Check slots assignments on self types (#19332)

Fixes #19331.

Author: sterliakov

mypy/checker.py

@@ -3752,6 +3752,9 @@ def check_assignment_to_slots(self, lvalue: Lvalue) -> None:
             return
 
         inst = get_proper_type(self.expr_checker.accept(lvalue.expr))
+        if isinstance(inst, TypeVarType) and inst.id.is_self():
+            # Unwrap self type
+            inst = get_proper_type(inst.upper_bound)
         if not isinstance(inst, Instance):
             return
         if inst.type.slots is None:

test-data/unit/check-slots.test

@@ -544,3 +544,19 @@ x = X()
 X.a  # E: "a" in __slots__ conflicts with class variable access
 x.a
 [builtins fixtures/tuple.pyi]
+
+[case testSlotsOnSelfType]
+from typing_extensions import Self
+
+class X:
+    __slots__ = ("foo",)
+    foo: int
+
+    def method1(self: Self) -> Self:
+        self.bar = 0  # E: Trying to assign name "bar" that is not in "__slots__" of type "__main__.X"
+        return self
+
+    def method2(self) -> Self:
+        self.bar = 0  # E: Trying to assign name "bar" that is not in "__slots__" of type "__main__.X"
+        return self
+[builtins fixtures/tuple.pyi]