Why do compilers give a warning about returning a reference to a local stack variable if it is undefined behaviour?

Question

The C++ standard states that returning reference to a local variable (on the stack) is undefined behaviour, so why do many (if not all) of the current compilers only

Answer 1

Because standard does not restrict us.

If you want to shoot to your own foot you can do it!

However lets see and example where it can be useful:

int &foo()
{
    int y;
}

bool stack_grows_forward()
{
    int &p=foo();
    int my_p;
    return &my_p < &p;
}