At least with macOS Sequoia 15.5 and Xcode 16.3: $ cat test.cc #include <locale.h> #include <string.h> #include <xlocale.h> int main(void) { locale_t l = newlocale(LC_ALL_MASK, "el_GR.UTF-8", 0); strxfrm_l(NULL, "ό", 0, l); return 0; } $ c99 test.c && ./a.out Assertion failed: (p->val == key), function lookup_substsearch, file collate.c, line 596. Abort trap: 6