Don't infer infinitely complex types in loops

Author: JukkaL

mypy/checker.py

@@ -618,7 +618,7 @@ def accept_loop(
                 widened_new = len(self.widened_vars)
                 if (
                     (partials_new == partials_old)
-                    and not self.binder.last_pop_changed
+                    and (not self.binder.last_pop_changed or iter > 3)
                     and (widened_new == widened_old or iter > 1)
                 ):
                     break

test-data/unit/check-redefine2.test

@@ -1061,3 +1061,16 @@ if int():
 
 [out]
 tmp/b.py:1: error: --local-partial-types must be enabled if using --allow-redefinition-new
+
+[case testNewRedefineNestedLoopInfiniteExpansion]
+# flags: --allow-redefinition-new --local-partial-types
+def a(): ...
+
+def f() -> None:
+    while int():
+        x = a()
+
+        while int():
+            x = [x]
+
+    reveal_type(x) # N: Revealed type is "Union[Any, builtins.list[Any], builtins.list[Union[Any, builtins.list[Any]]], builtins.list[Union[Any, builtins.list[Any], builtins.list[Union[Any, builtins.list[Any]]]]], builtins.list[Union[Any, builtins.list[Any], builtins.list[Union[Any, builtins.list[Any]]], builtins.list[Union[Any, builtins.list[Any], builtins.list[Union[Any, builtins.list[Any]]]]]]]]"