In C++ mode (but not in C mode), g++ silently (without -Wpedantic) accepts both a definition of a struct with a flexible array member and an object of such a type without an initializer but issues a warning for accesses to the array (see below).  Debugging reveals that unlike the C front end, the C++ front end sets the bounds on flexible arrays to [0, SIZE_MAX].  The code in check_array_ref in tree-vrp.c then trips up on such an array because it first assumes that flexible arrays have no bounds, and further assumes that no array has an upper bound of SIZE_MAX when adding one to the bound.  When the computation wraps around to zero, the function incorrectly deduces that the array is empty and issues a warning.

It seems wrong for the C++ front end to set the upper bound to SIZE_MAX for any array.  First, because GCC itself assumes that no object is larger than SIZE_MAX / 2.  Second, when the size of the array element is greater than 1 as in the case below, even the most permissive upper bound cannot be SIZE_MAX.

$ cat u.cpp && /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -O2 -S -Wall -Wextra -o/dev/null u.cpp 
struct S {
    int n;
    int a[];
} s;

int i;

void f ()
{
    i = s.a [0];
}

u.cpp: In function ‘void f()’:
u.cpp:10:15: warning: array subscript is above array bounds [-Warray-bounds]
     i = s.a [0];
               ^