The error message uses 'incompatible types' with the notation '=?', which I"m nost sure is all that clear. Will users be familiar with '=' in generic notation enough to diagnose? I wish I had a better suggestion, maybe something about writing to a read only class.
 
class test
{
  class A {}
  class B extends A {}
  class List<T> { List<T> next; T t; }

  <A> List<? extends A> bad_reverse(List<? extends A> xs)
  {
    List<? extends A> tmp1 = xs, tmp2 = xs.next;
    tmp2.next = tmp1; // <<fail>> attempt to write to readonly class
    return tmp1;
  }
}

----------error message-----------
test.java:21: incompatible types
found : test.List<? extends A>
required: test.List<=? extends A>
    tmp2.next = tmp1; // <<fail>>
                ^