[Bug tree-optimization/99502] New: missing -Warray-bounds on partial out of bounds access in C++

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99502

            Bug ID: 99502
           Summary: missing -Warray-bounds on partial out of bounds access
                    in C++
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The program below triggers two instances of -Warray-bounds when compiled as C
code but only one when compiled as C++.  This is a regression introduced in 
r11-7563.

$ cat u.C && gcc -O2 -S -Wall u.C
typedef struct A { int i; } A;

char a[sizeof (A) - 1];

void f (A x)
{
  A *p = (A*)a;
  *p = x;          // -Warray-bounds (good)
}

void g (void)
{
  A *p = (A*)a;
  p->i = 0;        // missing warning in C++
}

u.C: In function ‘void f(A)’:
u.C:8:6: warning: array subscript ‘A[0]’ is partly outside array bounds of
‘char [3]’ [-Warray-bounds]
    8 |   *p = x;          // -Warray-bounds (good)
      |   ~~~^~~
u.C:3:6: note: while referencing ‘a’
    3 | char a[sizeof (A) - 1];
      |      ^