I think even deriving the argument might invoke undefined behaviour here, but I'm not certain.

If I understand correctly, it's undefined behaviour to do this:

    int *ptr = (int*)42;

As, in order to avoid undefined behaviour, ptr should only be assigned a valid address of an int, or the 'address' one past the end of an array of int, or NULL (logical zero).

It's possible things are different when the type is void*, I'm not certain.

Make it free(free) if you want it to be a valid pointer.

No, because the defined contract for free() is that you pass it a pointer that was previously returned from malloc(), that you don't try to free() it twice, etc.

That's documented undefined behavior.

Is it documented undefined behavior to feed bad IR into LLVM?

Of course. If there's an input checking mode, and you disable the checks, then you're guaranteeing that you won't supply invalid input.