a warning to implement

[Robert Dewar]

<<  Certain object representations need not represent a value of the
  object type.  If the stored value of the object has such a
  representation and is read by an lvalue expression that does not
  have character type, the behavior is undefined.  If such a
  representation is produced by a side effect thaty modifies all or
  any part of the object by an lvalue expression that does not have
  character type, the behavior is undefined.  Such a representation is
  called a trap represetation.
>>

You seem to be assuming that of necessity a char object can contain only
valid representations. This is definitely not true on all implementations.