The call to function f() in the test case below has undefined behavior because the array index is out of bounds.  As a result, it is required to be diagnosed (according to [expr.const] of C++ 14).  However, G++ silently accepts it.

See also bug 82872 for a problem in a related area.

#include <stddef.h>

struct S { int i, a[1]; };

constexpr int f ()
{
  return offsetof (S, a[__PTRDIFF_MAX__ / 2]);
}

constexpr int i = f ();


In contrast to the above, the following (also undefined) program is diagnosed as expected:

#include <stddef.h>

constexpr int f ()
{
  struct S { int a[1]; };

  return offsetof (S, a[__PTRDIFF_MAX__]);
}

constexpr int i = f ();
t.C:10:21:   in constexpr expansion of ‘f()’
t.C:10:22: error: overflow in constant expression [-fpermissive]
 constexpr int i = f ();
                      ^