https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92869
--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> --- #c5 is rejected with -std=c++2a starting with r263115 aka http://wg21.link/p1008r1 So, I believe it is correct to reject this in C++20, but I think neither of those papers have been a DR and so shouldn't affect C++17 or earlier compilation modes.
