No, mixing boxed and unboxed types is very typical for garbage collected languages. As the article shows, you can distinguish with a bit whether something is a

No, mixing boxed and unboxed types is very typical for garbage collected languages. As the article shows, you can distinguish with a bit whether something is a boxed type or not, so you do that during GC. For many GCs, you typically have another bit for marking (whether you need it depends on the GC technique -- copying GC may not need it). With modern 64-bit CPUs, usually the three lowest bits are free (because every struct will be 8-byte aligned and thus pointers end in 0b000) that you can use for tagging. This will give you 61 bit ints, which work just as described in the article.