fix False positive with @DataClass(frozen=True) and a ClassVar #2789 (#2792)

Summary:
Fixes #2789

stops ClassVar members from being classified as frozen dataclass fields.

preserves the existing ClassVar write restriction on instances, but no longer incorrectly rejects writes through a class object like `self.__class__.x = False` in a frozen dataclass.

Pull Request resolved: #2792

Test Plan: add test

Reviewed By: grievejia

Differential Revision: D96812149

Pulled By: rchen152

fbshipit-source-id: 467eac510e0a47f519c32a23db99f03dc4d12b3a

Author: asukaminato0721

pyrefly/lib/alt/class/class_field.rs

@@ -2118,9 +2118,12 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         {
             return Some(ReadOnlyReason::NamedTuple);
         }
+        // ClassVars in dataclasses are class attributes rather than frozen instance fields.
+        let is_classvar = annotation.is_some_and(|ann| ann.has_qualifier(&Qualifier::ClassVar));
         // Frozen dataclass fields (not methods) are read-only
         if let Some(dm) = metadata.dataclass_metadata()
             && dm.kws.frozen
+            && !is_classvar
             && dm.fields.contains(name)
         {
             let reason = if metadata.is_pydantic_model() {

pyrefly/lib/test/dataclasses.rs

@@ -792,6 +792,21 @@ C()
     "#,
 );
 
+testcase!(
+    test_frozen_classvar_class_assignment,
+    r#"
+import dataclasses
+from typing import ClassVar
+
+@dataclasses.dataclass(frozen=True)
+class C:
+    x: ClassVar[bool] = True
+
+    def set_x(self) -> None:
+        self.__class__.x = False
+    "#,
+);
+
 testcase!(
     test_hashable,
     r#"