This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Adopted at the November, 2020 meeting as part of paper P2029R4.]
According to 5.13.3 [lex.ccon] paragraphs 3-5, a Unicode character literal “containing multiple c-chars is ill-formed.” However, it is not clear in what phase of translation that restriction applies.
One possible resolution would be to add a note saying that the pp-token is formed according to the grammar and the restriction to a single c-char is checked in phase 7.