Object destructed props did not match sometimes

[cvlab]

v3.0.0-beta.20

Small bugs using object destructed dynamic props

<Component { ...{ b: 1 } } />
to
<Component { ...undefined } />
did not remove old prop

https://svelte.dev/repl/3757254ce74b375821ea525b818a5bc0?version=3.6.9

<Component { ...{ b: 1 } } />
to
<Component { ...{ } } />
did not remove b, b prop exists as undefined

https://svelte.dev/repl/431834e7d8d83e0634304572736621a6?version=3.6.9

[Rich-Harris]

Simpler repro: https://svelte.dev/repl/08385b2631684133b6fc987a1be21606?version=3.8.1

What's happening here is that $$self.$set looks like this:

function instance($$self, $$props, $$invalidate) {
	$$self.$set = $$new_props => {
		$$invalidate('$$props', $$props = assign(assign({}, $$props), $$new_props))
	};

	return {
		$$props,
		$$props: $$props = exclude_internal_props($$props)
	};
}

(Unrelated to this bug, but $$props is being output twice in the instance context.)

In other words, new props are always merged with the old ones. Normally this works, and allows partial updates, because the list of prop names is the same. But in the case of spread props where the list of prop names is not the same, it results in this buggy behaviour.

[Conduitry]

I don't think we want to consider the behavior beyond that which was fixed by #3306 a bug. Given a component instance, there is no way to unset a prop, even if spread props aren't involved at all. All you can do is set it to undefined. Closing, as the rest of this was fixed back in 3.12.0.