*       example both {@code Enum<?>} and {@code Enum<? extends Enum<?>>} canonicalize to {@code
   *       Enum<? extends Enum<E>}.
   *   <li>{@code canonicalize(t)} produces a "literal" supertype of t. For example: {@code Enum<?
   *       extends Enum<?>>} canonicalizes to {@code Enum<?>}, which is a supertype (if we disregard
   *       the upper bound is implicitly an Enum too).