stdc_first_trailing_one_uc
uint32_t idx = stdc_first_trailing_one_uc(
uint32_t idx = stdc_first_trailing_one_uc(
.so_uc = stdc_first_trailing_one_uc,
extern unsigned int stdc_first_trailing_one_uc(unsigned char) _C23_UNSEQ_ATTR;
char(*)[1]: stdc_first_trailing_one_uc(val), \