don't allow overriding a namedtuple element

conformance/third_party/conformance.exp

@@ -7455,6 +7455,16 @@
       "stop_column": 13,
       "stop_line": 59
     },
+    {
+      "code": -2,
+      "column": 5,
+      "concise_description": "Cannot override named tuple element `x`",
+      "description": "Cannot override named tuple element `x`",
+      "line": 79,
+      "name": "bad-override",
+      "stop_column": 6,
+      "stop_line": 79
+    },
     {
       "code": -2,
       "column": 19,

conformance/third_party/conformance.result

@@ -373,9 +373,7 @@
   ],
   "literals_parameterizations.py": [],
   "literals_semantics.py": [],
-  "namedtuples_define_class.py": [
-    "Line 79: Expected 1 errors"
-  ],
+  "namedtuples_define_class.py": [],
   "namedtuples_define_functional.py": [],
   "namedtuples_type_compat.py": [],
   "namedtuples_usage.py": [],

conformance/third_party/results.json

@@ -1,9 +1,9 @@
 {
   "total": 136,
-  "pass": 60,
-  "fail": 76,
-  "pass_rate": 0.44,
-  "differences": 356,
+  "pass": 61,
+  "fail": 75,
+  "pass_rate": 0.45,
+  "differences": 355,
   "passing": [
     "aliases_explicit.py",
     "aliases_newtype.py",
@@ -49,6 +49,7 @@
     "historical_positional.py",
     "literals_parameterizations.py",
     "literals_semantics.py",
+    "namedtuples_define_class.py",
     "namedtuples_define_functional.py",
     "namedtuples_type_compat.py",
     "namedtuples_usage.py",
@@ -114,7 +115,6 @@
     "generics_variance.py": 12,
     "literals_interactions.py": 2,
     "literals_literalstring.py": 3,
-    "namedtuples_define_class.py": 1,
     "narrowing_typeguard.py": 2,
     "narrowing_typeis.py": 4,
     "overloads_consistency.py": 2,

pyrefly/lib/alt/class/class_field.rs

@@ -1201,6 +1201,18 @@ 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),
+                    );
+                }
+            }
             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
+"#,
+);