as the comment in this block says:

// Cache is unreliable once we start adjusting labels

We can't let this isReachableFlowNodeWorker call here to populate the cache as that would create entries based on the temporary antecedent(s). Anything computed below this call can't be preserved, at least not without including the id of this ReduceLabel flow in the cache key or smth.

So this disables cache writes. As later on that would be used on the same shared flow but with "correct" antecedents (and that's the only result the compiler should cache and trust).

I'm still considering 2 things here:

• should this code maybe save & restore flowNodeReachable[getFlowNodeId(target)]?
• should reads be permanently disabled here? this would make the previous consideration obsolete

I have not been able to produce a test case that would prove either to be required so far but I have only spent a little bit of time on it.

cc @ahejlsberg who has been thinking about this sort of thing recently and I believe had ideas on how to simplify the flow node logic here

fwiw, Corsa still suffers from this (checked at microsoft/typescript-go@a92eff4 )