[Bug middle-end/91462] New: missing -Warray-bounds writing to an empty flexible array member in a ctor

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91462

            Bug ID: 91462
           Summary: missing -Warray-bounds writing to an empty flexible
                    array member in a ctor
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

In the test case below, the write to the empty flexible array member in Ax
isn't diagnosed even though the equivalent write in A0 is.

$ cat t.C && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout t.C
struct A0
{
  char n, a[0];

  A0 () { a[0] = 0; }   // -Warray-bounds (good)
};

void f (void*);

void g ()
{
  struct A0 a;
  f (&a);
}

struct Ax
{ 
  char n, a[];

  Ax () { a[0] = 0; }   // missing -Warray-bounds
};

void h ()
{
  struct Ax a;
  f (&a);
}

t.C: In function ‘void g()’:
t.C:5:14: warning: array subscript 0 is above array bounds of ‘char [0]’
[-Warray-bounds]
    5 |   A0 () { a[0] = 0; }
      |           ~~~^

;; Function g (_Z1gv, funcdef_no=3, decl_uid=2317, cgraph_uid=4,
symbol_order=3)

g ()
{
  struct A0 a;

  <bb 2> [local count: 1073741824]:
  a ={v} {CLOBBER};
  a.a[0] = 0;
  f (&a);
  a ={v} {CLOBBER};
  return;

}



;; Function h (_Z1hv, funcdef_no=7, decl_uid=2362, cgraph_uid=8,
symbol_order=7)

h ()
{
  struct Ax a;

  <bb 2> [local count: 1073741824]:
  a ={v} {CLOBBER};
  a.a[0] = 0;
  f (&a);
  a ={v} {CLOBBER};
  return;

}