Additional notes (November, 2023):

The first case of undefined behavior was addressed by paper P2621R2 (Undefined behavior in the lexer), adopted in June, 2023. The relevant part of 15.6.4 [cpp.concat] paragraph 3 now reads:

... [Note 1: Concatenation can form a universal-character-name (5.3.1 [lex.charset]). —end note] If the result is not a valid preprocessing token, the behavior is undefined. ...

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1410, by decision of the CWG and SG12 chairs.

(From National Body comment US 143 on the C++20 DIS.)

Subclause 15.6.4 [cpp.concat] paragraph 3 specifies:

If the result begins with a sequence matching the syntax of universal-character-name, the behavior is undefined. [Note: ... --end note] If the result is not a valid preprocessing token, the behavior is undefined.

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