Update binding prop in component trigger twice reaction.

[dishuostec]

Describe the bug
When update binding prop within a component, reactive statement in parent component processed twice.

App.svelte

<script>
	import Foo from './Foo.svelte'
	let data;
	let count = 0;
	$: console.log('data', ++count, data);
</script>

<Foo bind:data={data} />
{count}

Foo.svelte

<script>
	export let data;
	data = {foo:null};
</script>

Logs

"data" 1 undefined
"data" 2 ▶ Object { foo: null }
"data" 3 ▶ Object { foo: null }

To Reproduce
It seems 3.16.5 introduced the bug.
https://svelte.dev/repl/b245bd85f01043e9b5c37270398f463c?version=3.16.5

3.16.4 is ok.
https://svelte.dev/repl/b245bd85f01043e9b5c37270398f463c?version=3.16.4

Expected behavior
Run reactive statement once.

[antony]

I'm not convinced this is a bug. By using ++count in your reactive statement, you are creating a reactive dependency on count, and also incrementing it once the statement is run. So the variable changes twice, from 0 to 1, and 1 to 2.

[dishuostec]

I removed count, it still processed twice.

[dishuostec]

https://svelte.dev/repl/b245bd85f01043e9b5c37270398f463c?version=3.16.4

"data" undefined
"data" ▶ Object { foo: null }

and https://svelte.dev/repl/b245bd85f01043e9b5c37270398f463c?version=3.16.5

"data" undefined
2x "data" ▶ Object { foo: null }

[tanhauhau]

Haven't take a deep look, but probably related to #3180

[antony]

@dishuostec thanks - that's a bit clearer now.

#3180 relates to stores, but if there is some shared logic there, then it makes sense that it might be the same root cause.

[Conduitry]

Fixed in 3.24.1 - https://svelte.dev/repl/b245bd85f01043e9b5c37270398f463c?version=3.24.1

[pwwang]

This is not fixed when the binding prop is used as an argument of a function:

https://svelte.dev/repl/5b37ba40c8854d86a9dab9ace0b6b861?version=3.24.1

[pzeinlinger]

@Conduitry I can confirm it's not fixed in 3.29.7 when binding prop is used as an argument as @pwwang states.