gcc/testsuite/gcc.c-torture/execute/960301-1.c

void abort (void);
void exit (int);

struct foo {
  unsigned : 12;
  unsigned field : 4;
} foo;
unsigned oldfoo;

int
bar (unsigned k)
{
  oldfoo = foo.field;
  foo.field = k;
  if (k)
    return 1;
  return 2;
}

int
main (void)
{
  if (bar (1U) != 1)
    abort ();
  exit (0);
}