Derived store from multiple stores won't get update

[trongthanh]

I'm experiencing a bug on my application that is very hard to reproduce to a minimal case on REPL. Anyway, after few hours of tweaking, I am able to make it happen on REPL and realize what's going on.

This bug happen to Svelte 3.5.4

REPL: https://svelte.dev/repl/012883fabed9405fab808be3fd613d5c?version=3.5.4

What does not work: Notice derived multi-store will not update and return initial value forever.

In summary:

// the source stores:
storeA = writable({counter: 0, name: 'Thanh'})
storeB = derived(storeA, $storeA => $storeA.name)

// somewhere down the path: increase counter, but name never change
storeA.update(current => {...current, counter: current.counter + 1});

// the troubled store:
storeC = derived([storeA, storeB], ([$storeA, $storeB]) => {
  // should compute stuff when either storeA or storeB updates, but it never update
});

Interestingly, it was working previously with older version of Svelte, specifically, 3.2.1 from my yarn.lock: https://svelte.dev/repl/012883fabed9405fab808be3fd613d5c?version=3.2.1

After reproducing this, I realize that deriving from storeA is enough because storeB also derive from it. (And that's my current work around).

If storeB is a readable from another value, this bug won't happen.

[Conduitry]

#3191 was actually a duplicate of this. Fixed in 3.6.9.