Allow final class returns for Self

Summary:
Returning a concrete class instance from a method annotated with `Self` is normally unsafe because `Self` can represent a subclass. Final classes are the sound exception: there can be no subclass for `Self` to stand for.

This allows an exact final class instance to satisfy Self while keeping non-final concrete class returns rejected. The rule is intentionally narrow so the removed `ClassType` <: `SelfType` behavior does not come back for subclassable classes.

Reviewed By: yangdanny97

Differential Revision: D104711160

fbshipit-source-id: cfa0eabd89fa117419db4380c20637f278774d87

Author: fangyi-zhou

pyrefly/lib/solver/subset.rs

@@ -2026,6 +2026,11 @@ impl<'a, Ans: LookupAnswer> Subset<'a, Ans> {
             {
                 self.is_subset_eq(&got, want)
             }
+            (Type::ClassType(got), Type::SelfType(want))
+                if got == want && self.type_order.is_final(got.class_object()) =>
+            {
+                Ok(())
+            }
             (Type::SelfType(_), Type::SelfType(_)) => Ok(()),
             (Type::SelfType(got), _) => self.is_subset_eq(&Type::ClassType(got.clone()), want),
             (Type::Tuple(l), Type::Tuple(u)) => self.is_subset_tuple(l, u),

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_protocol()
     }
 
+    pub fn is_final(self, cls: &Class) -> bool {
+        self.0.get_metadata_for_class(cls).is_final()
+    }
+
     pub fn is_new_type(self, cls: &Class) -> bool {
         self.0.get_metadata_for_class(cls).is_new_type()
     }

pyrefly/lib/test/typing_self.rs

@@ -209,6 +209,22 @@ class Shape:
 "#,
 );
 
+testcase!(
+    test_self_return_final_concrete_class,
+    r#"
+from typing import Self, final
+
+@final
+class Shape:
+    def method(self) -> Self:
+        return Shape()
+
+    @classmethod
+    def cls_method(cls) -> Self:
+        return Shape()
+    "#,
+);
+
 testcase!(
     test_self_in_class_body_expression,
     r#"