[lex.ccon] What is the single code unit for an ordinary character literal or wide character literal? CWG2779

As the special rules specified in [lex.ccon]#1, that is:

A non-encodable character literal is a character-literal whose c-char-sequence consists of a single c-char that is not a numeric-escape-sequence and that specifies a character that either lacks representation in the literal's associated character encoding or that cannot be encoded as a single code unit.

The Unicode standard specifies how large a code unit for UTF8, UTF16, and UTF32 respectively. Which has a similar meaning as stated in wiki Character_encoding. However, it does not state how large the code unit for the encoding of the execution (wide-)character set. So, in this case, how to determine whether a code point value for a character in an ordinary or wide character literal can be encoded as a single code unit for the corresponding kind character literal?

Is it a good idea to change the wording "cannot be encoded as a single code unit" to "cannot be represented by an object with the type of the corresponding kind character-literal"?

Start with [lex.ccon] paragraph 1 and the wording discussed in CWG2779. Research how the execution and wide-character encodings define a code unit, then assess whether the proposed object-type wording precisely resolves the ambiguity. Done means reaching and documenting a decided wording or interpretation.