Fine-grained: Fix hang due to nested class (#4728)

This works around the problem but doesn't fully resolve the underlying
issue.

Fixes #4612.

Author: JukkaL

mypy/semanal.py

@@ -1066,8 +1066,15 @@ def setup_class_def_analysis(self, defn: ClassDef) -> None:
             if kind == LDEF:
                 # We need to preserve local classes, let's store them
                 # in globals under mangled unique names
-                local_name = defn.info._fullname + '@' + str(defn.line)
-                defn.info._fullname = self.cur_mod_id + '.' + local_name
+                #
+                # TODO: Putting local classes into globals breaks assumptions in fine-grained
+                #     incremental mode and we should avoid it.
+                if '@' not in defn.info._fullname:
+                    local_name = defn.info._fullname + '@' + str(defn.line)
+                    defn.info._fullname = self.cur_mod_id + '.' + local_name
+                else:
+                    # Preserve name from previous fine-grained incremental run.
+                    local_name = defn.info._fullname
                 defn.fullname = defn.info._fullname
                 self.globals[local_name] = node
 

test-data/unit/fine-grained.test

@@ -2832,3 +2832,22 @@ class C(Enum):
 ==
 a.py:4: error: Argument 1 to "f" has incompatible type "C"; expected "int"
 a.py:5: error: Argument 1 to "f" has incompatible type "C"; expected "int"
+
+[case testRefreshNestedClassWithSelfReference]
+import a
+[file a.py]
+import b
+
+def f(self) -> None:
+    b.y
+    class C:
+        z: C
+[file b.py]
+y = 0
+[file b.py.2]
+y = ''
+[file b.py.3]
+y = 0
+[out]
+==
+==