Wow, that issue is kind of mindblowing. To me, it proves that making GC observable rips a large hole in the semantics. The damage was ameliorated somewhat with

Wow, that issue is kind of mindblowing. To me, it proves that making GC observable rips a large hole in the semantics. The damage was ameliorated somewhat with [[KeptAlive]], but the destruction is not contained in any understandable or bounded way, and we'll be bumping into it for years to come. It feels like we managed to add something a lot like C and C++'s undefined behavior into JS. I thought it was just an issue of nondeterminism, but that issue proves that it could be a lot more problematic than that. I don't find the suggested optimizations particularly far-fetched, especially in a trace-compiled or otherwise specialized subset of the code. Something like inlining in a way that looks for all of the things provable locally even if they may not be true globally; it could totally do things like build alias sets of values and optimize down `w.deref() === obj` to `w.deref() !== undefined`.