From 6395bc43293b83aebfadb7ba201d27c0343ebc04 Mon Sep 17 00:00:00 2001
From: hauntsaninja <hauntsaninja@gmail.com>
Date: Tue, 30 Dec 2025 22:41:12 -0800
Subject: [PATCH 1/3] Treat NotImplemented as a singleton type

---
 mypy/typeops.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mypy/typeops.py b/mypy/typeops.py
index 3a229f9201aa..00431f02fa5e 100644
--- a/mypy/typeops.py
+++ b/mypy/typeops.py
@@ -34,6 +34,7 @@
 from mypy.state import state
 from mypy.types import (
     ELLIPSIS_TYPE_NAMES,
+    NOT_IMPLEMENTED_TYPE_NAMES,
     AnyType,
     CallableType,
     ExtraAttrs,
@@ -996,8 +997,10 @@ def is_singleton_identity_type(typ: ProperType) -> bool:
     if isinstance(typ, NoneType):
         return True
     if isinstance(typ, Instance):
-        return (typ.type.is_enum and len(typ.type.enum_members) == 1) or (
-            typ.type.fullname in ELLIPSIS_TYPE_NAMES
+        return (
+            (typ.type.is_enum and len(typ.type.enum_members) == 1)
+            or (typ.type.fullname in ELLIPSIS_TYPE_NAMES)
+            or (typ.type.fullname in NOT_IMPLEMENTED_TYPE_NAMES)
         )
     if isinstance(typ, LiteralType):
         return typ.is_enum_literal() or isinstance(typ.value, bool)

From e94106759373e5a9e988737f315ac99cb9f15f24 Mon Sep 17 00:00:00 2001
From: hauntsaninja <hauntsaninja@gmail.com>
Date: Fri, 16 Jan 2026 13:39:59 -0800
Subject: [PATCH 2/3] test

---
 test-data/unit/check-narrowing.test | 7 +++++++
 test-data/unit/lib-stub/types.pyi   | 2 ++
 2 files changed, 9 insertions(+)

diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 237271558ac6..c9c0153d8f0a 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -2714,7 +2714,14 @@ reveal_type(t.foo)  # N: Revealed type is "__main__.C"
 [builtins fixtures/property.pyi]
 
 [case testNarrowingNotImplemented]
+# flags: --python-version 3.13
 from __future__ import annotations
+import types
+
+def foo(x: types.NotImplementedType | str):
+    if x is not NotImplemented:
+        reveal_type(x)  # N: Revealed type is "builtins.str"
+
 from typing_extensions import Self
 
 class X:
diff --git a/test-data/unit/lib-stub/types.pyi b/test-data/unit/lib-stub/types.pyi
index 3f713c31e417..6670b7cace0d 100644
--- a/test-data/unit/lib-stub/types.pyi
+++ b/test-data/unit/lib-stub/types.pyi
@@ -19,3 +19,5 @@ if sys.version_info >= (3, 10):
 
     class UnionType:
         def __or__(self, x) -> UnionType: ...
+
+    class NotImplementedType: ...

From c2cc9a91ed858e359784495db023e229c7cd87c1 Mon Sep 17 00:00:00 2001
From: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Date: Sat, 17 Jan 2026 12:35:14 -0800
Subject: [PATCH 3/3] Update test-data/unit/check-narrowing.test

---
 test-data/unit/check-narrowing.test | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index c9c0153d8f0a..00e018e6aec1 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -2714,7 +2714,7 @@ reveal_type(t.foo)  # N: Revealed type is "__main__.C"
 [builtins fixtures/property.pyi]
 
 [case testNarrowingNotImplemented]
-# flags: --python-version 3.13
+# flags: --python-version 3.10
 from __future__ import annotations
 import types