Array assignment reactivity not working; adding a line-end comment bizarrely fixes it

[paulcalcraft]

Describe the bug

In this short example, propdata is incorrectly invalidated when triggering reactivity on editedData with editedData = editedData, which keeps resetting the array to its default value whenever you try to add to it. This problem strangely goes away if you add a comment to the end of the reactivity line. (Change line 13 to editedData = editedData // this works.)

<script>
    // copy this script to https://svelte.dev/repl/
    // (github login on svelte.dev is not working for me, so I cannot save on the REPL)
    export let propdata = [1, 1]
    console.log("initial propdata", propdata)
    $: editedData = [...propdata]
    
    function add() {
        console.log("clicked")
        editedData.push(1)
	console.log("element added, editedData set to (no reactivity)", editedData)
        console.log("triggering reactivity on editedData", editedData)
        editedData = editedData
    }
    $: console.log("$: propdata set to", propdata)
    $: console.log("$: editedData set to", editedData)
</script>
<button on:click={add}>Add</button>
{JSON.stringify(editedData)}

The JS output for add() without the comment is:

function add() {
	console.log("clicked");
	editedData.push(1);
	console.log("element added, editedData set to (no reactivity)", editedData);
	console.log("triggering reactivity on editedData", editedData);
	($$invalidate(0, editedData), $$invalidate(2, propdata));
}

We see that propdata is incorrectly invalidated alongside editedData.

The JS output for add() with the comment (working correctly) is:

function add() {
	console.log("clicked");
	editedData.push(1);
	console.log("element added, editedData set to (no reactivity)", editedData);
	console.log("triggering reactivity on editedData", editedData);
	$$invalidate(0, editedData = editedData); // this works
}

Only editedData is invalidated.

The problem doesn't occur if you use editedData = [...editedData, 1], but my understanding is that reactivity should also work with push and reassign.

Reproduction

Code for repro is here, just copy to svelte.dev REPL:
https://gist.github.com/paulcalcraft/5e9ea299cf2e37d0105295889e1a9eb8

Unfortunately I'm not able to login via github to svelte.dev REPL so I can't save the REPL, hence the gist. But that's another bug report for another time.

Logs

Console output after clicking Add twice, showing Add button does not update editedData:

"initial propdata" (2) [ 1 ,1 ]
"$: propdata set to" (2) [ 1 ,1 ]
"$: editedData set to" (2) [ 1 ,1 ]
"clicked"
"element added, editedData set to (no reactivity)" (3) [ 1 ,1 ,1 ]
"triggering reactivity on editedData" (3) [ 1 ,1 ,1 ]
"$: propdata set to" (2) [ 1 ,1 ]
"$: editedData set to" (2) [ 1 ,1 ]
"clicked"
"element added, editedData set to (no reactivity)" (3) [ 1 ,1 ,1 ]
"triggering reactivity on editedData" (3) [ 1 ,1 ,1 ]
"$: propdata set to" (2) [ 1 ,1 ]
"$: editedData set to" (2) [ 1 ,1 ]

System Info

System:
    OS: Windows 10 10.0.22621
    CPU: (8) x64 Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz
    Memory: 3.76 GB / 15.81 GB
  Binaries:
    Node: 18.16.1 - C:\Program Files\nodejs\node.EXE
    npm: 9.7.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 116.0.5845.188
    Edge: Spartan (44.22621.2283.0), Chromium (116.0.1938.81)
    Internet Explorer: 11.0.22621.1
  npmPackages:
    svelte: ^4.0.5 => 4.2.0 

The problem is also currently reproducing on the live svelte.dev REPL.

Severity

annoyance

[Prinzhorn]

https://github.com/search?q=repo%3Asveltejs%2Fsvelte+comment+reactivity&type=issues
I didn't look closely at your code, does #9088 resemble your issue?

[paulcalcraft]

Thanks @Prinzhorn - sorry for not searching. Closing as duplicate of #9088

Maybe my repro will still be helpful to anyone working on the bug.