Workaround parenthesised context manager issue (#16949)

Fixes #16945

Author: hauntsaninja

mypy/checker.py

@@ -4,7 +4,7 @@
 
 import itertools
 from collections import defaultdict
-from contextlib import contextmanager, nullcontext
+from contextlib import ExitStack, contextmanager
 from typing import (
     AbstractSet,
     Callable,
@@ -526,17 +526,11 @@ def check_second_pass(
                     # print("XXX in pass %d, class %s, function %s" %
                     #       (self.pass_num, type_name, node.fullname or node.name))
                     done.add(node)
-                    with (
-                        self.tscope.class_scope(active_typeinfo)
-                        if active_typeinfo
-                        else nullcontext()
-                    ):
-                        with (
-                            self.scope.push_class(active_typeinfo)
-                            if active_typeinfo
-                            else nullcontext()
-                        ):
-                            self.check_partial(node)
+                    with ExitStack() as stack:
+                        if active_typeinfo:
+                            stack.enter_context(self.tscope.class_scope(active_typeinfo))
+                            stack.enter_context(self.scope.push_class(active_typeinfo))
+                        self.check_partial(node)
             return True
 
     def check_partial(self, node: DeferredNodeType | FineGrainedDeferredNodeType) -> None: