@Override
    public @Nullable Void visitTypeVariable(TypeVariable t, ImmutableSet.Builder<TypeElement> p) {
      t.getLowerBound().accept(this, p);
      t.getUpperBound().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