You could do it. You could allocate something as garbage-collectible or not at allocation time. You probably would want them to be different types, though, so that the type system keeps track of whether you need to manually deallocate this particular thing, or whether the garbage collector will do so.

It could be done... but I'm not sure what it would gain you. It seems to me that knowing that it will do one or the other is better than having to think about which it will do in each instance.