An existing test case, org.hibernate.test.idclass.IdClassTest, maps an IdClass using an HBM mapping.

Specifically, the composite ID with IdClass is mapped as:

		<composite-id class="CustomerId" mapped="true">
			<key-property name="orgName" column="org_name"/>
			<key-property name="customerName" column="cust_name"/>
		</composite-id>

There is a comment and JPA metamodel population is disabled:

// the Customer entity has a composite id that is not embeddable ( not supported by JPA ).
configuration.setProperty( AvailableSettings.JPA_METAMODEL_POPULATION, "disabled" );

The comment is incorrect, as the mapping does, in fact, map an ID class: CustomerId

The IdClass is mapped by:
<composite-id class="CustomerId" mapped="true">

The test only passes because it sets:
hibernate.ejb.metamodel.population=disabled

When the property value is changed to ignoreUnsupported or enabled, the test fails due to:

org.hibernate.testing.junit4.CallbackException: org.hibernate.testing.junit4.BaseCoreFunctionalTestCase#buildSessionFactory
at org.hibernate.testing.junit4.TestClassMetadata.performCallbackInvocation(TestClassMetadata.java:204)
at org.hibernate.testing.junit4.TestClassMetadata.invokeCallback(TestClassMetadata.java:188)
at org.hibernate.testing.junit4.TestClassMetadata.performCallbacks(TestClassMetadata.java:182)
at org.hibernate.testing.junit4.TestClassMetadata.performBeforeClassCallbacks(TestClassMetadata.java:173)
at org.hibernate.testing.junit4.BeforeClassCallbackHandler.evaluate(BeforeClassCallbackHandler.java:25)
at org.hibernate.testing.junit4.AfterClassCallbackHandler.evaluate(AfterClassCallbackHandler.java:25)
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 sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.IllegalArgumentException: expecting IdClass mapping
at org.hibernate.metamodel.internal.AttributeFactory$3.resolveMember(AttributeFactory.java:978)
at org.hibernate.metamodel.internal.AttributeFactory$5.resolveMember(AttributeFactory.java:1036)
at org.hibernate.metamodel.internal.AttributeFactory.determineAttributeMetadata(AttributeFactory.java:458)
at org.hibernate.metamodel.internal.AttributeFactory.buildIdAttribute(AttributeFactory.java:128)
at org.hibernate.metamodel.internal.MetadataContext.buildIdClassAttributes(MetadataContext.java:360)
at org.hibernate.metamodel.internal.MetadataContext.applyIdMetadata(MetadataContext.java:290)
at org.hibernate.metamodel.internal.MetadataContext.wrapUp(MetadataContext.java:199)
at org.hibernate.metamodel.internal.MetamodelImpl.initialize(MetamodelImpl.java:230)
at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:295)
at org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:461)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:708)
at org.hibernate.testing.junit4.BaseCoreFunctionalTestCase.buildSessionFactory(BaseCoreFunctionalTestCase.java:113)
at org.hibernate.testing.junit4.BaseCoreFunctionalTestCase.buildSessionFactory(BaseCoreFunctionalTestCase.java:100)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.hibernate.testing.junit4.TestClassMetadata.performCallbackInvocation(TestClassMetadata.java:201)
... 15 more