#47341 breaks knockout.mapping tests on Definitely Typed #47371

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1. Nathan Shively-Sanders (sandersn)

   assigned

   Anders Hejlsberg (ahejlsberg)

   on Jan 10, 2022
2. Nathan Shively-Sanders (sandersn)

   added this to the TypeScript 4.6.0 milestone on Jan 10, 2022
3. ahejlsberg commented on Jan 10, 2022

   

   Anders Hejlsberg (ahejlsberg)

   on Jan 10, 2022

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   Hmm, the repro also fails with TS 4.5.4. Am I missing something?
4. ahejlsberg commented on Jan 10, 2022

   

   Anders Hejlsberg (ahejlsberg)

   on Jan 10, 2022

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   Actually, here's a repro that only fails with #47341.

   interface Kow<T> {
       (): void;
       (value: T): void;
   }
   declare var src: Kow<true> | Kow<false>;
   var tgt: Kow<boolean> = src;

   But really, that ought to fail, it just happened not to previously because we cross-related the signatures. I mean, src might reference a Kow<true> which shouldn't be callable with a boolean.
5. ahejlsberg commented on Jan 10, 2022

   

   Anders Hejlsberg (ahejlsberg)

   on Jan 10, 2022

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   And, just to add to the bizarreness of our current behavior, we error if the type is written as an aliased type instead of an interface:

   type Kow<T> = {
       (): void;
       (value: T): void;
   }

   The two obviously should behave the same.
6. sandersn commented on Jan 10, 2022

   

   Nathan Shively-Sanders (sandersn)

   on Jan 10, 2022

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   So it sounds like main is in a better state than 4.5.4, right? If that's the case then this issue should be closed.
7. ahejlsberg commented on Jan 11, 2022

   

   Anders Hejlsberg (ahejlsberg)

   on Jan 11, 2022

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   Yes, I'd say main has better and more consistent behavior.
8. Anders Hejlsberg (ahejlsberg)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Jan 11, 2022
9. Nathan Shively-Sanders (sandersn)

   closed this as completedon Jan 11, 2022
10. microsoft

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