Fix uninitialized @@var reads silently skipping downstream type checking

core/src/analyzer/dispatch.rs

@@ -137,12 +137,17 @@ pub(crate) fn dispatch_simple(genv: &mut GlobalEnv, lenv: &mut LocalEnv, node: &
     }
 
     // Class variable read: @@name
+    // Ruby: uninitialized class variables raise NameError at runtime.
+    // We fall back to nil so downstream method calls are still type-checked
+    // rather than silently skipped. This may produce nil-union types when
+    // reads precede writes in source order.
     if let Some(cvar_read) = node.as_class_variable_read_node() {
         let cvar_name = bytes_to_name(cvar_read.name().as_slice());
-        return match install_class_var_read(genv, &cvar_name) {
-            Some(vtx) => DispatchResult::Vertex(vtx),
-            None => DispatchResult::NotHandled,
-        };
+        let vtx = install_class_var_read(genv, &cvar_name).unwrap_or_else(|| {
+            let nil_vtx = genv.new_source(Type::Nil);
+            install_class_var_write(genv, cvar_name, nil_vtx)
+        });
+        return DispatchResult::Vertex(vtx);
     }
 
     // Global variable read: $name

core/src/env/scope.rs

@@ -326,10 +326,9 @@ impl ScopeManager {
             .find(|scope| matches!(&scope.kind, ScopeKind::Class { .. }))
             .map(|scope| scope.id);
         if let Some(scope_id) = class_scope_id {
-            self.scopes
-                .get_mut(&scope_id)
-                .expect("scope id from walk_scopes must exist")
-                .set_class_var(name, vtx);
+            if let Some(scope) = self.scopes.get_mut(&scope_id) {
+                scope.set_class_var(name, vtx);
+            }
         }
     }
 }

test/class_variable_test.rb

@@ -41,10 +41,6 @@ def log(msg)
     assert_no_check_errors(source)
   end
 
-  # ============================================
-  # Error Detection
-  # ============================================
-
   def test_class_variable_write_before_read
     source = <<~RUBY
       class Config
@@ -60,6 +56,10 @@ def env
     assert_no_check_errors(source)
   end
 
+  # ============================================
+  # Error Detection
+  # ============================================
+
   def test_class_variable_type_error
     source = <<~RUBY
       class Counter
@@ -86,4 +86,53 @@ def run
     RUBY
     assert_check_error(source, method_name: 'length', receiver_type: 'Integer')
   end
+
+  def test_class_variable_class_body_write_type_error
+    source = <<~RUBY
+      class Counter
+        @@count = 42
+
+        def display
+          @@count.upcase
+        end
+      end
+    RUBY
+    assert_check_error(source, method_name: 'upcase', receiver_type: 'Integer')
+  end
+
+  def test_class_variable_isolation_between_classes
+    source = <<~RUBY
+      class StringHolder
+        def setup
+          @@value = "hello"
+        end
+
+        def run
+          @@value.upcase
+        end
+      end
+
+      class IntHolder
+        def setup
+          @@value = 42
+        end
+
+        def run
+          @@value.upcase
+        end
+      end
+    RUBY
+    assert_check_error(source, method_name: 'upcase', receiver_type: 'Integer')
+  end
+
+  def test_class_variable_in_module_no_crash
+    source = <<~RUBY
+      module Config
+        @@setting = "production"
+      end
+    RUBY
+    # Module-scoped @@var is not yet supported (v0.2.0 limitation).
+    # The checker should not crash.
+    assert_no_check_errors(source)
+  end
 end