Sequence error show & tell.

$ cat a.c

#include <stdio.h>

int main()
{
  int q = 1;
  q = q++ + ++q * --q - q--;
  printf("a = %d\n", q);

  return 0;
}

$ gcc-9 a.c
$ ./a.out
a = 3
$ clang a.c
a.c:7:8: warning: multiple unsequenced modifications to 'q' [-Wunsequenced]
  q = q++ + ++q * --q - q--;
       ^    ~~
1 warning generated.
$ ./a.out
a = 5