{#if a.deep != null} runs when a.deep is null if used as a transition parameter

[rChaoz]

Describe the bug

Did my best with the title. Can't describe this very well, please see the reproduction as I almost died getting it.

In short, this code...

{#if s.drawer}
  {s.drawer.toString()}
{/if}

...crashes with s.drawer is undefined. The contents of the if are evaluated even when the if is false, but only when s.drawer is used as the argument of a transition (transition:whatever={s.drawer}).

Reproduction

REPL

<script>
	const s = $state({})
	
	function slide() {
			return { duration: 150 }
	}
	
	function open() {
		s.drawer = { id: 123 }
	}
	
	function close() {
		delete s.drawer
	}
</script>

Hit Open, then Close. It crashes and the drawer remains open but the Close
button cannot be interacted with anymore (see console).
<br><br>

{#if s.drawer}
    {@const id = s.drawer.id}
    {#if id}
	Hi, I'm the drawer.
	<!-- Remove the "={id}" and the error goes away -->
        <button transition:slide|global onclick={close}>Close</button>
    {/if}
{/if}

<button onclick={open}>Open</button>

Regression

This is also a regression. The same bug happens in legacy mode, but the code works in v4.

v5 REPL | v4 REPL

Severity

annoyance