don't allow overriding a namedtuple element

pyrefly/lib/alt/class/class_field.rs

@@ -1146,6 +1146,19 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
 
         for (parent, parent_metadata) in parents {
             parent_has_any = parent_has_any || parent_metadata.has_base_any();
+            //don't allow overriding a namedtuple element
+            if let Some(named_tuple_metadata) = parent_metadata.named_tuple_metadata() {
+                if named_tuple_metadata.elements.contains(name) {
+                    self.error(
+                        errors,
+                        range,
+                        ErrorKind::BadOverride,
+                        None,
+                        format!("Cannot override named tuple element `{}`", name),
+                    );
+                    return;
+                }
+            }
             let Some(want_member) = self.get_class_member(parent.class_object(), name) else {
                 continue;
             };

pyrefly/lib/test/named_tuple.rs

@@ -256,3 +256,20 @@ class Foo(NamedTuple):
     y: str # E: NamedTuple field 'y' without a default may not follow NamedTuple field with a default
 "#,
 );
+
+testcase!(
+    test_named_tuple_override_error,
+    r#"
+from typing import NamedTuple
+
+class A(NamedTuple):
+    x: int
+
+class B(A):
+    x: int  # E: Cannot override named tuple element `x`
+    y: int
+
+class C(B):
+    y: int  # OK
+"#,
+);