mbrtowc does not work on 4-bytes characters

[bhaible]

Unicode characters in the range U+10000 .. U+10FFFF are, in UTF-8, encoded as sequences of 4 bytes. In a UTF-8 locale, mbrtowc should be able to combine these bytes to a single character. POSIX specification: ​https://pubs.opengroup.org/onlinepubs/9799919799/functions/mbrtowc.html

This does not work in Haiku hrev57823 (from 2024-07-15).

How to reproduce: Compile and run the attached test program.

gcc -Wall foo.c
./a.out

Expected output:

ret = 1, wc = 0x0001F403

Actual output:

ret = -1, wc = 0x0BADFACE
a.out: foo.c:92:main: ret == 1

In Gnulib, most of the Unicode support is based on mbrtowc. Due to this bug, 31 tests of the test suite fail.

[bhaible]

test case foo.c

[waddlesplash]

Fixed in hrev58063 +beta5.