Testcase:

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>

int main(void)
{
  char a[8], b[8];
  char *a_ptr = a+8;
  char *b_ptr = b;
  printf("a_ptr = %p, b_ptr = %p\n", a_ptr, b_ptr);
  if (a_ptr != b_ptr)
  {
    printf("a_ptr != b_ptr\n");
  }
  else
  {
    printf("a_ptr == b_ptr\n");
  }
  
  
  if ((uintptr_t)a_ptr != (uintptr_t)b_ptr)
  {
    printf("(uintptr_t)a_ptr != (uintptr_t)b_ptr\n");
  }
  else
  {
    printf("(uintptr_t)a_ptr == (uintptr_t)b_ptr\n");
  }
  return EXIT_SUCCESS;
}

Checked with option gcc test.c -std=c18 -Wall -Wextra -O2
Output:

a_ptr = 0x7ffc0a1dea38, b_ptr = 0x7ffc0a1dea38
a_ptr != b_ptr
(uintptr_t)a_ptr == (uintptr_t)b_ptr

I think this is wrong. According to latest available C18 draft:

https://web.archive.org/web/20181230041359if_/http://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf#subsection.6.5.9

> Two pointers compare equal if and only if both are null pointers, both are pointers to the same object(including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space. .111)

> 111)Two objects may be adjacent in memory because they are adjacent elements of a larger array or adjacent members of a structure with no padding between them, or because the implementation chose to place them so, even though they are unrelated. If prior invalid pointer operations (such as accesses outside array bounds) produced undefined behavior, subsequent comparisons also produce undefined behavior.

Here we have the case when "one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space" AND "implementation chose to place them so, even though they are unrelated".