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

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)

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: ...