Svelte 5: Type issues with derived store / page store

[rChaoz]

Describe the bug

Just upgraded to Svelte 5 and I'm getting type errors for some stores-only code (.ts file).

Reproduction

import { derived, readable, writable, type Updater, type Writable } from "svelte/store"
import { page } from "$app/stores"
import { browser } from "$app/environment"
import deepEquals from "fast-deep-equal"

let oldState: App.PageState = {}

export const pageState = derived<typeof page, App.PageState>(
    page,
    (_, set) => {
        if (!browser) return
        setTimeout(() => {
            const newState = history.state["sveltekit:states"] ?? {}
            if (!deepEquals(oldState, newState)) {
                oldState = newState
                set(newState)
            }
        })
    },
    {},
)

Workaround

Manually type the arrow function - replace _, set with:

_: typeof page extends Readable<infer U> ? U : never, set: (value: App.PageState) => void)

Logs

$ tsc --noEmit true
src/lib/stores.ts:13:6 - error TS7006: Parameter '_' implicitly has an 'any' type.

13     (_, set) => {
        ~

src/lib/stores.ts:13:9 - error TS7006: Parameter 'set' implicitly has an 'any' type.

13     (_, set) => {

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (20) x64 12th Gen Intel(R) Core(TM) i7-12700KF
    Memory: 13.10 GB / 31.86 GB
  Binaries:
    Node: 20.8.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 4.1.1 - C:\Program Files\nodejs\yarn.CMD
    npm: 10.2.1 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.1266.0)
    Internet Explorer: 11.0.19041.1566

Severity

annoyance

[CaptainCodeman]

I get it when using svelte-check: sveltejs/language-tools#2359

It could be a regression: #8699