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 comments US 144 and US 145 on the C++20 DIS.)
Subclause 15.7 [cpp.line] paragraph 3 specifies:
If the digit sequence specifies zero or a number greater than 2147483647, the behavior is undefined.
Subclause 15.7 [cpp.line] paragraph 5 specifies:
If the directive resulting after all replacements does not match one of the two previous forms, the behavior is undefined; otherwise, the result is processed as appropriate.
Undefined behavior during lexing is not acceptable. This ought to be ill-formed.
Additional notes (January, 2023):
Forwarded to SG12 with paper issue 1411, by decision of the CWG and SG12 chairs.