This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[ Resolved by paper P2621R2 (Undefined behavior in the lexer), adopted in June, 2023. ]
(From National Body comment US 027 on the C++20 DIS.)
Subclause 5.4 [lex.pptoken] paragraph 2 specifies:
If a U+0027 apostrophe or a U+0022 quotation mark character matches the last category, 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 1406, by decision of the CWG and SG12 chairs.