overflow_fix_in_listextend.patch: I doubt Python supports the kinds of platform where this overflow would be possible. It may require pointers smaller than 32 bits, or char objects larger than 8 bits. Perhaps we could just add a comment explaining we assume the overflow cannot happen.

It seems list objects will hold one pointer for each element, but the overflow involves the number of elements. Python defines PY_SSIZE_T_MAX as PY_SIZE_MAX // 2. For the overflow to happen we would need

m + n > PY_SSIZE_T_MAX

Assuming a “flat” address space that can allocate up to PY_SIZE_MAX bytes _in total_, the total number of elements cannot exceed

m + n == PY_SIZE_MAX // sizeof (PyObject *)

So in this scenario, the overflow cannot happen unless sizeof (PyObject *) == 1.

Considering things like the 16-bit segmented Intel “large” memory model (which I doubt Python is compatible with), each list could _independently_ be up to PY_SIZE_MAX bytes. Therefore the total number of elements may reach

m + n == PY_SIZE_MAX // sizeof (PyObject *) * 2

So even in this case, sizeof (PyObject *) == 4 (large model) is fine, but anything less (e.g. 16-bit char, or 1-byte segment + 2-byte offset) might overflow.