@Override
    public @Nullable Void visitWildcard(WildcardType t, ImmutableSet.Builder<TypeElement> p) {
      TypeMirror extendsBound = t.getExtendsBound();
      if (extendsBound != null) {
        extendsBound.accept(this, p);
      }
      TypeMirror superBound = t.getSuperBound();
      if (superBound != null) {
        superBound.accept(this, p);
      }
      return null;
    }
Returns the set of {@linkplain TypeElement types} that are referenced by the given {@link
TypeMirror}.
common/src/main/java/com/google/auto/common/MoreTypes.java