Historically, @Configuration classes that did not declare @Bean
methods were allowed to be abstract. However, the changes made in
76a6b9ea79 introduced a regression that prevents such classes from
being abstract, resulting in a BeanInstantiationException. This change
in behavior is caused by the fact that such a @Configuration class is
no longer replaced by a concrete subclass created dynamically by CGLIB.
This commit restores support for abstract @Configuration classes
without @Bean methods by modifying the "no enhancement required" check
in ConfigurationClassParser.
See gh-34486
Closes gh-34663
044258f085