[Accepted at the June, 2021 meeting as part of paper P1949R7 (C++ Identifier Syntax using Unicode Standard Annex 31).]

According to 5.11 [lex.name] paragraph 1,

Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646 falls into one of the ranges specified in _N4606_.E.1 [charname.allowed].

However, identifier-nondigit is also used in the grammar for pp-number. Should this restriction also be understood to apply in that non-identifier context?