svelte:boundary doesn't catch errors inside a @const tag

[therealsujitk]

Describe the bug

When an error is thrown inside the {@const} tag, <svelte:boundary> doesn't catch it.

This snippet works as expected:

<script>
  function fail() {
    throw new Error('💣');
  }
</script>

<h1>Hello!</h1>

<svelte:boundary>
  <span>{fail()}</span>

  {#snippet failed()}
    <p>Somethning went wrong :(</p>
  {/snippet}
</svelte:boundary>

However, this doesn't:

<script>
  function fail() {
    throw new Error('💣');
  }
</script>

<h1>Hello!</h1>

<svelte:boundary>
  {@const result = fail()}
  <span>{result}</span>

  {#snippet failed()}
    <p>Somethning went wrong :(</p>
  {/snippet}
</svelte:boundary>

Reproduction

https://svelte.dev/playground/5bcbda8a665d489fa8d36977a7ebb551?version=5.20.2

Logs

System Info

System:
    OS: macOS 15.3.1
    CPU: (10) arm64 Apple M2 Pro
    Memory: 264.11 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.15.1 - /opt/homebrew/bin/node
    npm: 10.7.0 - /opt/homebrew/bin/npm
    pnpm: 8.15.4 - /opt/homebrew/bin/pnpm
    bun: 1.1.18 - /opt/homebrew/bin/bun
  Browsers:
    Brave Browser: 131.1.73.91
    Safari: 18.3
  npmPackages:
    svelte: ^5.20.2 => 5.20.2

Severity

annoyance

[adiguba]

Hello,

Note that the code is only present on dev mode, as it add an extra $.get(result)

Generated code :

		const result = $.derived_safe_equal(fail);

+		$.get(result);

Source :

svelte/packages/svelte/src/compiler/phases/3-transform/client/visitors/ConstTag.js

Lines 30 to 34 in 3c4a8d4

	// we need to eagerly evaluate the expression in order to hit any
	// 'Cannot access x before initialization' errors
	if (dev) {
	context.state.init.push(b.stmt(b.call('$.get', declaration.id)));
	}

I think I can fix that...