test.c

#include <stdio.h>

// The file with the unchecked interface
#include <stdlib.h>

// The Redeclaration with the correct type
void *calloc(size_t nmemb, size_t size) : byte_count(nmemb * size);

#pragma BOUNDS_CHECKED ON

int main(void) {
  _Array_ptr<int> arr : count(4) = calloc(4, sizeof(int));
  
  for (int i = 0; i < 4; i++) 
    _Unchecked {
      printf("%d\n", arr[i]);
    }
  
  return 0;
}

I updated it with the right keyword names and including the stdio header. Oops. But you got the idea.