> probably meaning on an address that’s a multiple of sizeof(int), but who knows

Sigh. s/sizeof(int)/_Alignof(int)/.

There are good reasons for an implementation to have sizeof(int) = _Alignof(int) and not a mere multiple of it, but if you are going to discuss subtle points and UB, just stick to the language guarantees.

> But let’s say you have a modern machine, where NULL is a pointer to address zero, and you actually have an object there.

You don't program in C on such a machine. Or maybe memory is virtualized, and it does not matter that your object lives at physical address zero, as long as you can map a non-zero virtual address to it.

> So how do you print an uid_t?

    if ((uid_t)-1 < (uid_t)0) {
        // uid_t is signed
        printf("%" PRIdMAX, (intmax_t)id);
    } else {
        // uid_t is unsigned
        printf("%" PRIuMAX, (uintmax_t)id);
    }

> It’s not rare for the denominator to come from untrusted input.

It's not rare for the array index to come from untrusted input.

It's not rare for the supposedly valid UTF-8 string to come from untrusted input.

...

Why single out division? This problem affects every partially defined operation. In the case of division at least, everyone learned in school that thou shalt not divide by zero. Adding two untrusted integers and forgetting that signed overflow is UB, not defined as a modulo? Your average programmer is much less likely to see that coming.

    > unsigned char a = 0xff;
    > unsigned char b = 1;
    > unsigned char zero = 0;
    > bool overflowed = (a + b) == zero;
    >
    > unsigned char a = 0x80;
    > uint64_t b = a << 24;

Please. Convert your operands to wide enough types before the operation. Convert your results back to narrow enough types to compensate for integer promotion to wider types than you would have liked. Do that consistently, and you're good.

Here:

    unsigned char a = 0xff;
    unsigned char b = 1;
    unsigned char zero = 0;
    bool overflowed = (unsigned char)(a + b) == zero;

    unsigned char a = 0x80;
    uint64_t b = (uint32_t)a << 24;