Describe the bug

{@const} inside a slot consumer that references a let: slot prop causes a runtime ReferenceError: Cannot access '<name>' before initialization (temporal dead zone). The issue reproduces on 5.55.1, so the fix from #16585 does not cover this variant.

The original issue (#16585) was reproduced with let:index used directly in a {@const} expression ({@const item = list[index]}). This variant fails when {@const} creates an object shorthand that captures the slot prop: {@const props = { row }} where row comes from let:row.

Reproduction

<!-- Parent.svelte -->
<script>
  export let data = [{ id: 1, label: 'A' }, { id: 2, label: 'B' }]
</script>

{#each data as row}
  <slot name="row" {row}></slot>
{/each}

<!-- Consumer.svelte -->
<Parent>
  <tr slot="row" let:row>
    {@const props = { row, onClick: () => console.log(row) }}
    <td>{props.row.label}</td>
  </tr>
</Parent>

Runtime error:

ReferenceError: Cannot access 'row' before initialization

REPL

Logs

System Info

System:
    OS: macOS 26.3.1
    CPU: (12) arm64 Apple M3 Pro
    Memory: 97.25 MB / 18.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.14.1 - /Users/xxx/.nvm/versions/node/v24.14.1/bin/node
    npm: 11.11.0 - /Users/xxx/.nvm/versions/node/v24.14.1/bin/npm
    pnpm: 10.33.0 - /Users/xxx/.nvm/versions/node/v24.14.1/bin/pnpm
  Browsers:
    Chrome: 147.0.7727.56
  npmPackages:
    svelte: ^5.55.1 => 5.55.1

Severity

blocking an upgrade