Array index UB issue.

$ cat indexudef.c

#include <stdio.h>

#define MAX 10

int main() {
  int a[MAX];

  for (size_t i = 0; i < MAX; ++i) {
    a[i] = i++;
    printf("%3d", a[i]);
  }
  putchar('\n');
}

$ gcc-11 -Wall -pedantic -std=c11 -o ../bin/indexudefG indexudef.c
indexudef.c: In function 'main':
indexudef.c:10:11: warning: operation on 'i' may be undefined [-Wsequence-point]
   10 |   a[i] = i++;
      |          ~^~
$ ../bin/indexudefG
  0  2  4  6  8
$ clang -Wall -pedantic -std=c11 -o ../bin/indexudefC indexudef.c
indexudef.c:10:11: warning: unsequenced modification and access to 'i' [-Wunsequenced]
  a[i] = i++;
    ~     ^
1 warning generated.
$ ../bin/indexudefC
  0  2  4  6  8