From b7723410a7eafe3d435066157d27460a73c7deb7 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Sat, 28 Feb 2026 10:40:14 +0300
Subject: [PATCH 1/2] [stubtest] Fix crash on instances with redefined
 `__class__`

---
 mypy/stubtest.py          |  4 ++++
 mypy/test/teststubtest.py | 43 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/mypy/stubtest.py b/mypy/stubtest.py
index 9dd630d61d4a..5973d82f2383 100644
--- a/mypy/stubtest.py
+++ b/mypy/stubtest.py
@@ -2049,6 +2049,10 @@ def _named_type(name: str) -> mypy.types.Instance:
         return mypy.types.TupleType(items, fallback)
 
     fallback = mypy.types.Instance(type_info, [anytype() for _ in type_info.type_vars])
+    if type(runtime) != runtime.__class__:
+        # Since `__class__` is redefined for an instance, we can't trust
+        # its `isinstance` checks, it can be dynamic. See #20919
+        return fallback
 
     value: bool | int | str
     if isinstance(runtime, enum.Enum) and isinstance(runtime.name, str):
diff --git a/mypy/test/teststubtest.py b/mypy/test/teststubtest.py
index 4023a191b548..4c95e7648030 100644
--- a/mypy/test/teststubtest.py
+++ b/mypy/test/teststubtest.py
@@ -1564,6 +1564,49 @@ def f(): return 3
             error=None,
         )
 
+    @collect_cases
+    def test_proxy_object(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            class LazyObject:
+                def __init__(self, func: object) -> None: ...
+                def __bool__(self) -> bool: ...
+            """,
+            runtime="""
+            class LazyObject:
+                def __init__(self, func):
+                    self.__dict__["_wrapped"] = None
+                    self.__dict__["_setupfunc"] = func
+                def _setup(self):
+                    self.__dict__["_wrapped"] = self._setupfunc()
+                @property
+                def __class__(self):
+                    if self._wrapped is None:
+                        self._setup()
+                    return type(self._wrapped)
+                def __bool__(self):
+                    if self._wrapped is None:
+                        self._setup()
+                    return bool(self._wrapped)
+            """,
+            error='test_module.LazyObject.__class__',
+        )
+        yield Case(
+            stub="""
+            def default_value() -> bool: ...
+
+            DEFAULT_VALUE: bool
+            """,
+            runtime="""
+            def default_value():
+                return True
+
+            DEFAULT_VALUE = LazyObject(default_value)
+            bool(DEFAULT_VALUE)  # evaluate the lazy object
+            """,
+            error='test_module.DEFAULT_VALUE',
+        )
+
     @collect_cases
     def test_all_at_runtime_not_stub(self) -> Iterator[Case]:
         yield Case(

From 64af2c6314f991cfeacc9705d683f27c24680c28 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sat, 28 Feb 2026 08:00:58 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/test/teststubtest.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mypy/test/teststubtest.py b/mypy/test/teststubtest.py
index 4c95e7648030..d373b1560271 100644
--- a/mypy/test/teststubtest.py
+++ b/mypy/test/teststubtest.py
@@ -1589,7 +1589,7 @@ def __bool__(self):
                         self._setup()
                     return bool(self._wrapped)
             """,
-            error='test_module.LazyObject.__class__',
+            error="test_module.LazyObject.__class__",
         )
         yield Case(
             stub="""
@@ -1604,7 +1604,7 @@ def default_value():
             DEFAULT_VALUE = LazyObject(default_value)
             bool(DEFAULT_VALUE)  # evaluate the lazy object
             """,
-            error='test_module.DEFAULT_VALUE',
+            error="test_module.DEFAULT_VALUE",
         )
 
     @collect_cases