Fix scoping for generators in named expressions in classes (#9248)

Closes #9230.

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_24.py

@@ -0,0 +1,8 @@
+"""Test for accessing class members within a generator."""
+
+
+class Class:
+    items = []
+
+    if len(replacements := {item[1] for item in items}) > 1:
+        pass

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -1745,10 +1745,13 @@ impl<'a> Checker<'a> {
             return;
         }
 
+        // If the expression is the left-hand side of a walrus operator, then it's a named
+        // expression assignment.
         if self
             .semantic
             .current_expressions()
-            .any(Expr::is_named_expr_expr)
+            .filter_map(Expr::as_named_expr_expr)
+            .any(|parent| parent.target.as_ref() == expr)
         {
             self.add_binding(id, expr.range(), BindingKind::NamedExprAssignment, flags);
             return;

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -142,6 +142,7 @@ mod tests {
     #[test_case(Rule::UndefinedName, Path::new("F821_21.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_22.ipynb"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_23.py"))]
+    #[test_case(Rule::UndefinedName, Path::new("F821_24.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_0.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_1.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_2.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F821_F821_24.py.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+