-Wrestrict triggers for a strcpy call from a string of unknown length and unknown offset but not the same call with a constant offset, even though the latter seems more likely to overlap than the former (the smaller the offset the more likely the cooy is to overlap).  In any case, unlike for memcpy, for string functions -Wrestrict is specifically meant to warn even for possible overlaps (i.e., when copying into the same array) so this is a bug, not just a missing feature or design choice.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-wrestrict=/dev/stdout z.c
char a[32];

void f (int i)
{
  __builtin_strcpy (a, a + i);   // -Wrestrict (good)
}

void g (void)
{
  __builtin_strcpy (a, a + 1);   // missing -Wrestrict
}


;; Function f (f, funcdef_no=0, decl_uid=1959, cgraph_uid=0, symbol_order=1)

z.c: In function ‘f’:
z.c:5:3: warning: ‘__builtin_strcpy’ accessing 1 byte at offsets 0 and [0, 32] may overlap 1 byte at offset 0 [-Wrestrict]
   __builtin_strcpy (a, a + i);   // -Wrestrict (good)
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
f (int i)
{
  sizetype _1;
  const char * _2;

  <bb 2> [local count: 1073741825]:
  _1 = (sizetype) i_3(D);
  _2 = &a + _1;
  __builtin_strcpy (&a, _2);
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1962, cgraph_uid=1, symbol_order=2)

g ()
{
  <bb 2> [local count: 1073741825]:
  __builtin_strcpy (&a, &MEM[(void *)&a + 1B]);
  return;

}