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 148 and US 149 on the C++20 DIS.)
Subclause 15.11 [cpp.predefined] paragraph 4 specifies:
If any of the pre-defined macro names in this subclause, or the identifier defined, is the subject of a #define or a #undef preprocessing directive, 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 1412, by decision of the CWG and SG12 chairs.