In 5.1, the failure occurs when an HBM mapping that maps an IdClass is used with EntityManager.

That failure occurs when building the EntityManagerFactory:

javax.persistence.PersistenceException: Unable to build entity manager factory
at org.hibernate.jpa.HibernatePersistenceProvider.createEntityManagerFactory(HibernatePersistenceProvider.java:66)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:55)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:39)
at org.hibernate.bugs.JPAUnitTestCase.init(JPAUnitTestCase.java:20)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:78)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:212)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:68)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.IllegalArgumentException: expecting IdClass mapping
at org.hibernate.jpa.internal.metamodel.AttributeFactory$3.resolveMember(AttributeFactory.java:966)
at org.hibernate.jpa.internal.metamodel.AttributeFactory$5.resolveMember(AttributeFactory.java:1024)
at org.hibernate.jpa.internal.metamodel.AttributeFactory.determineAttributeMetadata(AttributeFactory.java:449)
at org.hibernate.jpa.internal.metamodel.AttributeFactory.buildIdAttribute(AttributeFactory.java:128)
at org.hibernate.jpa.internal.metamodel.MetadataContext.buildIdClassAttributes(MetadataContext.java:346)
at org.hibernate.jpa.internal.metamodel.MetadataContext.applyIdMetadata(MetadataContext.java:278)
at org.hibernate.jpa.internal.metamodel.MetadataContext.wrapUp(MetadataContext.java:199)
at org.hibernate.jpa.internal.metamodel.MetamodelImpl.buildMetamodel(MetamodelImpl.java:82)
at org.hibernate.jpa.internal.EntityManagerFactoryImpl.<init>(EntityManagerFactoryImpl.java:118)
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:889)
at org.hibernate.jpa.HibernatePersistenceProvider.createEntityManagerFactory(HibernatePersistenceProvider.java:58)
... 30 more