a new thing accepting a subset of values is fine. an existing thing suddenly changing its current invariants (all objects can be weakly held) is not. this would apply to weakmap, weakset, weakref, and finalizationregistry - that's a lot of things to have changed invariants.

if browsers were OK with a isWeakable predicate - proposed for precisely this problem when non-global symbols became allowed - then it'd have been OK to change, but sans that predicate, it's not

If we have prototypes and WeakMaps, something like SourceFile could have a statements getter that could return a Proxy for a struct like

shared struct NodeArrayData {
  pos; // number
  end; // number
  items; // SharedArray<Node>
}

where the proxy emulates NodeArray and redirects numeric indexed access to items. The performance would be abysmal though.

If a SharedArray were essentially like a shared struct, maybe we could leverage subclassing somehow?

shared struct NodeArray extends SharedArray {
  pos = -1;
  end = -1;
  constructor(length) {
    super(length);
  }
}