[SERVER] writable $derived in class results invalid setter expression

[PuruVJ]

Describe the bug

I have this code for Box

export class Box<T> {
    #getter: () => T;
    #setter?: (value: T) => void;

    #derived = $derived.by(() => {
        let val = $state(this.#getter());

        return {
            get current() {
                return val;
            },
            set current(value) {
                val = value;
            }
        };
    });

    constructor(getter: () => T, setter?: (value: T) => void) {
        this.#getter = getter;
        this.#setter = setter;

        $effect(() => {
            this.#setter?.($state.snapshot(this.#derived.current) as T);
        });
    }

    get current(): T {
        return this.#derived.current;
    }

    set current(value: T) {
        this.#derived.current = value;
    }
}

Then, I tried to leverage writable $derived and rewrote to this:

export class Box<T> {
    #getter: () => T;
    #setter?: (value: T) => void;

    #derived = $derived.by(() => this.#getter());

    constructor(getter: () => T, setter?: (value: T) => void) {
        this.#getter = getter;
        this.#setter = setter;

        $effect(() => {
            this.#setter?.($state.snapshot(this.#derived) as T);
        });
    }

    get current(): T {
        return this.#derived;
    }

    set current(value: T) {
        this.#derived = value;
    }
}

This gave the issue when generated on server

Reason: This is generating the following invalid code when generate: "server":

class Box {
		#getter;
		#setter;
		#derived = $.once(() => this.#getter());

		constructor(getter, setter) {
			this.#getter = getter;
			this.#setter = setter;
		}

		get current() {
			return this.#derived();
		}

		set current(value) {
			this.#derived() = value;
		}
	}

The line this.#derived() = value; is wrong.

Reproduction

https://svelte.dev/playground/hello-world?version=5.25.6#H4sIAAAAAAAAE31SwW6DMAz9FSvrIUgVu9OWajvtA3obO2TgtkhZgmJDO1X8-0yBwdi0KIfk-fk9J_ZNOfOBKlEvaK2Hiw-2AI1FyVhEaq2OpUVSyetN8WfV8TpA8CHrqapiatByh70bwr_w3DtGxyKjtpSHsmKwxp12mWLKVJq5jPFa-cCQW0MEz_66PaRwyxzIejghM4YEdAS7FA6bAaY7vBe8MbbGBA73eOPLQigDqcBQNljADlbDUQOfS4oHVR1F32QpkzjUOfugF55r-MctGivt1lxcXPvDZhmnMU5jfGKs8HjEnHVvPZNepu9jvSI2jDE5U9HZs-7Dw0sjMCRlzszb8dKOhlIf5HUI0h4dyaPmfgG5Dg5-aC7zaZY_fcyv_5jacCdNKrK3j_1MpCCjIoPAKpEuYPsmN1PaS-kKlRyNJWy_ACjOwAGtAgAA

Go to compile settings, turn on generate: server

Logs

System Info

System:
    OS: macOS 15.4
    CPU: (10) arm64 Apple M1 Pro
    Memory: 97.44 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - ~/.volta/tools/image/node/22.14.0/bin/node
    npm: 10.9.2 - ~/.volta/tools/image/node/22.14.0/bin/npm
    pnpm: 9.2.0 - ~/.volta/bin/pnpm
    bun: 1.2.4 - ~/.bun/bin/bun
  Browsers:
    Edge: 134.0.3124.95
    Safari: 18.4
  npmPackages:
    svelte: 5.25.6 => 5.25.6

Severity

annoyance

[dummdidumm]

We already noticed this during #15628 and marked it as a follow-up, but didn't think about the static-code-generation-side of things. Related #14977

[paoloricciuti]

This was technically fixed in #15820 although it introduced #15960 so let's see how it goes lol