#163 Raising error in case wrong decorator type is given

processor/src/main/java/org/mapstruct/ap/processor/MapperCreationProcessor.java

@@ -173,6 +173,15 @@ public class MapperCreationProcessor implements ModelElementProcessor<List<Sourc
 
         TypeElement decoratorElement = (TypeElement) typeUtils.asElement( decoratorPrism.value() );
 
+        if ( !typeUtils.isAssignable( decoratorElement.asType(), element.asType() ) ) {
+            messager.printMessage(
+                Kind.ERROR,
+                String.format( "Specified decorator type is no subtype of the annotated mapper type." ),
+                element,
+                decoratorPrism.mirror
+            );
+        }
+
         List<MappingMethod> mappingMethods = new ArrayList<MappingMethod>( methods.size() );
 
         for ( SourceMethod mappingMethod : methods ) {

processor/src/test/java/org/mapstruct/ap/test/decorator/DecoratorTest.java

@@ -19,10 +19,14 @@
 package org.mapstruct.ap.test.decorator;
 
 import java.util.Calendar;
+import javax.tools.Diagnostic.Kind;
 
 import org.mapstruct.ap.testutil.IssueKey;
 import org.mapstruct.ap.testutil.MapperTestBase;
 import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.compilation.annotation.CompilationResult;
+import org.mapstruct.ap.testutil.compilation.annotation.Diagnostic;
+import org.mapstruct.ap.testutil.compilation.annotation.ExpectedCompilationOutcome;
 import org.testng.annotations.Test;
 
 import static org.fest.assertions.Assertions.assertThat;
@@ -120,4 +124,22 @@ public class DecoratorTest extends MapperTestBase {
         assertThat( personDto.getAddress() ).isNotNull();
         assertThat( personDto.getAddress().getAddressLine() ).isEqualTo( "42 Ocean View Drive" );
     }
+
+    @Test
+    @WithClasses({
+        ErroneousPersonMapper.class,
+        ErroneousPersonMapperDecorator.class
+    })
+    @ExpectedCompilationOutcome(
+        value = CompilationResult.FAILED,
+        diagnostics = {
+            @Diagnostic(type = ErroneousPersonMapper.class,
+                kind = Kind.ERROR,
+                line = 27,
+                messageRegExp = "Specified decorator type is no subtype of the annotated mapper type")
+        }
+    )
+    public void shouldRaiseErrorInCaseWrongDecoratorTypeIsGiven() {
+
+    }
 }

processor/src/test/java/org/mapstruct/ap/test/decorator/ErroneousPersonMapper.java

@@ -0,0 +1,35 @@
+/**
+ *  Copyright 2012-2014 Gunnar Morling (http://www.gunnarmorling.de/)
+ *  and/or other contributors as indicated by the @authors tag. See the
+ *  copyright.txt file in the distribution for a full listing of all
+ *  contributors.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.mapstruct.ap.test.decorator;
+
+import org.mapstruct.DecoratedWith;
+import org.mapstruct.Mapper;
+import org.mapstruct.ReportingPolicy;
+import org.mapstruct.factory.Mappers;
+
+@Mapper(unmappedTargetPolicy = ReportingPolicy.IGNORE)
+@DecoratedWith(ErroneousPersonMapperDecorator.class)
+public interface ErroneousPersonMapper {
+
+    ErroneousPersonMapper INSTANCE = Mappers.getMapper( ErroneousPersonMapper.class );
+
+    PersonDto personToPersonDto(Person person);
+
+    AddressDto addressToAddressDto(Address address);
+}

processor/src/test/java/org/mapstruct/ap/test/decorator/ErroneousPersonMapperDecorator.java

@@ -0,0 +1,22 @@
+/**
+ *  Copyright 2012-2014 Gunnar Morling (http://www.gunnarmorling.de/)
+ *  and/or other contributors as indicated by the @authors tag. See the
+ *  copyright.txt file in the distribution for a full listing of all
+ *  contributors.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.mapstruct.ap.test.decorator;
+
+public abstract class ErroneousPersonMapperDecorator {
+}