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1. Tom Sherman (tom-sherman)

   changed the title [-]Potential regression in 4.2 - Calling `.map` on `T[] | Y[]` is now callback but leads to an implicit any error[/-] [+]Potential regression in 4.2 - Calling `.map` on `T[] | Y[]` is now callable but leads to an implicit any error[/+] on Feb 4, 2021
2. weswigham commented on Feb 4, 2021

   

   Wesley Wigham (weswigham)

   on Feb 4, 2021

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   It's not a regression (definitionally, going from can't call at all to implicitly any is a less restrictive error - now you can explicitly annotate it and actually resolve the call), and we already have #42620 open to improve the experience further~
3. thorn0 commented on Feb 4, 2021

   

   Georgii Dolzhykov (thorn0)

   on Feb 4, 2021

   Last edited by thorn0

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   But why does it allow specifically map (and some)? every and filter are still errors.

   related issue: #35045
4. weswigham commented on Feb 4, 2021

   

   Wesley Wigham (weswigham)

   on Feb 4, 2021

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   every and filter have multiple overloads (with differing numbers of type parameters) which makes the union call difficult to resolve, while map only has one overload, so combining the signatures is much more straightforward.

   👍React with 👍1Reacted by Georgii Dolzhykov
5. thorn0 commented on Feb 4, 2021

   

   Georgii Dolzhykov (thorn0)

   on Feb 4, 2021

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   Is there any chance for every to work for unions of read-only array types in the foreseeable future?
6. weswigham commented on Feb 4, 2021

   

   Wesley Wigham (weswigham)

   on Feb 4, 2021

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   Not in the immediate future, no. The readonly-ness isn't what matters for our ability to check it - the overload count/genericness of the signature is. We've been slowly expanding how much of that we can check, but I can't make any claims as to how far we'll go or by when.
7. thorn0 commented on Feb 4, 2021

   

   Georgii Dolzhykov (thorn0)

   on Feb 4, 2021

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   I see, thanks. For some reason, I thought these methods are disallowed because of type safety. E.g., what can be pushed to an array if it's type is string[] | number[]? So it looked like readonly-ness might help. But now I see I misunderstood the problem.
8. thorn0 commented on Feb 4, 2021

   

   Georgii Dolzhykov (thorn0)

   on Feb 4, 2021

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   Anyway, wouldn't it be a helpful trick to implicitly transform readonly string[] | readonly number[] to readonly (string | number)[] for the purposes of signature merging?
9. tom-sherman commented on Feb 4, 2021

   

   Tom Sherman (tom-sherman)

   on Feb 4, 2021

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   [...] and we already have #42620 open to improve the experience further~

   Wesley Wigham (@weswigham) It's not clear to me how your PR relates to this issue, do you have a playground link I can poke around with?
10. thorn0 commented on Feb 4, 2021

    

    Georgii Dolzhykov (thorn0)

    on Feb 4, 2021

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    Tom Sherman (@tom-sherman) Previously, such calls were disallowed because merging signatures for generic overloaded functions is a tricky procedure, and it wasn't implemented. Now, in some cases, it became possible. After merging that PR, it'll be possible in more cases. Hopefully, we'll read more on that in the release blog post.
11. Ryan Cavanaugh (RyanCavanaugh)

    added

    Experience EnhancementNoncontroversial enhancementsNoncontroversial enhancements

    SuggestionAn idea for TypeScriptAn idea for TypeScript

    on Feb 5, 2021
12. Ryan Cavanaugh (RyanCavanaugh)

    added this to the Backlog milestone on Feb 5, 2021
13. thorn0 commented on Aug 26, 2022

    

    Georgii Dolzhykov (thorn0)

    on Aug 26, 2022

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    Now that it works for map, I wonder if this issue should be renamed with a mention of filter and every in the title, That'd make it more discoverable.
14. thorn0 commented on Aug 26, 2022

    

    Georgii Dolzhykov (thorn0)

    on Aug 26, 2022

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    An isolated real-world case to demo how this issue is a pain:
    playground

    import * as Babel from "@babel/types";
    import { TSESTree } from "@typescript-eslint/typescript-estree";
    type Node = Babel.Node | TSESTree.Node;
    
    declare const node: Node;
    
    if (
        (node.type === "GenericTypeAnnotation" || node.type === "TSTypeReference") &&
        (!node.typeParameters ||
            node.typeParameters.params.length <= 2 && node.typeParameters.params.every(n => true))
    ) { }

    every causes the following error:

    This expression is not callable.
      Each member of the union type '{ <S extends TSType>(predicate: (value: TSType, index: number, array: TSType[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: TSType, index: number, array: TSType[]) => unknown, thisArg?: any): boolean; <S extends TSType>(predicate: (value: TSType, index: number, array: TSType[]) => value is S, thi...' has signatures, but none of those signatures are compatible with each other.(2349)
    
15. jakebailey commented on Jun 20, 2023

    

    Jake Bailey (jakebailey)

    on Jun 20, 2023

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    Closing as fixed in #53489; reading the PR, it has this case as a test.
16. Jake Bailey (jakebailey)

    closed this as completedon Jun 20, 2023