Different type information when using @typescript-eslint with a compat layer

[sheremet-va]

I wanted to introduce this library to the Vitest monorepo: vitest-dev/vitest#6884

But I cannot figure out the inconsistencies that I encountered and would appreciate some help:

Whatever I do in https://github.com/sheremet-va/vitest/blob/4b068e337c4782167c7aa4be59ad73ee2b96be9f/packages/spy/src/index.ts causes tsslint to think that I always return any even if I cast it or when the type is already known 🤔

[sheremet-va]

I don't know if this is an issue with the plugin itself or the language server 👀 Reoading the server didn't help, unfortunately

[johnsoncodehk]

This seems to be a bug in @typescript-eslint/no-unsafe-return and it can be reproduced in the TS-ESLint playground.

https://typescript-eslint.io/play/#ts=5.7.2&fileType=.ts&code=C4TwDgpgBACgTgewMYQCYFc7QLxQBQB0RAhnAOYDOAXFMQHYgDaAugJRTYB8tDAULwDN0dJMACWCOlAh0AFvRQBlMCAA8AFWkAPYDNQVYiFBiyc87AN68oUUJCjqAShGCY6B3M9dw668BA1Oa3wAN2IAGxonFzcKdi4oMPDeAF9%2BAHp0qABZMToxAFsIqCwwRAxRCTpeO2gAQQ4eEF48JJo6%2BO4k3iA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tieQEMAZolrRE%2BWNCbooiaNA7RI4MAF8QKoA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Inconsistencies are usually caused by ESLint using a TS program that is inconsistent with tsserver.

[sheremet-va]

Good to know! Thank you for looking into this ❤️

[JoshuaKGoldberg]

Nit: I don't know that we can conclude this is a bug in typescript-eslint 😅. We'd happily take an issue but:

For enhanceSpy: ReturnType<T> can be any. Procedure's return type is any, so the constraint of T isn't making the return anything more narrow than any.

[sheremet-va]

Ah, you might be right! The default Procedure has the return type as any 🤔

[johnsoncodehk]

@JoshuaKGoldberg Yes this is most likely my misunderstanding of the no-unsafe-return rule.