Allow type checkers to generate an error when list[Self] instance variables are accessed

Author: davidhalter

conformance/results/mypy/generics_self_advanced.toml

@@ -7,14 +7,14 @@ Does not retain `Self` when accessing attribute through `type[Self]`.
 """
 output = """
 generics_self_advanced.py:35: error: Expression is of type "ChildB", not "Self"  [assert-type]
-generics_self_advanced.py:38: error: Expression is of type "ChildB", not "Self"  [assert-type]
-generics_self_advanced.py:42: error: Expression is of type "type[ChildB]", not "type[Self]"  [assert-type]
-generics_self_advanced.py:48: error: Expression is of type "ChildB", not "Self"  [assert-type]
+generics_self_advanced.py:41: error: Expression is of type "ChildB", not "Self"  [assert-type]
+generics_self_advanced.py:45: error: Expression is of type "type[ChildB]", not "type[Self]"  [assert-type]
+generics_self_advanced.py:51: error: Expression is of type "ChildB", not "Self"  [assert-type]
 """
 conformance_automated = "Fail"
 errors_diff = """
 Line 35: Unexpected errors ['generics_self_advanced.py:35: error: Expression is of type "ChildB", not "Self"  [assert-type]']
-Line 38: Unexpected errors ['generics_self_advanced.py:38: error: Expression is of type "ChildB", not "Self"  [assert-type]']
-Line 42: Unexpected errors ['generics_self_advanced.py:42: error: Expression is of type "type[ChildB]", not "type[Self]"  [assert-type]']
-Line 48: Unexpected errors ['generics_self_advanced.py:48: error: Expression is of type "ChildB", not "Self"  [assert-type]']
+Line 41: Unexpected errors ['generics_self_advanced.py:41: error: Expression is of type "ChildB", not "Self"  [assert-type]']
+Line 45: Unexpected errors ['generics_self_advanced.py:45: error: Expression is of type "type[ChildB]", not "type[Self]"  [assert-type]']
+Line 51: Unexpected errors ['generics_self_advanced.py:51: error: Expression is of type "ChildB", not "Self"  [assert-type]']
 """

conformance/results/zuban/generics_self_advanced.toml

@@ -2,5 +2,5 @@ conformance_automated = "Pass"
 errors_diff = """
 """
 output = """
-generics_self_advanced.py:45: error: Access to generic instance variables via class is ambiguous  [misc]
+generics_self_advanced.py:48: error: Access to generic instance variables via class is ambiguous  [misc]
 """

conformance/tests/generics_self_advanced.py

@@ -33,8 +33,11 @@ class ChildB(ParentB):
 
     def method2(self) -> None:
         assert_type(self, Self)
-        assert_type(self.a, list[Self])
-        assert_type(self.a[0], Self)
+        # Allow type checkers to error here, since Self definitions on
+        # non-final classes are unsound.
+        a = self.a  # E?
+        assert_type(a, list[Self])
+        assert_type(a[0], Self)
         assert_type(self.method1(), Self)
 
     @classmethod