Bindings don't work inside {#each reactiveThing as thing}

[Rich-Harris]

In a case like this...

{#each filtered as todo}
	<label class:done={todo.done}>
		<input type="checkbox" bind:checked={todo.done}>
		{todo.text}
	</label>
{/each}

<p>completed {todos.filter(t => t.done).length} of {todos.length}</p>

...where filtered is derived from todos, toggling an individual input doesn't affect anything else that depends on todos. Demo here.

That's because this code gets generated:

function input_change_handler_1({ todo, each_value, todo_index }) {
	each_value[todo_index].done = this.checked;
	$$invalidate('filtered', filtered);
}

We should instead invalidate the ultimate dependencies of filtered rather than filtered itself:

function input_change_handler_1({ todo, each_value, todo_index }) {
	each_value[todo_index].done = this.checked;
-	$$invalidate('filtered', filtered);
+	$$invalidate('todos', todos);
+	$$invalidate('hideDone', hideDone);
}

(While hideDone doesn't need to be invalidated here, I'm not certain we can reliably determine that statically. Needs more thought.)

[mrobakowski]

Hi, I had a shot at this and managed to get the compiler to generate 3 invalidates - one for each dependency and one for the reactive declaration. Some tests didn't pass if I emitted invalidates only for the dependencies. Here's the commit: mrobakowski@8e470fa

[mrobakowski]

Also, I believe I found another bug while trying to reduce the example https://svelte.dev/repl?version=3.1.0&gist=c4b48ccfb869a6915a3def99b3fbbd2d. With my commit name is properly invalidated, but the text isn't updated (possibly because compiler doesn't consider name as ever written to?).

[Rich-Harris]

Thank you! I adapted your solution in #2694. Reopening this (autoclosed) issue so that we can address the second bug you found — you're right, the compiler thinks name is never assigned to or mutated, so it generates update code like this...

p(changed, ctx) {
  if ((changed.foo) && t5_value !== (t5_value = ctx.foo.name.w)) {
    set_data(t5, t5_value);
  }
}

...which means name can never update.