Incidentally, AFAICT C23 now allows stashing log2 alignof(max_align_t) bits in pointers in a portable (if awkward) manner: for example, for a char pointer p, th

Incidentally, AFAICT C23 now allows stashing log2 alignof(max_align_t) bits in pointers in a portable (if awkward) manner: for example, for a char pointer p, the lowest such bit can be retrieved with memalignment(p)<2 and masked off with p-(memalignment(p)<2). (C++’s std::aligned would be less awkward, but, well, memalignment() is what we get.) You still can’t portably store pointers and integers (or doubles) in the same place and distinguish them, though.