[Moved to DR at the May, 2015 meeting.]

Proposed resolution (April, 2015):

Change the grammar in 5.10 [lex.name] as follows:

identifier-nondigit:
nondigit
universal-character-name
other implementation-defined-characters

The grammar in 5.10 [lex.name] includes the production,

identifier-nondigit:
nondigit
universal-character-name
other implementation-defined characters

The rule for “other implementation-defined characters” is a holdover from before the introduction of universal-character-names, intended to allow the use of non-ASCII national characters in identifiers. However, since all characters outside the basic source character set are now conceptually mapped to universal-character-names in translation phase 1, there is no longer a need for such a provision and it should be removed.