Excessive invalidation in ArrowFunctionExpression results in infinite loop

[timhall]

Bit of strange one, but here is a repro: https://svelte.dev/repl/67a7fb3e15fb4991916b9f6947c1e82a?version=3.2.2 (Note: will likely cause an infinite loop)

Here is the input:

<script>
  const a = Promise.resolve('a');
  let b = Promise.resolve('b');
	
  let a_value, b_value;
  $: a.then(value => a_value = value);
  $: b.then(value => b_value = value);
  $: b.then(value => { b_value = value });
</script>

and here is the relevant output:

function instance($$self, $$props, $$invalidate) {
  const a = Promise.resolve('a');
  let b = Promise.resolve('b');
	
  let a_value, b_value;

  $$self.$$.update = ($$dirty = { b: 1 }) => {
    if ($$dirty.b) { b.then(value => { const $$result = b_value = value; $$invalidate('b_value', b_value), $$invalidate('b', b); return $$result; }); }
    // Note the extra invalidate of b here ----------------------------------------------------------------^
    if ($$dirty.b) { b.then(value => { $$invalidate('b_value', b_value = value) }); }
  };

  a.then(value => { const $$result = a_value = value; $$invalidate('a_value', a_value); return $$result; });

  return {};
}

My guess is something related to mutation tracking (possibly tangentially related to #2728)

[Conduitry]

Before I realized there was already an issue for this, I had explored a little more into what seemed to be causing this in #2873.

[Panya]

It was fixed in 3.4.4 and above.

[Conduitry]

This is not fixed. Svelte will now no longer include invalidations if the variables are not referenced anywhere else, but if you add {b_value} to the template, you'll see the invalidations for both b_value and b.

[Conduitry]

It looks like this would also be fixed by #3533.

[Conduitry]

Fixed in 3.10.1.