Many of these same points were raised by others at the last C9X meeting. This occurs very early, even before trigraph replacements. Some UCNs are valid in identifiers, but not others.
Problems Here are some problems with UCNs as they appear in the current draft.
The current draft assumes that source file characters can be transliterated to Unicode and back again without loss of information. This assumption is incorrect and unrealistic. This is not what the programmer likely intended, and C9X must not require this weird sort of case-folding. The current draft requires that the implementation maintain translation tables from the source and execution character set to Unicode, in order to properly stringize strings. This requirement is unrealistic.