Describe the bug

On initial load/SSR, private class fields that use $derived break with the following:
TypeError: Cannot read properties of undefined (reading '<var name>')

Introduced in [email protected]

Reproduction

<script lang="ts">
    class SomeState {
        opts: { baz: boolean };
        #foo = $derived.by(() => this.opts.baz);

        constructor(opts: { baz: boolean }) {
            this.opts = opts;
        }
    }

    new SomeState({ baz: true });
</script>

Stackblitz: https://stackblitz.com/github/AdrianGonz97/dependent-on-private-derived-repro?file=README.md
GitHub Repo: https://github.com/AdrianGonz97/dependent-on-private-derived-repro

Logs

System Info

(StackBlitz)


  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    svelte: ^5.0.0 => 5.31.1

Severity

blocking an upgrade