Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1407, by decision of the CWG and SG12 chairs.

(From National Body comment US 130 on the C++20 DIS.)

Subclause 15.2 [cpp.cond] paragraph 10 specifies:

If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined.

Undefined behavior during lexing is not acceptable. The behavior ought to be well-defined, ill-formed, or conditionally-supported.