So they're unboxed in certain situations, but otherwise boxed.
With NaN-boxing you would do this for Int64s. They would need boxing when you need a single value, but you can have one or more tags for vectors of them, in which case the elements can be stored unboxed.
My argument is you would probably use FP64 more frequently than Int64s. For most common operations involving int, Int48 would suffice. In the cases where you need full Int64 (cryptography, serialization, etc), you commonly need vectors of Int64 anyway.
With NaN-boxing you would do this for Int64s. They would need boxing when you need a single value, but you can have one or more tags for vectors of them, in which case the elements can be stored unboxed.
My argument is you would probably use FP64 more frequently than Int64s. For most common operations involving int, Int48 would suffice. In the cases where you need full Int64 (cryptography, serialization, etc), you commonly need vectors of Int64 anyway.