additional test for Duplicate interface

When the language automatically adds a supertype

org.eclipse.xtext.xbase.testlanguages/src/org/eclipse/xtext/xbase/testlanguages/jvmmodel/JvmGenericTypeValidatorTestLangJmvModelInferrer.java

@@ -137,14 +137,14 @@ protected void inferClassWithSuperTypes(MyClassWithSuperTypes myClass, IJvmDecla
 			(JvmGenericType it) -> {
 				jvmTypesBuilder.setDocumentation(it, jvmTypesBuilder.getDocumentation(myClass));
 				var superTypes = myClass.getSuperTypes();
+				it.getSuperTypes().add(typeReferences.getTypeForName(Serializable.class, myClass));
 				for (int i = 0; i < superTypes.size(); i++) {
 					if (i == 0) {
 						jvmTypesBuilder.setSuperClass(it, superTypes.get(i));
 					} else {
 						jvmTypesBuilder.addSuperInterface(it, superTypes.get(i));
 					}
 				}
-				it.getSuperTypes().add(typeReferences.getTypeForName(Serializable.class, myClass));
 			}));
 	}
 

org.eclipse.xtext.xbase.tests/src/org/eclipse/xtext/xbase/tests/validation/JvmGenericTypeValidatorTest.java

@@ -248,6 +248,20 @@ public class JvmGenericTypeValidatorTest {
 				"Duplicate interface Serializable for the type Foo");
 	}
 
+	@Test public void testDuplicateImplementedInterfacesWithImplicitInterface() throws Exception {
+		// this class construct automatically has Serializable as a supertype
+		var source = "import java.io.Serializable\n"
+				+ "classWithSuperTypes Foo superTypes "
+				+ "Object, java.io.Serializable, Cloneable, Serializable {}";
+		var model = parse(source);
+		validationHelper.assertError(model, MY_CLASS_WITH_SUPER_TYPES, DUPLICATE_INTERFACE,
+				source.lastIndexOf("java.io.Serializable"), "java.io.Serializable".length(),
+				"Duplicate interface Serializable for the type Foo");
+		validationHelper.assertError(model, MY_CLASS_WITH_SUPER_TYPES, DUPLICATE_INTERFACE,
+				source.lastIndexOf("Serializable"), "Serializable".length(),
+				"Duplicate interface Serializable for the type Foo");
+	}
+
 	@Test public void testCheckSuperTypesWithClassWithSuperTypes() throws Exception {
 		// the first supertype must be a class, and then the other ones interfaces
 		var source = "classWithSuperTypes Foo superTypes Cloneable, Object {}";

org.eclipse.xtext.xbase/src/org/eclipse/xtext/xbase/validation/JvmGenericTypeValidator.java

@@ -191,8 +191,11 @@ protected void checkSuperTypes(EObject sourceType, JvmGenericType type) {
 		for (int i = 0; i < superTypes.size(); ++i) {
 			JvmTypeReference extendedType = superTypes.get(i);
 			var associated = getSuperTypeSourceElement(extendedType);
-			if (associated == null)
+			if (associated == null) {
+				// we still record this supertype for possible duplication checks
+				seen.add(extendedType.getIdentifier());
 				continue; // synthetic superclass (e.g., Object)
+			}
 			var eContainingFeature = associated.eContainingFeature();
 			int featureIndex = INSIGNIFICANT_INDEX;
 			multiFeatureIndex++;