Empty types and types with `never` as a key behave differently

[kazatsuyu]

🔎 Search Terms

Mapped type
unknown

🕗 Version & Regression Information

• This changed between versions 4.7.4 and 4.8.4

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.4.0-dev.20231206#code/CYUwxgNghgTiAEYD2A7AzgF3gVwFw5QGsUkB3FAbgCgqMBPABwQEF4BeeAbwG0BpeAJYp4KEADcQMALr4oKOgF8qAemXx1APQD8NVfABySeAFEYMJDCrJ0WKPhTYIEeAB8CoAGZCQwV1wXsONR66vDaumqm5pbWmPAARvaOzm7YKJ7evm6sHNjBaqHhQA

💻 Code

declare const u: unknown;

type A = {[K in never]: any}
//   ^?

// No Error
const a: null | undefined | {} = u;
//    ^?

// Error
const b: null | undefined | A = u;
//    ^?

🙁 Actual behavior

The types a and b appear to be the same, but only the assignment to b causes an error.

🙂 Expected behavior

I'm not sure which is better, I think it's either no error or both errors.

Additional information about the issue

Both were errors in version 4.7.4

[MartinJohns]

What is that mapped type supposed to do?

Regardless, this seems intentional: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-8.html#improved-intersection-reduction-union-compatibility-and-narrowing

TypeScript now recognizes this, and allows assignments from unknown to {} | null | undefined.

[kazatsuyu]

If the possible assignment of unknown to null | undefeind | {} is intentional, then the behavior of a is correct, but b should not be an error.

[jcalz]

I suggest simplifying the example to something like type A = {[K in never]: any} which reproduces the behavior without the added distraction of conditional types.

Relevant: #49119 and #49119 (comment)

[kazatsuyu]

Well, it certainly looks like the problem is not that I mapped {}, but that I created a type with never key.

[RyanCavanaugh]

I suppose the steelmanned version of the issue looks like this

export  {}
declare const u: unknown;
type Point = { x: number, y: number };

declare function foo<T, K extends keyof T>(obj: T, keys: K[], rest: Omit<T, K> | null | undefined): void;
const p: Point = { x: 0, y: 0 };
// Error, but should OK
foo(p, ["x", "y"], u);

[RyanCavanaugh]

I'm not committing to take a fix for this since it's pretty niche, but if it's straightforward and doesn't break/slow down something else, we can take a PR