discriminator work

- Handle discriminator as Fetch for entity ResultGraphNodes.  This allows us to make the distinction about whether to handle the discriminator as `Class` or as its "underlying" type when selecting it.  Fetches return the underlying type.  DomainResults return `Class`, or String for entity-named entity mappings

hibernate-core/src/main/java/org/hibernate/metamodel/mapping/EntityDiscriminatorMapping.java

@@ -73,10 +73,4 @@ public interface EntityDiscriminatorMapping extends VirtualModelPart, BasicValue
 		// can never be circular
 		return null;
 	}
-
-	<T> DomainResult<T> createUnderlyingDomainResult(
-			NavigablePath navigablePath,
-			TableGroup tableGroup,
-			String resultVariable,
-			DomainResultCreationState creationState);
 }

hibernate-core/src/main/java/org/hibernate/metamodel/mapping/internal/AbstractEntityDiscriminatorMapping.java

@@ -88,23 +88,6 @@ public abstract class AbstractEntityDiscriminatorMapping implements EntityDiscri
 		);
 	}
 
-	@Override
-	public <T> DomainResult<T> createUnderlyingDomainResult(
-			NavigablePath navigablePath,
-			TableGroup tableGroup,
-			String resultVariable,
-			DomainResultCreationState creationState) {
-		final SqlSelection sqlSelection = resolveSqlSelection( tableGroup, true, creationState );
-
-		//noinspection unchecked
-		return new BasicResult(
-				sqlSelection.getValuesArrayPosition(),
-				resultVariable,
-				mappingType.getUnderlyingType().getJavaTypeDescriptor(),
-				navigablePath
-		);
-	}
-
 	@Override
 	public void applySqlSelections(
 			NavigablePath navigablePath,

hibernate-core/src/test/java/org/hibernate/orm/test/discriminator/DiscriminatorQueryUsageTests.java

@@ -0,0 +1,100 @@
+/*
+ * Hibernate, Relational Persistence for Idiomatic Java
+ *
+ * License: GNU Lesser General Public License (LGPL), version 2.1 or later
+ * See the lgpl.txt file in the root directory or http://www.gnu.org/licenses/lgpl-2.1.html
+ */
+package org.hibernate.orm.test.discriminator;
+
+import javax.persistence.Tuple;
+
+import org.hibernate.testing.orm.junit.DomainModel;
+import org.hibernate.testing.orm.junit.SessionFactory;
+import org.hibernate.testing.orm.junit.SessionFactoryScope;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.assertj.core.api.Assertions;
+
+/**
+ * @author Steve Ebersole
+ */
+@DomainModel(
+		xmlMappings = "org/hibernate/orm/test/discriminator/Person.hbm.xml"
+)
+@SessionFactory
+public class DiscriminatorQueryUsageTests {
+	private Long steveId;
+
+	@Test
+	public void testUsageAsSelection(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			final Tuple resultTuple = session
+					.createQuery( "select p.id as id, type(p) as type from Person p", Tuple.class )
+					.uniqueResult();
+
+			Assertions.assertThat( resultTuple.get( "id" ) ).isEqualTo( steveId );
+			Assertions.assertThat( resultTuple.get( "type" ) ).isEqualTo( Employee.class );
+		} );
+	}
+
+	@Test
+	public void testUsageAsPredicate(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			final Long id = session.createQuery( "select p.id from Person p where type(p) = Employee", Long.class ).uniqueResult();
+			Assertions.assertThat( id ).isEqualTo( steveId );
+		} );
+	}
+
+	@Test
+	public void testUsageAsPredicateOfUnderlyingType(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			final Long id = session.createQuery( "select p.id from Person p where type(p) = 'P'", Long.class ).uniqueResult();
+			Assertions.assertThat( id ).isEqualTo( steveId );
+		} );
+	}
+
+	@Test
+	public void testUsageAsPredicateWithParam(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			final Long id = session.createQuery( "select p.id from Person p where type(p) = :type", Long.class )
+					.setParameter( "type", Employee.class )
+					.uniqueResult();
+			Assertions.assertThat( id ).isEqualTo( steveId );
+		} );
+	}
+
+	@Test
+	public void testUsageAsPredicateWithParamOfUnderlyingType(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			final Long id = session.createQuery( "select p.id from Person p where type(p) = :type", Long.class )
+					.setParameter( "type", "P" )
+					.uniqueResult();
+			Assertions.assertThat( id ).isEqualTo( steveId );
+		} );
+	}
+
+	@BeforeEach
+	public void createTestData(SessionFactoryScope scope) {
+		final Employee created = scope.fromTransaction( (session) -> {
+			final Employee e = new Employee();
+			e.setName( "Steve" );
+			e.setSex( 'M' );
+			e.setTitle( "Gangster of love" );
+
+			session.save( e );
+
+			return e;
+		} );
+
+		steveId = created.getId();
+	}
+
+	@AfterEach
+	public void dropTestData(SessionFactoryScope scope) {
+		scope.inTransaction( (session) -> {
+			session.createQuery( "delete from Person" ).executeUpdate();
+		} );
+	}
+}