Unexpected return.type.incompatible: Set<V extends @KeyFor...> vs Set<V> on `return map.keySet()`

Here's a part of the class in question

public class DefaultDirectedGraph<V, E extends DefaultEdge>
    implements DirectedGraph<V, E> {
  final Set<E> edges = new LinkedHashSet<>();
  final Map<V, VertexInfo<V, E>> vertexMap = new LinkedHashMap<>();
  final @NotOnlyInitialized EdgeFactory<V, E> edgeFactory;

  @Override public Set<V> vertexSet() { // implement DirectedGraph nethod
    return vertexMap.keySet();
  }

checker-framework issues error as follows:

calcite/core/src/main/java/org/apache/calcite/util/graph/DefaultDirectedGraph.java:161: error: [return.type.incompatible] incompatible types in return.
    return vertexMap.keySet();
                           ^
  type of expression: Set<V extends @KeyFor("this.vertexMap") Object>
  method return type: Set<V extends Object>

I think it might be related to https://github.com/typetools/checker-framework/issues/1653, however, the difference here is that Set<V> signature is specified for a public API.

The method itself looks very much java.util.Map<K, V>.keySet() which return Set<V>.

Does that mean the methods like that always have to suppress return.type.incompatible error?

Start with the reported vertexSet() method in calcite/core/src/main/java/org/apache/calcite/util/graph/DefaultDirectedGraph.java and compare its Map.keySet() return with the related Checker Framework issue 1653. Trace how the checker handles the @KeyFor type on the returned set; done means establishing whether this diagnostic is expected and documenting or fixing the behavior.