This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
(From National Body comment US 142 on the C++20 DIS.)
Subclause 15.6.3 [cpp.stringize] paragraph 2 specifies:
If the replacement that results is not a valid character string literal, the behavior is undefined.
Undefined behavior during lexing is not acceptable. This ought to be ill-formed.
Additional notes (January, 2023):
Forwarded to SG12 with paper issue 1409, by decision of the CWG and SG12 chairs.