In #3994 I found that MapStruct currently generates invalide copy constructors when trying to map from Collection/Map with <? extends T> or <? super T> bounds to itself.

I fixed the issue by using the diamond operator for SetterWrapperForCollectionsAndMapsWithNullCheck on the constructor.

I found that the same issue is present in ExistingInstanceSetterWrapperForCollectionsAndMaps.ftl. I didn't include that in this PR, as MapStruct tries to use addAll(Collection<? extends E> c)/putAll(Map<? extends K, ? extends V> m). It is impossible to add a super bounded to to an extend bounded one, example

if ( extendTypes.getSimpleObjectsCollection() != null ) {
        Collection<? super SimpleObject> collection = types.getSimpleObjectsCollection();
        if ( collection != null ) {
            extendTypes.getSimpleObjectsCollection().clear();
            extendTypes.getSimpleObjectsCollection().addAll( collection ); // Error
        }
    }
    else {
        Collection<? super SimpleObject> collection = types.getSimpleObjectsCollection();
        if ( collection != null ) {
            extendTypes.setSimpleObjectsCollection( new ArrayList<SimpleObject>( collection ) );
        }
    }
}

I am not sure what the correct solution would be:

• Only override the values with a copy constructor (linke in the else branch)
• Reject the complete mapping as impossible