In Cygwin 3.5.3, the attached program has an assertion failure in line 24:
bytes is not (size_t)-2.

How to reproduce:
$ gcc -Wall foo.c
$ ./a

I think this is a bug, because
  - ISO C 23 § 7.30.1.5 talks about "completing" a character, not
    "representing" an (entire) character.
  - The test passes on glibc, musl libc, FreeBSD 14.0, Solaris 11.4.

Bruno