Fix type narrowing with not isinstance(x, cls) in classmethods

Adjust get_type_range_of_type to set is_upper_bound=False for Self types, allowing precise narrowing in negative checks. Fixes #21271.

Author: sanyam-brudite

mypy/checker.py

@@ -8173,10 +8173,13 @@ def get_type_range_of_type(self, typ: Type) -> TypeRange | None:
             # Type[A] means "any type that is a subtype of A" rather than "precisely type A"
             # we indicate this by setting is_upper_bound flag
             is_upper_bound = True
-            if isinstance(typ.item, NoneType):
+            item = get_proper_type(typ.item)
+            if isinstance(item, TypeVarType) and item.id.is_self():
+                is_upper_bound = False
+            elif isinstance(item, NoneType):
                 # except for Type[None], because "'NoneType' is not an acceptable base type"
                 is_upper_bound = False
-            if isinstance(typ.item, Instance) and typ.item.type.is_final:
+            elif isinstance(item, Instance) and item.type.is_final:
                 is_upper_bound = False
             return TypeRange(typ.item, is_upper_bound=is_upper_bound)
         if isinstance(typ, AnyType):

test-data/unit/check-isinstance.test

@@ -3322,3 +3322,16 @@ def f2(x: A | None, t: type[A]):
     else:
         reveal_type(x)  # N: Revealed type is "None"
 [builtins fixtures/isinstancelist.pyi]
+
+[case testNarrowingSelfInClassMethod]
+# flags: --warn-unreachable
+from typing import Self, Union
+
+class Foo:
+    @classmethod
+    def foo(cls, x: Union[Self, float]) -> None:
+        if isinstance(x, cls):
+            reveal_type(x)  # N: Revealed type is "Self`0"
+        else:
+            reveal_type(x)  # N: Revealed type is "builtins.float"
+[builtins fixtures/isinstancelist.pyi]