@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;
        }
Returns true if casting to the given type will elicit an unchecked warning from the compiler.
Only generic types such as {@code List<String>} produce such warnings. There will be no warning
if the type's only generic parameters are simple wildcards, as in {@code Map<?, ?>}.
value/src/main/java/com/google/auto/value/processor/TypeSimplifier.java