cache type only takes unknown as key

[fregante]

I'm trying to write a local filesystem cache:

const fetchDocument = pMemoize(async (url: string): Promise<string> => {
	const request = await fetch(url, {
		headers: {
			Accept: 'text/html',
		},
	});
	return request.text();
}, {
	cache: {
		async get(url: string): Promise<string | undefined> {
			try {
				return await readFile(`./test/.cache/${filenamify(url)}.html`, 'utf8');
			} catch {
				return undefined;
			}
		},
		async set(url: string, contents: string): Promise<void> {
			await writeFile(`./test/.cache/${filenamify(url)}.html`, contents);
		},
		async has(url: string): Promise<boolean> {
			return Boolean(await this.get(url));
		}
	}
});

Every cache method is failing with:

Type '(url: string) => Promise<string | undefined>' is not assignable to type '(key: unknown) => string | Promise<string | undefined> | undefined'.
  Types of parameters 'url' and 'key' are incompatible.
    Type 'unknown' is not assignable to type 'string'.ts(2322)

[fregante]

Interestingly, if I move the cache to its own object it's fine…

import pMemoize from 'p-memoize';
import filenamify from 'filenamify';
import {writeFile, mkdir, unlink, readFile} from 'node:fs/promises';

const fsCache = {
	async get(path: string): Promise<string | undefined> {
		try {
			return await readFile(path, 'utf8');
		} catch {
			return undefined;
		}
	},
	async set(path: string, contents: string): Promise<void> {
		await writeFile(path, contents);
	},
	async has(path: string): Promise<boolean> {
		return Boolean(await this.get(path));
	},
	async delete(path: string): Promise<void> {
		await unlink(path);
	},
} as const;
const fetchDocument = pMemoize(async (url: string): Promise<string> => {
	const request = await fetch(url, {
		headers: {
			Accept: 'text/html',
		},
	});
	return request.text();
}, {
	cacheKey: ([url]) => `./test/.cache/${filenamify(url)}.html`,
	cache: fsCache,
});

[sindresorhus]

It seems to me like a TypeScript limitation. I looked into this today, but failed to find a solution.