The following code has a subtle bug:

#include <assert.h>

class Blah
{
public:
  const unsigned int &myfoo;
  Blah(const unsigned int &p):
    myfoo(p) {}
  const unsigned int getfoo()
  {
    return myfoo;
  }
};

int main()
{
  int foo(42);
  Blah baz(foo);

  ++foo;

  assert(foo == baz.getfoo());

  return 0;
}

When "foo" is passed to the constructor for "baz", as foo is not unsigned a temporary is made and a constant reference to it is passed to Blah.

No warnings for this code appear in 4.2.0 at the highest warning levels. Would it be possible to add a warning, such as "reference is bound to converted temporary"? I'm not sure how many false-positives would be generated, but I do know this bug too a long time to find for someone.