type guard narrowing to a type with type with `any` in type parameters replaces `any` with `unknown`

[eps1lon]

🔎 Search Terms

type guard parameter any unknown

🕗 Version & Regression Information

• This changed between versions 4.9.6 and 5.0.4

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.5.0-dev.20240321#code/JYOwLgpgTgZghgYwgAgEIHsAeAeAKgPmQG8AoZc5ANzgBsBXCALmVxIF8SSY6QExh0IZMADOGHAQAUAIyzMeAaxDoA7iACUzWZmEi0WPIVIVkUCGDpQhYKA3acEgkWGQg6AW2nRxzcdkXKaoQAvMTU9EwAjBwkwDDIkqJ+bp7Q+JIpXlDi6urEZBSZ3lgAdOF2JgD0lSYmAHoA-PZxCUkGcCAAnulF2Vi5+Sa94mW0FRTVtRSN7EA

💻 Code

interface Box<T> {
    value: T
}

function isBox<T>(box: unknown): box is Box<T> {
    return true
}

const numberBox: Box<unknown> = {value:1}

if (isBox<number>(numberBox)) {
    numberBox.value
    //        ^?
}
if (isBox<any>(numberBox)) {
    numberBox.value
    //        ^?
}

🙁 Actual behavior

if (isBox<any>(numberBox)) {
    numberBox.value
    //        ^? (property) Box<unknown>.value: unknown
}

🙂 Expected behavior

if (isBox<any>(numberBox)) {
    numberBox.value
    //        ^? (property) Box<any>.value: any
}

Additional information about the issue

Maybe caused by #52282 which also caused #53178

[Andarist]

Likely this is working as intended. There is a subtype relationship between Box<unknown> is a strict subtype of Box<any> and thus it's preferred when narrowing here. If the predicate's type would be preferred here then you would end up with Box<any> here and that's not desirable:

interface Box<T> {
  value: T;
}

function isBox(box: unknown): box is Box<any> {
  return true;
}

declare class Dog {
  bark(): void;
}

declare const smth: Box<number> | Dog;

if (isBox(smth)) {
  smth.value;
  //   ^? Box<number>
}

[eps1lon]

If the predicate's type would be preferred here then you would end up with Box here and that's not desirable:

If the type guard is typed as box is Box<any> then the any is clearly desired.

It's fine if TypeScript tries to guess missing intent. But here we authored it as "narrow this to any" so TypeScript shouldn't override it.

[Andarist]

Narrowing is more like a filtering operation and not like a cast/assignment. Otherwise, you likely wouldn't be able to express "check if it's a Box and if it's a Box keep the type intact". We can check how this doesn't infer the type argument:

interface Box<T> {
  value: T;
}

function isBox<T>(box: unknown): box is Box<T> {
  return true;
}

declare class Dog {
  bark(): void;
}

declare const smth: Box<number> | Dog;

if (isBox(smth)) {
  // ^? function isBox<unknown>(box: unknown): box is Box<unknown>
  smth.value;
  //   ^? Box<number>
}

[RyanCavanaugh]

If the type guard is typed as box is Box<any> then the any is clearly desired.

It really isn't. People in general hate any appearing in their code unless it's via some extremely direct incantation.

This has been the behavior since at least 3.3 and I don't think this is a) surprising, since we haven't gotten other reports on it or b) a welcome change to all the people who wrote declarations of the form is F<any> when they maybe should have written is F<unknown> but will now see an infectious any they didn't want.

[RyanCavanaugh]

I missed the regression part since I think I was working off the other example. I'll bisect to ensure we're on the same page here.

[RyanCavanaugh]

Yeah, the 4.9.5 behavior is just inconsistent for no obvious reason:

interface Box<T> {
    value: T
}

function isBox(box: unknown): box is Box<any> {
    return true
}

declare const box1: string | Box<unknown>;
if (isBox(box1)) {
    box1.value
    //     ^?
    //     any
}

declare const box2: string | Box<{} | null | undefined>;
if (isBox(box2)) {
    box2.value 
    //     ^?
    //     {} | null | undefined
}

declare const box3: string | Box<string>;
if (isBox(box3)) {
    box3.value
    //     ^?
    //     string
}

It doesn't make sense to narrow from unknown to any but not any of the other types.

[fatcerberus]

note that IIRC Array.isArray(x) is typed as x is Array<any> - and obviously you don't want your values typed as e.g. number[] | number to be "narrowed" to any[]

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.