Here, the union is being used as a space-saving structure. We only need to store one pointer, but it can be of different types. We access the same one that we most recently stored.
Most of the code surrounding this won't contain assignments into the union, so it won't impact optimization.
In an interpreted language I wrote this kind of union is initialized when a function object comes to life, and then not mutated again.
The union is necessary because a struct would blow up the size of the object significantly (and then it wouldn't fit into the GC heap cell size, requiring an additional piece of malloced memory).
Most of the code surrounding this won't contain assignments into the union, so it won't impact optimization.
In an interpreted language I wrote this kind of union is initialized when a function object comes to life, and then not mutated again.
The union is necessary because a struct would blow up the size of the object significantly (and then it wouldn't fit into the GC heap cell size, requiring an additional piece of malloced memory).