
             # We don't know whether the name will be there, since the namespace
             # is incomplete. Defer the current target.
-            self.mark_incomplete(imported_id, context)
+            self.mark_incomplete(
+                imported_id, context, module_public=module_public, module_hidden=module_hidden
+            )
             return
         message = 'Module "{}" has no attribute "{}"'.format(import_id, source_id)
         # Suggest alternatives, if any match is found.
diff --git a/test-data/unit/check-incremental.test b/test-data/unit/check-incremental.test
index 342850ab24d0..1d2262ab303d 100644
--- a/test-data/unit/check-incremental.test
+++ b/test-data/unit/check-incremental.test
@@ -5585,3 +5585,28 @@ bar.x + 0
 x = 0
 [rechecked]
 [stale]
+
+[case testExplicitReexportImportCycleWildcard]
+# flags: --no-implicit-reexport
+import pkg.a
+[file pkg/__init__.pyi]
+
+[file pkg/a.pyi]
+MYPY = False
+if MYPY:
+    from pkg.b import B
+
+[file pkg/b.pyi]
+import pkg.a
+MYPY = False
+if MYPY:
+    from pkg.c import C
+class B:
+    pass
+
+[file pkg/c.pyi]
+from pkg.a import *
+class C:
+    pass
+[rechecked]
+[stale]