@Override
    public Boolean visitUnknown(TypeMirror t, Void p) {
      // We don't know whether casting is unchecked for this mysterious type but assume it is,
      // so we will insert a possibly unnecessary @SuppressWarnings("unchecked").
      return true;
    }
Visitor that tells whether a type is erased, in the sense of {@link #castIsUnchecked}. Each
visitX method returns true if its input parameter is true or if the type being visited is
erased.
common/src/main/java/com/google/auto/common/MoreTypes.java