Adding check for non-final class and method returning Self

Author: edmondop

pyrefly/lib/solver/subset.rs

@@ -1004,7 +1004,8 @@ impl<'a, Ans: LookupAnswer> Subset<'a, Ans> {
             }
             (Type::ClassType(got), Type::SelfType(want)) => ok_or(
                 self.type_order
-                    .has_superclass(got.class_object(), want.class_object()),
+                    .has_superclass(got.class_object(), want.class_object())
+                    && self.type_order.is_final(want.class_object()),
                 SubsetError::Other,
             ),
             (Type::Type(box Type::ClassType(got)), Type::SelfType(want)) => ok_or(

pyrefly/lib/solver/type_order.rs

@@ -85,6 +85,10 @@ impl<'a, Ans: LookupAnswer> TypeOrder<'a, Ans> {
         self.0.get_metadata_for_class(cls).is_new_type()
     }
 
+    pub fn is_final(self, cls: &Class) -> bool {
+        self.0.get_metadata_for_class(cls).is_final()
+    }
+
     pub fn get_protocol_member_names(self, cls: &Class) -> SmallSet<Name> {
         let meta = self.0.get_metadata_for_class(cls);
         if let Some(proto) = meta.protocol_metadata() {

pyrefly/lib/test/simple.rs

@@ -1785,3 +1785,14 @@ X = type("X", (object,), {"foo": "bar"})  # E: No matching overload
 assert_type(X, type)  # E: assert_type(Any, type)
     "#,
 );
+
+testcase!(
+    test_nonfinal_class_self_return_type,
+    r#"
+from typing import Self
+
+class NonFinalClass:
+    def test(self) -> Self:
+        return NonFinalClass()  # E: Returned type `NonFinalClass` is not assignable to declared return type `Self@NonFinalClass`
+    "#,
+);