When custom AttributeConverter has no explicit "implements AttributeConverter" (it is subclass of some base implementation of AttributeConverter) I got exception as below.

When I explicitly add "implements AttributeConverter" everything is ok, so there is easy workaround.

I created test case:
https://github.com/wjtk/mytests-jpa-spring
on branch "hibernate-bug", in class: wkr.testy.jpa.converters.ColorOneLetterConverter.

Exception
Caused by: org.hibernate.AssertionFailure: Could not extract ParameterizedType representation of AttributeConverter definition from AttributeConverter implementation class [wkr.testy.jpa.converters.ColorOneLetterConverter]
at org.hibernate.cfg.AttributeConverterDefinition.extractAttributeConverterParameterizedType(AttributeConverterDefinition.java:94)
at org.hibernate.cfg.AttributeConverterDefinition.<init>(AttributeConverterDefinition.java:52)
at org.hibernate.cfg.Configuration.addAttributeConverter(Configuration.java:2690)
at org.hibernate.cfg.Configuration.addAttributeConverter(Configuration.java:2641)
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.buildHibernateConfiguration(EntityManagerFactoryBuilderImpl.java:1129)
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl$4.perform(EntityManagerFactoryBuilderImpl.java:846)
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl$4.perform(EntityManagerFactoryBuilderImpl.java:843)
at org.hibernate.boot.registry.classloading.internal.ClassLoaderServiceImpl.withTccl(ClassLoaderServiceImpl.java:399)
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:842)
at org.hibernate.jpa.HibernatePersistenceProvider.createContainerEntityManagerFactory(HibernatePersistenceProvider.java:150)
at org.hibernate.ejb.HibernatePersistence.createContainerEntityManagerFactory(HibernatePersistence.java:67)
at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:318)
at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:318)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1612)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1549)
... 53 more