Allow a ClassVar to be overridden with a nested class def and vice versa

Summary: Fixes #2543.

Reviewed By: yangdanny97

Differential Revision: D95405904

fbshipit-source-id: 24c3dd55eca1073e9c60b6368e1abebb40308d1e

Author: rchen152

pyrefly/lib/alt/class/class_field.rs

@@ -4167,9 +4167,19 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             (ClassAttribute::NoAccess(_), _) => return Err(Box::new(AttrSubsetError::NoAccess)),
             _ => {}
         }
-        let got_is_classvar = matches!(got, ClassAttribute::ReadOnly(_, ReadOnlyReason::ClassVar));
-        let want_is_classvar =
-            matches!(want, ClassAttribute::ReadOnly(_, ReadOnlyReason::ClassVar));
+        // Both ClassVar and ClassObjectInitializedOnBody represent class-level read-only
+        // attributes, so they are compatible for override purposes.
+        let is_classvar_compatible = |attr: &ClassAttribute| {
+            matches!(
+                attr,
+                ClassAttribute::ReadOnly(
+                    _,
+                    ReadOnlyReason::ClassVar | ReadOnlyReason::ClassObjectInitializedOnBody
+                )
+            )
+        };
+        let got_is_classvar = is_classvar_compatible(got);
+        let want_is_classvar = is_classvar_compatible(want);
         if got_is_classvar != want_is_classvar {
             return Err(Box::new(AttrSubsetError::ClassVarMismatch {
                 got_is_classvar,

pyrefly/lib/test/class_overrides.rs

@@ -72,6 +72,34 @@ class E(B):
  "#,
 );
 
+testcase!(
+    test_override_classvar_with_nested_class,
+    r#"
+from typing import ClassVar
+
+class Endpoint: ...
+
+class Base:
+    endpoint_cls: ClassVar[type[Endpoint]] = Endpoint
+
+class Child(Base):
+    class endpoint_cls(Endpoint): ...
+"#,
+);
+
+testcase!(
+    test_override_nested_class_with_classvar,
+    r#"
+from typing import ClassVar
+
+class Base:
+    class endpoint_cls: ...
+
+class Child(Base):
+    endpoint_cls: ClassVar[type[Base.endpoint_cls]] = type("endpoint_cls", (Base.endpoint_cls,), {})
+"#,
+);
+
 testcase!(
     test_override_final_var,
     r#"