From b2fb8c8f33df65b6490da5db78ffd3d9ad584f3b Mon Sep 17 00:00:00 2001
From: sanyamk23 <sanyamk23@gmail.com>
Date: Tue, 15 Sep 2026 01:19:07 +0530
Subject: [PATCH] Fix type narrowing with not isinstance(x, cls) in
 classmethods

Rebased onto latest master; dropped the stale actions/checkout and
setup-python version bumps from the original commit since upstream
has since pinned newer action versions itself.
---
 mypy/checker.py                     |  3 +++
 test-data/unit/check-narrowing.test | 15 +++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index 33ed5387554d8..c3e9c087ea66e 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -8397,6 +8397,9 @@ def get_type_range_of_type(self, typ: Type) -> TypeRange | None:
                 is_upper_bound = False
             if isinstance(typ.item, Instance) and typ.item.type.is_final:
                 is_upper_bound = False
+            if isinstance(typ.item, TypeVarType) and typ.item.id.is_self():
+                # except for Self types, because isinstance checks for exact type
+                is_upper_bound = False
             return TypeRange(typ.item, is_upper_bound=is_upper_bound)
         if isinstance(typ, AnyType):
             return TypeRange(typ, is_upper_bound=False)
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 29f4cd47929d9..02d696a4e4608 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -3052,6 +3052,21 @@ def check_d(arg: D[T]) -> None:
 [builtins fixtures/tuple.pyi]
 
 
+[case testIsinstanceNarrowingWithSelfTypeInClassmethod]
+from typing_extensions import Self
+
+class Foo:
+    @classmethod
+    def foo_1(cls, x: Self | float) -> None:
+        assert isinstance(x, cls)
+        reveal_type(x)  # N: Revealed type is "Self`0"
+
+    @classmethod
+    def foo_2(cls, x: Self | float) -> None:
+        assert not isinstance(x, cls)
+        reveal_type(x)  # N: Revealed type is "builtins.float"
+[builtins fixtures/isinstancelist.pyi]
+
 [case testNarrowingUnionMixins]
 # flags: --strict-equality --warn-unreachable
 class Base: ...