Throw UnsupportedOperationException when a projected value cannot be returned

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-commons</artifactId>
-	<version>2.5.0-SNAPSHOT</version>
+	<version>2.5.0-GH-2290-SNAPSHOT</version>
 
 	<name>Spring Data Core</name>
 

src/main/java/org/springframework/data/mapping/context/PersistentPropertyPathFactory.java

@@ -222,8 +222,7 @@ private Pair<DefaultPersistentPropertyPath<P>, E> getPair(DefaultPersistentPrope
 			return null;
 		}
 
-		TypeInformation<?> type = property.getTypeInformation().getRequiredActualType();
-		return Pair.of(path.append(property), iterator.hasNext() ? context.getRequiredPersistentEntity(type) : entity);
+		return Pair.of(path.append(property), iterator.hasNext() ? context.getRequiredPersistentEntity(property) : entity);
 	}
 
 	private <T> Collection<PersistentPropertyPath<P>> from(TypeInformation<T> type, Predicate<? super P> filter,
@@ -236,6 +235,12 @@ private <T> Collection<PersistentPropertyPath<P>> from(TypeInformation<T> type,
 		}
 
 		E entity = context.getRequiredPersistentEntity(actualType);
+		return from(entity, filter, traversalGuard, basePath);
+	}
+
+	private Collection<PersistentPropertyPath<P>> from(E entity, Predicate<? super P> filter, Predicate<P> traversalGuard,
+			DefaultPersistentPropertyPath<P> basePath) {
+
 		Set<PersistentPropertyPath<P>> properties = new HashSet<>();
 
 		PropertyHandler<P> propertyTester = persistentProperty -> {
@@ -254,7 +259,7 @@ private <T> Collection<PersistentPropertyPath<P>> from(TypeInformation<T> type,
 			}
 
 			if (traversalGuard.and(IS_ENTITY).test(persistentProperty)) {
-				properties.addAll(from(typeInformation, filter, traversalGuard, currentPath));
+				properties.addAll(from(context.getPersistentEntity(persistentProperty), filter, traversalGuard, currentPath));
 			}
 		};
 

src/main/java/org/springframework/data/projection/ProjectingMethodInterceptor.java

@@ -27,7 +27,6 @@
 
 import org.aopalliance.intercept.MethodInterceptor;
 import org.aopalliance.intercept.MethodInvocation;
-
 import org.springframework.core.CollectionFactory;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.data.util.ClassTypeInformation;
@@ -45,6 +44,7 @@
  *
  * @author Oliver Gierke
  * @author Mark Paluch
+ * @author Christoph Strobl
  * @since 1.10
  */
 class ProjectingMethodInterceptor implements MethodInterceptor {
@@ -98,14 +98,22 @@ protected Object potentiallyConvertResult(TypeInformation<?> type, @Nullable Obj
 			return null;
 		}
 
-		if (type.isCollectionLike() && !ClassUtils.isPrimitiveArray(type.getType())) {
+		Class<?> targetType = type.getType();
+
+		if (type.isCollectionLike() && !ClassUtils.isPrimitiveArray(targetType)) {
 			return projectCollectionElements(asCollection(result), type);
 		} else if (type.isMap()) {
 			return projectMapValues((Map<?, ?>) result, type);
-		} else if (conversionRequiredAndPossible(result, type.getType())) {
-			return conversionService.convert(result, type.getType());
+		} else if (ClassUtils.isAssignable(targetType, result.getClass())) {
+			return result;
+		} else if (conversionService.canConvert(result.getClass(), targetType)) {
+			return conversionService.convert(result, targetType);
+		} else if (targetType.isInterface()) {
+			return getProjection(result, targetType);
 		} else {
-			return getProjection(result, type.getType());
+			throw new UnsupportedOperationException(
+					String.format("Cannot project %s to %s. Target type is not an interface and no matching Converter found!",
+							ClassUtils.getDescriptiveType(result), ClassUtils.getQualifiedName(targetType)));
 		}
 	}
 
@@ -155,28 +163,11 @@ private Map<Object, Object> projectMapValues(Map<?, ?> sources, TypeInformation<
 	}
 
 	@Nullable
-	private Object getProjection(Object result, Class<?> returnType) {
+	private Object getProjection(@Nullable Object result, Class<?> returnType) {
 		return result == null || ClassUtils.isAssignable(returnType, result.getClass()) ? result
 				: factory.createProjection(returnType, result);
 	}
 
-	/**
-	 * Returns whether the source object needs to be converted to the given target type and whether we can convert it at
-	 * all.
-	 *
-	 * @param source can be {@literal null}.
-	 * @param targetType must not be {@literal null}.
-	 * @return
-	 */
-	private boolean conversionRequiredAndPossible(Object source, Class<?> targetType) {
-
-		if (source == null || targetType.isInstance(source)) {
-			return false;
-		}
-
-		return conversionService.canConvert(source.getClass(), targetType);
-	}
-
 	/**
 	 * Turns the given value into a {@link Collection}. Will turn an array into a collection an wrap all other values into
 	 * a single-element collection.

src/test/java/org/springframework/data/projection/ProjectingMethodInterceptorUnitTests.java

@@ -19,6 +19,7 @@
 import static org.mockito.ArgumentMatchers.*;
 import static org.mockito.Mockito.*;
 
+import java.math.BigInteger;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
@@ -42,6 +43,7 @@
  * @author Oliver Gierke
  * @author Saulo Medeiros de Araujo
  * @author Mark Paluch
+ * @author Christoph Strobl
  */
 @ExtendWith(MockitoExtension.class)
 class ProjectingMethodInterceptorUnitTests {
@@ -95,6 +97,20 @@ void considersPrimitivesAsWrappers() throws Throwable {
 		verify(factory, times(0)).createProjection((Class<?>) any(), any());
 	}
 
+	@Test // GH-2290
+	void failsForNonConvertibleTypes() throws Throwable {
+
+		MethodInterceptor methodInterceptor = new ProjectingMethodInterceptor(factory, interceptor, conversionService);
+
+		when(invocation.getMethod()).thenReturn(Helper.class.getMethod("getBoolean"));
+		when(interceptor.invoke(invocation)).thenReturn(BigInteger.valueOf(1));
+
+		assertThatThrownBy(() -> methodInterceptor.invoke(invocation)) //
+				.isInstanceOf(UnsupportedOperationException.class) //
+				.hasMessageContaining("BigInteger") //
+				.hasMessageContaining("boolean");
+	}
+
 	@Test // DATAREST-394, DATAREST-408
 	@SuppressWarnings("unchecked")
 	void appliesProjectionToNonEmptySets() throws Throwable {
@@ -213,6 +229,8 @@ interface Helper {
 
 		long getPrimitive();
 
+		boolean getBoolean();
+
 		Collection<HelperProjection> getHelperCollection();
 
 		List<HelperProjection> getHelperList();