Report error for result types with wildcard extends bound

[hduelme]

MapStruct does not support result types with an extends bound. For example

import java.util.List;
import org.mapstruct.Mapper;

@Mapper
public interface BoundMapper {
    List<? extends Number> toExtends(List<Integer> in);
}

is invalide, because of the wildcard extends bound ? extends . MapStruct reports this as

Can't generate mapping method for a wildcard extends bound result.

Currently the plugin doesn't report this as an error.

One possible QuickFix could be removing the wildcard extends bound ? extends .