When trying to compile this code:

  if (INT_ADD_OVERFLOW ((ptrdiff_t) (bool_header_size - header_size),
                        total_payload_bytes))
    memory_full (SIZE_MAX);

gcc produces this warning:

alloc.c:2051: warning: assuming signed overflow does not occur when
assuming that (X + c) >= X is always true [-Wstrict-overflow]

Is INT_ADD_OVERFLOW actually safe?