Issue #8297 has been reported by dunric (David Unric). ---------------------------------------- Bug #8297: extend & inherited class variable issue https://bugs.ruby-lang.org/issues/8297 Author: dunric (David Unric) Status: Open Priority: Normal Assignee: Category: core Target version: ruby -v: 2.0.0p0 Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN =begin By the current documentation, Object#extend method has to (only) add instance methods of a module given as a parameter. In the following example, the class ((*C*)) is extended with module ((*M*)). By ((*class_variables*)) method sent to singleton class of C also did inherit class variable ((*@@xyz*)). However when inherited ((*@@xyz*)) is accessed, ((*NameError*)) exception is raised as it is was not initialized. (({module M @@xyz = 123 end M.class_variables # [:@@xyz] M.class_variable_get :@@xyz # 123 , so far so good class C extend M end ((*p C.singleton_class.class_variables # [:@@xyz] p C.singleton_class.class_variable_get :@@xyz # NameError exception*)) })) Either ((*class_variables*)) returns invalid array - ie. ((*@@xyz*)) was not inherited at all or ((*class_variable_get*)) ignores class variables inherited from module (when sent to a singleton). =end Prior Ruby versions like 1.9.3p392 does not suffer this issue as return with Module#class_variables returns an empty array. -- http://bugs.ruby-lang.org/