Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1411, by decision of the CWG and SG12 chairs.

(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.