BTW there's a compromise with unboxing that also has benefits, which is embedding. An [@embed] annotation could turn an array into an embedding array, for example. This would mean that an array would have boxed members within it ie. not accessible via pointers. The advantages are better cache performance and that the GC could be instructed when the array is completely flat ie. an embedded array without any pointers could be skipped by the GC in the mark phase. It would have full polymorphism capability.

The down side is that deallocating the array without deallocating the embedded structure would be tricky. When deallocating, you have to check every member to see if it should be deallocated as well. If not, you copy the member (minor heap) or reallocate the member where it is in memory (major heap).