fix: Type of instance variable narrowed incorrectly #20736

mypy/checker.py

@@ -8321,6 +8321,18 @@ def conditional_types(
         proposed_type = make_simplified_union(proposed_items)
 
     if isinstance(proper_type, AnyType):
+        # When current type is Any, we need to be careful about narrowing.
+        # For isinstance checks, proposed_type is typically an Instance, and we should narrow.
+        # For identity checks (is/is not), proposed_type is typically a FunctionLike (type object),
+        # and we should NOT narrow because Any could be anything.
+        proposed_proper = proposed_type
+        if (
+            isinstance(proposed_proper, (FunctionLike, Overloaded))
+            and proposed_proper.is_type_obj()
+        ):
+            # Identity check against a type object - don't narrow Any
+            return current_type, current_type
+        # isinstance check or other cases - narrow as before
         return proposed_type, current_type
     if isinstance(proposed_type, AnyType):
         # We don't really know much about the proposed type, so we shouldn't

test-data/unit/check-narrowing.test

@@ -3439,12 +3439,12 @@ else:
 from typing import Any
 
 def foo(y: object):
-    if y.__class__ == int:
+    if y.__class__ == int:  # E: "object" has no attribute "__class__"
         reveal_type(y) # N: Revealed type is "builtins.int"
     else:
         reveal_type(y) # N: Revealed type is "builtins.object"
 
-    if y.__class__ is int:
+    if y.__class__ is int:  # E: "object" has no attribute "__class__"
         reveal_type(y) # N: Revealed type is "builtins.int"
     else:
         reveal_type(y) # N: Revealed type is "builtins.object"
@@ -3460,6 +3460,36 @@ def bar(y: Any):
         reveal_type(y) # N: Revealed type is "builtins.int"
     else:
         reveal_type(y) # N: Revealed type is "Any"
+
+
+[case testAnyIdentityTypeObjectNoNarrowing]
+# flags: --strict-equality
+from typing import Any
+
+class A:
+    foo: Any
+
+def f(x: Any, a: A) -> None:
+    if x is list:
+        reveal_type(x)  # N: Revealed type is "Any"
+    else:
+        reveal_type(x)  # N: Revealed type is "Any"
+
+    if x is not list:
+        reveal_type(x)  # N: Revealed type is "Any"
+    else:
+        reveal_type(x)  # N: Revealed type is "Any"
+
+    if a.foo is list:
+        reveal_type(a.foo)  # N: Revealed type is "Any"
+    else:
+        reveal_type(a.foo)  # N: Revealed type is "Any"
+
+    if a.foo is not list:
+        a.foo = []
+
+    reveal_type(a.foo)  # N: Revealed type is "Any"
+    a.foo.append(1)
 [builtins fixtures/dict-full.pyi]
 
 [case testNarrowTypeVarType]