Restore all_are_submodules import logic as workaround for #4498

mypy/build.py

@@ -713,16 +713,22 @@ def correct_rel_imp(imp: Union[ImportFrom, ImportAll]) -> str:
                 elif isinstance(imp, ImportFrom):
                     cur_id = correct_rel_imp(imp)
                     pos = len(res)
+                    all_are_submodules = True
                     # Also add any imported names that are submodules.
                     pri = import_priority(imp, PRI_MED)
                     for name, __ in imp.names:
                         sub_id = cur_id + '.' + name
                         if self.is_module(sub_id):
                             res.append((pri, sub_id, imp.line))
+                        else:
+                            all_are_submodules = False
                     # Add cur_id as a dependency, even if all of the
                     # imports are submodules. Processing import from will try
                     # to look through cur_id, so we should depend on it.
-                    pri = import_priority(imp, PRI_HIGH)
+                    # As a workaround for for some bugs in cycle handling (#4498),
+                    # if all of the imports are submodules, do the import at a lower
+                    # priority.
+                    pri = import_priority(imp, PRI_HIGH if not all_are_submodules else PRI_LOW)
                     res.insert(pos, ((pri, cur_id, imp.line)))
                 elif isinstance(imp, ImportAll):
                     pri = import_priority(imp, PRI_HIGH)

test-data/unit/check-modules.test

@@ -1001,6 +1001,48 @@ from foo import bar
 [file foo/bar.py]
 pass
 
+[case testImportReExportFromChildrenInCycle1]
+# cmd: mypy -m project.root project.study.a project.neighbor
+[file project/__init__.py]
+from project.study import CustomType
+x = 10
+[file project/root.py]
+[file project/study/__init__.py]
+from project.study.a import CustomType
+[file project/study/a.py]
+from project import root
+# TODO (#4498): This test is basically testing the `all_are_submodules` logic
+# in build, which skips generating a dependenecy to a module if
+# everything in it is a submodule. But that is still all just a
+# workaround for bugs in cycle handling. If we uncomment the next
+# line, we'll still break:
+# from project import x
+CustomType = str
+[file project/neighbor/__init__.py]
+from project.study import CustomType
+def m(arg: CustomType) -> str:
+    return 'test'
+
+[case testImportReExportFromChildrenInCycle2]
+# cmd: mypy -m project project.b project.ba project.c
+# See comments in above test about this being a workaround.
+[file foo.py]
+def get_foo() -> int: return 12
+
+[file project/ba.py]
+from . import b
+b.FOO
+
+[file project/b.py]
+import foo
+from . import c
+FOO = foo.get_foo()
+
+[file project/c.py]
+
+[file project/__init__.py]
+from . import ba
+
 [case testSuperclassInImportCycle]
 import a
 import d