Treat NotImplemented as a singleton type

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)

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 3.10
 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:

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: ...