Only the first of the two functions is diagnosed for attempting to construct an int object in an array of size 1.  The equivalent attempt in g() involving a multidimensional array with insufficient size is not diagnosed, either by -Wplacement-new or later by -Warray-bounds (or -Wstringop-overflow, although this warning is only meant to trigger for char accesses).

$ cat t.C && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout t.C
void* operator new (__SIZE_TYPE__, void *p) { return p; }

char a[1];
char b[1][1];

void f ()
{
  new (&a[0]) int (123);     // warning (good)
}

void g ()
{
  new (&b[0][0]) int (456);  // no warning
}
t.C: In function ‘void f()’:
t.C:8:8: warning: placement new constructing an object of type ‘int’ and size ‘4’ in a region of type ‘char [1]’ and size ‘1’ [-Wplacement-new=]
    8 |   new (&a[0]) int (123);     // warning (good)
      |        ^~~~~

;; Function operator new (_ZnwmPv, funcdef_no=0, decl_uid=2337, cgraph_uid=1, symbol_order=0)

operator new (long unsigned int D.2335, void * p)
{
  <bb 2> [local count: 1073741824]:
  return p_1(D);

}


t.C:8:23: warning: writing 4 bytes into a region of size 1 [-Wstringop-overflow=]
    8 |   new (&a[0]) int (123);     // warning (good)
      |                       ^
t.C:3:6: note: at offset 0 to object ‘a’ with size 1 declared here
    3 | char a[1];
      |      ^

;; Function f (_Z1fv, funcdef_no=1, decl_uid=2341, cgraph_uid=2, symbol_order=3)

f ()
{
  <bb 2> [local count: 1073741824]:
  MEM[(int *)&a] = 123;
  return;

}



;; Function g (_Z1gv, funcdef_no=2, decl_uid=2346, cgraph_uid=3, symbol_order=4)

g ()
{
  <bb 2> [local count: 1073741824]:
  MEM[(int *)&b] = 456;
  return;

}