type guard narrowing to a type with type with any in type parameters replaces any with unknown #57892

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1. Andarist commented on Mar 21, 2024

   

   Mateusz Burzyński (Andarist)

   on Mar 21, 2024

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   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>
   }

   👍React with 👍1Reacted by Ryan Cavanaugh
2. eps1lon commented on Mar 21, 2024

   

   Sebastian "Sebbie" Silbermann (eps1lon)

   on Mar 21, 2024

   Last edited by eps1lon

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   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.
3. Andarist commented on Mar 21, 2024

   

   Mateusz Burzyński (Andarist)

   on Mar 21, 2024

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   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>
   }
4. RyanCavanaugh commented on Mar 21, 2024

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Mar 21, 2024

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   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.
5. Ryan Cavanaugh (RyanCavanaugh)

   added

   Not a DefectThis behavior is one of several equally-correct optionsThis behavior is one of several equally-correct options

   on Mar 21, 2024
6. RyanCavanaugh commented on Mar 21, 2024

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Mar 21, 2024

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   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.
7. RyanCavanaugh commented on Mar 21, 2024

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Mar 21, 2024

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   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.
8. fatcerberus commented on Mar 22, 2024

   

   Bruce Pascoe (fatcerberus)

   on Mar 22, 2024

   Last edited by fatcerberus

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   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[]
9. typescript-bot commented on Mar 25, 2024

   

   TypeScript Bot (typescript-bot)

   on Mar 25, 2024

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   This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.
10. TypeScript Bot (typescript-bot)

    closed this as not plannedon Mar 25, 2024
11. microsoft

    locked as resolved and limited conversation to collaborators on Oct 22, 2025